# pdftract-3gq3: PDF String Literal Lexer Implementation

## Summary

Implemented PDF string literal lexer with octal escapes and balanced parentheses.

## What Was Done

### 1. Verified Existing Implementation

The `lex_literal_string()` function in `/home/coding/pdftract/crates/pdftract-core/src/parser/lexer/mod.rs` (lines 535-656) already implements all required functionality:

- **Escape sequences**: `\n`, `\r`, `\t`, `\b`, `\f`, `\\`, `\(`, `\)`
- **Octal escapes**: `\ddd` consuming 1-3 octal digits
- **Line continuation**: `\<newline>` (LF, CR, CRLF)
- **Nested balanced parens**: Depth tracking with `depth: usize`
- **Out-of-range octals**: Truncated with `STRUCT_INVALID_OCTAL` diagnostic
- **Unterminated strings**: `STRUCT_UNTERMINATED_STRING` diagnostic
- **Unknown escapes**: Emit literal character per PDF spec

### 2. Added Proptests

Added two property tests to the lexer test module (lines 1305-1397):

1. **`proptest_string_never_panics_on_random_bytes`**: Verifies that random byte sequences starting with `(` never panic
2. **`proptest_valid_string_roundtrips`**: Verifies that valid `(...)` strings produce non-empty `Token::String` output

## Acceptance Criteria

All acceptance criteria PASS:

### Critical Tests

| Test | Input | Expected Output | Status |
|------|-------|-----------------|--------|
| Balanced parens | `(foo (bar) baz)` | `b"foo (bar) baz"` | PASS |
| Octal escape | `(abc\101)` | `b"abcA"` | PASS |
| Octal with non-octal | `(abc\10A)` | `b"abc\x08A"` | PASS |
| Line continuation | `(abc\<LF>def)` | `b"abcdef"` | PASS |
| Unterminated | `(unterminated` | Partial bytes + diagnostic | PASS |

### Proptests

| Property | Status |
|----------|--------|
| Random bytes starting with `(` never panic | PASS |
| Valid `(...)` round-trips to non-empty `Token::String` | PASS |

### INV-8 Compliance

No `unwrap()`, `expect()`, or `panic!` in the lexer code. All errors are emitted as diagnostics.

## Test Results

```
test result: ok. 54 passed; 0 failed; 0 ignored; 0 measured
```

All 54 lexer tests pass, including 23 string literal tests, 2 proptests, and 29 other lexer tests.

## Files Modified

- `/home/coding/pdftract/crates/pdftract-core/src/parser/lexer/mod.rs`: Added proptests (lines 1305-1397)

## Implementation Notes

The existing implementation correctly handles:

1. **Paren depth tracking**: Starts at `depth = 1` after opening `(`, increments on `(`, decrements on `)`. Only terminates when `depth == 0`.

2. **Octal escape parsing**: Greedily consumes up to 3 octal digits (0-7). Non-octal digits terminate the escape sequence and are treated as literal text.

3. **Line ending normalization**: Per PDF spec 7.3.4.2, bare `\r` inside strings is NOT normalized to `\n` - the implementation emits `\r` literally. This is spec-compliant as the normalization applies to line endings in the PDF file structure, not within string literals.

4. **Out-of-range octals**: Values > 255 are truncated via `(value & 0xFF)` and a diagnostic is emitted.

5. **Unknown escapes**: Emit the escaped character literally (e.g., `\q` → `q`) with no diagnostic, as permitted by PDF spec 7.3.4.2.