From ededdd75c97588855b822349c53a22cabbd070f4 Mon Sep 17 00:00:00 2001
From: "Dylan M. Taylor" <dylan@dylanmtaylor.com>
Date: Fri, 13 Nov 2026 19:51:49 +0000
Subject: [PATCH] gif: expose first-frame data byte count

---
 crates/gif/src/lib.rs   | 23 +++++++++++++++++++++++
 crates/gif/src/tests.rs |  9 +++++++++
 2 files changed, 32 insertions(+)

diff --git a/crates/gif/src/lib.rs b/crates/gif/src/lib.rs
index 56b6767fb..1cec284ca 100644
--- a/crates/gif/src/lib.rs
+++ b/crates/gif/src/lib.rs
@@ -253,6 +253,29 @@ pub fn first_frame_decoded_pixels(bytes: &[u8]) -> Option<u64> {
     Some(pixels)
 }
 
+/// Counts compressed image-data bytes in the first frame's sub-blocks, excluding its LZW code-size
+/// byte and all later blocks. A truncated final sub-block contributes only the bytes present.
+pub fn first_frame_compressed_bytes(bytes: &[u8]) -> Option<usize> {
+    let parsed = scan(bytes, 1)?;
+    let frame = parsed.frames.first()?;
+    let packed = *bytes.get(frame.offset.checked_add(9)?)?;
+    let mut pos = frame.offset.checked_add(10)?.checked_add(table_len(packed))?.checked_add(1)?;
+    let mut total = 0usize;
+    loop {
+        let length = usize::from(*bytes.get(pos)?);
+        pos = pos.checked_add(1)?;
+        if length == 0 {
+            return Some(total);
+        }
+        let end = pos.checked_add(length)?;
+        total = total.checked_add(end.min(bytes.len()).saturating_sub(pos))?;
+        if end > bytes.len() {
+            return Some(total);
+        }
+        pos = end;
+    }
+}
+
 /// Whether a file of `file_len` bytes can plausibly hold `declared_bytes` of decoded picture: at
 /// most `max_ratio` decoded bytes for each file byte, plus 1 MiB. A decoder that sets up the whole
 /// logical screen before it reads a frame should be given only files that pass this.
diff --git a/crates/gif/src/tests.rs b/crates/gif/src/tests.rs
index a75743b19..6a776eb50 100644
--- a/crates/gif/src/tests.rs
+++ b/crates/gif/src/tests.rs
@@ -54,6 +54,15 @@ fn animated(delays_cs: &[u32], repeat: Option<image::codecs::gif::Repeat>) -> Ve
     out
 }
 
+#[test]
+fn first_frame_compressed_bytes_exclude_extensions_after_the_image_data() {
+    let mut bytes = gif(&[[1, 2, 3], [4, 5, 6]], None, None);
+    assert_eq!(first_frame_compressed_bytes(&bytes), Some(2));
+    bytes.splice(bytes.len() - 1..bytes.len() - 1, [0x21, 0xFE, 1, b'x', 0]);
+    assert_eq!(first_frame_compressed_bytes(&bytes), Some(2));
+    assert_eq!(first_frame_compressed_bytes(b"not a gif"), None);
+}
+
 #[test]
 fn the_first_frames_table_and_transparent_entry() {
     let g = [[255, 0, 0], [0, 255, 0], [0, 0, 255], [255, 255, 255]];