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fix: correct arithmetic coder interval normalisation #106

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Merged
danieleades merged 1 commit into from
Dec 9, 2025
Merged

fix: correct arithmetic coder interval normalisation #106

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2 changes: 1 addition & 1 deletion src/common.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -17,7 +17,7 @@ where
{
pub fn new(precision: u32) -> Self {
let low = B::ZERO;
let high = B::ONE << precision;
let high = (B::ONE << precision) - B::ONE;

Self {
precision,
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6 changes: 3 additions & 3 deletions src/decoder.rs
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Expand Up @@ -209,13 +209,13 @@ where
fn normalise(&mut self) -> io::Result<()> {
while self.state.high < self.state.half() || self.state.low >= self.state.half() {
if self.state.high < self.state.half() {
self.state.high <<= 1;
self.state.high = (self.state.high << 1) + B::ONE;
self.state.low <<= 1;
self.x <<= 1;
} else {
// self.low >= self.half()
self.state.low = (self.state.low - self.state.half()) << 1;
self.state.high = (self.state.high - self.state.half()) << 1;
self.state.high = ((self.state.high - self.state.half()) << 1) + B::ONE;
self.x = (self.x - self.state.half()) << 1;
}

Expand All @@ -228,7 +228,7 @@ where
&& self.state.high < (self.state.three_quarter())
{
self.state.low = (self.state.low - self.state.quarter()) << 1;
self.state.high = (self.state.high - self.state.quarter()) << 1;
self.state.high = ((self.state.high - self.state.quarter()) << 1) + B::ONE;
self.x = (self.x - self.state.quarter()) << 1;

if self.input.next_bit()? == Some(true) {
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6 changes: 3 additions & 3 deletions src/encoder.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -201,12 +201,12 @@ where
while self.state.high < self.state.half() || self.state.low >= self.state.half() {
if self.state.high < self.state.half() {
self.emit(false)?;
self.state.high <<= 1;
self.state.high = (self.state.high << 1) + B::ONE;
self.state.low <<= 1;
} else {
self.emit(true)?;
self.state.low = (self.state.low - self.state.half()) << 1;
self.state.high = (self.state.high - self.state.half()) << 1;
self.state.high = ((self.state.high - self.state.half()) << 1) + B::ONE;
}
}

Expand All @@ -215,7 +215,7 @@ where
{
self.pending += 1;
self.state.low = (self.state.low - self.state.quarter()) << 1;
self.state.high = (self.state.high - self.state.quarter()) << 1;
self.state.high = ((self.state.high - self.state.quarter()) << 1) + B::ONE;
}

Ok(())
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