Display Mapping System

Layered coordinate conversion and code folding for Editor/Input.

Architecture

Buffer (Rope)              Logical text
    ↓
WrapMap                    Soft-wrapping (buffer_line ↔ wrap_row)
    ↓
FoldMap                    Fold projection (wrap_row ↔ display_row)
    ↓
DisplayMap                 Public facade (BufferPos ↔ DisplayPos)

Coordinate Systems

Modules

DisplayMap — Public facade

• buffer_pos_to_display_pos() / display_pos_to_buffer_pos()
• set_fold_candidates(), toggle_fold(), is_folded_at(), clear_folds()
• on_text_changed(), on_layout_changed(), set_font()
• adjust_folds_for_edit() — incremental fold/candidate line-delta adjustment
• update_fold_candidates_for_edit() — region-scoped candidate extraction after edits

WrapMap — Soft-wrapping layer

Built on TextWrapper. Provides buffer ↔ wrap coordinate mapping with prefix sum cache for O(1) line lookups.

FoldMap — Fold projection layer

Maintains visible_wrap_rows and reverse mapping. When no folds are active, uses identity mapping (wrap_row == display_row) without Vec allocation.

FoldRange / folding — Fold extraction

• extract_fold_ranges(tree) — full tree traversal (initial load only)
• extract_fold_ranges_in_range(tree, byte_range) — region-scoped traversal for edits, skips subtrees outside range