[Velli20]

Summary

Opening a document re-merged each page's inherited /Resources at every level of the page tree. Pages that share one /Resources dictionary also each got their own deep copy. Moving the PdfPage values themselves was cheap (about 120 bytes per move, no heap data copied).

• Resources::merged_with_parent skips needless merges. It returns None when the parent defines nothing the child lacks, so the child's Arc is kept. It used to return Some every time.
• Inheritance happens in one pass. PageTreeBranch reads each /Pages node exactly as written, then PageTree walks down the tree once from the root. Each node combines with its ancestors once, each page is merged at most once, and each page is moved once into the final Vec.
• Shared merges. A ResourceMerges cache, keyed on the (own, inherited) Arc pointers, gives pages that share /Resources under the same ancestors one shared merged copy.

One test assertion changed: leaf_page_child_entries_take_priority_during_inheritance now expects the child's own Arc back, because there the parent adds nothing.

Measurements

Release profile (opt-level "z", fat LTO), base vs. change alternating over 5 rounds, best time per document:

Document	Before	After	Allocations before → after
Generated 2,000-page tree, 3 levels, shared page /Resources	38.2 ms	11.6 ms	581k → 158k
examples/assets/* (8 documents)		under 1.5% change (noise)	±1

Test plan

• cargo test
• cargo clippy --workspace --all-targets -- -D warnings, cargo fmt --check
• PDFium corpus, read-only: 0 documents read worse or better than the base commit
• PDFium corpus, full render: all 1,126 pages have the same status, metrics and draw count as the base commit
• pdf.js corpus (needs a local PDFium library; CI covers it)

🤖 Generated with Claude Code