136 lines
3.8 KiB
Rust
136 lines
3.8 KiB
Rust
/// Wraps a slice and enables reading arbitrary amounts of bits
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/// from that slice.
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pub struct BitReader<'s> {
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idx: usize, //index counts bits already read
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source: &'s [u8],
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}
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impl<'s> BitReader<'s> {
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pub fn new(source: &'s [u8]) -> BitReader<'s> {
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BitReader { idx: 0, source }
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}
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pub fn bits_left(&self) -> usize {
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self.source.len() * 8 - self.idx
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}
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pub fn bits_read(&self) -> usize {
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self.idx
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}
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pub fn return_bits(&mut self, n: usize) {
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if n > self.idx {
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panic!("Cant return this many bits");
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}
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self.idx -= n;
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}
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pub fn get_bits(&mut self, n: usize) -> Result<u64, GetBitsError> {
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if n > 64 {
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return Err(GetBitsError::TooManyBits {
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num_requested_bits: n,
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limit: 64,
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});
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}
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if self.bits_left() < n {
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return Err(GetBitsError::NotEnoughRemainingBits {
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requested: n,
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remaining: self.bits_left(),
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});
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}
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let old_idx = self.idx;
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let bits_left_in_current_byte = 8 - (self.idx % 8);
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let bits_not_needed_in_current_byte = 8 - bits_left_in_current_byte;
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//collect bits from the currently pointed to byte
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let mut value = u64::from(self.source[self.idx / 8] >> bits_not_needed_in_current_byte);
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if bits_left_in_current_byte >= n {
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//no need for fancy stuff
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//just mask all but the needed n bit
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value &= (1 << n) - 1;
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self.idx += n;
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} else {
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self.idx += bits_left_in_current_byte;
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//n spans over multiple bytes
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let full_bytes_needed = (n - bits_left_in_current_byte) / 8;
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let bits_in_last_byte_needed = n - bits_left_in_current_byte - full_bytes_needed * 8;
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assert!(
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bits_left_in_current_byte + full_bytes_needed * 8 + bits_in_last_byte_needed == n
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);
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let mut bit_shift = bits_left_in_current_byte; //this many bits are already set in value
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assert!(self.idx % 8 == 0);
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//collect full bytes
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for _ in 0..full_bytes_needed {
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value |= u64::from(self.source[self.idx / 8]) << bit_shift;
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self.idx += 8;
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bit_shift += 8;
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}
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assert!(n - bit_shift == bits_in_last_byte_needed);
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if bits_in_last_byte_needed > 0 {
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let val_las_byte =
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u64::from(self.source[self.idx / 8]) & ((1 << bits_in_last_byte_needed) - 1);
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value |= val_las_byte << bit_shift;
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self.idx += bits_in_last_byte_needed;
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}
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}
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assert!(self.idx == old_idx + n);
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Ok(value)
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}
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}
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#[derive(Debug)]
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#[non_exhaustive]
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pub enum GetBitsError {
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TooManyBits {
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num_requested_bits: usize,
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limit: u8,
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},
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NotEnoughRemainingBits {
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requested: usize,
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remaining: usize,
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},
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}
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#[cfg(feature = "std")]
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impl std::error::Error for GetBitsError {}
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impl core::fmt::Display for GetBitsError {
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fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
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match self {
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GetBitsError::TooManyBits {
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num_requested_bits,
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limit,
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} => {
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write!(
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f,
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"Cant serve this request. The reader is limited to {} bits, requested {} bits",
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limit, num_requested_bits,
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)
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}
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GetBitsError::NotEnoughRemainingBits {
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requested,
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remaining,
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} => {
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write!(
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f,
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"Can\'t read {} bits, only have {} bits left",
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requested, remaining,
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)
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}
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}
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}
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}
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