postbag 0.4.3

Postbag is a high-performance serde codec for Rust that provides efficient data encoding with configurable levels of forward and backward compatibility.
Documentation 
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use std::mem::size_of;

/// Returns the maximum number of bytes required to encode T.
pub const fn varint_max<T: Sized>() -> usize {
    const BITS_PER_BYTE: usize = 8;
    const BITS_PER_VARINT_BYTE: usize = 7;

    // How many data bits do we need for this type?
    let bits = size_of::<T>() * BITS_PER_BYTE;

    // We add (BITS_PER_VARINT_BYTE - 1), to ensure any integer divisions
    // with a remainder will always add exactly one full byte, but
    // an evenly divided number of bits will be the same
    let roundup_bits = bits + (BITS_PER_VARINT_BYTE - 1);

    // Apply division, using normal "round down" integer division
    roundup_bits / BITS_PER_VARINT_BYTE
}

/// Returns the maximum value stored in the last encoded byte.
pub const fn max_of_last_byte<T: Sized>() -> u8 {
    let max_bits = size_of::<T>() * 8;
    let extra_bits = max_bits % 7;
    (1 << extra_bits) - 1
}

pub fn varint_u16(n: u16, out: &mut [u8; varint_max::<u16>()]) -> &mut [u8] {
    let mut value = n;
    for i in 0..varint_max::<u16>() {
        out[i] = value.to_le_bytes()[0];
        if value < 128 {
            return &mut out[..=i];
        }

        out[i] |= 0x80;
        value >>= 7;
    }
    debug_assert_eq!(value, 0);
    &mut out[..]
}

pub fn varint_u32(n: u32, out: &mut [u8; varint_max::<u32>()]) -> &mut [u8] {
    let mut value = n;
    for i in 0..varint_max::<u32>() {
        out[i] = value.to_le_bytes()[0];
        if value < 128 {
            return &mut out[..=i];
        }

        out[i] |= 0x80;
        value >>= 7;
    }
    debug_assert_eq!(value, 0);
    &mut out[..]
}

pub fn varint_u64(n: u64, out: &mut [u8; varint_max::<u64>()]) -> &mut [u8] {
    let mut value = n;
    for i in 0..varint_max::<u64>() {
        out[i] = value.to_le_bytes()[0];
        if value < 128 {
            return &mut out[..=i];
        }

        out[i] |= 0x80;
        value >>= 7;
    }
    debug_assert_eq!(value, 0);
    &mut out[..]
}

pub fn varint_u128(n: u128, out: &mut [u8; varint_max::<u128>()]) -> &mut [u8] {
    let mut value = n;
    for i in 0..varint_max::<u128>() {
        out[i] = value.to_le_bytes()[0];
        if value < 128 {
            return &mut out[..=i];
        }

        out[i] |= 0x80;
        value >>= 7;
    }
    debug_assert_eq!(value, 0);
    &mut out[..]
}

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

    #[test]
    fn usize_varint_encode() {
        let mut buf = [0; varint_max::<usize>()];
        let res = varint_u64(1, &mut buf);

        assert_eq!(&[1], res);

        let res = varint_u64(u64::MAX, &mut buf);

        assert_eq!(&[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x01], res);
    }
}