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use failure::{bail, Error};

pub fn decode(buf: &[u8]) -> Result<(usize, u64), Error> {
    let mut value = 0u64;
    let mut m = 1u64;
    let mut offset = 0usize;
    for _i in 0..8 {
        if offset >= buf.len() {
            bail!["buffer supplied to varint decoding too small"]
        }
        let byte = buf[offset];
        offset += 1;
        value += m * u64::from(byte & 127);
        m *= 128;
        if byte & 128 == 0 {
            break;
        }
    }
    Ok((offset, value))
}

pub fn encode(value: u64, buf: &mut [u8]) -> Result<usize, Error> {
    return encode_with_offset(value, buf, 0);
}

pub fn encode_with_offset(value: u64, buf: &mut [u8], offset: usize) -> Result<usize, Error> {
    let len = length(value);
    if buf.len() < len {
        bail!["buffer is too small to write varint"]
    }
    let mut v = value;
    let mut off = offset;
    while v > 127 {
        buf[off] = (v as u8) | 128;
        off += 1;
        v >>= 7;
    }
    buf[off] = v as u8;
    Ok(len)
}

pub fn length(value: u64) -> usize {
    let msb = (64 - value.leading_zeros()) as usize;
    (msb.max(1) + 6) / 7
}