#[derive(Debug, PartialEq)] pub enum Error { IncompleteNumber, Overflow, } /// Convert a list of numbers to a stream of bytes encoded with variable length encoding. pub fn to_bytes(values: &[u32]) -> Vec { let mut res = vec![]; for value in values { res.append(&mut to_bytes_single(*value)); } res } fn to_bytes_single(mut value: u32) -> Vec { // over allocates, but avoids growth let mut res = Vec::with_capacity(4); // 0 must be handled specially, because we need to push one byte if value == 0 { return vec![0]; } while value > 0 { // take the lower 7 bits let mut tmp = (value & 0x7f) as u8; // remove them from the original value value >>= 7; // set continuation bit if !res.is_empty() { tmp |= 0x80; } res.push(tmp); } // order is wrong due to the way we pushed the data onto it res.reverse(); res } // Alternative solution with hardcoded borders // /// Convert a list of numbers to a stream of bytes encoded with variable length encoding. // pub fn to_bytes(values: &[u32]) -> Vec { // let mut res = vec![]; // // for &value in values { // if value <= 0x7f { // res.push(value as u8); // } else if value <= 0x3fff { // res.push(((value >> 7) & 0xff) as u8 | 0x80); // res.push((value & 0x7f) as u8); // } else if value <= 0x1f_ffff { // res.push(((value >> 14) & 0xff) as u8 | 0x80); // res.push(((value >> 7) & 0xff) as u8 | 0x80); // res.push((value & 0x7f) as u8); // } else if value <= 0x0fff_ffff { // res.push(((value >> 21) & 0xff) as u8 | 0x80); // res.push(((value >> 14) & 0xff) as u8 | 0x80); // res.push(((value >> 7) & 0xff) as u8 | 0x80); // res.push((value & 0x7f) as u8); // } else { // res.push(((value >> 28) & 0xff) as u8 | 0x80); // res.push(((value >> 21) & 0xff) as u8 | 0x80); // res.push(((value >> 14) & 0xff) as u8 | 0x80); // res.push(((value >> 7) & 0xff) as u8 | 0x80); // res.push((value & 0x7f) as u8); // } // } // res // } /// Given a stream of bytes, extract all numbers which are encoded in there. pub fn from_bytes(bytes: &[u8]) -> Result, Error> { let mut res = vec![]; let mut tmp = 0; for (i, b) in bytes.iter().enumerate() { // test if first 7 bit are set, to check for overflow if (tmp & 0xfe_00_00_00) > 0 { return Err(Error::Overflow); } // append bytes of b to tmp tmp = (tmp << 7) | u32::from(b & 0x7f); if 0x80 & b == 0 { // continuation bit not set, number if complete res.push(tmp); tmp = 0; } else { // check for incomplete bytes if i + 1 == bytes.len() { // the next index would be past the end, // i.e. there are no more bytes. return Err(Error::IncompleteNumber); } } } Ok(res) }