pub const EC_SYM_BITS: u32 = 8;
pub const EC_CODE_BITS: u32 = 32;
pub const EC_SYM_MAX: u32 = (1 << EC_SYM_BITS) - 1;
pub const EC_CODE_SHIFT: u32 = EC_CODE_BITS - EC_SYM_BITS - 1;
pub const EC_CODE_TOP: u32 = 1 << (EC_CODE_BITS - 1);
pub const EC_CODE_BOT: u32 = EC_CODE_TOP >> EC_SYM_BITS;
pub const EC_CODE_EXTRA: u32 = (EC_CODE_BITS - 2) % EC_SYM_BITS + 1;
pub const BITRES: i32 = 3;
#[derive(Clone)]
pub struct RangeCoder {
pub buf: Vec<u8>,
pub storage: u32,
pub end_offs: u32,
pub end_window: u32,
pub nend_bits: i32,
pub nbits_total: i32,
pub offs: u32,
pub rng: u32,
pub val: u64,
pub ext: u32,
pub rem: i32,
pub error: i32,
}
impl RangeCoder {
pub fn new_encoder(size: u32) -> Self {
RangeCoder {
buf: vec![0; size as usize],
storage: size,
end_offs: 0,
end_window: 0,
nend_bits: 0,
nbits_total: 33,
offs: 0,
rng: 1 << 31,
val: 0,
ext: 0,
rem: -1,
error: 0,
}
}
pub fn new_decoder(buf: Vec<u8>) -> Self {
let storage = buf.len() as u32;
let mut rc = RangeCoder {
buf,
storage,
end_offs: 0,
end_window: 0,
nend_bits: 0,
nbits_total: (EC_CODE_BITS + 1
- ((EC_CODE_BITS - EC_CODE_EXTRA) / EC_SYM_BITS) * EC_SYM_BITS)
as i32,
offs: 0,
rng: 1 << EC_CODE_EXTRA,
val: 0,
ext: 0,
rem: 0,
error: 0,
};
rc.rem = rc.read_byte() as i32;
rc.val = (rc.rng - 1 - (rc.rem as u32 >> (EC_SYM_BITS - EC_CODE_EXTRA))) as u64;
rc.normalize_decoder();
rc
}
fn normalize_decoder(&mut self) {
while self.rng <= EC_CODE_BOT {
self.nbits_total += EC_SYM_BITS as i32;
self.rng <<= EC_SYM_BITS;
if self.rng == 0 {
debug_assert!(false, "normalize_decoder: rng=0 after shift, corrupt bitstream");
self.error = 1;
self.rng = 1;
return;
}
let sym = self.rem;
self.rem = self.read_byte() as i32;
let combined_sym = ((sym << EC_SYM_BITS) | self.rem) >> (EC_SYM_BITS - EC_CODE_EXTRA);
self.val = ((self.val << EC_SYM_BITS)
+ (EC_SYM_MAX & !combined_sym as u32) as u64)
& (EC_CODE_TOP as u64 - 1);
}
}
fn read_byte(&mut self) -> u8 {
if self.offs < self.storage {
let b = self.buf[self.offs as usize];
self.offs += 1;
b
} else {
0
}
}
pub fn enc_uint(&mut self, fl: u32, ft: u32) {
if ft > (1 << 8) {
let mut ft = ft - 1;
let s = 32 - ft.leading_zeros() as i32 - 8;
self.enc_bits(fl & ((1 << s) - 1), s as u32);
let fl = fl >> s;
ft >>= s;
ft += 1;
self.encode(fl, fl + 1, ft);
} else if ft > 1 {
self.encode(fl, fl + 1, ft);
}
}
pub fn dec_uint(&mut self, ft: u32) -> u32 {
if ft > (1 << 8) {
let mut ft = ft - 1;
let s = 32 - ft.leading_zeros() as i32 - 8;
let r = self.dec_bits(s as u32);
ft >>= s;
ft += 1;
let fs = self.decode(ft);
self.update(fs, fs + 1, ft);
(fs << s) | r
} else if ft > 1 {
let fs = self.decode(ft);
self.update(fs, fs + 1, ft);
fs
} else {
0
}
}
pub fn enc_bits(&mut self, val: u32, bits: u32) {
if bits == 0 {
return;
}
let mut window = self.end_window;
let mut used = self.nend_bits;
if (used as u32) + bits > EC_CODE_BITS {
while used >= EC_SYM_BITS as i32 {
self.write_byte_at_end((window & EC_SYM_MAX) as u8);
window >>= EC_SYM_BITS;
used -= EC_SYM_BITS as i32;
}
}
window |= (val & ((1 << bits) - 1)) << used;
used += bits as i32;
self.end_window = window;
self.nend_bits = used;
self.nbits_total += bits as i32;
}
pub fn dec_bits(&mut self, bits: u32) -> u32 {
if bits == 0 {
return 0;
}
let mut window = self.end_window;
let mut used = self.nend_bits;
if used < bits as i32 {
loop {
let byte = if self.end_offs < self.storage {
self.end_offs += 1;
self.buf[(self.storage - self.end_offs) as usize]
} else {
0
};
window |= (byte as u32) << used;
used += 8;
if used > 32 - 8 {
break;
}
}
}
let ret = window & ((1 << bits) - 1);
self.end_window = window >> bits;
self.nend_bits = used - bits as i32;
self.nbits_total += bits as i32;
ret
}
pub fn tell_frac(&self) -> i32 {
static CORRECTION: [u32; 8] = [35733, 38967, 42495, 46340, 50535, 55109, 60097, 65535];
let nbits = self.nbits_total << BITRES;
let mut l = 32 - self.rng.leading_zeros() as i32;
let r = self.rng >> (l - 16);
let mut b = (r >> 12) - 8;
if b < 7 && r > CORRECTION[b as usize] {
b += 1;
}
l = (l << 3) + b as i32;
nbits - l
}
pub fn tell(&self) -> i32 {
(self.tell_frac() + 7) >> 3
}
fn write_byte(&mut self, value: u8) {
if self.offs + self.end_offs < self.storage {
self.buf[self.offs as usize] = value;
self.offs += 1;
} else {
self.error = 1;
}
}
fn carry_out(&mut self, c: i32) {
if c != EC_SYM_MAX as i32 {
let carry = c >> EC_SYM_BITS;
if self.rem >= 0 {
self.write_byte((self.rem + carry) as u8);
}
if self.ext > 0 {
let sym = (EC_SYM_MAX as i32 + carry) & EC_SYM_MAX as i32;
for _ in 0..self.ext {
self.write_byte(sym as u8);
}
self.ext = 0;
}
self.rem = c & EC_SYM_MAX as i32;
} else {
self.ext += 1;
}
}
pub fn encode(&mut self, fl: u32, fh: u32, ft: u32) {
if ft == 0 {
return;
}
let r = self.rng / ft;
if fl > 0 {
self.val += (self.rng - r * (ft - fl)) as u64;
self.rng = r * (fh - fl);
} else {
self.rng -= r * (ft - fh);
}
self.normalize_encoder();
}
fn normalize_encoder(&mut self) {
if self.rng == 0 {
self.error = 1;
self.rng = 1;
return;
}
while self.rng <= EC_CODE_BOT {
self.carry_out((self.val >> EC_CODE_SHIFT) as i32);
self.val = (self.val << EC_SYM_BITS) & (EC_CODE_TOP as u64 - 1);
self.rng <<= EC_SYM_BITS;
self.nbits_total = self.nbits_total.wrapping_add(EC_SYM_BITS as i32);
}
}
pub fn encode_bit_logp(&mut self, val: bool, logp: u32) {
let s = self.rng >> logp;
let r = self.rng - s;
if val {
self.val += r as u64;
self.rng = s;
} else {
self.rng = r;
}
self.normalize_encoder();
}
pub fn encode_icdf(&mut self, s: i32, icdf: &[u8], ftb: u32) {
let r = self.rng >> ftb;
if s > 0 {
let val = icdf[(s - 1) as usize] as u32;
self.val += (self.rng as u64) - (r as u64 * val as u64);
let lower = icdf.get(s as usize).copied().unwrap_or(0) as u32;
let diff = val - lower;
debug_assert!(
diff > 0,
"encode_icdf: zero-probability symbol s={s}, icdf={icdf:?}, ftb={ftb} \
(icdf[{prev}]={val} == icdf[{s}]={lower})",
prev = s - 1,
);
self.rng = r * diff;
} else {
let val = icdf[s as usize] as u32;
let full = 1u32 << ftb;
debug_assert!(
val < full,
"encode_icdf: zero-probability symbol s=0, icdf={icdf:?}, ftb={ftb} \
(icdf[0]={val} == 2^ftb={full}, symbol has zero probability)"
);
self.rng -= r * val;
}
self.normalize_encoder();
}
pub fn decode_bit_logp(&mut self, logp: u32) -> bool {
let s = self.rng >> logp;
let ret = self.val < s as u64;
if !ret {
self.val -= s as u64;
self.rng -= s;
} else {
self.rng = s;
}
self.normalize_decoder();
ret
}
pub fn decode_icdf(&mut self, icdf: &[u8], ftb: u32) -> i32 {
let mut s = self.rng;
let d = self.val as u32;
let r = s >> ftb;
let mut ret = -1;
let mut t;
loop {
ret += 1;
t = s;
s = r * (icdf[ret as usize] as u32);
if d >= s {
break;
}
}
self.val = (d - s) as u64;
self.rng = t - s;
self.normalize_decoder();
ret
}
pub fn decode(&mut self, ft: u32) -> u32 {
let r = self.rng / ft;
self.ext = r;
let s = (self.val / r as u64) as u32;
ft - ft.min(s + 1)
}
pub fn update(&mut self, fl: u32, fh: u32, ft: u32) {
let s = self.ext * (ft - fh);
self.val -= s as u64;
self.rng = if fl > 0 {
self.ext * (fh - fl)
} else {
self.rng - s
};
self.normalize_decoder();
}
pub fn laplace_encode(&mut self, value: &mut i32, fs: u32, decay: i32) {
let mut val = *value;
let mut fl = 0;
let mut fs_val = fs;
if val != 0 {
let s = if val < 0 { -1 } else { 0 };
val = (val + s) ^ s;
fl = fs_val;
fs_val = self.laplace_get_freq1(fs_val, decay);
let mut i = 1;
while fs_val > 0 && i < val {
fs_val *= 2;
fl += fs_val + 2;
fs_val = ((fs_val as i32 * decay) >> 15) as u32;
i += 1;
}
if fs_val == 0 {
let ndi_max = 32768 - fl + 1 - 1;
let ndi_max = (ndi_max as i32 - s) >> 1;
let di = (val - i).min(ndi_max - 1);
fl += (2 * di + 1 + s) as u32;
fs_val = 1u32.min(32768 - fl);
*value = (i + di + s) ^ s;
} else {
fs_val += 1;
fl += fs_val & (!s as u32);
}
}
self.encode(fl, fl + fs_val, 1 << 15);
}
fn laplace_get_freq1(&self, fs0: u32, decay: i32) -> u32 {
let ft = 32768 - (2 * 16) - fs0;
((ft as i32 * (16384 - decay)) >> 15) as u32
}
pub fn laplace_decode(&mut self, fs: u32, decay: i32) -> i32 {
let fm = self.decode(1 << 15);
let mut fl = 0;
let mut fs_val = fs;
let mut val = 0;
if fm >= fs_val {
val += 1;
fl = fs_val;
fs_val = self.laplace_get_freq1(fs_val, decay) + 1;
while fs_val > 1 && fm >= fl + 2 * fs_val {
fs_val *= 2;
fl += fs_val;
fs_val = (((fs_val as i32 - 2) * decay) >> 15) as u32 + 1;
val += 1;
}
if fs_val <= 1 {
let di = (fm - fl) >> 1;
val += di as i32;
fl += 2 * di;
}
if fm < fl + fs_val {
val = -val;
} else {
fl += fs_val;
}
}
self.update(fl, fl + fs_val.min(32768 - fl), 1 << 15);
val
}
fn write_byte_at_end(&mut self, value: u8) {
if self.offs + self.end_offs < self.storage {
self.end_offs += 1;
let idx = (self.storage - self.end_offs) as usize;
self.buf[idx] = value;
} else {
self.error = 1;
}
}
pub fn patch_initial_bits(&mut self, val: u32, nbits: u32) {
let shift = EC_SYM_BITS - nbits;
let mask = ((1u32 << nbits) - 1) << shift;
if self.offs > 0 {
self.buf[0] = ((self.buf[0] as u32 & !mask) | (val << shift)) as u8;
} else if self.rem >= 0 {
self.rem = ((self.rem as u32 & !mask) | (val << shift)) as i32;
} else if self.rng <= (EC_CODE_TOP >> nbits) {
let mask64 = (mask as u64) << EC_CODE_SHIFT;
self.val = (self.val & !mask64) | ((val as u64) << (EC_CODE_SHIFT + shift));
} else {
self.error = -1;
}
}
pub fn done(&mut self) {
let ilog = 32 - self.rng.leading_zeros();
let mut l = (EC_CODE_BITS - ilog) as i32;
let mut msk = (EC_CODE_TOP as u64 - 1) >> l;
let mut end = (self.val + msk) & !msk;
if (end | msk) >= self.val + self.rng as u64 {
l += 1;
msk >>= 1;
end = (self.val + msk) & !msk;
}
while l > 0 {
self.carry_out((end >> EC_CODE_SHIFT) as i32);
end = (end << EC_SYM_BITS) & (EC_CODE_TOP as u64 - 1);
l -= EC_SYM_BITS as i32;
}
if self.rem >= 0 || self.ext > 0 {
self.carry_out(0);
}
let mut window = self.end_window;
let mut used = self.nend_bits;
while used >= EC_SYM_BITS as i32 {
self.write_byte_at_end((window & EC_SYM_MAX) as u8);
window >>= EC_SYM_BITS;
used -= EC_SYM_BITS as i32;
}
if self.error == 0 {
for i in self.offs..(self.storage - self.end_offs) {
self.buf[i as usize] = 0;
}
if used > 0 {
if self.end_offs >= self.storage {
self.error = -1;
} else {
let idx = (self.storage - self.end_offs - 1) as usize;
self.buf[idx] |= window as u8;
self.end_offs += 1;
}
}
}
}
pub fn finish(&mut self) -> Vec<u8> {
self.done();
let mut result = Vec::with_capacity((self.offs + self.end_offs) as usize);
result.extend_from_slice(&self.buf[0..self.offs as usize]);
result.extend_from_slice(
&self.buf[(self.storage - self.end_offs) as usize..self.storage as usize],
);
result
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_laplace() {
let mut enc = RangeCoder::new_encoder(100);
let mut val = -3;
let fs = 100 << 7;
let decay = 120 << 6;
enc.laplace_encode(&mut val, fs, decay);
enc.done();
assert_eq!(enc.offs, 1);
assert_eq!(enc.buf[0], 224);
let mut dec = RangeCoder::new_decoder(enc.buf[..enc.offs as usize].to_vec());
let decoded_val = dec.laplace_decode(fs, decay);
assert_eq!(decoded_val, -3);
}
#[test]
fn test_icdf_consistency() {
let mut enc = RangeCoder::new_encoder(1024);
let icdf = [2, 1, 0];
enc.encode_icdf(0, &icdf, 2);
enc.encode_icdf(1, &icdf, 2);
enc.encode_icdf(2, &icdf, 2);
enc.done();
let data = enc.buf[..enc.offs as usize].to_vec();
let mut dec = RangeCoder::new_decoder(data);
let s0 = dec.decode_icdf(&icdf, 2);
let s1 = dec.decode_icdf(&icdf, 2);
let s2 = dec.decode_icdf(&icdf, 2);
assert_eq!(s0, 0);
assert_eq!(s1, 1);
assert_eq!(s2, 2);
}
#[test]
fn test_icdf_last_symbol_no_oob() {
let icdf: &[u8] = &[170, 85, 0];
let ftb = 8u32;
for sym in 0..3i32 {
let mut enc = RangeCoder::new_encoder(256);
enc.encode_icdf(sym, icdf, ftb); enc.done();
let data = enc.buf[..enc.offs as usize].to_vec();
let mut dec = RangeCoder::new_decoder(data);
let decoded = dec.decode_icdf(icdf, ftb);
assert_eq!(decoded, sym, "往返失败: 编码 symbol={sym} 解码得 {decoded}");
}
}
#[test]
fn test_icdf_decode_terminates() {
let icdf: &[u8] = &[192, 128, 64, 0];
let ftb = 8u32;
let symbols = [0i32, 1, 2, 3];
let mut enc = RangeCoder::new_encoder(256);
for &s in &symbols {
enc.encode_icdf(s, icdf, ftb);
}
enc.done();
let data = enc.buf[..enc.offs as usize].to_vec();
let mut dec = RangeCoder::new_decoder(data);
for &expected in &symbols {
let got = dec.decode_icdf(icdf, ftb);
assert_eq!(got, expected, "解码器输出 {got}，期望 {expected}");
}
}
#[test]
fn test_bits_only() {
let mut enc = RangeCoder::new_encoder(1024);
enc.enc_bits(1, 1);
enc.enc_bits(5, 3);
enc.enc_bits(7, 3);
enc.enc_bits(0, 2);
let data = enc.finish();
let mut dec = RangeCoder::new_decoder(data);
let b1 = dec.dec_bits(1);
let b2 = dec.dec_bits(3);
let b3 = dec.dec_bits(3);
let b4 = dec.dec_bits(2);
assert_eq!(b1, 1);
assert_eq!(b2, 5);
assert_eq!(b3, 7);
assert_eq!(b4, 0);
}
#[test]
fn test_interleaved_bits_entropy() {
let mut enc = RangeCoder::new_encoder(1024);
enc.enc_bits(1, 1);
enc.encode(10, 20, 100);
enc.enc_bits(5, 3);
enc.encode(50, 60, 100);
let data = enc.finish();
let mut dec = RangeCoder::new_decoder(data);
let b1 = dec.dec_bits(1);
let d1 = dec.decode(100);
dec.update(10, 20, 100);
let b2 = dec.dec_bits(3);
let d2 = dec.decode(100);
dec.update(50, 60, 100);
assert_eq!(b1, 1);
assert!((10..20).contains(&d1), "d1={} expected in [10, 20)", d1);
assert_eq!(b2, 5);
assert!((50..60).contains(&d2), "d2={} expected in [50, 60)", d2);
}
}