use core::cmp::min;
use core::num::NonZeroU64;

use ::bytes::{Buf, BufMut};

use crate::DecodeError;

/// Encodes an integer value into LEB128 variable length format, and writes it to the buffer.
/// The buffer must have enough remaining space (maximum 10 bytes).
#[inline]
pub fn encode_varint(mut value: u64, buf: &mut impl BufMut) {
    // Varints are never more than 10 bytes
    for _ in 0..10 {
        if value < 0x80 {
            buf.put_u8(value as u8);
            break;
        } else {
            buf.put_u8(((value & 0x7F) | 0x80) as u8);
            value >>= 7;
        }
    }
}

/// Returns the encoded length of the value in LEB128 variable length format.
/// The returned value will be between 1 and 10, inclusive.
#[inline]
pub const fn encoded_len_varint(value: u64) -> usize {
    // Based on [VarintSize64][1].
    // [1]: https://github.com/protocolbuffers/protobuf/blob/v28.3/src/google/protobuf/io/coded_stream.h#L1744-L1756
    // Safety: value | 1 is non-zero.
    let log2value = unsafe { NonZeroU64::new_unchecked(value | 1) }.ilog2();
    ((log2value * 9 + (64 + 9)) / 64) as usize
}

/// Decodes a LEB128-encoded variable length integer from the buffer.
#[inline]
pub fn decode_varint(buf: &mut impl Buf) -> Result<u64, DecodeError> {
    let bytes = buf.chunk();
    let len = bytes.len();
    if len == 0 {
        return Err(DecodeError::new("invalid varint"));
    }

    let byte = bytes[0];
    if byte < 0x80 {
        buf.advance(1);
        Ok(u64::from(byte))
    } else if len > 10 || bytes[len - 1] < 0x80 {
        let (value, advance) = decode_varint_slice(bytes)?;
        buf.advance(advance);
        Ok(value)
    } else {
        decode_varint_slow(buf)
    }
}

/// Decodes a LEB128-encoded variable length integer from the slice, returning the value and the
/// number of bytes read.
///
/// Based loosely on [`ReadVarint64FromArray`][1] with a varint overflow check from
/// [`ConsumeVarint`][2].
///
/// ## Safety
///
/// The caller must ensure that `bytes` is non-empty and either `bytes.len() >= 10` or the last
/// element in bytes is < `0x80`.
///
/// [1]: https://github.com/google/protobuf/blob/3.3.x/src/google/protobuf/io/coded_stream.cc#L365-L406
/// [2]: https://github.com/protocolbuffers/protobuf-go/blob/v1.27.1/encoding/protowire/wire.go#L358
#[inline]
fn decode_varint_slice(bytes: &[u8]) -> Result<(u64, usize), DecodeError> {
    // Fully unrolled varint decoding loop. Splitting into 32-bit pieces gives better performance.

    // Use assertions to ensure memory safety, but it should always be optimized after inline.
    assert!(!bytes.is_empty());
    assert!(bytes.len() > 10 || bytes[bytes.len() - 1] < 0x80);

    let mut b: u8 = unsafe { *bytes.get_unchecked(0) };
    let mut part0: u32 = u32::from(b);
    if b < 0x80 {
        return Ok((u64::from(part0), 1));
    };
    part0 -= 0x80;
    b = unsafe { *bytes.get_unchecked(1) };
    part0 += u32::from(b) << 7;
    if b < 0x80 {
        return Ok((u64::from(part0), 2));
    };
    part0 -= 0x80 << 7;
    b = unsafe { *bytes.get_unchecked(2) };
    part0 += u32::from(b) << 14;
    if b < 0x80 {
        return Ok((u64::from(part0), 3));
    };
    part0 -= 0x80 << 14;
    b = unsafe { *bytes.get_unchecked(3) };
    part0 += u32::from(b) << 21;
    if b < 0x80 {
        return Ok((u64::from(part0), 4));
    };
    part0 -= 0x80 << 21;
    let value = u64::from(part0);

    b = unsafe { *bytes.get_unchecked(4) };
    let mut part1: u32 = u32::from(b);
    if b < 0x80 {
        return Ok((value + (u64::from(part1) << 28), 5));
    };
    part1 -= 0x80;
    b = unsafe { *bytes.get_unchecked(5) };
    part1 += u32::from(b) << 7;
    if b < 0x80 {
        return Ok((value + (u64::from(part1) << 28), 6));
    };
    part1 -= 0x80 << 7;
    b = unsafe { *bytes.get_unchecked(6) };
    part1 += u32::from(b) << 14;
    if b < 0x80 {
        return Ok((value + (u64::from(part1) << 28), 7));
    };
    part1 -= 0x80 << 14;
    b = unsafe { *bytes.get_unchecked(7) };
    part1 += u32::from(b) << 21;
    if b < 0x80 {
        return Ok((value + (u64::from(part1) << 28), 8));
    };
    part1 -= 0x80 << 21;
    let value = value + ((u64::from(part1)) << 28);

    b = unsafe { *bytes.get_unchecked(8) };
    let mut part2: u32 = u32::from(b);
    if b < 0x80 {
        return Ok((value + (u64::from(part2) << 56), 9));
    };
    part2 -= 0x80;
    b = unsafe { *bytes.get_unchecked(9) };
    part2 += u32::from(b) << 7;
    // Check for u64::MAX overflow. See [`ConsumeVarint`][1] for details.
    // [1]: https://github.com/protocolbuffers/protobuf-go/blob/v1.27.1/encoding/protowire/wire.go#L358
    if b < 0x02 {
        return Ok((value + (u64::from(part2) << 56), 10));
    };

    // We have overrun the maximum size of a varint (10 bytes) or the final byte caused an overflow.
    // Assume the data is corrupt.
    Err(DecodeError::new("invalid varint"))
}

/// Decodes a LEB128-encoded variable length integer from the buffer, advancing the buffer as
/// necessary.
///
/// Contains a varint overflow check from [`ConsumeVarint`][1].
///
/// [1]: https://github.com/protocolbuffers/protobuf-go/blob/v1.27.1/encoding/protowire/wire.go#L358
#[inline(never)]
#[cold]
fn decode_varint_slow(buf: &mut impl Buf) -> Result<u64, DecodeError> {
    let mut value = 0;
    for count in 0..min(10, buf.remaining()) {
        let byte = buf.get_u8();
        value |= u64::from(byte & 0x7F) << (count * 7);
        if byte <= 0x7F {
            // Check for u64::MAX overflow. See [`ConsumeVarint`][1] for details.
            // [1]: https://github.com/protocolbuffers/protobuf-go/blob/v1.27.1/encoding/protowire/wire.go#L358
            if count == 9 && byte >= 0x02 {
                return Err(DecodeError::new("invalid varint"));
            } else {
                return Ok(value);
            }
        }
    }

    Err(DecodeError::new("invalid varint"))
}

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

    #[test]
    fn varint() {
        fn check(value: u64, encoded: &[u8]) {
            // Small buffer.
            let mut buf = Vec::with_capacity(1);
            encode_varint(value, &mut buf);
            assert_eq!(buf, encoded);

            // Large buffer.
            let mut buf = Vec::with_capacity(100);
            encode_varint(value, &mut buf);
            assert_eq!(buf, encoded);

            assert_eq!(encoded_len_varint(value), encoded.len());

            // See: https://github.com/tokio-rs/prost/pull/1008 for copying reasoning.
            let mut encoded_copy = encoded;
            let roundtrip_value = decode_varint(&mut encoded_copy).expect("decoding failed");
            assert_eq!(value, roundtrip_value);

            let mut encoded_copy = encoded;
            let roundtrip_value =
                decode_varint_slow(&mut encoded_copy).expect("slow decoding failed");
            assert_eq!(value, roundtrip_value);
        }

        check(2u64.pow(0) - 1, &[0x00]);
        check(2u64.pow(0), &[0x01]);

        check(2u64.pow(7) - 1, &[0x7F]);
        check(2u64.pow(7), &[0x80, 0x01]);
        check(300, &[0xAC, 0x02]);

        check(2u64.pow(14) - 1, &[0xFF, 0x7F]);
        check(2u64.pow(14), &[0x80, 0x80, 0x01]);

        check(2u64.pow(21) - 1, &[0xFF, 0xFF, 0x7F]);
        check(2u64.pow(21), &[0x80, 0x80, 0x80, 0x01]);

        check(2u64.pow(28) - 1, &[0xFF, 0xFF, 0xFF, 0x7F]);
        check(2u64.pow(28), &[0x80, 0x80, 0x80, 0x80, 0x01]);

        check(2u64.pow(35) - 1, &[0xFF, 0xFF, 0xFF, 0xFF, 0x7F]);
        check(2u64.pow(35), &[0x80, 0x80, 0x80, 0x80, 0x80, 0x01]);

        check(2u64.pow(42) - 1, &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x7F]);
        check(2u64.pow(42), &[0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01]);

        check(
            2u64.pow(49) - 1,
            &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x7F],
        );
        check(
            2u64.pow(49),
            &[0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01],
        );

        check(
            2u64.pow(56) - 1,
            &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x7F],
        );
        check(
            2u64.pow(56),
            &[0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01],
        );

        check(
            2u64.pow(63) - 1,
            &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x7F],
        );
        check(
            2u64.pow(63),
            &[0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01],
        );

        check(
            u64::MAX,
            &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x01],
        );
    }

    const U64_MAX_PLUS_ONE: &[u8] = &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x02];

    #[test]
    fn varint_overflow() {
        let mut copy = U64_MAX_PLUS_ONE;
        decode_varint(&mut copy).expect_err("decoding u64::MAX + 1 succeeded");
    }

    #[test]
    fn variant_slow_overflow() {
        let mut copy = U64_MAX_PLUS_ONE;
        decode_varint_slow(&mut copy).expect_err("slow decoding u64::MAX + 1 succeeded");
    }
}
