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|
pub use super::bit_reader::GetBitsError;
use byteorder::ByteOrder;
use byteorder::LittleEndian;
pub struct BitReaderReversed<'s> {
idx: isize, //index counts bits already read
source: &'s [u8],
bit_container: u64,
bits_in_container: u8,
}
impl<'s> BitReaderReversed<'s> {
pub fn bits_remaining(&self) -> isize {
self.idx + self.bits_in_container as isize
}
pub fn new(source: &'s [u8]) -> BitReaderReversed<'_> {
BitReaderReversed {
idx: source.len() as isize * 8,
source,
bit_container: 0,
bits_in_container: 0,
}
}
/// We refill the container in full bytes, shifting the still unread portion to the left, and filling the lower bits with new data
#[inline(always)]
fn refill_container(&mut self) {
let byte_idx = self.byte_idx() as usize;
let retain_bytes = (self.bits_in_container + 7) / 8;
let want_to_read_bits = 64 - (retain_bytes * 8);
// if there are >= 8 byte left to read we go a fast path:
// The slice is looking something like this |U..UCCCCCCCCR..R| Where U are some unread bytes, C are the bytes in the container, and R are already read bytes
// What we do is, we shift the container by a few bytes to the left by just reading a u64 from the correct position, rereading the portion we did not yet return from the conainer.
// Technically this would still work for positions lower than 8 but this guarantees that enough bytes are in the source and generally makes for less edge cases
if byte_idx >= 8 {
self.refill_fast(byte_idx, retain_bytes, want_to_read_bits)
} else {
// In the slow path we just read however many bytes we can
self.refill_slow(byte_idx, want_to_read_bits)
}
}
#[inline(always)]
fn refill_fast(&mut self, byte_idx: usize, retain_bytes: u8, want_to_read_bits: u8) {
let load_from_byte_idx = byte_idx - 7 + retain_bytes as usize;
let refill = LittleEndian::read_u64(&self.source[load_from_byte_idx..]);
self.bit_container = refill;
self.bits_in_container += want_to_read_bits;
self.idx -= want_to_read_bits as isize;
}
#[cold]
fn refill_slow(&mut self, byte_idx: usize, want_to_read_bits: u8) {
let can_read_bits = isize::min(want_to_read_bits as isize, self.idx);
let can_read_bytes = can_read_bits / 8;
match can_read_bytes {
8 => {
self.bit_container = LittleEndian::read_u64(&self.source[byte_idx - 7..]);
self.bits_in_container += 64;
self.idx -= 64;
}
6..=7 => {
self.bit_container <<= 48;
self.bits_in_container += 48;
self.bit_container |= LittleEndian::read_u48(&self.source[byte_idx - 5..]);
self.idx -= 48;
}
4..=5 => {
self.bit_container <<= 32;
self.bits_in_container += 32;
self.bit_container |=
u64::from(LittleEndian::read_u32(&self.source[byte_idx - 3..]));
self.idx -= 32;
}
2..=3 => {
self.bit_container <<= 16;
self.bits_in_container += 16;
self.bit_container |=
u64::from(LittleEndian::read_u16(&self.source[byte_idx - 1..]));
self.idx -= 16;
}
1 => {
self.bit_container <<= 8;
self.bits_in_container += 8;
self.bit_container |= u64::from(self.source[byte_idx]);
self.idx -= 8;
}
_ => {
panic!("This cannot be reached");
}
}
}
/// Next byte that should be read into the container
/// Negative values mean that the source buffer as been read into the container completetly.
fn byte_idx(&self) -> isize {
(self.idx - 1) / 8
}
#[inline(always)]
pub fn get_bits(&mut self, n: u8) -> Result<u64, GetBitsError> {
if n == 0 {
return Ok(0);
}
if self.bits_in_container >= n {
return Ok(self.get_bits_unchecked(n));
}
self.get_bits_cold(n)
}
#[cold]
fn get_bits_cold(&mut self, n: u8) -> Result<u64, GetBitsError> {
if n > 56 {
return Err(GetBitsError::TooManyBits {
num_requested_bits: usize::from(n),
limit: 56,
});
}
let signed_n = n as isize;
if self.bits_remaining() <= 0 {
self.idx -= signed_n;
return Ok(0);
}
if self.bits_remaining() < signed_n {
let emulated_read_shift = signed_n - self.bits_remaining();
let v = self.get_bits(self.bits_remaining() as u8)?;
debug_assert!(self.idx == 0);
let value = v << emulated_read_shift;
self.idx -= emulated_read_shift;
return Ok(value);
}
while (self.bits_in_container < n) && self.idx > 0 {
self.refill_container();
}
debug_assert!(self.bits_in_container >= n);
//if we reach this point there are enough bits in the container
Ok(self.get_bits_unchecked(n))
}
#[inline(always)]
pub fn get_bits_triple(
&mut self,
n1: u8,
n2: u8,
n3: u8,
) -> Result<(u64, u64, u64), GetBitsError> {
let sum = n1 as usize + n2 as usize + n3 as usize;
if sum == 0 {
return Ok((0, 0, 0));
}
if sum > 56 {
// try and get the values separatly
return Ok((self.get_bits(n1)?, self.get_bits(n2)?, self.get_bits(n3)?));
}
let sum = sum as u8;
if self.bits_in_container >= sum {
let v1 = if n1 == 0 {
0
} else {
self.get_bits_unchecked(n1)
};
let v2 = if n2 == 0 {
0
} else {
self.get_bits_unchecked(n2)
};
let v3 = if n3 == 0 {
0
} else {
self.get_bits_unchecked(n3)
};
return Ok((v1, v2, v3));
}
self.get_bits_triple_cold(n1, n2, n3, sum)
}
#[cold]
fn get_bits_triple_cold(
&mut self,
n1: u8,
n2: u8,
n3: u8,
sum: u8,
) -> Result<(u64, u64, u64), GetBitsError> {
let sum_signed = sum as isize;
if self.bits_remaining() <= 0 {
self.idx -= sum_signed;
return Ok((0, 0, 0));
}
if self.bits_remaining() < sum_signed {
return Ok((self.get_bits(n1)?, self.get_bits(n2)?, self.get_bits(n3)?));
}
while (self.bits_in_container < sum) && self.idx > 0 {
self.refill_container();
}
debug_assert!(self.bits_in_container >= sum);
//if we reach this point there are enough bits in the container
let v1 = if n1 == 0 {
0
} else {
self.get_bits_unchecked(n1)
};
let v2 = if n2 == 0 {
0
} else {
self.get_bits_unchecked(n2)
};
let v3 = if n3 == 0 {
0
} else {
self.get_bits_unchecked(n3)
};
Ok((v1, v2, v3))
}
#[inline(always)]
fn get_bits_unchecked(&mut self, n: u8) -> u64 {
let shift_by = self.bits_in_container - n;
let mask = (1u64 << n) - 1u64;
let value = self.bit_container >> shift_by;
self.bits_in_container -= n;
let value_masked = value & mask;
debug_assert!(value_masked < (1 << n));
value_masked
}
pub fn reset(&mut self, new_source: &'s [u8]) {
self.idx = new_source.len() as isize * 8;
self.source = new_source;
self.bit_container = 0;
self.bits_in_container = 0;
}
}
|
