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|
#[derive(Debug)]
pub enum BitReaderMode {
BE,
LE,
LE16MSB,
LE32MSB,
}
#[derive(Debug)]
pub enum BitReaderError {
BitstreamEnd,
TooManyBitsRequested,
InvalidValue,
}
use self::BitReaderError::*;
pub type BitReaderResult<T> = Result<T, BitReaderError>;
#[derive(Debug)]
pub struct BitReader<'a> {
cache: u64,
bits: u8,
pos: usize,
end: usize,
src: &'a [u8],
mode: BitReaderMode,
}
impl<'a> BitReader<'a> {
pub fn new(src: &'a [u8], size: usize, mode: BitReaderMode) -> Self {
if src.len() < size { panic!("size is less than needed"); }
BitReader{ cache: 0, pos: 0, bits: 0, end: size, src: src, mode: mode }
}
pub fn tell(&self) -> usize {
self.pos * 8 - (self.bits as usize)
}
pub fn left(&self) -> isize {
((self.end as isize) - (self.pos as isize)) * 8 + (self.bits as isize)
}
fn fill32be(&mut self, src: &[u8]) {
let nw = (((src[0] as u32) << 24) |
((src[1] as u32) << 16) |
((src[2] as u32) << 8) |
((src[3] as u32) << 0)) as u64;
self.cache |= nw << (32 - self.bits);
}
fn fill32le16(&mut self, src: &[u8]) {
let nw = (((src[1] as u32) << 24) |
((src[0] as u32) << 16) |
((src[3] as u32) << 8) |
((src[2] as u32) << 0)) as u64;
self.cache |= nw << (32 - self.bits);
}
fn fill32le32(&mut self, src: &[u8], lsb: bool) {
let nw = (((src[3] as u32) << 24) |
((src[2] as u32) << 16) |
((src[1] as u32) << 8) |
((src[0] as u32) << 0)) as u64;
if lsb {
self.cache |= nw << self.bits;
} else {
self.cache |= nw << (32 - self.bits);
}
}
#[inline(always)]
fn refill(&mut self) -> BitReaderResult<()> {
if self.pos >= self.end { return Err(BitstreamEnd) }
while self.bits <= 32 {
if self.pos + 4 <= self.end {
let buf = &self.src[self.pos..];
match self.mode {
BitReaderMode::BE => self.fill32be (buf),
BitReaderMode::LE16MSB => self.fill32le16(buf),
BitReaderMode::LE => self.fill32le32(buf, true),
BitReaderMode::LE32MSB => self.fill32le32(buf, false),
}
self.pos += 4;
self.bits += 32;
} else {
let mut buf: [u8; 4] = [0, 0, 0, 0];
let mut newbits: u8 = 0;
for i in 0..3 {
if self.pos < self.end {
buf[i] = self.src[self.pos];
self.pos = self.pos + 1;
newbits += 8;
}
}
if newbits == 0 { break; }
match self.mode {
BitReaderMode::BE => self.fill32be (&buf),
BitReaderMode::LE16MSB => self.fill32le16(&buf),
BitReaderMode::LE => self.fill32le32(&buf, true),
BitReaderMode::LE32MSB => self.fill32le32(&buf, false),
}
self.bits += newbits;
}
}
Ok(())
}
#[inline(always)]
fn read_cache(&mut self, nbits: u8) -> u32 {
let res = match self.mode {
BitReaderMode::LE => ((1u64 << nbits) - 1) & self.cache,
_ => (self.cache as u64) >> (64 - nbits),
};
res as u32
}
fn read_cache_s(&mut self, nbits: u8) -> i32 {
let res = match self.mode {
BitReaderMode::LE => ((self.cache as i64) << (64 - nbits)) >> (64 - nbits),
_ => (self.cache as i64) >> (64 - nbits),
};
res as i32
}
#[inline(always)]
fn skip_cache(&mut self, nbits: u8) {
match self.mode {
BitReaderMode::LE => self.cache >>= nbits,
_ => self.cache <<= nbits,
};
self.bits -= nbits;
}
#[inline(always)]
fn reset_cache(&mut self) {
self.bits = 0;
self.cache = 0;
}
#[inline(always)]
pub fn read(&mut self, nbits: u8) -> BitReaderResult<u32> {
if nbits == 0 { return Ok(0) }
if nbits > 32 { return Err(TooManyBitsRequested) }
if self.bits < nbits {
if let Err(err) = self.refill() { return Err(err) }
if self.bits < nbits { return Err(BitstreamEnd) }
}
let res = self.read_cache(nbits);
self.skip_cache(nbits);
Ok(res)
}
pub fn read_s(&mut self, nbits: u8) -> BitReaderResult<i32> {
if nbits == 0 || nbits > 32 { return Err(TooManyBitsRequested) }
if self.bits < nbits {
if let Err(err) = self.refill() { return Err(err) }
if self.bits < nbits { return Err(BitstreamEnd) }
}
let res = self.read_cache_s(nbits);
self.skip_cache(nbits);
Ok(res)
}
#[inline(always)]
pub fn read_bool(&mut self) -> BitReaderResult<bool> {
if self.bits < 1 {
if let Err(err) = self.refill() { return Err(err) }
if self.bits < 1 { return Err(BitstreamEnd) }
}
let res = self.read_cache(1);
self.skip_cache(1);
Ok(res == 1)
}
#[inline(always)]
pub fn peek(&mut self, nbits: u8) -> u32 {
if nbits > 32 { return 0 }
if self.bits < nbits { let _ = self.refill(); }
self.read_cache(nbits)
}
#[inline(always)]
pub fn skip(&mut self, nbits: u32) -> BitReaderResult<()> {
if self.bits as u32 >= nbits {
self.skip_cache(nbits as u8);
return Ok(());
}
let mut skip_bits = nbits - (self.bits as u32);
self.reset_cache();
self.pos += ((skip_bits / 32) * 4) as usize;
skip_bits = skip_bits & 0x1F;
self.refill()?;
if skip_bits > 0 {
self.skip_cache(skip_bits as u8);
}
Ok(())
}
pub fn seek(&mut self, nbits: u32) -> BitReaderResult<()> {
if ((nbits + 7) >> 3) as usize > self.end { return Err(TooManyBitsRequested); }
self.reset_cache();
self.pos = ((nbits / 32) * 4) as usize;
self.skip(nbits & 0x1F)
}
pub fn align(&mut self) {
let pos = self.bits & 7;
if pos != 0 {
self.skip_cache(pos);
}
}
}
pub fn reverse_bits(inval: u32, len: u8) -> u32 {
if len == 0 { return 0; }
const REV_TAB: [u8; 16] = [
0b0000, 0b1000, 0b0100, 0b1100, 0b0010, 0b1010, 0b0110, 0b1110,
0b0001, 0b1001, 0b0101, 0b1101, 0b0011, 0b1011, 0b0111, 0b1111,
];
let mut ret = 0;
let mut val = inval;
for _ in 0..8 {
ret = (ret << 4) | (REV_TAB[(val & 0xF) as usize] as u32);
val = val >> 4;
}
ret >> (32 - len)
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn br_works() {
const DATA: [u8; 18] = [0b00011011; 18];
let src = &DATA;
let mut br = BitReader::new(src, src.len(), BitReaderMode::LE16MSB);
for _ in 0..8 {
assert_eq!(br.read(16).unwrap(), 0x1B1B);
}
const DATA2: [u8; 1] = [ 0b00011011 ];
let src = &DATA2;
let mut br = BitReader::new(src, src.len(), BitReaderMode::LE);
assert_eq!(br.read_s(5).unwrap(), -5);
}
}
|
