From 42cdf44ca6bb0f6ad4604495ec9d24b3c5236ef6 Mon Sep 17 00:00:00 2001
From: steschu77 <steschu77@gmail.com>
Date: Fri, 14 Nov 2025 17:27:25 +0100
Subject: [PATCH] Improve coefficient sign decoding using the fact that range
 is always within 128..254.

* Add sign decoding to arithmetic decoder.
* Add narrower limits for asserting `range`.
* Improved flag decoding by limiting `shift` to be 0 (for range=128) or 1 (for range in 64..127)
---
 src/vp8.rs                    |  2 +-
 src/vp8_arithmetic_decoder.rs | 88 +++++++++++++++++++++++++++++++----
 2 files changed, 80 insertions(+), 10 deletions(-)

diff --git a/src/vp8.rs b/src/vp8.rs
index a8d48f7..33392fa 100644
--- a/src/vp8.rs
+++ b/src/vp8.rs
@@ -852,7 +852,7 @@ impl<R: Read> Vp8Decoder<R> {
                 2
             };
 
-            if decoder.read_flag().or_accumulate(&mut res) {
+            if decoder.read_sign().or_accumulate(&mut res) {
                 abs_value = -abs_value;
             }
 
diff --git a/src/vp8_arithmetic_decoder.rs b/src/vp8_arithmetic_decoder.rs
index 7f8c4be..a25d3e9 100644
--- a/src/vp8_arithmetic_decoder.rs
+++ b/src/vp8_arithmetic_decoder.rs
@@ -175,6 +175,16 @@ impl ArithmeticDecoder {
         self.cold_read_flag()
     }
 
+    // Do not inline this because inlining seems to worsen performance.
+    #[inline(never)]
+    pub(crate) fn read_sign(&mut self) -> BitResult<bool> {
+        if let Some(b) = self.fast().read_sign() {
+            return BitResult::ok(b);
+        }
+
+        self.cold_read_flag()
+    }
+
     // Do not inline this because inlining seems to worsen performance.
     #[inline(never)]
     pub(crate) fn read_literal(&mut self, n: u8) -> BitResult<u8> {
@@ -399,6 +409,11 @@ impl FastDecoder<'_> {
         self.commit_if_valid(value)
     }
 
+    fn read_sign(mut self) -> Option<bool> {
+        let value = self.fast_read_sign();
+        self.commit_if_valid(value)
+    }
+
     fn read_literal(mut self, n: u8) -> Option<u8> {
         let value = self.fast_read_literal(n);
         self.commit_if_valid(value)
@@ -448,6 +463,7 @@ impl FastDecoder<'_> {
         }
         debug_assert!(bit_count >= 0);
 
+        debug_assert!((128..=255).contains(&range));
         let probability = u32::from(probability);
         let split = 1 + (((range - 1) * probability) >> 8);
         let bigsplit = u64::from(split) << bit_count;
@@ -460,18 +476,18 @@ impl FastDecoder<'_> {
             range = split;
             false
         };
-        debug_assert!(range > 0);
 
         // Compute shift required to satisfy `range >= 128`.
         // Apply that shift to `range` and `self.bitcount`.
         //
         // Subtract 24 because we only care about leading zeros in the
         // lowest byte of `range` which is a `u32`.
+        debug_assert!((1..=254).contains(&range));
         let shift = range.leading_zeros().saturating_sub(24);
         range <<= shift;
         bit_count -= shift as i32;
-        debug_assert!(range >= 128);
 
+        debug_assert!((128..=254).contains(&range));
         self.uncommitted_state = State {
             chunk_index,
             value,
@@ -504,6 +520,7 @@ impl FastDecoder<'_> {
         }
         debug_assert!(bit_count >= 0);
 
+        debug_assert!((128..=255).contains(&range));
         let half_range = range / 2;
         let split = range - half_range;
         let bigsplit = u64::from(split) << bit_count;
@@ -516,18 +533,71 @@ impl FastDecoder<'_> {
             range = split;
             false
         };
-        debug_assert!(range > 0);
 
         // Compute shift required to satisfy `range >= 128`.
+        // A `range` of 64..127 requires a shift of 1. No shift if `range` is 128.
         // Apply that shift to `range` and `self.bitcount`.
-        //
-        // Subtract 24 because we only care about leading zeros in the
-        // lowest byte of `range` which is a `u32`.
-        let shift = range.leading_zeros().saturating_sub(24);
+        debug_assert!((64..=128).contains(&range));
+        let shift = if range == 0x80 { 0 } else { 1 };
         range <<= shift;
-        bit_count -= shift as i32;
-        debug_assert!(range >= 128);
+        bit_count -= shift;
+
+        debug_assert!((128..=254).contains(&range));
+        self.uncommitted_state = State {
+            chunk_index,
+            value,
+            range,
+            bit_count,
+        };
+        retval
+    }
+
+    fn fast_read_sign(&mut self) -> bool {
+        let State {
+            mut chunk_index,
+            mut value,
+            mut range,
+            mut bit_count,
+        } = self.uncommitted_state;
+
+        if bit_count < 0 {
+            let chunk = self.chunks.get(chunk_index).copied();
+            // We ignore invalid data inside the `fast_` functions,
+            // but we increase `chunk_index` below, so we can check
+            // whether we read invalid data in `commit_if_valid`.
+            let chunk = chunk.unwrap_or_default();
+
+            let v = u32::from_be_bytes(chunk);
+            chunk_index += 1;
+            value <<= 32;
+            value |= u64::from(v);
+            bit_count += 32;
+        }
+
+        // Range is only 255 at the start of decoding. After reading any symbol, it is guaranteed
+        // to be at most 254. Sign bits are never the first symbol in a bit stream.
+        debug_assert!((128..=254).contains(&range));
+        let half_range = range / 2;
+        let split = range - half_range;
+        let bigsplit = u64::from(split) << bit_count;
+
+        let retval = if let Some(new_value) = value.checked_sub(bigsplit) {
+            range = half_range;
+            value = new_value;
+            true
+        } else {
+            range = split;
+            false
+        };
+
+        // Compute shift required to satisfy `range >= 128`.
+        // Since `range` lies in 64..127 it always requires a shift of 1.
+        // Apply that shift to `range` and `self.bitcount`.
+        debug_assert!((64..=127).contains(&range));
+        range <<= 1;
+        bit_count -= 1;
 
+        debug_assert!((128..=254).contains(&range));
         self.uncommitted_state = State {
             chunk_index,
             value,