pdftract/notes/pdftract-5nbp.md

pdftract-5nbp: /Differences overlay handler verification

Summary

The /Differences overlay handler for font encodings was already fully implemented in crates/pdftract-core/src/font/encoding.rs. No code changes were required.

Implementation details

DifferencesOverlay struct (lines 128-234)

• entries: Vec<(u8, Arc<str>)> - sparse list of (code, glyph_name) overrides
• parse() method walks the array linearly, maintaining a cursor that resets on each integer
• get() method performs linear search (fine for <32 entries)

FontEncoding struct (lines 236-364)

• Combines base encoding with differences overlay
• parse_from_font() handles all encoding indirection patterns
• glyph_name_for() lookup order: differences first, base second

Acceptance criteria verification

PASS: [ 39 /quotesingle 96 /grave ] parsing

Test: test_differences_overlay_parse_simple

• Code 39 → "quotesingle"
• Code 96 → "grave"

PASS: [ 39 /a /b /c ] consecutive assignment

Test: test_differences_overlay_parse_consecutive

• Code 39 → "a"
• Code 40 → "b"
• Code 41 → "c"

PASS: Overlay precedence over base encoding

Tests: test_font_encoding_glyph_name_with_differences, test_font_encoding_lookup_order

• Overlaid codes return the override
• Non-overlaid codes return the base encoding entry

PASS: Unknown glyph names returned as-is

Test: test_font_encoding_unknown_glyph_name

• Arbitrary glyph names not in AGL are returned (not None)
• Resolver can attempt L3/L4 fallback with the name

Critical considerations

PASS: Multiple Differences blocks

Test: test_differences_overlay_parse_multiple_blocks

• Handles [ 39 /a /b 100 /x /y ] correctly

PASS: Out-of-range code clamping with diagnostics

Tests: test_differences_overlay_out_of_range_positive, test_differences_overlay_out_of_range_negative

• Codes < 0 clamped to 0
• Codes > 255 clamped to 255
• Emits DiagCode::FontEncodingDifferenceOutOfRange diagnostic

PASS: Implicit base encoding

• FontEncoding::parse_from_font() handles /Differences without /BaseEncoding
• Falls back to default_base parameter (Standard for Type1, built-in for TrueType)

Test results

running 28 tests
test result: ok. 28 passed; 0 failed; 0 ignored

Files

• crates/pdftract-core/src/font/encoding.rs - Full implementation
• crates/pdftract-core/src/font/mod.rs - Re-exports FontEncoding, DifferencesOverlay, NamedEncoding

No code changes required

The implementation was already complete and tested. This bead verified existing functionality.