// Copyright (C) 2022-2023 Parity Technologies (UK) Ltd. (admin@parity.io)
// This file is a part of the scale-value crate.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//         http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#![allow(clippy::enum_variant_names)]

use super::string_helpers;
use crate::prelude::*;
use crate::value_type::{BitSequence, Composite, Primitive, Value, Variant};
use core::num::ParseIntError;
use yap::{types::StrTokens, IntoTokens, TokenLocation, Tokens};

/// A struct which will try to parse a string into a [`Value`].
/// This can be configured with custom parsers to extend what we're able
/// to parse into a [`Value`].
pub struct FromStrBuilder {
    custom_parsers: Vec<CustomParser>,
}

type CustomParser = Box<dyn Fn(&mut &str) -> Option<Result<Value<()>, ParseError>> + 'static>;

impl FromStrBuilder {
    pub(crate) fn new() -> Self {
        FromStrBuilder { custom_parsers: Vec::new() }
    }

    /// Add a custom parser. A custom parser is a function which is given a mutable string
    /// reference and will:
    ///
    /// - return `None` if the string given is not applicable,
    /// - return `Some(Ok(value))` if we can successfully parse a value from the string. In
    ///   this case, the parser should update the `&mut str` it's given to consume however
    ///   much was parsed.
    /// - return `Some(Err(error))` if the string given looks like a match, but something went
    ///   wrong in trying to properly parse it. In this case, parsing will stop immediately and
    ///   this error will be returned, so use this sparingly, as other parsers may be able to
    ///   successfully parse what this one has failed to parse. No additional tokens will be consumed
    ///   if an error occurs.
    pub fn add_custom_parser<F>(mut self, f: F) -> Self
    where
        F: Fn(&mut &str) -> Option<Result<Value<()>, ParseError>> + 'static,
    {
        self.custom_parsers.push(Box::new(f));
        self
    }

    /// Attempt to parse the string provided into a value, returning any error that occurred while
    /// attempting to do so, as well as the rest of the string that was not consumed by this parsing.
    pub fn parse<'a>(&self, s: &'a str) -> (Result<Value<()>, ParseError>, &'a str) {
        let mut tokens = s.into_tokens();
        let res = parse_value(&mut tokens, &self.custom_parsers);
        let remaining = tokens.remaining();
        (res, remaining)
    }
}

/// An error parsing the provided string into a Value
#[derive(Debug, PartialEq, Eq)]
pub struct ParseError {
    /// Byte offset into the provided string that the error begins.
    pub start_loc: usize,
    /// Byte offset into the provided string that the error ends. Many errors begin at some
    /// point but do not have a known end position.
    pub end_loc: Option<usize>,
    /// Details about the error that occurred.
    pub err: ParseErrorKind,
}

#[cfg(feature = "std")]
impl std::error::Error for ParseError {}

impl ParseError {
    /// Construct a new `ParseError` for tokens at the given location.
    pub fn new_at<E: Into<ParseErrorKind>>(err: E, loc: usize) -> Self {
        Self { start_loc: loc, end_loc: None, err: err.into() }
    }
    /// Construct a new `ParseError` for tokens between the given locations.
    pub fn new_between<E: Into<ParseErrorKind>>(err: E, start: usize, end: usize) -> Self {
        Self { start_loc: start, end_loc: Some(end), err: err.into() }
    }
}

impl core::fmt::Display for ParseError {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        if let Some(end_loc) = self.end_loc {
            write!(f, "Error from char {} to {}: {}", self.start_loc, end_loc, self.err)
        } else {
            write!(f, "Error at char {}: {}", self.start_loc, self.err)
        }
    }
}

// Add handy helper methods to error-kinds
macro_rules! at_between {
    ($ty:ident) => {
        impl $ty {
            /// Error at a specific location with no specific end
            pub fn at(self, loc: usize) -> ParseError {
                ParseError::new_at(self, loc)
            }
            /// Error at a specific location for the next character
            pub fn at_one(self, loc: usize) -> ParseError {
                ParseError::new_between(self, loc, loc + 1)
            }
            /// Error between two locations.
            pub fn between(self, start: usize, end: usize) -> ParseError {
                ParseError::new_between(self, start, end)
            }
        }
    };
}

/// Details about the error that occurred.
#[derive(Debug, derive_more::Display, derive_more::From, PartialEq, Eq)]
#[allow(missing_docs)]
pub enum ParseErrorKind {
    #[display(fmt = "Expected a value")]
    ExpectedValue,
    #[from]
    Complex(ParseComplexError),
    #[from]
    Char(ParseCharError),
    #[from]
    String(ParseStringError),
    #[from]
    Number(ParseNumberError),
    #[from]
    BitSequence(ParseBitSequenceError),
    #[from]
    Custom(String),
}
at_between!(ParseErrorKind);

impl ParseErrorKind {
    /// Construct a custom error from a type implementing [`core::fmt::Display`].
    pub fn custom<T: core::fmt::Display>(value: T) -> Self {
        ParseErrorKind::Custom(format!("{value}"))
    }

    /// Construct a custom error from a type implementing [`Debug`].
    /// Prefer [`ParseErrorKind::custom`] where possible.
    pub fn custom_debug<T: core::fmt::Debug>(value: T) -> Self {
        ParseErrorKind::Custom(format!("{value:?}"))
    }
}

#[derive(Debug, Clone, PartialEq, Eq, derive_more::Display)]
#[allow(missing_docs)]
pub enum ParseComplexError {
    #[display(fmt = "The first character in a field name should be alphabetic")]
    InvalidStartingCharacterInIdent,
    #[display(
        fmt = "Field name is not valid (it should begin with an alphabetical char and then consist only of alphanumeric chars)"
    )]
    InvalidFieldName,
    #[display(fmt = "Missing field separator; expected {_0}")]
    MissingFieldSeparator(char),
    #[display(fmt = "Missing closing '{_0}'")]
    ExpectedCloserToMatch(char, usize),
}
at_between!(ParseComplexError);

#[derive(Debug, Clone, PartialEq, Eq, derive_more::Display)]
#[allow(missing_docs)]
pub enum ParseCharError {
    #[display(fmt = "Expected a single character")]
    ExpectedValidCharacter,
    #[display(fmt = "Expected an escape code to follow the '\\'")]
    ExpectedValidEscapeCode,
    #[display(fmt = "Expected a closing quote to match the opening quote at position {_0}")]
    ExpectedClosingQuoteToMatch(usize),
}
at_between!(ParseCharError);

#[derive(Debug, Clone, PartialEq, Eq, derive_more::Display)]
#[allow(missing_docs)]
pub enum ParseStringError {
    #[display(fmt = "Expected a closing quote to match the opening quote at position {_0}")]
    ExpectedClosingQuoteToMatch(usize),
    #[display(fmt = "Expected an escape code to follow the '\\'")]
    ExpectedValidEscapeCode,
}
at_between!(ParseStringError);

#[derive(Debug, Clone, PartialEq, Eq, derive_more::Display)]
#[allow(missing_docs)]
pub enum ParseNumberError {
    #[display(fmt = "Expected one or more digits")]
    ExpectedDigit,
    #[display(fmt = "Failed to parse digits into an integer: {_0}")]
    ParsingFailed(ParseIntError),
}
at_between!(ParseNumberError);

#[derive(Debug, Clone, PartialEq, Eq, derive_more::Display)]
#[allow(missing_docs)]
pub enum ParseBitSequenceError {
    #[display(fmt = "Expected a closing bracket ('>') to match the opening one at position {_0}")]
    ExpectedClosingBracketToMatch(usize),
    #[display(fmt = "Invalid character; expecting a 0 or 1")]
    InvalidCharacter,
}
at_between!(ParseBitSequenceError);

// Parse a value.
fn parse_value(
    t: &mut StrTokens,
    custom_parsers: &[CustomParser],
) -> Result<Value<()>, ParseError> {
    // Try any custom parsers first.
    if !custom_parsers.is_empty() {
        let s = t.remaining();
        let start_offset = t.offset();
        let cursor = &mut &*s;

        for parser in custom_parsers {
            if let Some(res) = parser(cursor) {
                match res {
                    Ok(value) => {
                        // Wind our StrTokens forward to take into account anything that
                        // was consumed. We do this rather than replacing it because we want
                        // to preserve the full string and offsets.
                        for _ in cursor.len()..s.len() {
                            t.next();
                        }
                        return Ok(value);
                    }
                    Err(e) => {
                        // Adjust locations in error to be relative to the big string,
                        // not the smaller slice that was passed to the custom parser.
                        return Err(ParseError {
                            start_loc: start_offset + e.start_loc,
                            end_loc: e.end_loc.map(|l| start_offset + l),
                            err: e.err,
                        });
                    }
                }
            }
        }
    }

    // Our parsers return `Result<Thing, Option<ParseError>>`, but in order to know
    // whether to try the next item, `one_of` expects `Option<T>`, so we transpose_err
    // to convert to the right shape.
    let val = yap::one_of!(t;
        transpose_err(parse_bool(t).map(Value::bool).ok_or(None)),
        transpose_err(parse_char(t).map(Value::char)),
        transpose_err(parse_string(t).map(Value::string)),
        transpose_err(parse_number(t).map(Value::primitive)),
        transpose_err(parse_named_composite(t, custom_parsers).map(|v| v.into())),
        transpose_err(parse_unnamed_composite(t, custom_parsers).map(|v| v.into())),
        transpose_err(parse_bit_sequence(t).map(Value::bit_sequence)),
        transpose_err(parse_variant(t, custom_parsers).map(|v| v.into())),
    );

    match val {
        Some(Ok(val)) => Ok(val),
        Some(Err(e)) => Err(e),
        None => Err(ParseError::new_at(ParseErrorKind::ExpectedValue, t.offset())),
    }
}

// Parse a named composite value like `{ foo: 123 }`.
//
// As with most of the parsers here, the error is optional. A `Some` error indicates that
// we're midway through parsing something and have run into an error. a `None` error indicates
// that we can see up front that the characters we're parsing aren't going to be the right shape,
// and can attempt to parse the characters into a different thing if we wish.
fn parse_named_composite(
    t: &mut StrTokens,
    custom_parsers: &[CustomParser],
) -> Result<Composite<()>, Option<ParseError>> {
    let start = t.offset();
    if !t.token('{') {
        return Err(None);
    }
    skip_whitespace(t);

    // No values? bail early.
    if t.token('}') {
        return Ok(Composite::Named(vec![]));
    }

    let vals = t
        .sep_by_err(
            |t| parse_field_name_and_value(t, custom_parsers),
            |t| skip_spaced_separator(t, ','),
        )
        .collect::<Result<_, _>>()?;

    skip_whitespace(t);
    if !t.token('}') {
        return Err(Some(ParseComplexError::ExpectedCloserToMatch('}', start).at_one(t.offset())));
    }
    Ok(Composite::Named(vals))
}

// Parse an unnamed composite value like `(true, 123)`
fn parse_unnamed_composite(
    t: &mut StrTokens,
    custom_parsers: &[CustomParser],
) -> Result<Composite<()>, Option<ParseError>> {
    let start = t.offset();
    if !t.token('(') {
        return Err(None);
    }
    skip_whitespace(t);

    // No values? bail early.
    if t.token(')') {
        return Ok(Composite::Unnamed(vec![]));
    }

    let vals = t
        .sep_by_err(|t| parse_value(t, custom_parsers), |t| skip_spaced_separator(t, ','))
        .collect::<Result<_, _>>()?;

    skip_whitespace(t);
    if !t.token(')') {
        return Err(Some(ParseComplexError::ExpectedCloserToMatch(')', start).at_one(t.offset())));
    }
    Ok(Composite::Unnamed(vals))
}

// Parse a variant like `Variant { hello: "there" }` or `Foo (123, true)`
fn parse_variant(
    t: &mut StrTokens,
    custom_parsers: &[CustomParser],
) -> Result<Variant<()>, Option<ParseError>> {
    let ident = match parse_optional_variant_ident(t) {
        Some(ident) => ident,
        None => return Err(None),
    };

    skip_whitespace(t);

    let composite = yap::one_of!(t;
        transpose_err(parse_named_composite(t, custom_parsers)),
        transpose_err(parse_unnamed_composite(t, custom_parsers))
    );

    match composite {
        Some(Ok(values)) => Ok(Variant { name: ident, values }),
        Some(Err(e)) => Err(Some(e)),
        None => Err(None),
    }
}

// Parse a sequence of bits like `<01101>` or `<>` into a bit sequence.
fn parse_bit_sequence(t: &mut StrTokens) -> Result<BitSequence, Option<ParseError>> {
    let start = t.offset();
    if !t.token('<') {
        return Err(None);
    }
    let bits = t.tokens_while(|&c| c == '0' || c == '1').map(|c| c == '1');
    let mut seq = BitSequence::new();
    for bit in bits {
        seq.push(bit);
    }
    if !t.token('>') {
        return Err(Some(
            ParseBitSequenceError::ExpectedClosingBracketToMatch(start)
                .between(t.offset(), t.offset() + 1),
        ));
    }
    Ok(seq)
}

// Parse a bool (`true` or `false`)
fn parse_bool(t: &mut StrTokens) -> Option<bool> {
    if t.tokens("true".chars()) {
        Some(true)
    } else if t.tokens("false".chars()) {
        Some(false)
    } else {
        None
    }
}

// Parse a char like `'a'`
fn parse_char(t: &mut StrTokens) -> Result<char, Option<ParseError>> {
    let start = t.offset();
    if !t.token('\'') {
        return Err(None);
    }
    let char = match t.next() {
        None => return Err(Some(ParseCharError::ExpectedValidCharacter.at_one(t.offset()))),
        Some(c) => c,
    };

    // If char is a backslash, it's an escape code and we
    // need to unescape it to find our inner char:
    let char = if char == '\\' {
        let escape_code = match t.next() {
            None => return Err(Some(ParseCharError::ExpectedValidEscapeCode.at_one(t.offset()))),
            Some(c) => c,
        };
        match string_helpers::from_escape_code(escape_code) {
            None => return Err(Some(ParseCharError::ExpectedValidEscapeCode.at_one(t.offset()))),
            Some(c) => c,
        }
    } else {
        char
    };

    if !t.token('\'') {
        return Err(Some(ParseCharError::ExpectedClosingQuoteToMatch(start).at_one(t.offset())));
    }
    Ok(char)
}

// Parse a number like `-123_456` or `234` or `+1234_5`
fn parse_number(t: &mut StrTokens) -> Result<Primitive, Option<ParseError>> {
    let start_loc = t.offset();
    let is_positive = t.token('+') || !t.token('-');

    // When we iterate numeric digits, prefix a sign as needed:
    let sign = if is_positive { "".chars() } else { "-".chars() };

    // Now, we expect a digit and then a mix of digits and underscores:
    let mut seen_n = false;
    let digits = t
        .tokens_while(|c| {
            if c.is_ascii_digit() {
                seen_n = true;
                true
            } else {
                seen_n && *c == '_'
            }
        })
        .filter(|c| c.is_ascii_digit());

    // Chain sign to digits and attempt to parse into a number.
    let n_str: String = sign.chain(digits).collect();
    let end_loc = t.offset();

    // Nothing was parsed; Return None.
    if end_loc == start_loc {
        return Err(None);
    }

    // No digits were parsed but a sign was; err.
    if !seen_n {
        return Err(Some(ParseNumberError::ExpectedDigit.between(end_loc, end_loc + 1)));
    }

    // Parse into a number as best we can:
    if is_positive {
        n_str
            .parse::<u128>()
            .map(Primitive::u128)
            .map_err(|e| Some(ParseNumberError::ParsingFailed(e).between(start_loc, end_loc)))
    } else {
        n_str
            .parse::<i128>()
            .map(Primitive::i128)
            .map_err(|e| Some(ParseNumberError::ParsingFailed(e).between(start_loc, end_loc)))
    }
}

// Parse a string like `"hello\n there"`
fn parse_string(t: &mut StrTokens) -> Result<String, Option<ParseError>> {
    let start = t.offset();
    if !t.token('"') {
        return Err(None);
    }

    let mut out: String = String::new();
    let mut next_is_escaped = false;
    loop {
        let pos = t.offset();
        let char = match t.next() {
            Some(c) => c,
            None => {
                return Err(Some(
                    ParseStringError::ExpectedClosingQuoteToMatch(start).at_one(t.offset()),
                ))
            }
        };

        match char {
            // Escape a char:
            '\\' if !next_is_escaped => {
                next_is_escaped = true;
            }
            // Handle escaped chars:
            c if next_is_escaped => match string_helpers::from_escape_code(c) {
                Some(c) => {
                    out.push(c);
                    next_is_escaped = false;
                }
                None => {
                    return Err(Some(
                        ParseStringError::ExpectedValidEscapeCode.between(pos, pos + 1),
                    ))
                }
            },
            // String has closed
            '"' => {
                break; // closing quote seen; done!
            }
            // All other chars pushed as-is.
            c => {
                out.push(c);
            }
        }
    }

    Ok(out)
}

// Parse a field in a named composite like `foo: 123` or `"hello there": 123`
fn parse_field_name_and_value(
    t: &mut StrTokens,
    custom_parsers: &[CustomParser],
) -> Result<(String, Value<()>), ParseError> {
    let name = parse_field_name(t)?;
    if !skip_spaced_separator(t, ':') {
        return Err(ParseComplexError::MissingFieldSeparator(':').at_one(t.offset()));
    }
    let value = parse_value(t, custom_parsers)?;
    Ok((name, value))
}

// Parse a field name in a named composite like `foo` or `"hello there"`
fn parse_field_name(t: &mut StrTokens) -> Result<String, ParseError> {
    let field_name = yap::one_of!(t;
        transpose_err(parse_string(t)),
        Some(parse_ident(t)),
    );

    match field_name {
        Some(Ok(name)) => Ok(name),
        Some(Err(e)) => Err(e),
        None => Err(ParseComplexError::InvalidFieldName.at(t.offset())),
    }
}

// Parse an ident used for the variant name, like `MyVariant` or the special case
// `i"My variant name"` for idents that are not normally valid variant names, but
// can be set in Value variants (this is to ensure that we can display and then parse
// as many user-generated Values as possible).
fn parse_optional_variant_ident(t: &mut StrTokens) -> Option<String> {
    fn parse_i_string(t: &mut StrTokens) -> Option<String> {
        if t.next()? != 'v' {
            return None;
        }
        parse_string(t).ok()
    }

    yap::one_of!(t;
        parse_i_string(t),
        parse_ident(t).ok()
    )
}

// Parse an ident like `foo` or `MyVariant`
fn parse_ident(t: &mut StrTokens) -> Result<String, ParseError> {
    let start = t.location();
    if t.skip_tokens_while(|c| c.is_alphabetic()) == 0 {
        return Err(ParseComplexError::InvalidStartingCharacterInIdent.at_one(start.offset()));
    }
    t.skip_tokens_while(|c| c.is_alphanumeric() || *c == '_');
    let end = t.location();

    let ident_str = t.slice(start, end).as_iter().collect();
    Ok(ident_str)
}

// Skip any whitespace characters
fn skip_whitespace(t: &mut StrTokens) {
    t.skip_tokens_while(|c| c.is_whitespace());
}

// Skip a provided separator, with optional spaces on either side
fn skip_spaced_separator(t: &mut StrTokens, s: char) -> bool {
    skip_whitespace(t);
    let is_sep = t.token(s);
    skip_whitespace(t);
    is_sep
}

// Turn a ` Result<T, Option<E>>` into `Option<Result<T, E>>`.
fn transpose_err<T, E>(r: Result<T, Option<E>>) -> Option<Result<T, E>> {
    match r {
        Ok(val) => Some(Ok(val)),
        Err(Some(e)) => Some(Err(e)),
        Err(None) => None,
    }
}

#[cfg(test)]
mod test {
    use crate::value;

    use super::*;

    fn from(s: &str) -> Result<Value<()>, ParseError> {
        let (res, remaining) = FromStrBuilder::new().parse(s);
        match res {
            Ok(value) => {
                assert_eq!(remaining.len(), 0, "was not expecting any unparsed output");
                Ok(value)
            }
            Err(e) => Err(e),
        }
    }

    #[test]
    fn parse_bools() {
        assert_eq!(from("true"), Ok(Value::bool(true)));
        assert_eq!(from("false"), Ok(Value::bool(false)));
    }

    #[test]
    fn parse_numbers() {
        assert_eq!(from("123"), Ok(Value::u128(123)));
        assert_eq!(from("1_234_56"), Ok(Value::u128(123_456)));
        assert_eq!(from("+1_234_56"), Ok(Value::u128(123_456)));
        assert_eq!(from("-123_4"), Ok(Value::i128(-1234)));
        assert_eq!(from("-abc"), Err(ParseNumberError::ExpectedDigit.between(1, 2)));
    }

    #[test]
    fn parse_chars() {
        assert_eq!(from("'a'"), Ok(Value::char('a')));
        assert_eq!(from("'😀'"), Ok(Value::char('😀')));
        assert_eq!(from("'\\n'"), Ok(Value::char('\n')));
        assert_eq!(from("'\\t'"), Ok(Value::char('\t')));
        assert_eq!(from("'\\\"'"), Ok(Value::char('"')));
        assert_eq!(from("'\\\''"), Ok(Value::char('\'')));
        assert_eq!(from("'\\r'"), Ok(Value::char('\r')));
        assert_eq!(from("'\\\\'"), Ok(Value::char('\\')));
        assert_eq!(from("'\\0'"), Ok(Value::char('\0')));
        assert_eq!(from("'a"), Err(ParseCharError::ExpectedClosingQuoteToMatch(0).at_one(2)));
    }

    #[test]
    fn parse_strings() {
        assert_eq!(from("\"\\n \\r \\t \\0 \\\"\""), Ok(Value::string("\n \r \t \0 \"")));
        assert_eq!(from("\"Hello there 😀\""), Ok(Value::string("Hello there 😀")));
        assert_eq!(from("\"Hello\\n\\t there\""), Ok(Value::string("Hello\n\t there")));
        assert_eq!(from("\"Hello\\\\ there\""), Ok(Value::string("Hello\\ there")));
        assert_eq!(
            from("\"Hello\\p there\""),
            Err(ParseStringError::ExpectedValidEscapeCode.between(7, 8))
        );
        assert_eq!(from("\"Hi"), Err(ParseStringError::ExpectedClosingQuoteToMatch(0).at_one(3)));
    }

    #[test]
    fn parse_unnamed_composites() {
        assert_eq!(
            from("(  true, 1234 ,\t\n\t \"Hello!\" )"),
            Ok(value!((true, 1234u32, "Hello!")))
        );
        assert_eq!(from("()"), Ok(value!(())));
        assert_eq!(from("(\n\n\t\t\n)"), Ok(value!(())));
    }

    #[test]
    fn parse_named_composites() {
        assert_eq!(
            from(
                "{
            hello: true,
            foo: 1234,
            \"Hello there 😀\": \"Hello!\"
        }"
            ),
            Ok(Value::named_composite([
                ("hello", Value::bool(true)),
                ("foo", Value::u128(1234)),
                ("Hello there 😀", Value::string("Hello!"))
            ]))
        );
    }

    #[test]
    fn parse_variants() {
        assert_eq!(
            from(
                "MyVariant {
            hello: true,
            foo: 1234,
            \"Hello there 😀\": \"Hello!\"
        }"
            ),
            Ok(Value::named_variant(
                "MyVariant",
                [
                    ("hello", Value::bool(true)),
                    ("foo", Value::u128(1234)),
                    ("Hello there 😀", Value::string("Hello!"))
                ]
            ))
        );

        assert_eq!(
            from("Foo (  true, 1234 ,\t\n\t \"Hello!\" )"),
            Ok(value!(Foo(true, 1234u32, "Hello!")))
        );

        assert_eq!(from("Foo()"), Ok(value!(Foo())));
        assert_eq!(from("Foo{}"), Ok(value!(Foo {})));
        assert_eq!(from("Foo( \t)"), Ok(value!(Foo())));
        assert_eq!(from("Foo{  }"), Ok(value!(Foo {})));

        // Parsing special "v" strings:
        assert_eq!(
            from("v\"variant name\" {  }"),
            Ok(Value::named_variant::<_, String, _>("variant name", []))
        );
    }

    #[test]
    fn parse_bit_sequences() {
        use scale_bits::bits;
        assert_eq!(
            from("<011010110101101>"),
            Ok(Value::bit_sequence(bits![0, 1, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 0, 1]))
        );
        assert_eq!(from("<01101>"), Ok(Value::bit_sequence(bits![0, 1, 1, 0, 1])));
        assert_eq!(from("<0>"), Ok(Value::bit_sequence(bits![0])));
        assert_eq!(from("<>"), Ok(Value::bit_sequence(bits![])));
    }

    #[test]
    fn custom_parsers() {
        let custom_parser = FromStrBuilder::new()
            // We add the ability to parse custom hex strings:
            .add_custom_parser(|s| {
                let mut toks = s.into_tokens();

                // Return None if this clearly isn't hex.
                if !toks.tokens("0x".chars()) {
                    return None;
                }

                let from = toks.location();
                let num_hex_chars = toks.skip_tokens_while(|c| {
                    c.is_numeric()
                        || ['a', 'b', 'c', 'd', 'e', 'f'].contains(&c.to_ascii_lowercase())
                });

                // Return an error if is _looks_ like hex but something isn't right about it.
                if num_hex_chars % 2 != 0 {
                    let e = ParseErrorKind::custom("Wrong number hex")
                        .between(from.offset(), toks.offset());
                    return Some(Err(e));
                }

                // For testing, we just dump the hex chars into a string.
                let hex: String = toks.slice(from, toks.location()).as_iter().collect();
                // Since we parsed stuff, we need to update the cursor to consume what we parsed.
                *s = toks.remaining();

                Some(Ok(Value::string(hex)))
            });

        let expected = [
            // Hex can be parsed as a part of values now!
            ("(1, 0x1234, true)", (Ok(value!((1u8, "1234", true))), "")),
            // Invalid hex emits the expected custom error:
            (
                "0x12345zzz",
                (Err(ParseErrorKind::custom("Wrong number hex").between(2, 7)), "0x12345zzz"),
            ),
            // Custom error locations are relative to the entire string:
            (
                "(true, 0x12345)",
                (Err(ParseErrorKind::custom("Wrong number hex").between(9, 14)), ", 0x12345)"),
            ),
        ];

        for (s, v) in expected {
            let (expected_res, expected_leftover) = (v.0, v.1);
            let (res, leftover) = custom_parser.parse(s);
            assert_eq!(res, expected_res, "result isn't what we expected for: {s}");
            assert_eq!(leftover, expected_leftover, "wrong number of leftover bytes for: {s}");
        }
    }
}