use num_traits::{NumCast, PrimInt};
use std::iter::Peekable;

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
pub enum Error {
    Overflow,
    Empty,
    InvalidDigit,
    CharsLeft,
}

struct ParseResult {
    result: u64,
    negative: bool,
    consumed_all: bool,
}

/// Helper to get the current char, or \0.
fn current<Chars>(chars: &mut Peekable<Chars>) -> char
where
    Chars: Iterator<Item = char>,
{
    match chars.peek() {
        Some(c) => *c,
        None => '\0',
    }
}

/// Parse the given \p src as an integer.
/// If mradix is not None, it is used as the radix; otherwise the radix is inferred:
///   - Leading 0x or 0X means 16.
///   - Leading 0 means 8.
///   - Otherwise 10.
/// The parse result contains the number as a u64, and whether it was negative.
fn fish_parse_radix<Chars>(ichars: Chars, mradix: Option<u32>) -> Result<ParseResult, Error>
where
    Chars: Iterator<Item = char>,
{
    if let Some(r) = mradix {
        assert!((2..=36).contains(&r), "fish_parse_radix: invalid radix {r}");
    }
    let chars = &mut ichars.peekable();

    // Skip leading whitespace.
    while current(chars).is_whitespace() {
        chars.next();
    }

    if chars.peek().is_none() {
        return Err(Error::Empty);
    }

    // Consume leading +/-.
    let mut negative;
    match current(chars) {
        '-' | '+' => {
            negative = current(chars) == '-';
            chars.next();
        }
        _ => negative = false,
    }

    // Determine the radix.
    let radix = if let Some(radix) = mradix {
        radix
    } else if current(chars) == '0' {
        chars.next();
        match current(chars) {
            'x' | 'X' => {
                chars.next();
                16
            }
            c if ('0'..='9').contains(&c) => 8,
            _ => {
                // Just a 0.
                return Ok(ParseResult {
                    result: 0,
                    negative: false,
                    consumed_all: chars.peek() == None,
                });
            }
        }
    } else {
        10
    };

    // Compute as u64.
    let mut consumed1 = false;
    let mut result: u64 = 0;
    while let Some(digit) = current(chars).to_digit(radix) {
        result = result
            .checked_mul(radix as u64)
            .and_then(|r| r.checked_add(digit as u64))
            .ok_or(Error::Overflow)?;
        chars.next();
        consumed1 = true;
    }

    // Did we consume at least one char?
    if !consumed1 {
        return Err(Error::InvalidDigit);
    }

    // Do not return -0.
    if result == 0 {
        negative = false;
    }
    let consumed_all = chars.peek() == None;
    Ok(ParseResult { result, negative, consumed_all })
}

/// Parse some iterator over Chars into some Integer type, optionally with a radix.
fn fish_wcstoi_impl<Int, Chars>(src: Chars, mradix: Option<u32>, consume_all: bool) -> Result<Int, Error>
where
    Chars: Iterator<Item = char>,
    Int: PrimInt,
{
    let bits = Int::zero().count_zeros();
    assert!(bits <= 64, "fish_wcstoi: Int must be <= 64 bits");
    let signed = Int::min_value() < Int::zero();

    let ParseResult {
        result, negative, consumed_all, ..
    } = fish_parse_radix(src, mradix)?;

    if !signed && negative {
        Err(Error::InvalidDigit)
    } else if consume_all && !consumed_all {
        Err(Error::CharsLeft)
    } else if !signed || !negative {
        match Int::from(result) {
            Some(r) => Ok(r),
            None => Err(Error::Overflow),
        }
    } else {
        assert!(signed && negative);
        // Signed type, so convert to s64.
        // Careful of the most negative value.
        if bits == 64 && result == 1 << 63 {
            return Ok(Int::min_value());
        }
        <i64 as NumCast>::from(result)
            .and_then(|r| r.checked_neg())
            .and_then(|r| Int::from(r))
            .ok_or(Error::Overflow)
    }
}

/// Convert the given wide string to an integer.
/// The semantics here match wcstol():
///  - Leading whitespace is skipped.
///  - 0 means octal, 0x means hex
///  - Leading + is supported.
pub fn fish_wcstoi<Int, Chars>(src: Chars) -> Result<Int, Error>
where
    Chars: Iterator<Item = char>,
    Int: PrimInt,
{
    fish_wcstoi_impl(src, None, false)
}

/// Convert the given wide string to an integer using the given radix.
/// Leading whitespace is skipped.
pub fn fish_wcstoi_radix<Int, Chars>(src: Chars, radix: u32) -> Result<Int, Error>
where
    Chars: Iterator<Item = char>,
    Int: PrimInt,
{
    fish_wcstoi_impl(src, Some(radix), false)
}

pub fn fish_wcstoi_radix_all<Int, Chars>(src: Chars, radix: Option<u32>, consume_all: bool) -> Result<Int, Error>
where
    Chars: Iterator<Item = char>,
    Int: PrimInt,
{
    fish_wcstoi_impl(src, radix, consume_all)
}

#[cfg(test)]
mod tests {
    use super::*;

    fn test_min_max<Int: PrimInt + std::fmt::Display + std::fmt::Debug>(min: Int, max: Int) {
        assert_eq!(fish_wcstoi(min.to_string().chars()), Ok(min));
        assert_eq!(fish_wcstoi(max.to_string().chars()), Ok(max));
    }

    #[test]
    fn tests() {
        let run1 = |s: &str| -> Result<i32, Error> { fish_wcstoi(s.chars()) };
        let run1_rad =
            |s: &str, radix: u32| -> Result<i32, Error> { fish_wcstoi_radix(s.chars(), radix) };
        assert_eq!(run1(""), Err(Error::Empty));
        assert_eq!(run1("   \n   "), Err(Error::Empty));
        assert_eq!(run1("0"), Ok(0));
        assert_eq!(run1("-0"), Ok(0));
        assert_eq!(run1("+0"), Ok(0));
        assert_eq!(run1("+-0"), Err(Error::InvalidDigit));
        assert_eq!(run1("-+0"), Err(Error::InvalidDigit));
        assert_eq!(run1("123"), Ok(123));
        assert_eq!(run1("+123"), Ok(123));
        assert_eq!(run1("-123"), Ok(-123));
        assert_eq!(run1("123"), Ok(123));
        assert_eq!(run1("+0x123"), Ok(291));
        assert_eq!(run1("-0x123"), Ok(-291));
        assert_eq!(run1("+0X123"), Ok(291));
        assert_eq!(run1("-0X123"), Ok(-291));
        assert_eq!(run1("+0123"), Ok(83));
        assert_eq!(run1("-0123"), Ok(-83));
        assert_eq!(run1("  345  "), Ok(345));
        assert_eq!(run1("  -345  "), Ok(-345));
        assert_eq!(run1("  x345"), Err(Error::InvalidDigit));
        assert_eq!(run1("456x"), Ok(456));
        assert_eq!(run1("456 x"), Ok(456));
        assert_eq!(run1("99999999999999999999999"), Err(Error::Overflow));
        assert_eq!(run1("-99999999999999999999999"), Err(Error::Overflow));
        // This is subtle. "567" in base 8 is "375" in base 10. The final "8" is not converted.
        assert_eq!(run1_rad("5678", 8), Ok(375));

        test_min_max(std::i8::MIN, std::i8::MAX);
        test_min_max(std::i16::MIN, std::i16::MAX);
        test_min_max(std::i32::MIN, std::i32::MAX);
        test_min_max(std::i64::MIN, std::i64::MAX);
        test_min_max(std::u8::MIN, std::u8::MAX);
        test_min_max(std::u16::MIN, std::u16::MAX);
        test_min_max(std::u32::MIN, std::u32::MAX);
        test_min_max(std::u64::MIN, std::u64::MAX);
    }
}