//! The lowest level parsing API in this package;
//! supports streaming input with a low memory footprint.
//! The memory requirement is `O(d)` where d is the nesting depth of `[]` or `{}` containers in the input.
//! Specifically `d/8` bytes are required for this purpose,
//! with some extra buffer according to the implementation of `std.ArrayList`.
//!
//! This scanner can emit partial tokens; see `std.json.Token`.
//! The input to this class is a sequence of input buffers that you must supply one at a time.
//! Call `feedInput()` with the first buffer, then call `next()` repeatedly until `error.BufferUnderrun` is returned.
//! Then call `feedInput()` again and so forth.
//! Call `endInput()` when the last input buffer has been given to `feedInput()`, either immediately after calling `feedInput()`,
//! or when `error.BufferUnderrun` requests more data and there is no more.
//! Be sure to call `next()` after calling `endInput()` until `Token.end_of_document` has been returned.
//!
//! Notes on standards compliance: https://datatracker.ietf.org/doc/html/rfc8259
//! * RFC 8259 requires JSON documents be valid UTF-8,
//!   but makes an allowance for systems that are "part of a closed ecosystem".
//!   I have no idea what that's supposed to mean in the context of a standard specification.
//!   This implementation requires inputs to be valid UTF-8.
//! * RFC 8259 contradicts itself regarding whether lowercase is allowed in \u hex digits,
//!   but this is probably a bug in the spec, and it's clear that lowercase is meant to be allowed.
//!   (RFC 5234 defines HEXDIG to only allow uppercase.)
//! * When RFC 8259 refers to a "character", I assume they really mean a "Unicode scalar value".
//!   See http://www.unicode.org/glossary/#unicode_scalar_value .
//! * RFC 8259 doesn't explicitly disallow unpaired surrogate halves in \u escape sequences,
//!   but vaguely implies that \u escapes are for encoding Unicode "characters" (i.e. Unicode scalar values?),
//!   which would mean that unpaired surrogate halves are forbidden.
//!   By contrast ECMA-404 (a competing(/compatible?) JSON standard, which JavaScript's JSON.parse() conforms to)
//!   explicitly allows unpaired surrogate halves.
//!   This implementation forbids unpaired surrogate halves in \u sequences.
//!   If a high surrogate half appears in a \u sequence,
//!   then a low surrogate half must immediately follow in \u notation.
//! * RFC 8259 allows implementations to "accept non-JSON forms or extensions".
//!   This implementation does not accept any of that.
//! * RFC 8259 allows implementations to put limits on "the size of texts",
//!   "the maximum depth of nesting", "the range and precision of numbers",
//!   and "the length and character contents of strings".
//!   This low-level implementation does not limit these,
//!   except where noted above, and except that nesting depth requires memory allocation.
//!   Note that this low-level API does not interpret numbers numerically,
//!   but simply emits their source form for some higher level code to make sense of.
//! * This low-level implementation allows duplicate object keys,
//!   and key/value pairs are emitted in the order they appear in the input.

const Scanner = @This();
const std = @import("std");

const Allocator = std.mem.Allocator;
const assert = std.debug.assert;
const BitStack = std.BitStack;

state: State = .value,
string_is_object_key: bool = false,
stack: BitStack,
value_start: usize = undefined,
utf16_code_units: [2]u16 = undefined,

input: []const u8 = "",
cursor: usize = 0,
is_end_of_input: bool = false,
diagnostics: ?*Diagnostics = null,

/// The allocator is only used to track `[]` and `{}` nesting levels.
pub fn initStreaming(allocator: Allocator) @This() {
    return .{
        .stack = BitStack.init(allocator),
    };
}
/// Use this if your input is a single slice.
/// This is effectively equivalent to:
/// ```
/// initStreaming(allocator);
/// feedInput(complete_input);
/// endInput();
/// ```
pub fn initCompleteInput(allocator: Allocator, complete_input: []const u8) @This() {
    return .{
        .stack = BitStack.init(allocator),
        .input = complete_input,
        .is_end_of_input = true,
    };
}
pub fn deinit(self: *@This()) void {
    self.stack.deinit();
    self.* = undefined;
}

pub fn enableDiagnostics(self: *@This(), diagnostics: *Diagnostics) void {
    diagnostics.cursor_pointer = &self.cursor;
    self.diagnostics = diagnostics;
}

/// Call this whenever you get `error.BufferUnderrun` from `next()`.
/// When there is no more input to provide, call `endInput()`.
pub fn feedInput(self: *@This(), input: []const u8) void {
    assert(self.cursor == self.input.len); // Not done with the last input slice.
    if (self.diagnostics) |diag| {
        diag.total_bytes_before_current_input += self.input.len;
        // This usually goes "negative" to measure how far before the beginning
        // of the new buffer the current line started.
        diag.line_start_cursor -%= self.cursor;
    }
    self.input = input;
    self.cursor = 0;
    self.value_start = 0;
}
/// Call this when you will no longer call `feedInput()` anymore.
/// This can be called either immediately after the last `feedInput()`,
/// or at any time afterward, such as when getting `error.BufferUnderrun` from `next()`.
/// Don't forget to call `next*()` after `endInput()` until you get `.end_of_document`.
pub fn endInput(self: *@This()) void {
    self.is_end_of_input = true;
}

pub const NextError = Error || Allocator.Error || error{BufferUnderrun};
pub const AllocError = Error || Allocator.Error || error{ValueTooLong};
pub const PeekError = Error || error{BufferUnderrun};
pub const SkipError = Error || Allocator.Error;
pub const AllocIntoArrayListError = AllocError || error{BufferUnderrun};

/// Equivalent to `nextAllocMax(allocator, when, default_max_value_len);`
/// This function is only available after `endInput()` (or `initCompleteInput()`) has been called.
/// See also `std.json.Token` for documentation of `nextAlloc*()` function behavior.
pub fn nextAlloc(self: *@This(), allocator: Allocator, when: AllocWhen) AllocError!Token {
    return self.nextAllocMax(allocator, when, default_max_value_len);
}

/// This function is only available after `endInput()` (or `initCompleteInput()`) has been called.
/// See also `std.json.Token` for documentation of `nextAlloc*()` function behavior.
pub fn nextAllocMax(self: *@This(), allocator: Allocator, when: AllocWhen, max_value_len: usize) AllocError!Token {
    assert(self.is_end_of_input); // This function is not available in streaming mode.
    const token_type = self.peekNextTokenType() catch |e| switch (e) {
        error.BufferUnderrun => unreachable,
        else => |err| return err,
    };
    switch (token_type) {
        .number, .string => {
            var value_list = std.array_list.Managed(u8).init(allocator);
            errdefer {
                value_list.deinit();
            }
            if (self.allocNextIntoArrayListMax(&value_list, when, max_value_len) catch |e| switch (e) {
                error.BufferUnderrun => unreachable,
                else => |err| return err,
            }) |slice| {
                return if (token_type == .number)
                    Token{ .number = slice }
                else
                    Token{ .string = slice };
            } else {
                return if (token_type == .number)
                    Token{ .allocated_number = try value_list.toOwnedSlice() }
                else
                    Token{ .allocated_string = try value_list.toOwnedSlice() };
            }
        },

        // Simple tokens never alloc.
        .object_begin,
        .object_end,
        .array_begin,
        .array_end,
        .true,
        .false,
        .null,
        .end_of_document,
        => return self.next() catch |e| switch (e) {
            error.BufferUnderrun => unreachable,
            else => |err| return err,
        },
    }
}

/// Equivalent to `allocNextIntoArrayListMax(value_list, when, default_max_value_len);`
pub fn allocNextIntoArrayList(self: *@This(), value_list: *std.array_list.Managed(u8), when: AllocWhen) AllocIntoArrayListError!?[]const u8 {
    return self.allocNextIntoArrayListMax(value_list, when, default_max_value_len);
}
/// The next token type must be either `.number` or `.string`. See `peekNextTokenType()`.
/// When allocation is not necessary with `.alloc_if_needed`,
/// this method returns the content slice from the input buffer, and `value_list` is not touched.
/// When allocation is necessary or with `.alloc_always`, this method concatenates partial tokens into the given `value_list`,
/// and returns `null` once the final `.number` or `.string` token has been written into it.
/// In case of an `error.BufferUnderrun`, partial values will be left in the given value_list.
/// The given `value_list` is never reset by this method, so an `error.BufferUnderrun` situation
/// can be resumed by passing the same array list in again.
/// This method does not indicate whether the token content being returned is for a `.number` or `.string` token type;
/// the caller of this method is expected to know which type of token is being processed.
pub fn allocNextIntoArrayListMax(self: *@This(), value_list: *std.array_list.Managed(u8), when: AllocWhen, max_value_len: usize) AllocIntoArrayListError!?[]const u8 {
    while (true) {
        const token = try self.next();
        switch (token) {
            // Accumulate partial values.
            .partial_number, .partial_string => |slice| {
                try appendSlice(value_list, slice, max_value_len);
            },
            .partial_string_escaped_1 => |buf| {
                try appendSlice(value_list, buf[0..], max_value_len);
            },
            .partial_string_escaped_2 => |buf| {
                try appendSlice(value_list, buf[0..], max_value_len);
            },
            .partial_string_escaped_3 => |buf| {
                try appendSlice(value_list, buf[0..], max_value_len);
            },
            .partial_string_escaped_4 => |buf| {
                try appendSlice(value_list, buf[0..], max_value_len);
            },

            // Return complete values.
            .number => |slice| {
                if (when == .alloc_if_needed and value_list.items.len == 0) {
                    // No alloc necessary.
                    return slice;
                }
                try appendSlice(value_list, slice, max_value_len);
                // The token is complete.
                return null;
            },
            .string => |slice| {
                if (when == .alloc_if_needed and value_list.items.len == 0) {
                    // No alloc necessary.
                    return slice;
                }
                try appendSlice(value_list, slice, max_value_len);
                // The token is complete.
                return null;
            },

            .object_begin,
            .object_end,
            .array_begin,
            .array_end,
            .true,
            .false,
            .null,
            .end_of_document,
            => unreachable, // Only .number and .string token types are allowed here. Check peekNextTokenType() before calling this.

            .allocated_number, .allocated_string => unreachable,
        }
    }
}

/// This function is only available after `endInput()` (or `initCompleteInput()`) has been called.
/// If the next token type is `.object_begin` or `.array_begin`,
/// this function calls `next()` repeatedly until the corresponding `.object_end` or `.array_end` is found.
/// If the next token type is `.number` or `.string`,
/// this function calls `next()` repeatedly until the (non `.partial_*`) `.number` or `.string` token is found.
/// If the next token type is `.true`, `.false`, or `.null`, this function calls `next()` once.
/// The next token type must not be `.object_end`, `.array_end`, or `.end_of_document`;
/// see `peekNextTokenType()`.
pub fn skipValue(self: *@This()) SkipError!void {
    assert(self.is_end_of_input); // This function is not available in streaming mode.
    switch (self.peekNextTokenType() catch |e| switch (e) {
        error.BufferUnderrun => unreachable,
        else => |err| return err,
    }) {
        .object_begin, .array_begin => {
            self.skipUntilStackHeight(self.stackHeight()) catch |e| switch (e) {
                error.BufferUnderrun => unreachable,
                else => |err| return err,
            };
        },
        .number, .string => {
            while (true) {
                switch (self.next() catch |e| switch (e) {
                    error.BufferUnderrun => unreachable,
                    else => |err| return err,
                }) {
                    .partial_number,
                    .partial_string,
                    .partial_string_escaped_1,
                    .partial_string_escaped_2,
                    .partial_string_escaped_3,
                    .partial_string_escaped_4,
                    => continue,

                    .number, .string => break,

                    else => unreachable,
                }
            }
        },
        .true, .false, .null => {
            _ = self.next() catch |e| switch (e) {
                error.BufferUnderrun => unreachable,
                else => |err| return err,
            };
        },

        .object_end, .array_end, .end_of_document => unreachable, // Attempt to skip a non-value token.
    }
}

/// Skip tokens until an `.object_end` or `.array_end` token results in a `stackHeight()` equal the given stack height.
/// Unlike `skipValue()`, this function is available in streaming mode.
pub fn skipUntilStackHeight(self: *@This(), terminal_stack_height: usize) NextError!void {
    while (true) {
        switch (try self.next()) {
            .object_end, .array_end => {
                if (self.stackHeight() == terminal_stack_height) break;
            },
            .end_of_document => unreachable,
            else => continue,
        }
    }
}

/// The depth of `{}` or `[]` nesting levels at the current position.
pub fn stackHeight(self: *const @This()) usize {
    return self.stack.bit_len;
}

/// Pre allocate memory to hold the given number of nesting levels.
/// `stackHeight()` up to the given number will not cause allocations.
pub fn ensureTotalStackCapacity(self: *@This(), height: usize) Allocator.Error!void {
    try self.stack.ensureTotalCapacity(height);
}

/// See `std.json.Token` for documentation of this function.
pub fn next(self: *@This()) NextError!Token {
    state_loop: while (true) {
        switch (self.state) {
            .value => {
                switch (try self.skipWhitespaceExpectByte()) {
                    // Object, Array
                    '{' => {
                        try self.stack.push(OBJECT_MODE);
                        self.cursor += 1;
                        self.state = .object_start;
                        return .object_begin;
                    },
                    '[' => {
                        try self.stack.push(ARRAY_MODE);
                        self.cursor += 1;
                        self.state = .array_start;
                        return .array_begin;
                    },

                    // String
                    '"' => {
                        self.cursor += 1;
                        self.value_start = self.cursor;
                        self.state = .string;
                        continue :state_loop;
                    },

                    // Number
                    '1'...'9' => {
                        self.value_start = self.cursor;
                        self.cursor += 1;
                        self.state = .number_int;
                        continue :state_loop;
                    },
                    '0' => {
                        self.value_start = self.cursor;
                        self.cursor += 1;
                        self.state = .number_leading_zero;
                        continue :state_loop;
                    },
                    '-' => {
                        self.value_start = self.cursor;
                        self.cursor += 1;
                        self.state = .number_minus;
                        continue :state_loop;
                    },

                    // literal values
                    't' => {
                        self.cursor += 1;
                        self.state = .literal_t;
                        continue :state_loop;
                    },
                    'f' => {
                        self.cursor += 1;
                        self.state = .literal_f;
                        continue :state_loop;
                    },
                    'n' => {
                        self.cursor += 1;
                        self.state = .literal_n;
                        continue :state_loop;
                    },

                    else => return error.SyntaxError,
                }
            },

            .post_value => {
                if (try self.skipWhitespaceCheckEnd()) return .end_of_document;

                const c = self.input[self.cursor];
                if (self.string_is_object_key) {
                    self.string_is_object_key = false;
                    switch (c) {
                        ':' => {
                            self.cursor += 1;
                            self.state = .value;
                            continue :state_loop;
                        },
                        else => return error.SyntaxError,
                    }
                }

                switch (c) {
                    '}' => {
                        if (self.stack.pop() != OBJECT_MODE) return error.SyntaxError;
                        self.cursor += 1;
                        // stay in .post_value state.
                        return .object_end;
                    },
                    ']' => {
                        if (self.stack.pop() != ARRAY_MODE) return error.SyntaxError;
                        self.cursor += 1;
                        // stay in .post_value state.
                        return .array_end;
                    },
                    ',' => {
                        switch (self.stack.peek()) {
                            OBJECT_MODE => {
                                self.state = .object_post_comma;
                            },
                            ARRAY_MODE => {
                                self.state = .value;
                            },
                        }
                        self.cursor += 1;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },

            .object_start => {
                switch (try self.skipWhitespaceExpectByte()) {
                    '"' => {
                        self.cursor += 1;
                        self.value_start = self.cursor;
                        self.state = .string;
                        self.string_is_object_key = true;
                        continue :state_loop;
                    },
                    '}' => {
                        self.cursor += 1;
                        _ = self.stack.pop();
                        self.state = .post_value;
                        return .object_end;
                    },
                    else => return error.SyntaxError,
                }
            },
            .object_post_comma => {
                switch (try self.skipWhitespaceExpectByte()) {
                    '"' => {
                        self.cursor += 1;
                        self.value_start = self.cursor;
                        self.state = .string;
                        self.string_is_object_key = true;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },

            .array_start => {
                switch (try self.skipWhitespaceExpectByte()) {
                    ']' => {
                        self.cursor += 1;
                        _ = self.stack.pop();
                        self.state = .post_value;
                        return .array_end;
                    },
                    else => {
                        self.state = .value;
                        continue :state_loop;
                    },
                }
            },

            .number_minus => {
                if (self.cursor >= self.input.len) return self.endOfBufferInNumber(false);
                switch (self.input[self.cursor]) {
                    '0' => {
                        self.cursor += 1;
                        self.state = .number_leading_zero;
                        continue :state_loop;
                    },
                    '1'...'9' => {
                        self.cursor += 1;
                        self.state = .number_int;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },
            .number_leading_zero => {
                if (self.cursor >= self.input.len) return self.endOfBufferInNumber(true);
                switch (self.input[self.cursor]) {
                    '.' => {
                        self.cursor += 1;
                        self.state = .number_post_dot;
                        continue :state_loop;
                    },
                    'e', 'E' => {
                        self.cursor += 1;
                        self.state = .number_post_e;
                        continue :state_loop;
                    },
                    else => {
                        self.state = .post_value;
                        return Token{ .number = self.takeValueSlice() };
                    },
                }
            },
            .number_int => {
                while (self.cursor < self.input.len) : (self.cursor += 1) {
                    switch (self.input[self.cursor]) {
                        '0'...'9' => continue,
                        '.' => {
                            self.cursor += 1;
                            self.state = .number_post_dot;
                            continue :state_loop;
                        },
                        'e', 'E' => {
                            self.cursor += 1;
                            self.state = .number_post_e;
                            continue :state_loop;
                        },
                        else => {
                            self.state = .post_value;
                            return Token{ .number = self.takeValueSlice() };
                        },
                    }
                }
                return self.endOfBufferInNumber(true);
            },
            .number_post_dot => {
                if (self.cursor >= self.input.len) return self.endOfBufferInNumber(false);
                switch (self.input[self.cursor]) {
                    '0'...'9' => {
                        self.cursor += 1;
                        self.state = .number_frac;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },
            .number_frac => {
                while (self.cursor < self.input.len) : (self.cursor += 1) {
                    switch (self.input[self.cursor]) {
                        '0'...'9' => continue,
                        'e', 'E' => {
                            self.cursor += 1;
                            self.state = .number_post_e;
                            continue :state_loop;
                        },
                        else => {
                            self.state = .post_value;
                            return Token{ .number = self.takeValueSlice() };
                        },
                    }
                }
                return self.endOfBufferInNumber(true);
            },
            .number_post_e => {
                if (self.cursor >= self.input.len) return self.endOfBufferInNumber(false);
                switch (self.input[self.cursor]) {
                    '0'...'9' => {
                        self.cursor += 1;
                        self.state = .number_exp;
                        continue :state_loop;
                    },
                    '+', '-' => {
                        self.cursor += 1;
                        self.state = .number_post_e_sign;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },
            .number_post_e_sign => {
                if (self.cursor >= self.input.len) return self.endOfBufferInNumber(false);
                switch (self.input[self.cursor]) {
                    '0'...'9' => {
                        self.cursor += 1;
                        self.state = .number_exp;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },
            .number_exp => {
                while (self.cursor < self.input.len) : (self.cursor += 1) {
                    switch (self.input[self.cursor]) {
                        '0'...'9' => continue,
                        else => {
                            self.state = .post_value;
                            return Token{ .number = self.takeValueSlice() };
                        },
                    }
                }
                return self.endOfBufferInNumber(true);
            },

            .string => {
                while (self.cursor < self.input.len) : (self.cursor += 1) {
                    switch (self.input[self.cursor]) {
                        0...0x1f => return error.SyntaxError, // Bare ASCII control code in string.

                        // ASCII plain text.
                        0x20...('"' - 1), ('"' + 1)...('\\' - 1), ('\\' + 1)...0x7F => continue,

                        // Special characters.
                        '"' => {
                            const result = Token{ .string = self.takeValueSlice() };
                            self.cursor += 1;
                            self.state = .post_value;
                            return result;
                        },
                        '\\' => {
                            const slice = self.takeValueSlice();
                            self.cursor += 1;
                            self.state = .string_backslash;
                            if (slice.len > 0) return Token{ .partial_string = slice };
                            continue :state_loop;
                        },

                        // UTF-8 validation.
                        // See http://unicode.org/mail-arch/unicode-ml/y2003-m02/att-0467/01-The_Algorithm_to_Valide_an_UTF-8_String
                        0xC2...0xDF => {
                            self.cursor += 1;
                            self.state = .string_utf8_last_byte;
                            continue :state_loop;
                        },
                        0xE0 => {
                            self.cursor += 1;
                            self.state = .string_utf8_second_to_last_byte_guard_against_overlong;
                            continue :state_loop;
                        },
                        0xE1...0xEC, 0xEE...0xEF => {
                            self.cursor += 1;
                            self.state = .string_utf8_second_to_last_byte;
                            continue :state_loop;
                        },
                        0xED => {
                            self.cursor += 1;
                            self.state = .string_utf8_second_to_last_byte_guard_against_surrogate_half;
                            continue :state_loop;
                        },
                        0xF0 => {
                            self.cursor += 1;
                            self.state = .string_utf8_third_to_last_byte_guard_against_overlong;
                            continue :state_loop;
                        },
                        0xF1...0xF3 => {
                            self.cursor += 1;
                            self.state = .string_utf8_third_to_last_byte;
                            continue :state_loop;
                        },
                        0xF4 => {
                            self.cursor += 1;
                            self.state = .string_utf8_third_to_last_byte_guard_against_too_large;
                            continue :state_loop;
                        },
                        0x80...0xC1, 0xF5...0xFF => return error.SyntaxError, // Invalid UTF-8.
                    }
                }
                if (self.is_end_of_input) return error.UnexpectedEndOfInput;
                const slice = self.takeValueSlice();
                if (slice.len > 0) return Token{ .partial_string = slice };
                return error.BufferUnderrun;
            },
            .string_backslash => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                switch (self.input[self.cursor]) {
                    '"', '\\', '/' => {
                        // Since these characters now represent themselves literally,
                        // we can simply begin the next plaintext slice here.
                        self.value_start = self.cursor;
                        self.cursor += 1;
                        self.state = .string;
                        continue :state_loop;
                    },
                    'b' => {
                        self.cursor += 1;
                        self.value_start = self.cursor;
                        self.state = .string;
                        return Token{ .partial_string_escaped_1 = [_]u8{0x08} };
                    },
                    'f' => {
                        self.cursor += 1;
                        self.value_start = self.cursor;
                        self.state = .string;
                        return Token{ .partial_string_escaped_1 = [_]u8{0x0c} };
                    },
                    'n' => {
                        self.cursor += 1;
                        self.value_start = self.cursor;
                        self.state = .string;
                        return Token{ .partial_string_escaped_1 = [_]u8{'\n'} };
                    },
                    'r' => {
                        self.cursor += 1;
                        self.value_start = self.cursor;
                        self.state = .string;
                        return Token{ .partial_string_escaped_1 = [_]u8{'\r'} };
                    },
                    't' => {
                        self.cursor += 1;
                        self.value_start = self.cursor;
                        self.state = .string;
                        return Token{ .partial_string_escaped_1 = [_]u8{'\t'} };
                    },
                    'u' => {
                        self.cursor += 1;
                        self.state = .string_backslash_u;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },
            .string_backslash_u => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                const c = self.input[self.cursor];
                switch (c) {
                    '0'...'9' => {
                        self.utf16_code_units[0] = @as(u16, c - '0') << 12;
                    },
                    'A'...'F' => {
                        self.utf16_code_units[0] = @as(u16, c - 'A' + 10) << 12;
                    },
                    'a'...'f' => {
                        self.utf16_code_units[0] = @as(u16, c - 'a' + 10) << 12;
                    },
                    else => return error.SyntaxError,
                }
                self.cursor += 1;
                self.state = .string_backslash_u_1;
                continue :state_loop;
            },
            .string_backslash_u_1 => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                const c = self.input[self.cursor];
                switch (c) {
                    '0'...'9' => {
                        self.utf16_code_units[0] |= @as(u16, c - '0') << 8;
                    },
                    'A'...'F' => {
                        self.utf16_code_units[0] |= @as(u16, c - 'A' + 10) << 8;
                    },
                    'a'...'f' => {
                        self.utf16_code_units[0] |= @as(u16, c - 'a' + 10) << 8;
                    },
                    else => return error.SyntaxError,
                }
                self.cursor += 1;
                self.state = .string_backslash_u_2;
                continue :state_loop;
            },
            .string_backslash_u_2 => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                const c = self.input[self.cursor];
                switch (c) {
                    '0'...'9' => {
                        self.utf16_code_units[0] |= @as(u16, c - '0') << 4;
                    },
                    'A'...'F' => {
                        self.utf16_code_units[0] |= @as(u16, c - 'A' + 10) << 4;
                    },
                    'a'...'f' => {
                        self.utf16_code_units[0] |= @as(u16, c - 'a' + 10) << 4;
                    },
                    else => return error.SyntaxError,
                }
                self.cursor += 1;
                self.state = .string_backslash_u_3;
                continue :state_loop;
            },
            .string_backslash_u_3 => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                const c = self.input[self.cursor];
                switch (c) {
                    '0'...'9' => {
                        self.utf16_code_units[0] |= c - '0';
                    },
                    'A'...'F' => {
                        self.utf16_code_units[0] |= c - 'A' + 10;
                    },
                    'a'...'f' => {
                        self.utf16_code_units[0] |= c - 'a' + 10;
                    },
                    else => return error.SyntaxError,
                }
                self.cursor += 1;
                if (std.unicode.utf16IsHighSurrogate(self.utf16_code_units[0])) {
                    self.state = .string_surrogate_half;
                    continue :state_loop;
                } else if (std.unicode.utf16IsLowSurrogate(self.utf16_code_units[0])) {
                    return error.SyntaxError; // Unexpected low surrogate half.
                } else {
                    self.value_start = self.cursor;
                    self.state = .string;
                    return partialStringCodepoint(self.utf16_code_units[0]);
                }
            },
            .string_surrogate_half => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                switch (self.input[self.cursor]) {
                    '\\' => {
                        self.cursor += 1;
                        self.state = .string_surrogate_half_backslash;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError, // Expected low surrogate half.
                }
            },
            .string_surrogate_half_backslash => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                switch (self.input[self.cursor]) {
                    'u' => {
                        self.cursor += 1;
                        self.state = .string_surrogate_half_backslash_u;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError, // Expected low surrogate half.
                }
            },
            .string_surrogate_half_backslash_u => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                switch (self.input[self.cursor]) {
                    'D', 'd' => {
                        self.cursor += 1;
                        self.utf16_code_units[1] = 0xD << 12;
                        self.state = .string_surrogate_half_backslash_u_1;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError, // Expected low surrogate half.
                }
            },
            .string_surrogate_half_backslash_u_1 => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                const c = self.input[self.cursor];
                switch (c) {
                    'C'...'F' => {
                        self.cursor += 1;
                        self.utf16_code_units[1] |= @as(u16, c - 'A' + 10) << 8;
                        self.state = .string_surrogate_half_backslash_u_2;
                        continue :state_loop;
                    },
                    'c'...'f' => {
                        self.cursor += 1;
                        self.utf16_code_units[1] |= @as(u16, c - 'a' + 10) << 8;
                        self.state = .string_surrogate_half_backslash_u_2;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError, // Expected low surrogate half.
                }
            },
            .string_surrogate_half_backslash_u_2 => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                const c = self.input[self.cursor];
                switch (c) {
                    '0'...'9' => {
                        self.cursor += 1;
                        self.utf16_code_units[1] |= @as(u16, c - '0') << 4;
                        self.state = .string_surrogate_half_backslash_u_3;
                        continue :state_loop;
                    },
                    'A'...'F' => {
                        self.cursor += 1;
                        self.utf16_code_units[1] |= @as(u16, c - 'A' + 10) << 4;
                        self.state = .string_surrogate_half_backslash_u_3;
                        continue :state_loop;
                    },
                    'a'...'f' => {
                        self.cursor += 1;
                        self.utf16_code_units[1] |= @as(u16, c - 'a' + 10) << 4;
                        self.state = .string_surrogate_half_backslash_u_3;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },
            .string_surrogate_half_backslash_u_3 => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                const c = self.input[self.cursor];
                switch (c) {
                    '0'...'9' => {
                        self.utf16_code_units[1] |= c - '0';
                    },
                    'A'...'F' => {
                        self.utf16_code_units[1] |= c - 'A' + 10;
                    },
                    'a'...'f' => {
                        self.utf16_code_units[1] |= c - 'a' + 10;
                    },
                    else => return error.SyntaxError,
                }
                self.cursor += 1;
                self.value_start = self.cursor;
                self.state = .string;
                const code_point = std.unicode.utf16DecodeSurrogatePair(&self.utf16_code_units) catch unreachable;
                return partialStringCodepoint(code_point);
            },

            .string_utf8_last_byte => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                switch (self.input[self.cursor]) {
                    0x80...0xBF => {
                        self.cursor += 1;
                        self.state = .string;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError, // Invalid UTF-8.
                }
            },
            .string_utf8_second_to_last_byte => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                switch (self.input[self.cursor]) {
                    0x80...0xBF => {
                        self.cursor += 1;
                        self.state = .string_utf8_last_byte;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError, // Invalid UTF-8.
                }
            },
            .string_utf8_second_to_last_byte_guard_against_overlong => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                switch (self.input[self.cursor]) {
                    0xA0...0xBF => {
                        self.cursor += 1;
                        self.state = .string_utf8_last_byte;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError, // Invalid UTF-8.
                }
            },
            .string_utf8_second_to_last_byte_guard_against_surrogate_half => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                switch (self.input[self.cursor]) {
                    0x80...0x9F => {
                        self.cursor += 1;
                        self.state = .string_utf8_last_byte;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError, // Invalid UTF-8.
                }
            },
            .string_utf8_third_to_last_byte => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                switch (self.input[self.cursor]) {
                    0x80...0xBF => {
                        self.cursor += 1;
                        self.state = .string_utf8_second_to_last_byte;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError, // Invalid UTF-8.
                }
            },
            .string_utf8_third_to_last_byte_guard_against_overlong => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                switch (self.input[self.cursor]) {
                    0x90...0xBF => {
                        self.cursor += 1;
                        self.state = .string_utf8_second_to_last_byte;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError, // Invalid UTF-8.
                }
            },
            .string_utf8_third_to_last_byte_guard_against_too_large => {
                if (self.cursor >= self.input.len) return self.endOfBufferInString();
                switch (self.input[self.cursor]) {
                    0x80...0x8F => {
                        self.cursor += 1;
                        self.state = .string_utf8_second_to_last_byte;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError, // Invalid UTF-8.
                }
            },

            .literal_t => {
                switch (try self.expectByte()) {
                    'r' => {
                        self.cursor += 1;
                        self.state = .literal_tr;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },
            .literal_tr => {
                switch (try self.expectByte()) {
                    'u' => {
                        self.cursor += 1;
                        self.state = .literal_tru;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },
            .literal_tru => {
                switch (try self.expectByte()) {
                    'e' => {
                        self.cursor += 1;
                        self.state = .post_value;
                        return .true;
                    },
                    else => return error.SyntaxError,
                }
            },
            .literal_f => {
                switch (try self.expectByte()) {
                    'a' => {
                        self.cursor += 1;
                        self.state = .literal_fa;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },
            .literal_fa => {
                switch (try self.expectByte()) {
                    'l' => {
                        self.cursor += 1;
                        self.state = .literal_fal;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },
            .literal_fal => {
                switch (try self.expectByte()) {
                    's' => {
                        self.cursor += 1;
                        self.state = .literal_fals;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },
            .literal_fals => {
                switch (try self.expectByte()) {
                    'e' => {
                        self.cursor += 1;
                        self.state = .post_value;
                        return .false;
                    },
                    else => return error.SyntaxError,
                }
            },
            .literal_n => {
                switch (try self.expectByte()) {
                    'u' => {
                        self.cursor += 1;
                        self.state = .literal_nu;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },
            .literal_nu => {
                switch (try self.expectByte()) {
                    'l' => {
                        self.cursor += 1;
                        self.state = .literal_nul;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },
            .literal_nul => {
                switch (try self.expectByte()) {
                    'l' => {
                        self.cursor += 1;
                        self.state = .post_value;
                        return .null;
                    },
                    else => return error.SyntaxError,
                }
            },
        }
        unreachable;
    }
}

/// Seeks ahead in the input until the first byte of the next token (or the end of the input)
/// determines which type of token will be returned from the next `next*()` call.
/// This function is idempotent, only advancing past commas, colons, and inter-token whitespace.
pub fn peekNextTokenType(self: *@This()) PeekError!TokenType {
    state_loop: while (true) {
        switch (self.state) {
            .value => {
                switch (try self.skipWhitespaceExpectByte()) {
                    '{' => return .object_begin,
                    '[' => return .array_begin,
                    '"' => return .string,
                    '-', '0'...'9' => return .number,
                    't' => return .true,
                    'f' => return .false,
                    'n' => return .null,
                    else => return error.SyntaxError,
                }
            },

            .post_value => {
                if (try self.skipWhitespaceCheckEnd()) return .end_of_document;

                const c = self.input[self.cursor];
                if (self.string_is_object_key) {
                    self.string_is_object_key = false;
                    switch (c) {
                        ':' => {
                            self.cursor += 1;
                            self.state = .value;
                            continue :state_loop;
                        },
                        else => return error.SyntaxError,
                    }
                }

                switch (c) {
                    '}' => return .object_end,
                    ']' => return .array_end,
                    ',' => {
                        switch (self.stack.peek()) {
                            OBJECT_MODE => {
                                self.state = .object_post_comma;
                            },
                            ARRAY_MODE => {
                                self.state = .value;
                            },
                        }
                        self.cursor += 1;
                        continue :state_loop;
                    },
                    else => return error.SyntaxError,
                }
            },

            .object_start => {
                switch (try self.skipWhitespaceExpectByte()) {
                    '"' => return .string,
                    '}' => return .object_end,
                    else => return error.SyntaxError,
                }
            },
            .object_post_comma => {
                switch (try self.skipWhitespaceExpectByte()) {
                    '"' => return .string,
                    else => return error.SyntaxError,
                }
            },

            .array_start => {
                switch (try self.skipWhitespaceExpectByte()) {
                    ']' => return .array_end,
                    else => {
                        self.state = .value;
                        continue :state_loop;
                    },
                }
            },

            .number_minus,
            .number_leading_zero,
            .number_int,
            .number_post_dot,
            .number_frac,
            .number_post_e,
            .number_post_e_sign,
            .number_exp,
            => return .number,

            .string,
            .string_backslash,
            .string_backslash_u,
            .string_backslash_u_1,
            .string_backslash_u_2,
            .string_backslash_u_3,
            .string_surrogate_half,
            .string_surrogate_half_backslash,
            .string_surrogate_half_backslash_u,
            .string_surrogate_half_backslash_u_1,
            .string_surrogate_half_backslash_u_2,
            .string_surrogate_half_backslash_u_3,
            => return .string,

            .string_utf8_last_byte,
            .string_utf8_second_to_last_byte,
            .string_utf8_second_to_last_byte_guard_against_overlong,
            .string_utf8_second_to_last_byte_guard_against_surrogate_half,
            .string_utf8_third_to_last_byte,
            .string_utf8_third_to_last_byte_guard_against_overlong,
            .string_utf8_third_to_last_byte_guard_against_too_large,
            => return .string,

            .literal_t,
            .literal_tr,
            .literal_tru,
            => return .true,
            .literal_f,
            .literal_fa,
            .literal_fal,
            .literal_fals,
            => return .false,
            .literal_n,
            .literal_nu,
            .literal_nul,
            => return .null,
        }
        unreachable;
    }
}

const State = enum {
    value,
    post_value,

    object_start,
    object_post_comma,

    array_start,

    number_minus,
    number_leading_zero,
    number_int,
    number_post_dot,
    number_frac,
    number_post_e,
    number_post_e_sign,
    number_exp,

    string,
    string_backslash,
    string_backslash_u,
    string_backslash_u_1,
    string_backslash_u_2,
    string_backslash_u_3,
    string_surrogate_half,
    string_surrogate_half_backslash,
    string_surrogate_half_backslash_u,
    string_surrogate_half_backslash_u_1,
    string_surrogate_half_backslash_u_2,
    string_surrogate_half_backslash_u_3,

    // From http://unicode.org/mail-arch/unicode-ml/y2003-m02/att-0467/01-The_Algorithm_to_Valide_an_UTF-8_String
    string_utf8_last_byte, // State A
    string_utf8_second_to_last_byte, // State B
    string_utf8_second_to_last_byte_guard_against_overlong, // State C
    string_utf8_second_to_last_byte_guard_against_surrogate_half, // State D
    string_utf8_third_to_last_byte, // State E
    string_utf8_third_to_last_byte_guard_against_overlong, // State F
    string_utf8_third_to_last_byte_guard_against_too_large, // State G

    literal_t,
    literal_tr,
    literal_tru,
    literal_f,
    literal_fa,
    literal_fal,
    literal_fals,
    literal_n,
    literal_nu,
    literal_nul,
};

fn expectByte(self: *const @This()) !u8 {
    if (self.cursor < self.input.len) {
        return self.input[self.cursor];
    }
    // No byte.
    if (self.is_end_of_input) return error.UnexpectedEndOfInput;
    return error.BufferUnderrun;
}

fn skipWhitespace(self: *@This()) void {
    while (self.cursor < self.input.len) : (self.cursor += 1) {
        switch (self.input[self.cursor]) {
            // Whitespace
            ' ', '\t', '\r' => continue,
            '\n' => {
                if (self.diagnostics) |diag| {
                    diag.line_number += 1;
                    // This will count the newline itself,
                    // which means a straight-forward subtraction will give a 1-based column number.
                    diag.line_start_cursor = self.cursor;
                }
                continue;
            },
            else => return,
        }
    }
}

fn skipWhitespaceExpectByte(self: *@This()) !u8 {
    self.skipWhitespace();
    return self.expectByte();
}

fn skipWhitespaceCheckEnd(self: *@This()) !bool {
    self.skipWhitespace();
    if (self.cursor >= self.input.len) {
        // End of buffer.
        if (self.is_end_of_input) {
            // End of everything.
            if (self.stackHeight() == 0) {
                // We did it!
                return true;
            }
            return error.UnexpectedEndOfInput;
        }
        return error.BufferUnderrun;
    }
    if (self.stackHeight() == 0) return error.SyntaxError;
    return false;
}

fn takeValueSlice(self: *@This()) []const u8 {
    const slice = self.input[self.value_start..self.cursor];
    self.value_start = self.cursor;
    return slice;
}
fn takeValueSliceMinusTrailingOffset(self: *@This(), trailing_negative_offset: usize) []const u8 {
    // Check if the escape sequence started before the current input buffer.
    // (The algebra here is awkward to avoid unsigned underflow,
    //  but it's just making sure the slice on the next line isn't UB.)
    if (self.cursor <= self.value_start + trailing_negative_offset) return "";
    const slice = self.input[self.value_start .. self.cursor - trailing_negative_offset];
    // When trailing_negative_offset is non-zero, setting self.value_start doesn't matter,
    // because we always set it again while emitting the .partial_string_escaped_*.
    self.value_start = self.cursor;
    return slice;
}

fn endOfBufferInNumber(self: *@This(), allow_end: bool) !Token {
    const slice = self.takeValueSlice();
    if (self.is_end_of_input) {
        if (!allow_end) return error.UnexpectedEndOfInput;
        self.state = .post_value;
        return Token{ .number = slice };
    }
    if (slice.len == 0) return error.BufferUnderrun;
    return Token{ .partial_number = slice };
}

fn endOfBufferInString(self: *@This()) !Token {
    if (self.is_end_of_input) return error.UnexpectedEndOfInput;
    const slice = self.takeValueSliceMinusTrailingOffset(switch (self.state) {
        // Don't include the escape sequence in the partial string.
        .string_backslash => 1,
        .string_backslash_u => 2,
        .string_backslash_u_1 => 3,
        .string_backslash_u_2 => 4,
        .string_backslash_u_3 => 5,
        .string_surrogate_half => 6,
        .string_surrogate_half_backslash => 7,
        .string_surrogate_half_backslash_u => 8,
        .string_surrogate_half_backslash_u_1 => 9,
        .string_surrogate_half_backslash_u_2 => 10,
        .string_surrogate_half_backslash_u_3 => 11,

        // Include everything up to the cursor otherwise.
        .string,
        .string_utf8_last_byte,
        .string_utf8_second_to_last_byte,
        .string_utf8_second_to_last_byte_guard_against_overlong,
        .string_utf8_second_to_last_byte_guard_against_surrogate_half,
        .string_utf8_third_to_last_byte,
        .string_utf8_third_to_last_byte_guard_against_overlong,
        .string_utf8_third_to_last_byte_guard_against_too_large,
        => 0,

        else => unreachable,
    });
    if (slice.len == 0) return error.BufferUnderrun;
    return Token{ .partial_string = slice };
}

fn partialStringCodepoint(code_point: u21) Token {
    var buf: [4]u8 = undefined;
    switch (std.unicode.utf8Encode(code_point, &buf) catch unreachable) {
        1 => return Token{ .partial_string_escaped_1 = buf[0..1].* },
        2 => return Token{ .partial_string_escaped_2 = buf[0..2].* },
        3 => return Token{ .partial_string_escaped_3 = buf[0..3].* },
        4 => return Token{ .partial_string_escaped_4 = buf[0..4].* },
        else => unreachable,
    }
}

/// Scan the input and check for malformed JSON.
/// On `SyntaxError` or `UnexpectedEndOfInput`, returns `false`.
/// Returns any errors from the allocator as-is, which is unlikely,
/// but can be caused by extreme nesting depth in the input.
pub fn validate(allocator: Allocator, s: []const u8) Allocator.Error!bool {
    var scanner = Scanner.initCompleteInput(allocator, s);
    defer scanner.deinit();

    while (true) {
        const token = scanner.next() catch |err| switch (err) {
            error.SyntaxError, error.UnexpectedEndOfInput => return false,
            error.OutOfMemory => return error.OutOfMemory,
            error.BufferUnderrun => unreachable,
        };
        if (token == .end_of_document) break;
    }

    return true;
}

/// The parsing errors are divided into two categories:
///  * `SyntaxError` is for clearly malformed JSON documents,
///    such as giving an input document that isn't JSON at all.
///  * `UnexpectedEndOfInput` is for signaling that everything's been
///    valid so far, but the input appears to be truncated for some reason.
/// Note that a completely empty (or whitespace-only) input will give `UnexpectedEndOfInput`.
pub const Error = error{ SyntaxError, UnexpectedEndOfInput };

/// Used by `json.reader`.
pub const default_buffer_size = 0x1000;

/// The tokens emitted by `std.json.Scanner` and `std.json.Reader` `.next*()` functions follow this grammar:
/// ```
///  <document> = <value> .end_of_document
///  <value> =
///    | <object>
///    | <array>
///    | <number>
///    | <string>
///    | .true
///    | .false
///    | .null
///  <object> = .object_begin ( <string> <value> )* .object_end
///  <array> = .array_begin ( <value> )* .array_end
///  <number> = <It depends. See below.>
///  <string> = <It depends. See below.>
/// ```
///
/// What you get for `<number>` and `<string>` values depends on which `next*()` method you call:
///
/// ```
/// next():
///  <number> = ( .partial_number )* .number
///  <string> = ( <partial_string> )* .string
///  <partial_string> =
///    | .partial_string
///    | .partial_string_escaped_1
///    | .partial_string_escaped_2
///    | .partial_string_escaped_3
///    | .partial_string_escaped_4
///
/// nextAlloc*(..., .alloc_always):
///  <number> = .allocated_number
///  <string> = .allocated_string
///
/// nextAlloc*(..., .alloc_if_needed):
///  <number> =
///    | .number
///    | .allocated_number
///  <string> =
///    | .string
///    | .allocated_string
/// ```
///
/// For all tokens with a `[]const u8`, `[]u8`, or `[n]u8` payload, the payload represents the content of the value.
/// For number values, this is the representation of the number exactly as it appears in the input.
/// For strings, this is the content of the string after resolving escape sequences.
///
/// For `.allocated_number` and `.allocated_string`, the `[]u8` payloads are allocations made with the given allocator.
/// You are responsible for managing that memory. `json.Reader.deinit()` does *not* free those allocations.
///
/// The `.partial_*` tokens indicate that a value spans multiple input buffers or that a string contains escape sequences.
/// To get a complete value in memory, you need to concatenate the values yourself.
/// Calling `nextAlloc*()` does this for you, and returns an `.allocated_*` token with the result.
///
/// For tokens with a `[]const u8` payload, the payload is a slice into the current input buffer.
/// The memory may become undefined during the next call to `json.Scanner.feedInput()`
/// or any `json.Reader` method whose return error set includes `json.Error`.
/// To keep the value persistently, it recommended to make a copy or to use `.alloc_always`,
/// which makes a copy for you.
///
/// Note that `.number` and `.string` tokens that follow `.partial_*` tokens may have `0` length to indicate that
/// the previously partial value is completed with no additional bytes.
/// (This can happen when the break between input buffers happens to land on the exact end of a value. E.g. `"[1234"`, `"]"`.)
/// `.partial_*` tokens never have `0` length.
///
/// The recommended strategy for using the different `next*()` methods is something like this:
///
/// When you're expecting an object key, use `.alloc_if_needed`.
/// You often don't need a copy of the key string to persist; you might just check which field it is.
/// In the case that the key happens to require an allocation, free it immediately after checking it.
///
/// When you're expecting a meaningful string value (such as on the right of a `:`),
/// use `.alloc_always` in order to keep the value valid throughout parsing the rest of the document.
///
/// When you're expecting a number value, use `.alloc_if_needed`.
/// You're probably going to be parsing the string representation of the number into a numeric representation,
/// so you need the complete string representation only temporarily.
///
/// When you're skipping an unrecognized value, use `skipValue()`.
pub const Token = union(enum) {
    object_begin,
    object_end,
    array_begin,
    array_end,

    true,
    false,
    null,

    number: []const u8,
    partial_number: []const u8,
    allocated_number: []u8,

    string: []const u8,
    partial_string: []const u8,
    partial_string_escaped_1: [1]u8,
    partial_string_escaped_2: [2]u8,
    partial_string_escaped_3: [3]u8,
    partial_string_escaped_4: [4]u8,
    allocated_string: []u8,

    end_of_document,
};

/// This is only used in `peekNextTokenType()` and gives a categorization based on the first byte of the next token that will be emitted from a `next*()` call.
pub const TokenType = enum {
    object_begin,
    object_end,
    array_begin,
    array_end,
    true,
    false,
    null,
    number,
    string,
    end_of_document,
};

/// To enable diagnostics, declare `var diagnostics = Diagnostics{};` then call `source.enableDiagnostics(&diagnostics);`
/// where `source` is either a `std.json.Reader` or a `std.json.Scanner` that has just been initialized.
/// At any time, notably just after an error, call `getLine()`, `getColumn()`, and/or `getByteOffset()`
/// to get meaningful information from this.
pub const Diagnostics = struct {
    line_number: u64 = 1,
    line_start_cursor: usize = @as(usize, @bitCast(@as(isize, -1))), // Start just "before" the input buffer to get a 1-based column for line 1.
    total_bytes_before_current_input: u64 = 0,
    cursor_pointer: *const usize = undefined,

    /// Starts at 1.
    pub fn getLine(self: *const @This()) u64 {
        return self.line_number;
    }
    /// Starts at 1.
    pub fn getColumn(self: *const @This()) u64 {
        return self.cursor_pointer.* -% self.line_start_cursor;
    }
    /// Starts at 0. Measures the byte offset since the start of the input.
    pub fn getByteOffset(self: *const @This()) u64 {
        return self.total_bytes_before_current_input + self.cursor_pointer.*;
    }
};

/// See the documentation for `std.json.Token`.
pub const AllocWhen = enum { alloc_if_needed, alloc_always };

/// For security, the maximum size allocated to store a single string or number value is limited to 4MiB by default.
/// This limit can be specified by calling `nextAllocMax()` instead of `nextAlloc()`.
pub const default_max_value_len = 4 * 1024 * 1024;

/// All `next*()` methods here handle `error.BufferUnderrun` from `std.json.Scanner`, and then read from the reader.
pub const Reader = struct {
    scanner: Scanner,
    reader: *std.Io.Reader,

    /// The allocator is only used to track `[]` and `{}` nesting levels.
    pub fn init(allocator: Allocator, io_reader: *std.Io.Reader) @This() {
        return .{
            .scanner = Scanner.initStreaming(allocator),
            .reader = io_reader,
        };
    }
    pub fn deinit(self: *@This()) void {
        self.scanner.deinit();
        self.* = undefined;
    }

    /// Calls `std.json.Scanner.enableDiagnostics`.
    pub fn enableDiagnostics(self: *@This(), diagnostics: *Diagnostics) void {
        self.scanner.enableDiagnostics(diagnostics);
    }

    pub const NextError = std.Io.Reader.Error || Error || Allocator.Error;
    pub const SkipError = Reader.NextError;
    pub const AllocError = Reader.NextError || error{ValueTooLong};
    pub const PeekError = std.Io.Reader.Error || Error;

    /// Equivalent to `nextAllocMax(allocator, when, default_max_value_len);`
    /// See also `std.json.Token` for documentation of `nextAlloc*()` function behavior.
    pub fn nextAlloc(self: *@This(), allocator: Allocator, when: AllocWhen) Reader.AllocError!Token {
        return self.nextAllocMax(allocator, when, default_max_value_len);
    }
    /// See also `std.json.Token` for documentation of `nextAlloc*()` function behavior.
    pub fn nextAllocMax(self: *@This(), allocator: Allocator, when: AllocWhen, max_value_len: usize) Reader.AllocError!Token {
        const token_type = try self.peekNextTokenType();
        switch (token_type) {
            .number, .string => {
                var value_list = std.array_list.Managed(u8).init(allocator);
                errdefer {
                    value_list.deinit();
                }
                if (try self.allocNextIntoArrayListMax(&value_list, when, max_value_len)) |slice| {
                    return if (token_type == .number)
                        Token{ .number = slice }
                    else
                        Token{ .string = slice };
                } else {
                    return if (token_type == .number)
                        Token{ .allocated_number = try value_list.toOwnedSlice() }
                    else
                        Token{ .allocated_string = try value_list.toOwnedSlice() };
                }
            },

            // Simple tokens never alloc.
            .object_begin,
            .object_end,
            .array_begin,
            .array_end,
            .true,
            .false,
            .null,
            .end_of_document,
            => return try self.next(),
        }
    }

    /// Equivalent to `allocNextIntoArrayListMax(value_list, when, default_max_value_len);`
    pub fn allocNextIntoArrayList(self: *@This(), value_list: *std.array_list.Managed(u8), when: AllocWhen) Reader.AllocError!?[]const u8 {
        return self.allocNextIntoArrayListMax(value_list, when, default_max_value_len);
    }
    /// Calls `std.json.Scanner.allocNextIntoArrayListMax` and handles `error.BufferUnderrun`.
    pub fn allocNextIntoArrayListMax(self: *@This(), value_list: *std.array_list.Managed(u8), when: AllocWhen, max_value_len: usize) Reader.AllocError!?[]const u8 {
        while (true) {
            return self.scanner.allocNextIntoArrayListMax(value_list, when, max_value_len) catch |err| switch (err) {
                error.BufferUnderrun => {
                    try self.refillBuffer();
                    continue;
                },
                else => |other_err| return other_err,
            };
        }
    }

    /// Like `std.json.Scanner.skipValue`, but handles `error.BufferUnderrun`.
    pub fn skipValue(self: *@This()) Reader.SkipError!void {
        switch (try self.peekNextTokenType()) {
            .object_begin, .array_begin => {
                try self.skipUntilStackHeight(self.stackHeight());
            },
            .number, .string => {
                while (true) {
                    switch (try self.next()) {
                        .partial_number,
                        .partial_string,
                        .partial_string_escaped_1,
                        .partial_string_escaped_2,
                        .partial_string_escaped_3,
                        .partial_string_escaped_4,
                        => continue,

                        .number, .string => break,

                        else => unreachable,
                    }
                }
            },
            .true, .false, .null => {
                _ = try self.next();
            },

            .object_end, .array_end, .end_of_document => unreachable, // Attempt to skip a non-value token.
        }
    }
    /// Like `std.json.Scanner.skipUntilStackHeight()` but handles `error.BufferUnderrun`.
    pub fn skipUntilStackHeight(self: *@This(), terminal_stack_height: usize) Reader.NextError!void {
        while (true) {
            return self.scanner.skipUntilStackHeight(terminal_stack_height) catch |err| switch (err) {
                error.BufferUnderrun => {
                    try self.refillBuffer();
                    continue;
                },
                else => |other_err| return other_err,
            };
        }
    }

    /// Calls `std.json.Scanner.stackHeight`.
    pub fn stackHeight(self: *const @This()) usize {
        return self.scanner.stackHeight();
    }
    /// Calls `std.json.Scanner.ensureTotalStackCapacity`.
    pub fn ensureTotalStackCapacity(self: *@This(), height: usize) Allocator.Error!void {
        try self.scanner.ensureTotalStackCapacity(height);
    }

    /// See `std.json.Token` for documentation of this function.
    pub fn next(self: *@This()) Reader.NextError!Token {
        while (true) {
            return self.scanner.next() catch |err| switch (err) {
                error.BufferUnderrun => {
                    try self.refillBuffer();
                    continue;
                },
                else => |other_err| return other_err,
            };
        }
    }

    /// See `std.json.Scanner.peekNextTokenType()`.
    pub fn peekNextTokenType(self: *@This()) Reader.PeekError!TokenType {
        while (true) {
            return self.scanner.peekNextTokenType() catch |err| switch (err) {
                error.BufferUnderrun => {
                    try self.refillBuffer();
                    continue;
                },
                else => |other_err| return other_err,
            };
        }
    }

    fn refillBuffer(self: *@This()) std.Io.Reader.Error!void {
        const input = self.reader.peekGreedy(1) catch |err| switch (err) {
            error.ReadFailed => return error.ReadFailed,
            error.EndOfStream => return self.scanner.endInput(),
        };
        self.reader.toss(input.len);
        self.scanner.feedInput(input);
    }
};

const OBJECT_MODE = 0;
const ARRAY_MODE = 1;

fn appendSlice(list: *std.array_list.Managed(u8), buf: []const u8, max_value_len: usize) !void {
    const new_len = std.math.add(usize, list.items.len, buf.len) catch return error.ValueTooLong;
    if (new_len > max_value_len) return error.ValueTooLong;
    try list.appendSlice(buf);
}

/// For the slice you get from a `Token.number` or `Token.allocated_number`,
/// this function returns true if the number doesn't contain any fraction or exponent components, and is not `-0`.
/// Note, the numeric value encoded by the value may still be an integer, such as `1.0`.
/// This function is meant to give a hint about whether integer parsing or float parsing should be used on the value.
/// This function will not give meaningful results on non-numeric input.
pub fn isNumberFormattedLikeAnInteger(value: []const u8) bool {
    if (std.mem.eql(u8, value, "-0")) return false;
    return std.mem.findAny(u8, value, ".eE") == null;
}

test {
    _ = @import("./scanner_test.zig");
}
