From f7f18f483483140f1ad0ff0edbe0fb550086b80f Mon Sep 17 00:00:00 2001
From: Gabriel Schneider <gbrls@0x4200.cafe>
Date: Sun, 2 Aug 2026 23:18:45 -0300
Subject: cache MuPDF display lists for repeated renders

---
 src/pdf_bridge.c | 64 +++++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 61 insertions(+), 3 deletions(-)

(limited to 'src')

diff --git a/src/pdf_bridge.c b/src/pdf_bridge.c
index 0fcf74c4..b880cd21 100644
--- a/src/pdf_bridge.c
+++ b/src/pdf_bridge.c
@@ -14,21 +14,28 @@ struct pardes_pdf_document {
     fz_document *doc;
     int page_count;
 
-    /* One UI thread owns a document. Page switches invalidate this pair. */
+    /* One UI thread owns a document. Page switches invalidate this cache. */
     int cached_page_number;
     fz_page *cached_page;
     fz_stext_page *cached_text;
+    fz_display_list *cached_display_list;
     fz_rect cached_bounds;
+
+    /* Promote only a page which is rendered twice without changing pages. */
+    int display_list_candidate_page_number;
 };
 
 static void
 pardes_pdf_drop_cached_page(pardes_pdf_document *document)
 {
+    fz_drop_display_list(document->ctx, document->cached_display_list);
     fz_drop_stext_page(document->ctx, document->cached_text);
     fz_drop_page(document->ctx, document->cached_page);
+    document->cached_display_list = NULL;
     document->cached_text = NULL;
     document->cached_page = NULL;
     document->cached_page_number = -1;
+    document->display_list_candidate_page_number = -1;
 }
 
 static int
@@ -88,6 +95,45 @@ pardes_pdf_cache_page(
     document->cached_bounds = bounds;
 }
 
+/*
+ * Build only after one successful direct render of the still-cached page.
+ * This keeps cold renders and sequential document passes on their original
+ * path instead of recording a list which the next page would immediately
+ * evict. The temporary list is committed only after its device has closed;
+ * any partially recorded list is dropped before the exception propagates.
+ */
+static void
+pardes_pdf_cache_display_list(pardes_pdf_document *document)
+{
+    fz_context *ctx = document->ctx;
+    fz_display_list *list = NULL;
+    fz_device *device = NULL;
+
+    if (document->cached_display_list != NULL)
+        return;
+
+    fz_var(list);
+    fz_var(device);
+    fz_try(ctx)
+    {
+        list = fz_new_display_list(ctx, document->cached_bounds);
+        device = fz_new_list_device(ctx, list);
+        fz_run_page(ctx, document->cached_page, device, fz_identity, NULL);
+        fz_close_device(ctx, device);
+    }
+    fz_always(ctx)
+    {
+        fz_drop_device(ctx, device);
+    }
+    fz_catch(ctx)
+    {
+        fz_drop_display_list(ctx, list);
+        fz_rethrow(ctx);
+    }
+
+    document->cached_display_list = list;
+}
+
 static float
 pardes_pdf_clamp_unit(float value)
 {
@@ -187,6 +233,7 @@ pardes_pdf_open(const char *path, int *page_count)
     if (state == NULL)
         return NULL;
     state->cached_page_number = -1;
+    state->display_list_candidate_page_number = -1;
 
     ctx = fz_new_context(NULL, NULL, PARDES_PDF_STORE_LIMIT_BYTES);
     if (ctx == NULL) {
@@ -344,8 +391,16 @@ pardes_pdf_render_with_highlights(
          * premultiplied transparent samples, while Pardes consumes straight
          * RGBA after the Zig-side RGB expansion.
          */
-        pixmap = fz_new_pixmap_from_page(
-            ctx, document->cached_page, ctm, fz_device_rgb(ctx), 0);
+        if (document->cached_display_list != NULL ||
+            document->display_list_candidate_page_number == page_number) {
+            pardes_pdf_cache_display_list(document);
+            pixmap = fz_new_pixmap_from_display_list(
+                ctx, document->cached_display_list, ctm,
+                fz_device_rgb(ctx), 0);
+        } else {
+            pixmap = fz_new_pixmap_from_page(
+                ctx, document->cached_page, ctm, fz_device_rgb(ctx), 0);
+        }
         if (fz_pixmap_width(ctx, pixmap) < 1 ||
             fz_pixmap_height(ctx, pixmap) < 1)
             fz_throw(ctx, FZ_ERROR_FORMAT, "PDF page rendered empty");
@@ -398,6 +453,9 @@ pardes_pdf_render_with_highlights(
         return PARDES_PDF_ERROR;
     }
 
+    if (document->cached_display_list == NULL)
+        document->display_list_candidate_page_number = page_number;
+
     out->handle = pixmap;
     out->samples = fz_pixmap_samples(ctx, pixmap);
     out->width = fz_pixmap_width(ctx, pixmap);
-- 
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