kte/docs/DEVELOPER_GUIDE.md

kte Developer Guide

Welcome to kte development! This guide will help you understand the codebase, make changes, and contribute effectively.

Table of Contents

1. Architecture Overview
2. Core Components
3. Code Organization
4. Building and Testing
5. Making Changes
6. Code Style
7. Common Tasks

Architecture Overview

kte follows a clean separation of concerns with three main layers:

┌─────────────────────────────────────────┐
│  Frontend Layer (Terminal/ImGui/Qt)     │
│  - TerminalFrontend / ImGuiFrontend     │
│  - InputHandler + Renderer interfaces   │
└─────────────────────────────────────────┘
                    ↓
┌─────────────────────────────────────────┐
│  Command Layer                          │
│  - Command registry and execution       │
│  - All editing operations               │
└─────────────────────────────────────────┘
                    ↓
┌─────────────────────────────────────────┐
│  Core Model Layer                       │
│  - Editor (top-level state)             │
│  - Buffer (document model)              │
│  - PieceTable (text storage)            │
│  - UndoSystem (undo/redo)               │
│  - SwapManager (crash recovery)         │
└─────────────────────────────────────────┘

Design Principles

• Frontend Independence: Core editing logic is independent of UI. Frontends implement Frontend, InputHandler, and Renderer interfaces.
• Command Pattern: All editing operations go through the command system, enabling consistent undo/redo and testing.
• Piece Table: Efficient text storage using a piece table data structure that avoids copying large buffers.
• Lazy Materialization: Text is materialized on-demand to minimize memory allocations.

Core Components

Editor (Editor.h/.cc)

The top-level editor state container. Manages:

• Multiple buffers
• Editor modes (normal, k-command prefix, prompts)
• Kill ring (clipboard history)
• Universal argument state
• Search state
• Status messages
• Swap file management

Key Insight: Editor is primarily a state holder with many getter/setter pairs. It doesn't contain editing logic - that's in commands.

Buffer (Buffer.h/.cc)

Represents an open document. Manages:

• File I/O (open, save, external modification detection)
• Cursor position and viewport offsets
• Mark (selection start point)
• Visual line mode state
• Syntax highlighting integration
• Undo system integration
• Swap recording integration

Key Insight: Buffer wraps a PieceTable and provides a higher-level interface. The nested Buffer::Line class is a legacy wrapper that has been largely phased out in favor of direct PieceTable operations.

Line Access APIs: Buffer provides three ways to access line content:

• GetLineView(row) - Zero-copy string_view (fastest, 11x faster than Rows())
• GetLineString(row) - Returns std::string copy (1.7x faster than Rows())
• Rows() - Materializes all lines into cache (legacy, avoid in new code)

See docs/BENCHMARKS.md for detailed performance analysis and usage guidance.

PieceTable (PieceTable.h/.cc)

The core text storage data structure. Provides:

• Efficient insert/delete operations without copying entire buffer
• Line-based queries (line count, get line, line ranges)
• Position conversion (byte offset ↔ line/column)
• Substring extraction
• Search functionality
• Automatic consolidation to prevent piece fragmentation

Key Insight: PieceTable uses lazy materialization - the full text is only assembled when Data() is called. Most operations work directly on the piece list.

UndoSystem (UndoSystem.h/.cc, UndoTree.h/.cc, UndoNode.h/.cc)

Implements undo/redo with a tree structure supporting:

• Linear undo/redo
• Branching history (future enhancement)
• Checkpointing and compaction
• Memory-efficient node pooling

Key Insight: The undo system records operations at the PieceTable level, not at the command level.

Command System (Command.h/.cc)

All editing operations are implemented as commands:

• File operations (save, open, close)
• Navigation (move cursor, page up/down, word movement)
• Editing (insert, delete, kill, yank)
• Search and replace
• Buffer management
• Configuration (syntax, theme, font)

Key Insight: Command.cc is currently a monolithic 5000-line file. This is the biggest maintainability challenge in the codebase.

Frontend Abstraction

Three interfaces define the frontend contract:

• Frontend (Frontend.h): Top-level lifecycle (Init/Step/Shutdown)
• InputHandler (InputHandler.h): Converts UI events to commands
• Renderer (Renderer.h): Draws the editor state

Implementations:

• Terminal: ncurses-based (TerminalFrontend, TerminalInputHandler, TerminalRenderer)
• ImGui: Dear ImGui-based (ImGuiFrontend, ImGuiInputHandler, ImGuiRenderer)
• Qt: Qt-based (QtFrontend, QtInputHandler, QtRenderer)
• Test: Programmatic testing (TestFrontend, TestInputHandler, TestRenderer)

Code Organization

Directory Structure

kte/
├── *.h, *.cc           # Core implementation (root level)
├── main.cc             # Entry point
├── docs/               # Documentation
│   ├── ke.md          # Original ke editor reference (keybindings)
│   ├── swap.md        # Swap file design
│   ├── syntax.md      # Syntax highlighting
│   ├── themes.md      # Theme system
│   └── plans/         # Design documents
├── tests/              # Test suite
│   ├── Test.h         # Minimal test framework
│   ├── TestRunner.cc  # Test runner
│   └── test_*.cc      # Individual test files
├── syntax/             # Syntax highlighting engines
├── fonts/              # Embedded fonts for GUI
├── themes/             # Color themes
└── ext/                # External dependencies (imgui)

File Naming Conventions

• Headers: ComponentName.h
• Implementation: ComponentName.cc
• Tests: test_feature_name.cc

Key Files by Size

Large files that may need attention:

• Command.cc (4995 lines) - Needs refactoring: Consider splitting into logical groups
• Swap.cc (1300 lines) - Crash recovery system (migrated to direct PieceTable operations)
• QtFrontend.cc (985 lines) - Qt integration
• ImGuiRenderer.cc (930 lines) - ImGui rendering
• PieceTable.cc (800 lines) - Core data structure
• Buffer.cc (763 lines) - Document model

Building and Testing

Build System

kte uses CMake with multiple build profiles:

# Debug build (terminal only)
cmake -S . -B cmake-build-debug -DCMAKE_BUILD_TYPE=Debug
cmake --build cmake-build-debug

# Release build with GUI
cmake -S . -B cmake-build-release -DCMAKE_BUILD_TYPE=Release -DBUILD_GUI=ON
cmake --build cmake-build-release

# Build specific target
cmake --build cmake-build-debug --target kte_tests

CMake Targets

• kte - Terminal editor executable
• kge - GUI editor executable (when BUILD_GUI=ON)
• kte_tests - Test suite
• imgui - Dear ImGui library (when BUILD_GUI=ON)

Running Tests

# Build and run all tests
cmake --build cmake-build-debug --target kte_tests && ./cmake-build-debug/kte_tests

# Run tests with verbose output
./cmake-build-debug/kte_tests

Test Organization

The test suite uses a minimal custom framework (Test.h):

TEST(TestName) {
    // Test body
    ASSERT_EQ(actual, expected);
    ASSERT_TRUE(condition);
    EXPECT_TRUE(condition);  // Non-fatal
}

Test files by category:

• Core Data Structures:

  • test_piece_table.cc - PieceTable operations, line indexing, random edits
  • test_buffer_rows.cc - Buffer row operations
  • test_buffer_io.cc - File I/O (open, save, SaveAs)

• Editing Operations:

  • test_command_semantics.cc - Command execution
  • test_kkeymap.cc - Keybinding system
  • test_visual_line_mode.cc - Visual line selection

• Search and Replace:

  • test_search.cc - Search functionality
  • test_search_replace_flow.cc - Interactive search/replace

• Text Reflow:

  • test_reflow_paragraph.cc - Paragraph reformatting
  • test_reflow_indented_bullets.cc - Indented list handling

• Undo System:

  • test_undo.cc - Undo/redo operations

• Swap Files (Crash Recovery):

  • test_swap_recorder.cc - Recording operations
  • test_swap_writer.cc - Writing swap files
  • test_swap_replay.cc - Replaying operations
  • test_swap_recovery_prompt.cc - Recovery UI
  • test_swap_cleanup.cc - Cleanup logic
  • test_swap_git_editor.cc - Git editor integration

• Performance and Migration:

  • test_benchmarks.cc - Performance benchmarks for core operations
  • test_migration_coverage.cc - Buffer::Line migration validation

• Integration Tests:

  • test_daily_workflows.cc - Real-world editing scenarios
  • test_daily_driver_harness.cc - Workflow test infrastructure

Total: 98 tests across 22 test files. See docs/BENCHMARKS.md for performance benchmark results.

Writing Tests

When adding new functionality:

1. Add a test first - Write a failing test that demonstrates the desired behavior
2. Use descriptive names - Test names should explain what's being validated
3. Test edge cases - Empty buffers, EOF, beginning of file, etc.
4. Use TestFrontend - For integration tests, use the programmatic test frontend

Example test structure:

TEST(Feature_Behavior_Scenario) {
    // Setup
    Buffer buf;
    buf.insert_text(0, 0, "test content\n");
    
    // Exercise
    buf.delete_text(0, 5, 4);
    
    // Verify
    ASSERT_EQ(buf.GetLineString(0), std::string("test\n"));
}

Making Changes

Development Workflow

1. Understand the change scope:

   • Pure UI change? → Modify frontend only
   • New editing operation? → Add command in Command.cc
   • Core data structure? → Modify PieceTable or Buffer

2. Find relevant code:

   • Use git grep or IDE search to find similar functionality
   • Check Command.cc for existing command patterns
   • Look at tests to understand expected behavior

3. Make the change:

   • Follow existing code style (see below)
   • Add or update tests
   • Update documentation if needed

4. Test thoroughly:

   • Run the full test suite
   • Manually test in both terminal and GUI (if applicable)
   • Test edge cases (empty files, large files, EOF, etc.)

Common Pitfalls

• Don't modify Buffer::Rows() directly - Use the PieceTable API ( insert_text, delete_text, etc.) to ensure undo and swap recording work correctly.
• Prefer efficient line access - Use GetLineView() for read-only access (11x faster than Rows()), or GetLineString() when you need a copy. Avoid Rows() in new code.
• Remember to invalidate caches - If you modify PieceTable internals, ensure line index and materialization caches are invalidated.
• Cursor visibility - After editing operations, call ensure_cursor_visible() to update viewport offsets.
• Undo boundaries - Use buf.Undo()->BeginGroup() and EndGroup() to group related operations.
• GetLineView() lifetime - The returned string_view is only valid until the next buffer modification. Use immediately or copy to std::string.

Code Style

kte uses C++20 with these conventions:

Naming

• Classes/Structs: PascalCase (e.g., PieceTable, Buffer)
• Functions/Methods: PascalCase (e.g., GetLine, Insert)
• Variables: snake_case with trailing underscore for members ( e.g., total_size_, line_index_)
• Constants: snake_case or UPPER_CASE depending on context
• Private members: Trailing underscore (e.g., pieces_, dirty_)

Formatting

• Indentation: Tabs (width 8 in most files, but follow existing style)
• Braces: Opening brace on same line for functions, control structures
• Line length: No strict limit, but keep reasonable (~100-120 chars)
• Includes: Group by category (system, external, project) with blank lines between

Comments

• File headers: Brief description of the file's purpose
• Function comments: Explain non-obvious behavior, not what the code obviously does
• Inline comments: Explain why, not what
• TODO comments: Use TODO: prefix for future work

Example:

// Consolidate small pieces to prevent fragmentation.
// This is a heuristic: we only consolidate when piece count exceeds
// a threshold, and we cap the bytes processed per consolidation run.
void maybeConsolidate() {
    if (pieces_.size() < piece_limit_)
        return;
    // ... implementation
}

Common Tasks

Adding a New Command

1. Define the command function in Command.cc:

bool cmd_my_feature(CommandContext &ctx) {
    Editor &ed = ctx.ed;
    Buffer *buf = ed.CurrentBuffer();
    if (!buf) return false;
    
    // Implement the command
    buf->insert_text(buf->Cury(), buf->Curx(), "text");
    
    return true;
}

2. Register the command in InstallDefaultCommands():

CommandRegistry::Register({
    CommandId::MyFeature,
    "my-feature",
    "Description of what it does",
    cmd_my_feature
});

3. Add keybinding in the appropriate InputHandler (e.g., TerminalInputHandler.cc).

4. Write tests in tests/test_command_semantics.cc or a new test file.

Adding a New Frontend

1. Implement the three interfaces:

   • Frontend - Lifecycle management
   • InputHandler - Event → Command translation
   • Renderer - Draw the editor state

2. Study existing implementations:

   • TerminalFrontend - Simplest, good starting point
   • ImGuiFrontend - More complex, shows GUI patterns

3. Register in main.cc to make it selectable.

Modifying the PieceTable

The PieceTable is performance-critical. When making changes:

1. Understand the piece list - Each piece references a range in either original_ or add_ buffer
2. Maintain invariants:
   • total_size_ must match sum of piece lengths
   • Line index must be invalidated on content changes
   • Version must increment on mutations
3. Test thoroughly - Use test_piece_table.cc random edit test as a reference model
4. Profile if needed - Large file performance is a key goal

Adding Syntax Highlighting

1. Create a new highlighter in syntax/ directory:

   • Inherit from HighlighterEngine
   • Implement HighlightLine() method

2. Register in HighlighterRegistry ( syntax/HighlighterRegistry.cc)

3. Add file extension mapping in the registry

4. Test with sample files of that language

Debugging Tips

• Use the test frontend - Write a test that reproduces the issue
• Enable assertions - Build in Debug mode
• Check swap files - Look in /tmp/kte-swap-* for recorded operations
• Print debugging - Use std::cerr (stdout is used by ncurses)
• GDB/LLDB - Standard debuggers work fine with kte

Getting Help

• Read the code - kte is designed to be understandable; follow the data flow
• Check existing tests - Tests often show how to use APIs correctly
• Look at git history - See how similar features were implemented
• Read design docs - Check docs/plans/ for design rationale

Future Improvements

Areas where the codebase could be improved:

1. Split Command.cc - Break into logical groups (editing, navigation, file ops, etc.)
2. Complete Buffer::Line migration - A few legacy editing functions in Command.cc still use Buffer::Rows() directly (see lines 86-90 comment)
3. Add more inline documentation - Especially for complex algorithms
4. Improve test coverage - Add more edge case tests (current: 98 tests)
5. Performance profiling - Continue monitoring performance with benchmark suite
6. API documentation - Consider adding Doxygen-style comments

---

Welcome aboard! Start small, read the code, and don't hesitate to ask questions.