use std::slice::SliceIndex;
use std::{error, fmt};

use base64;
use encode_unicode::{error::InvalidUtf16Tuple, Utf16Char, Utf8Char};
use lexical_core;
use serde::de::{
    self, Deserialize, DeserializeOwned, DeserializeSeed, Deserializer, EnumAccess,
    IntoDeserializer, MapAccess, SeqAccess, VariantAccess, Visitor,
};

use super::super::LegacyF64;

/// Error code and byte offset describing a deserialization failure
#[derive(PartialEq, Eq, Debug, Clone)]
pub struct DecodeJsonError {
    /// Reason decoding failed
    pub code: ErrorCode,

    /// Byte offset at which the decoding failure occurred
    pub position: usize,
}

/// Everything that can go wrong during deserialization.
#[derive(PartialEq, Eq, Debug, Clone)]
pub enum ErrorCode {
    /// Expected more data but the input ended.
    UnexpectedEndOfInput,
    /// A generic syntax error. Any valid json would have been ok, but alas...
    Syntax,
    /// Expected a comma (`,`) to separate collection elements.
    Comma,
    /// Expected a colon (`:`) to separate a key from a value.
    Colon,
    /// Expected a decimal digit. Didn't get one. Sad times.
    Digit,
    /// Expected hexadecimal digit as part of a unicode escape sequence in a string.
    HexDigit,
    /// Expected a unicode escape (because we just parsed a unicode escape of a leading
    /// surrogate codepoint).
    UnicodeEscape,
    /// Could not merge two unicode escapes into a single code point.
    SurrogatePair(InvalidUtf16Tuple),
    /// A unicode escape encoded a trailing surrogate codepoint without a preceding
    /// leading surrogate codepoint.
    TrailingSurrogate,
    /// A string contained an unescaped control code point.
    UnescapedControlCodePoint,
    /// A string contained a backslash followed by a non-escape character.
    InvalidEscape,
    /// A string literal contains a non-utf8 byte sequence.
    InvalidUtf8String,
    /// A number is valid json but it evaluates to -0 or an infinity
    InvalidNumber,
    /// The input contained valid json followed by at least one non-whitespace byte.
    TrailingCharacters,
    /// Attempted to parse a number as an `i8` that was out of bounds.
    OutOfBoundsI8,
    /// Attempted to parse a number as an `i16` that was out of bounds.
    OutOfBoundsI16,
    /// Attempted to parse a number as an `i32` that was out of bounds.
    OutOfBoundsI32,
    /// Attempted to parse a number as an `i64` that was less than -2^53 or greater than 2^53.
    OutOfBoundsI64,
    /// Attempted to parse a number as an `u8` that was out of bounds.
    OutOfBoundsU8,
    /// Attempted to parse a number as an `u16` that was out of bounds.
    OutOfBoundsU16,
    /// Attempted to parse a number as an `u32` that was out of bounds.
    OutOfBoundsU32,
    /// Attempted to parse a number as an `u64` that was greater than 2^53.
    OutOfBoundsU64,
    /// Chars are represented as strings that contain one unicode scalar value.
    NotAChar,
    /// Attempted to read a string as base64-encoded bytes, but the string was not valid base64.
    Base64(base64::DecodeError),
    /// Expected a boolean, found something else.
    ExpectedBool,
    /// Expected a number, found something else.
    ExpectedNumber,
    /// Expected a string, found something else.
    ExpectedString,
    /// Expected null, found something else.
    ExpectedNull,
    /// Expected an array, found something else.
    ExpectedArray,
    /// Expected an object, found something else.
    ExpectedObject,
    /// Expected an enum, found something else.
    ExpectedEnum,
    /// Custom, stringly-typed error.
    Message(String),
}

impl fmt::Display for DecodeJsonError {
    fn fmt(&self, f: &mut fmt::Formatter) -> std::result::Result<(), fmt::Error> {
        fmt::Debug::fmt(&self.code, f)
    }
}

impl error::Error for DecodeJsonError {}

impl de::Error for DecodeJsonError {
    fn custom<T: fmt::Display>(msg: T) -> Self {
        DecodeJsonError {
            code: ErrorCode::Message(msg.to_string()),
            position: 0, // TODO
        }
    }
}

/// A structure that deserializes json encoded legacy message values.
///
/// https://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf
pub struct JsonDeserializer<'de> {
    input: &'de [u8],
    position: usize,
}

impl<'de> JsonDeserializer<'de> {
    /// Check whether there are no non-whitespace tokens up until the end of the input.
    pub fn end(&mut self) -> Result<(), DecodeJsonError> {
        match self.peek_ws() {
            Ok(_) => self.fail(ErrorCode::TrailingCharacters),
            Err(DecodeJsonError {
                code: ErrorCode::UnexpectedEndOfInput,
                position: _,
            }) => Ok(()),
            Err(e) => Err(e),
        }
    }

    fn slice<I: SliceIndex<[u8]>>(&self, i: I) -> &'de I::Output {
        &self.input[i]
    }

    /// Reference to portion of buffer yet to be deserialiced
    pub fn rest(&self) -> &'de [u8] {
        self.slice(self.position()..)
    }

    /// Current byte offset of buffer being deserialized
    pub fn position(&self) -> usize {
        self.position
    }

    fn fail<T>(&self, code: ErrorCode) -> Result<T, DecodeJsonError> {
        Err(DecodeJsonError {
            code,
            position: self.position(),
        })
    }

    fn fail_at_position<T>(&self, code: ErrorCode, position: usize) -> Result<T, DecodeJsonError> {
        Err(DecodeJsonError { code, position })
    }
}

/// Try to parse data from the input. Validates that there are no trailing non-whitespace bytes.
pub fn from_slice<'de, T>(input: &'de [u8]) -> Result<T, DecodeJsonError>
where
    T: DeserializeOwned,
{
    let mut de = JsonDeserializer::from_slice(input);
    match Deserialize::deserialize(&mut de) {
        Ok(t) => de.end().map(|_| t),
        Err(e) => Err(e),
    }
}

/// Try to parse data from the input, returning the remaining input when done.
pub fn from_slice_partial<'de, T>(input: &'de [u8]) -> Result<(T, &'de [u8]), DecodeJsonError>
where
    T: DeserializeOwned,
{
    let mut de = JsonDeserializer::from_slice(input);
    match Deserialize::deserialize(&mut de) {
        Ok(t) => Ok((t, de.rest())),
        Err(e) => Err(e),
    }
}

fn is_ws(byte: u8) -> bool {
    byte == 0x09 || byte == 0x0A || byte == 0x0D || byte == 0x20
}

fn is_digit(byte: u8) -> bool {
    byte.is_ascii_digit()
}

fn is_hex_digit(byte: u8) -> bool {
    byte.is_ascii_hexdigit()
}

impl<'de> JsonDeserializer<'de> {
    /// Creates a `Deserializer` from a `&[u8]`.
    pub fn from_slice(input: &'de [u8]) -> Self {
        JsonDeserializer { input, position: 0 }
    }

    // Advance the input slice by some number of bytes.
    fn advance(&mut self, offset: usize) {
        self.position += offset;
    }

    // Consumes the next byte and returns it.
    fn next(&mut self) -> Result<u8, DecodeJsonError> {
        if let Some(c) = self.input.get(self.position()) {
            self.advance(1);
            Ok(*c)
        } else {
            self.fail(ErrorCode::UnexpectedEndOfInput)
        }
    }

    // Consumes the expected byt, gives the given error if it is something else
    fn expect(&mut self, expected: u8, err: ErrorCode) -> Result<(), DecodeJsonError> {
        let pos = self.position();
        if self.next()? == expected {
            Ok(())
        } else {
            self.fail_at_position(err, pos)
        }
    }

    // Same as expect, but using a predicate.
    fn expect_pred(&mut self, pred: fn(u8) -> bool, err: ErrorCode) -> Result<(), DecodeJsonError> {
        let pos = self.position();
        if pred(self.next()?) {
            Ok(())
        } else {
            self.fail_at_position(err, pos)
        }
    }

    // Returns the next byte without consuming it.
    fn peek(&self) -> Result<u8, DecodeJsonError> {
        if let Some(c) = self.input.get(self.position()) {
            Ok(*c)
        } else {
            self.fail(ErrorCode::UnexpectedEndOfInput)
        }
    }

    // Returns the next byte without consuming it, or signals end of input as `None`.
    fn peek_or_end(&self) -> Option<u8> {
        self.input.get(self.position()).map(|b| *b)
    }

    // Skips values while the predicate returns true.
    fn skip(&mut self, pred: fn(u8) -> bool) -> () {
        loop {
            match self.peek_or_end() {
                None => return,
                Some(peeked) => {
                    if pred(peeked) {
                        self.advance(1);
                    } else {
                        return;
                    }
                }
            }
        }
    }

    fn skip_ws(&mut self) -> () {
        self.skip(is_ws)
    }

    // Consumes as much whitespace as possible, then peeks at the next non-whitespace byte.
    fn peek_ws(&mut self) -> Result<u8, DecodeJsonError> {
        self.skip_ws();
        self.peek()
    }

    fn expect_ws(&mut self, exp: u8, err: ErrorCode) -> Result<(), DecodeJsonError> {
        self.skip_ws();
        self.expect(exp, err)
    }

    fn expect_bytes(&mut self, exp: &[u8], err: ErrorCode) -> Result<(), DecodeJsonError> {
        if self.rest().starts_with(exp) {
            self.advance(exp.len());
            Ok(())
        } else {
            self.fail(err)
        }
    }

    // Parses the four characters of a unicode escape sequence and returns the codepoint they
    // encode. Json only allows escaping codepoints in the BMP, that's why it fits into a `u16`.
    fn parse_unicode_escape(&mut self) -> Result<u16, DecodeJsonError> {
        let start = self.position();

        for _ in 0..4 {
            self.expect_pred(is_hex_digit, ErrorCode::HexDigit)?;
        }

        u16::from_str_radix(
            unsafe { std::str::from_utf8_unchecked(&self.slice(start..start + 4)) },
            16,
        )
        .map_err(|_| unreachable!("We already checked for valid input"))
    }

    fn parse_bool(&mut self) -> Result<bool, DecodeJsonError> {
        match self.expect_bytes(b"true", ErrorCode::ExpectedBool) {
            Ok(()) => Ok(true),
            Err(_) => self
                .expect_bytes(b"false", ErrorCode::ExpectedBool)
                .map(|_| false),
        }
    }

    fn parse_number_except(
        &mut self,
        pred: fn(f64) -> bool,
        err: ErrorCode,
    ) -> Result<f64, DecodeJsonError> {
        let pos = self.position();
        let f = self.parse_number()?;
        if pred(f) {
            Ok(f)
        } else {
            self.fail_at_position(err, pos)
        }
    }

    fn parse_number(&mut self) -> Result<f64, DecodeJsonError> {
        let start = self.position();

        // trailing `-`
        match self.peek() {
            Ok(0x2D) => self.advance(1),
            Ok(_) => {}
            Err(_) => return self.fail(ErrorCode::ExpectedNumber),
        }

        let next = self.next()?;
        match next {
            // first digit `0` must be followed by `.`
            0x30 => {}
            // first digit nonzero, may be followed by more digits until the `.`
            0x31..=0x39 => self.skip(is_digit),
            _ => return self.fail_at_position(ErrorCode::ExpectedNumber, start),
        }

        // `.`, followed by many1 digits
        if let Some(0x2E) = self.peek_or_end() {
            self.advance(1);
            self.expect_pred(is_digit, ErrorCode::Digit)?;
            self.skip(is_digit);
        }

        // `e` or `E`, followed by an optional sign and many1 digits
        match self.peek_or_end() {
            Some(0x45) | Some(0x65) => {
                self.advance(1);

                // optional `+` or `-`
                if self.peek()? == 0x2B || self.peek()? == 0x2D {
                    self.advance(1);
                }

                // many1 digits
                self.expect_pred(is_digit, ErrorCode::Digit)?;
                self.skip(is_digit);
            }
            _ => {}
        }

        // done parsing the number, convert it to a rust value
        let f: f64 = lexical_core::parse(self.slice(start..self.position())).unwrap(); // We already checked that the input is a valid number

        if LegacyF64::is_valid(f) {
            Ok(f)
        } else {
            self.fail_at_position(ErrorCode::InvalidNumber, start)
        }
    }

    // Return a slice beginning and ending with 0x22 (`"`)
    fn parse_naive_string(&mut self) -> Result<&'de [u8], DecodeJsonError> {
        self.expect(0x22, ErrorCode::ExpectedString)?;
        let start = self.position();

        while self.next()? != 0x22 {
            // noop
        }

        Ok(self.slice(start..self.position()))
    }

    fn parse_string(&mut self) -> Result<String, DecodeJsonError> {
        self.expect(0x22, ErrorCode::ExpectedString).unwrap();

        let mut decoded = String::new();

        loop {
            match self.peek()? {
                // terminating `"`, return the decoded string
                0x22 => {
                    self.advance(1);
                    return Ok(decoded);
                }

                // `\` introduces an escape sequence
                0x5C => {
                    let pos = self.position();
                    self.advance(1);

                    match self.next()? {
                        // single character escape sequences
                        0x22 => decoded.push_str("\u{22}"), // `\"`
                        0x5C => decoded.push_str("\u{5C}"), // `\\`
                        0x2F => decoded.push_str("\u{2F}"), // `\/`
                        0x62 => decoded.push_str("\u{08}"), // `\b`
                        0x66 => decoded.push_str("\u{0C}"), // `\f`
                        0x6E => decoded.push_str("\u{0A}"), // `\n`
                        0x72 => decoded.push_str("\u{0D}"), // `\r`
                        0x74 => decoded.push_str("\u{09}"), // `\t`

                        // unicode escape sequences
                        0x75 => {
                            let cp = self.parse_unicode_escape()?;

                            match code_unit_type(cp) {
                                CodeUnitType::Valid => decoded
                                    .push(unsafe { std::char::from_u32_unchecked(cp as u32) }),

                                CodeUnitType::LeadingSurrogate => {
                                    // the unicode escape was for a leading surrogate, which
                                    // must be followed by another unicode escape which is a
                                    // trailing surrogate
                                    self.expect(0x5C, ErrorCode::UnicodeEscape)?;
                                    self.expect(0x75, ErrorCode::UnicodeEscape)?;
                                    let cp2 = self.parse_unicode_escape()?;

                                    match Utf16Char::from_tuple((cp, Some(cp2))) {
                                        Ok(c) => decoded.push(c.into()),
                                        Err(e) => {
                                            return self
                                                .fail_at_position(ErrorCode::SurrogatePair(e), pos)
                                        }
                                    }
                                }

                                CodeUnitType::TrailingSurrogate => {
                                    return self.fail_at_position(ErrorCode::TrailingSurrogate, pos)
                                }
                            }
                        }

                        // Nothing else may follow an unescaped `\`
                        _ => return self.fail_at_position(ErrorCode::InvalidEscape, pos),
                    }
                }

                // the control code points must be escaped
                0x00..=0x1F => return self.fail(ErrorCode::UnescapedControlCodePoint),

                // a regular utf8-encoded code point (unless it is malformed)
                _ => match Utf8Char::from_slice_start(self.rest()) {
                    Err(_) => return self.fail(ErrorCode::InvalidUtf8String),
                    Ok((_, len)) => unsafe {
                        decoded.push_str(std::str::from_utf8_unchecked(&self.rest()[..len]));
                        self.advance(len);
                    },
                },
            }
        }
    }

    fn parse_null(&mut self) -> Result<(), DecodeJsonError> {
        self.expect_bytes(b"null", ErrorCode::ExpectedNull)
    }
}

// Every utf16 code unit (a `u16`) falls into one of these categories.
enum CodeUnitType {
    // A valid code point in the BMP: either between 0x0000 and 0xD7FF (inclusive)
    // or between 0xE000 and 0xFFFF (inclusive)
    Valid,
    // Leading surrogate: between 0xD800 and 0xDBFF (inclusive)
    LeadingSurrogate,
    // Trailing surrogate: between 0xDC00 and 0xDFFF (inclusive)
    TrailingSurrogate,
}

// Maps a `u16` to its `CodeUnitType`.
fn code_unit_type(c: u16) -> CodeUnitType {
    match c {
        0x0000..=0xD7FF | 0xE000..=0xFFFF => CodeUnitType::Valid,
        0xD800..=0xDBFF => CodeUnitType::LeadingSurrogate,
        0xDC00..=0xDFFF => CodeUnitType::TrailingSurrogate,
    }
}

impl<'a, 'de> Deserializer<'de> for &'a mut JsonDeserializer<'de> {
    type Error = DecodeJsonError;

    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        match self.peek_ws()? {
            0x6E => {
                if self.rest()[1..].starts_with(b"ull") {
                    self.advance(4);
                    visitor.visit_unit()
                } else {
                    self.fail(ErrorCode::Syntax)
                }
            }
            0x66 => {
                if self.rest()[1..].starts_with(b"alse") {
                    self.advance(5);
                    visitor.visit_bool(false)
                } else {
                    self.fail(ErrorCode::Syntax)
                }
            }
            0x74 => {
                if self.rest()[1..].starts_with(b"rue") {
                    self.advance(4);
                    visitor.visit_bool(true)
                } else {
                    self.fail(ErrorCode::Syntax)
                }
            }
            0x22 => self.deserialize_str(visitor),
            0x5B => self.deserialize_seq(visitor),
            0x7B => self.deserialize_map(visitor),
            0x2D | 0x30..=0x39 => self.deserialize_f64(visitor),
            _ => self.fail(ErrorCode::Syntax),
        }
    }

    fn deserialize_bool<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        visitor.visit_bool(self.parse_bool()?)
    }

    fn deserialize_i8<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let f = self.parse_number_except(
            |n| n < std::i8::MIN as f64 || n > std::i8::MAX as f64,
            ErrorCode::OutOfBoundsI8,
        )?;
        visitor.visit_i8(f as i8)
    }

    fn deserialize_i16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let f = self.parse_number()?;
        if f < std::i16::MIN as f64 || f > std::i16::MAX as f64 {
            self.fail(ErrorCode::OutOfBoundsI16)
        } else {
            visitor.visit_i16(f as i16)
        }
    }

    fn deserialize_i32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let f = self.parse_number()?;
        if f < std::i32::MIN as f64 || f > std::i32::MAX as f64 {
            self.fail(ErrorCode::OutOfBoundsI32)
        } else {
            visitor.visit_i32(f as i32)
        }
    }

    fn deserialize_i64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let f = self.parse_number()?;
        if f < -9007199254740992.0f64 || f > 9007199254740992.0f64 {
            self.fail(ErrorCode::OutOfBoundsI64)
        } else {
            visitor.visit_i64(f as i64)
        }
    }

    fn deserialize_u8<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let f = self.parse_number()?;
        if f > std::u8::MAX as f64 {
            self.fail(ErrorCode::OutOfBoundsU8)
        } else {
            visitor.visit_u8(f as u8)
        }
    }

    fn deserialize_u16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let f = self.parse_number()?;
        if f > std::u16::MAX as f64 {
            self.fail(ErrorCode::OutOfBoundsU16)
        } else {
            visitor.visit_u16(f as u16)
        }
    }

    fn deserialize_u32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let f = self.parse_number()?;
        if f > std::u32::MAX as f64 {
            self.fail(ErrorCode::OutOfBoundsU32)
        } else {
            visitor.visit_u32(f as u32)
        }
    }

    fn deserialize_u64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let f = self.parse_number()?;
        if f > 9007199254740992.0f64 {
            self.fail(ErrorCode::OutOfBoundsU64)
        } else {
            visitor.visit_u64(f as u64)
        }
    }

    fn deserialize_f32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        visitor.visit_f32(self.parse_number()? as f32)
    }

    fn deserialize_f64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        visitor.visit_f64(self.parse_number()?)
    }

    fn deserialize_char<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let pos = self.position();
        let s = self.parse_string()?;
        let mut chars = s.chars();

        match chars.next() {
            None => self.fail_at_position(ErrorCode::NotAChar, pos),
            Some(c) => match chars.next() {
                None => visitor.visit_char(c),
                Some(_) => self.fail_at_position(ErrorCode::NotAChar, pos),
            },
        }
    }

    fn deserialize_str<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        // We can't reference json strings directly since they contain escape sequences.
        // For the conversion, we need to allocate an owned buffer, so always do owned
        // deserialization.
        self.deserialize_string(visitor)
    }

    fn deserialize_string<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        visitor.visit_string(self.parse_string()?)
    }

    fn deserialize_bytes<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        // We can't reference bytes directly since they are stored as base64 strings.
        // For the conversion, we need to allocate an owned buffer, so always do owned
        // deserialization.
        self.deserialize_byte_buf(visitor)
    }

    fn deserialize_byte_buf<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let pos = self.position();
        match base64::decode(self.parse_naive_string()?) {
            Ok(buf) => visitor.visit_byte_buf(buf),
            Err(e) => self.fail_at_position(ErrorCode::Base64(e), pos),
        }
    }

    fn deserialize_option<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        if self.rest().starts_with(b"null") {
            self.advance(4);
            visitor.visit_none()
        } else {
            visitor.visit_some(self)
        }
    }

    fn deserialize_unit<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        self.parse_null()?;
        visitor.visit_unit()
    }

    fn deserialize_unit_struct<V>(
        self,
        _name: &'static str,
        visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        self.deserialize_unit(visitor)
    }

    fn deserialize_newtype_struct<V>(
        self,
        _name: &'static str,
        visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        visitor.visit_newtype_struct(self)
    }

    fn deserialize_seq<V>(mut self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        self.expect(0x5B, ErrorCode::ExpectedArray)?;
        let value = visitor.visit_seq(CollectionAccessor::new(&mut self))?;
        self.expect_ws(0x5D, ErrorCode::Syntax)?; // Can't fail
        Ok(value)
    }

    fn deserialize_tuple<V>(self, _len: usize, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        self.deserialize_seq(visitor)
    }

    fn deserialize_tuple_struct<V>(
        self,
        _name: &'static str,
        _len: usize,
        visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        self.deserialize_seq(visitor)
    }

    fn deserialize_map<V>(mut self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        self.expect(0x7B, ErrorCode::ExpectedObject)?;
        let value = visitor.visit_map(CollectionAccessor::new(&mut self))?;
        self.expect_ws(0x7D, ErrorCode::Syntax)?; // Can't fail
        Ok(value)
    }

    fn deserialize_struct<V>(
        self,
        _name: &'static str,
        _fields: &'static [&'static str],
        visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        self.deserialize_map(visitor)
    }

    fn deserialize_enum<V>(
        self,
        _name: &'static str,
        _variants: &'static [&'static str],
        visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let pos = self.position();
        if self.peek()? == 0x22 {
            // Visit a unit variant.
            visitor.visit_enum(self.parse_string()?.into_deserializer())
        } else if self.next()? == 0x7B {
            // Visit a newtype variant, tuple variant, or struct variant.
            let value = visitor.visit_enum(Enum::new(self))?;
            self.expect_ws(0x7D, ErrorCode::Syntax)?; // Can't fail
            Ok(value)
        } else {
            self.fail_at_position(ErrorCode::ExpectedEnum, pos)
        }
    }

    fn deserialize_identifier<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        self.deserialize_str(visitor)
    }

    fn deserialize_ignored_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        self.deserialize_any(visitor)
    }
}

struct CollectionAccessor<'a, 'de> {
    des: &'a mut JsonDeserializer<'de>,
    first: bool,
}

impl<'a, 'de> CollectionAccessor<'a, 'de> {
    fn new(des: &'a mut JsonDeserializer<'de>) -> CollectionAccessor<'a, 'de> {
        CollectionAccessor { des, first: true }
    }
}

impl<'a, 'de> SeqAccess<'de> for CollectionAccessor<'a, 'de> {
    type Error = DecodeJsonError;

    fn next_element_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
    where
        T: DeserializeSeed<'de>,
    {
        // Array ends at `]`
        if let 0x5D = self.des.peek_ws()? {
            return Ok(None);
        }

        // expect `,` before every item except the first
        if self.first {
            self.first = false;
        } else {
            self.des.expect_ws(0x2C, ErrorCode::Comma)?;
        }

        self.des.peek_ws()?;

        seed.deserialize(&mut *self.des).map(Some)
    }
}

impl<'a, 'de> MapAccess<'de> for CollectionAccessor<'a, 'de> {
    type Error = DecodeJsonError;

    fn next_key_seed<K>(&mut self, seed: K) -> Result<Option<K::Value>, Self::Error>
    where
        K: DeserializeSeed<'de>,
    {
        // Object ends at `}`
        if let 0x7D = self.des.peek_ws()? {
            return Ok(None);
        }

        // expect `,` before every item except the first
        if self.first {
            self.first = false;
        } else {
            self.des.expect_ws(0x2C, ErrorCode::Comma)?;
        }

        self.des.peek_ws()?;
        seed.deserialize(&mut *self.des).map(Some)
    }

    fn next_value_seed<V>(&mut self, seed: V) -> Result<V::Value, Self::Error>
    where
        V: DeserializeSeed<'de>,
    {
        self.des.expect_ws(0x3A, ErrorCode::Colon)?; // `:`

        self.des.peek_ws()?;
        seed.deserialize(&mut *self.des)
    }
}

struct Enum<'a, 'de> {
    des: &'a mut JsonDeserializer<'de>,
}

impl<'a, 'de> Enum<'a, 'de> {
    fn new(des: &'a mut JsonDeserializer<'de>) -> Self {
        Enum { des }
    }
}

impl<'a, 'de> EnumAccess<'de> for Enum<'a, 'de> {
    type Error = DecodeJsonError;
    type Variant = Self;

    fn variant_seed<V>(self, seed: V) -> Result<(V::Value, Self::Variant), Self::Error>
    where
        V: DeserializeSeed<'de>,
    {
        self.des.peek_ws()?;
        let val = seed.deserialize(&mut *self.des)?;
        self.des.expect_ws(0x3A, ErrorCode::Colon)?; // `:`

        self.des.peek_ws()?;
        Ok((val, self))
    }
}

impl<'a, 'de> VariantAccess<'de> for Enum<'a, 'de> {
    type Error = DecodeJsonError;

    fn unit_variant(self) -> Result<(), Self::Error> {
        eprintln!("wtf is this");
        self.des.fail(ErrorCode::ExpectedString)
    }

    fn newtype_variant_seed<T>(self, seed: T) -> Result<T::Value, Self::Error>
    where
        T: DeserializeSeed<'de>,
    {
        seed.deserialize(self.des)
    }

    fn tuple_variant<V>(self, _len: usize, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        de::Deserializer::deserialize_seq(self.des, visitor)
    }

    // Struct variants are represented in JSON as `{ NAME: { K: V, ... } }` so
    // deserialize the inner map here.
    fn struct_variant<V>(
        self,
        _fields: &'static [&'static str],
        visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        de::Deserializer::deserialize_map(self.des, visitor)
    }
}