From 432e77cada6a4a634710b09f1a91c124b51d1e45 Mon Sep 17 00:00:00 2001
From: Sergio Gonzalez Martin <sergiog@microsoft.com>
Date: Wed, 12 Aug 2026 15:34:42 +0000
Subject: [PATCH] fix(jpeg): preserve progressive arithmetic AC scans

Backport of https://github.com/etemesi254/zune-image/pull/427.

---
diff --git a/third_party/rust/chromium_crates_io/vendor/zune-jpeg-v0_5/src/bitstream_arith.rs b/third_party/rust/chromium_crates_io/vendor/zune-jpeg-v0_5/src/bitstream_arith.rs
index 4c7bb2ee64b29..9acadf4b00b8a 100644
--- a/third_party/rust/chromium_crates_io/vendor/zune-jpeg-v0_5/src/bitstream_arith.rs
+++ b/third_party/rust/chromium_crates_io/vendor/zune-jpeg-v0_5/src/bitstream_arith.rs
@@ -22,6 +22,7 @@
 //! layout, etc.
 //!
 use alloc::format;
+use alloc::string::ToString;
 
 use zune_core::bytestream::{ZByteReaderTrait, ZReader};
 
@@ -898,9 +899,12 @@ impl BitStream for BitStreamArithmetic {
                     // This entry is zero
                     pos += 1;
                     if pos > self.spec_end {
-                        return Err(DecodeErrors::ArithmeticDecode(
-                            "Overrun while decoding initial AC coefficients, did not find end of band".to_string()
-                        ));
+                        // Match libjpeg-turbo's JWRN_ARITH_BAD_CODE path: keep
+                        // coefficients decoded so far, stop reading this scan,
+                        // skip its remaining MCUs, and reinitialize arithmetic
+                        // state at the next SOS. libjpeg does the same by
+                        // setting `ct = -1`; later MCU calls become no-ops.
+                        return Ok(false);
                     }
                 } else {
                     break;
@@ -914,7 +918,7 @@ impl BitStream for BitStreamArithmetic {
             block[t_pos] = (symbol as i16).wrapping_mul(bit);
             pos += 1;
         }
-        return Ok(true);
+        Ok(true)
     }
 
     #[allow(clippy::too_many_lines, clippy::op_ref)]
diff --git a/third_party/rust/chromium_crates_io/vendor/zune-jpeg-v0_5/src/mcu_prog.rs b/third_party/rust/chromium_crates_io/vendor/zune-jpeg-v0_5/src/mcu_prog.rs
index dd300f2f5fae7..fa36243031721 100644
--- a/third_party/rust/chromium_crates_io/vendor/zune-jpeg-v0_5/src/mcu_prog.rs
+++ b/third_party/rust/chromium_crates_io/vendor/zune-jpeg-v0_5/src/mcu_prog.rs
@@ -556,7 +556,11 @@ impl<T: ZByteReaderTrait> JpegDecoder<T> {
                         .unwrap();
 
                     let ac_table = B::get_ac_table(&mut self.entropy_tables, ac_pos)?;
-                    stream.decode_mcu_ac_first(&mut self.stream, ac_table, data)?;
+                    if !stream.decode_mcu_ac_first(&mut self.stream, ac_table, data)? {
+                        // Arithmetic bad-code termination is scan-wide, matching
+                        // libjpeg's no-op handling for the remaining MCUs.
+                        return Ok(());
+                    }
                 }
 
                 self.todo -= 1;
-- 
2.50.1