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uniform-height-large-files.md

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Uniform Height Mode for Large Files

Task: 29
Version: v0.2.8
Status: Done

Overview

Files with more than 100,000 lines activate uniform height mode in ViewState. This avoids O(N) memory allocation for per-line vectors (wrap_info, cumulative_heights, cumulative_visual_rows), keeping large file performance smooth.

How It Works

Threshold

The constant LARGE_FILE_THRESHOLD in view.rs is set to 100,000 lines. configure_for_file_size(total_lines) is called every frame in FerriteEditor::ui().

When Activated

When total_lines >= 100,000:

  • use_uniform_heights is set to true
  • All O(N) per-line vectors are cleared and shrunk to free memory
  • set_line_wrap_info() becomes a no-op (no per-line height data stored)
  • rebuild_height_cache() returns immediately
  • get_line_height() returns the single line_height value for all lines
  • get_line_y_offset(line) returns line * line_height (no cumulative array)
  • y_offset_to_line(y) returns y / line_height (no binary search)
  • total_content_height() returns total_lines * line_height

Word Wrap Override

The previous line overlap regression was caused by word-wrapped galleys being taller than the uniform line_height. A line wrapping to 2 visual rows would produce a 40px galley, but uniform spacing allocated only 20px.

Fix: When uniform heights are active, word wrap is force-disabled for the frame via effective_wrap_enabled = self.wrap_enabled && !self.view.uses_uniform_heights(). This local variable replaces self.wrap_enabled in:

  • View wrap configuration (enable_wrap/disable_wrap)
  • The render loop's wrap vs non-wrap branch
  • Height cache rebuild calls
  • Horizontal scrollbar visibility

The user's wrap preference (self.wrap_enabled) is preserved — wrap re-enables automatically if the file drops below the threshold.

Cursor Positioning

calculate_cursor_x() uses self.view.is_wrap_enabled() (the actual runtime state) rather than self.wrap_enabled (the user preference), so cursor positioning automatically respects the uniform-height override.

Performance Characteristics

Metric Without Uniform With Uniform (100K+ lines)
Memory for height vectors O(N) (~2.4 MB for 100K lines) O(1) (0 bytes)
get_line_y_offset O(1) array lookup O(1) multiplication
y_offset_to_line O(log N) binary search O(1) division
rebuild_height_cache O(N) or O(delta) No-op
Word wrap Available Force-disabled

Key Files

File Changes
src/editor/ferrite/view.rs configure_for_file_size() re-enabled with threshold + vector cleanup; rebuild_height_cache() early-returns in uniform mode
src/editor/ferrite/editor.rs effective_wrap_enabled computed after configure_for_file_size(); used in wrap setup, render loop, scrollbar, and cursor positioning

Tests

12 new tests in view.rs covering:

  • Activation/deactivation at threshold
  • Vector cleanup on activation
  • Uniform get_line_height, get_line_y_offset, total_content_height
  • set_line_wrap_info is a no-op
  • rebuild_height_cache is a no-op
  • y_offset_to_line correctness
  • scroll_by with uniform heights
  • Deactivation when file shrinks below threshold