PDF: look up page images by hash instead of scanning every earlier image

Page::image_rgba compared each new picture with all earlier pictures on
the page, so N distinct pictures cost N(N-1)/2 comparisons. Keep a
page-local hash-to-index map (pixels still compared with ImageData::same)
and add a regression test with 3,000 distinct pictures.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/pdf/src/lib.rs

@@ -434,6 +434,11 @@ pub struct Page {
     content: String,
     fonts: [bool; FONT_SLOTS],
     images: Vec<ImageData>,
+    /// Indexes into `images`, by content hash; a hash match is confirmed with [`ImageData::same`].
+    image_index: HashMap<u64, Vec<usize>>,
+    /// Pixel comparisons made while looking up duplicate pictures (test instrumentation).
+    #[cfg(test)]
+    image_comparisons: usize,
     /// Style operators already in effect at the end of `content`.
     style: Style,
     /// One entry per accepted [`Page::save`], restored by [`Page::restore`].
@@ -793,9 +798,20 @@ impl Page {
         }
         let img = ImageData::new(pw, ph, rgb, alpha);
         // The same picture drawn again on this page shares one XObject.
-        let idx = match self.images.iter().position(|i| i.same(&img)) {
+        let candidates = self.image_index.entry(img.hash).or_default();
+        #[cfg(test)]
+        let comparisons = &mut self.image_comparisons;
+        let known = candidates.iter().copied().find(|&i| {
+            #[cfg(test)]
+            {
+                *comparisons += 1;
+            }
+            self.images.get(i).is_some_and(|o| o.same(&img))
+        });
+        let idx = match known {
             Some(i) => i,
             None => {
+                candidates.push(self.images.len());
                 self.images.push(img);
                 self.images.len() - 1
             }
@@ -934,6 +950,9 @@ impl PdfDoc {
             content: String::new(),
             fonts: [false; FONT_SLOTS],
             images: Vec::new(),
+            image_index: HashMap::new(),
+            #[cfg(test)]
+            image_comparisons: 0,
             links: Vec::new(),
             style: Style::default(),
             saved: Vec::new(),

crates/pdf/src/tests.rs

@@ -571,6 +571,26 @@ fn different_images_are_not_merged() {
     assert_eq!(pdf.windows(15).filter(|w| *w == b"/Subtype /Image").count(), 6, "three distinct pictures, each with a soft mask (alpha 50)");
 }
 
+/// O4: many distinct pictures on one page are looked up by hash, not by comparing each new
+/// picture with every earlier one (which made dense picture exports quadratic).
+#[test]
+fn many_distinct_images_on_one_page_are_not_compared_pairwise() {
+    const N: u32 = 3000;
+    let mut d = PdfDoc::new();
+    let p = d.add_page(612.0, 792.0);
+    for i in 0..N {
+        let [a, b, c, _] = i.to_le_bytes();
+        p.image_rgba(Rect::new(0.0, 0.0, 1.0, 1.0), &[a, b, c, 255], [1, 1]);
+    }
+    // A repeat of the first picture still shares its XObject.
+    p.image_rgba(Rect::new(5.0, 5.0, 1.0, 1.0), &[0, 0, 0, 255], [1, 1]);
+    assert_eq!(p.images.len(), N as usize);
+    assert!(p.content.ends_with("/Im1 Do Q\n"), "the repeated picture reuses /Im1");
+    assert!(p.image_comparisons <= 1, "{} pixel comparisons for {N} distinct pictures", p.image_comparisons);
+    let pdf = d.finish();
+    assert_eq!(pdf.windows(15).filter(|w| *w == b"/Subtype /Image").count(), N as usize);
+}
+
 /// O4: equal hashes alone never merge images; the pixels are compared.
 #[test]
 fn hash_collisions_do_not_merge_images() {