Pad/doc/editor_implementation_plan.md

# Editor Implementation Plan

## 1. Design Principles

- **Single-Owner Pattern**: The logic goroutine is the exclusive authority for modifying the editor buffer, cursor, and selection state.
- **Asynchronous I/O**: File saving and loading operations are delegated to the worker pool. The UI never blocks on disk operations.
- **Buffer-Based Editing**: For performance, the editor will eventually utilize a gap buffer or rope data structure. For MVP, string manipulation is acceptable if performance targets are met.
- **Event-Driven**: Keyboard input, mouse interactions, and menu actions are passed through the existing `inputChan` to the logic goroutine.

## 2. Core State Structure

The `State` struct in `internal/editor/state.go` will be extended to track active editing session data:

```go
type EditorState struct {
    Buffer          string        // Document content (or gap buffer)
    CursorPosition  int           // Byte offset
    SelectionStart  int           // -1 if no selection
    SelectionEnd    int           // -1 if no selection
    UndoStack       []EditCommand // For undo/redo
    Dirty           bool          // Needs save
}
```

## 3. Implementation Phases

### Phase 1: Basic Buffer Management & Cursor
- [ ] Define `EditorState` (Cursor, Selection, Dirty flag, GlyphLayout).
- [ ] Implement `GlyphLayout` capture in `drawWrappedText` and feedback via `layoutChan`.
- [ ] Implement cursor navigation (Arrow keys: Left, Right, Up, Down) using GlyphLayout.
- [ ] Implement basic buffer updates (Insert character, Delete/Backspace).
- [ ] Implement cursor display and blink animation.
- [ ] Update `EditorLayout` to position cursor from GlyphLayout (no fixed-width approximation).

### Phase 2: File IO Integration
- [ ] Implement `SaveFile` handler: Dispatch `WriteFileTask` to worker pool.
- [ ] Implement `LoadFile` handler: Dispatch `ReadFileTask` to worker pool (existing in `logic.go`).
- [ ] Status bar integration: Show "Saving..." indicator, "Modified" status.

### Phase 3: Text Editing Operations
- [ ] Implement Cut/Copy/Paste interactions.
- [ ] Implement multi-line text navigation (Home/End, PageUp/PageDown).
- [ ] Implement text selection display and mouse interaction.
- [ ] Implement soft-wrap toggle (already exists, need to verify logic).

### Phase 4: Polish & Advanced Features
- [ ] Undo/Redo stack implementation.
- [ ] Performance testing with large files (>1MB).

---

## 4. Interaction Flow (Example: Keyboard/Mouse Input)

1. **User** performs an action (types a character, taps an icon).
2. **UI (Renderer)** registers the element's interaction (`event.Op` for keys, `gesture.Add` for mouse) within its clip context.
3. **Main Loop** captures the event (using `key.Filter` for keys, `gesture.Update` for mouse).
4. **Main Loop** generates an `InputEvent` and sends it to `inputChan`.
5. **Logic Goroutine** receives the event, triggers the corresponding `Handler` in `TheState`.
6. **Logic Goroutine** modifies state, and sends updated elements back through `frameChan`.
7. **Renderer** paints the new frame.

---

## 5. Performance Targets

| Operation | Target | Mechanism |
|---|---|---|
| Typing latency | < 16ms | Single-owner logic update, immediate frame redraw |
| File Load (1MB) | < 100ms | Async worker pool read, background loading |
| File Save | < 100ms | Async worker pool write |
| Undo/Redo | < 10ms | O(1) or O(N) memory-based buffer update |

---

## 7. Input Handling Mechanism

### 7.1 Mouse/Touch Input
- Uses `gesture.Click` and `gesture.Scroll`.
- Registered via `event.Op` and gesture `Add()` within the clip context of the element.
- Processed by `Renderer.CheckGestures` and dispatched via `InputEvent` to the logic goroutine.

### 7.2 Keyboard Input
- **Registration**: Elements register as input handlers using `event.Op(gtx.Ops, elementID)`. This tags the current clip context for input routing.
- **Focus**: Focus is managed by the logic goroutine. When an element is focused, `key.FocusCmd{Tag: elementID}` is submitted to the operation stack.
- **Filtering**: Keyboard events are processed in the main loop using `key.Filter{Focus: focusedElementID}` to ensure events are only routed to the active element.
- **Routing**: `key.Event` (for key presses) and `key.EditEvent` (for text input) are converted into `InputEvent` structs and sent to the logic goroutine via `inputChan` for state updates.

---

## 8. Glyph Layout Architecture

### 8.1 Problem
The editor must know the exact screen position of every character after shaping and word wrapping. This is required for:
- Accurate cursor rendering (no fixed-width approximation)
- Correct cursor navigation (arrow keys respect visual line boundaries)
- Scroll-aware positioning (cursor follows text when viewport changes)
- Screen resize resilience (cursor tracks text as wrap points shift)
- Future features: mouse click-to-place, text selection, search highlight positioning

### 8.2 Core Principle
**The renderer is the single source of truth for where characters appear on screen.** Logic never guesses positions — it reads the layout the renderer already computed during the draw pass.

### 8.3 Data Structure

```go
type GlyphLayout struct {
    ByteOffsets []int  // byte offset of each glyph in the buffer
    X           []Dp   // screen X (Dp) of each glyph, relative to text region origin
    Y           []Dp   // screen Y (Dp) of each glyph, relative to text region origin
    Advance     []Dp   // advance width (Dp) of each glyph
}
```

Each index `i` represents one glyph. `ByteOffsets[i]` is the byte position in the buffer, `(X[i], Y[i])` is its screen location, and `Advance[i]` is its width. The slice length equals the total number of glyphs (one per rune).

**Derived values:**
- `LastLineY` = `Y[len(Y)-1]` (last glyph's baseline Y) — replaces the separate `lastLineY` feedback
- Visual line breaks = any index `i` where `Y[i] > Y[i-1]`
- Cursor at byte offset `b` → binary search `ByteOffsets` for exact match, then read `(X[idx], Y[idx])`

Storing all X advances is not expensive — a single page of text is a tiny amount of memory.

### 8.4 Capture Point

`Renderer.drawWrappedText` already iterates every glyph via `r.shp.NextGlyph()`. It captures layout data during that loop and returns it:

```go
func (r *Renderer) drawWrappedText(...) GlyphLayout {
    // ... existing shaping setup ...
    r.shp.LayoutString(params, str)

    var layout GlyphLayout
    byteOffset := 0
    for g, ok := r.shp.NextGlyph(); ok; g, ok = r.shp.NextGlyph() {
        glyphX := Dp(float32(g.Xdot >> 6)) - scrollOffset
        glyphY := Dp(float32(g.Ydot))

        layout.ByteOffsets = append(layout.ByteOffsets, byteOffset)
        layout.X = append(layout.X, glyphX)
        layout.Y = append(layout.Y, glyphY)
        layout.Advance = append(layout.Advance, Dp(float32(g.Advance >> 6)))

        byteOffset += utf8.RuneLen(runeAtThisGlyph)
        // ... existing draw logic ...
    }
    r.lastLineY = layout.Y[len(layout.Y)-1] // derived, not separate
    return layout
}
```

`TextField.Draw` stores the returned layout on the renderer for the main loop to pick up.

### 8.5 Feedback Path

A new channel carries the full layout from renderer to logic:

```go
layoutChan chan GlyphLayout  // in Logic struct
```

Main loop sends `r.GlyphLayout()` after each draw frame. Logic stores it in `State.Editor.GlyphLayout`. The old `lastLineYChan` is removed — `LastLineY` is derived from `GlyphLayout.Y[len-1]`.

### 8.6 Editor State Consumption

`EditorState` gains the layout field:

```go
type EditorState struct {
    Buffer         string
    CursorPosition int
    GlyphLayout    GlyphLayout
    // ...
}
```

**Cursor rendering** in `EditorLayout`:
```go
func EditorLayout(...) []ui.Element {
    layout := TheState.Editor.GlyphLayout
    pos := TheState.Editor.CursorPosition

    idx := sort.Search(len(layout.ByteOffsets), func(i int) bool {
        return layout.ByteOffsets[i] >= pos
    })

    var cursorX, cursorY Dp
    if idx < len(layout.ByteOffsets) {
        cursorX = editorRegion.X + layout.X[idx]
        cursorY = editorRegion.Y + layout.Y[idx]
    } else {
        // cursor at end of buffer — place after last glyph
        cursorX = editorRegion.X + layout.X[len(layout.X)-1] + layout.Advance[len(layout.Advance)-1]
        cursorY = editorRegion.Y + layout.Y[len(layout.Y)-1]
    }
    // ... create cursor element ...
}
```

**Cursor movement** (`HandleCursorMove`):
```go
func HandleCursorMove(delta int) {
    layout := TheState.Editor.GlyphLayout
    pos := TheState.Editor.CursorPosition

    idx := findGlyphIndex(layout, pos) // binary search ByteOffsets

    if delta > 0 {
        idx++ // next glyph
    } else {
        idx-- // previous glyph
    }

    if idx >= 0 && idx < len(layout.ByteOffsets) {
        TheState.Editor.CursorPosition = layout.ByteOffsets[idx]
    }
}
```

**Arrow-up / arrow-down** (future): find current glyph's Y, search for the glyph with the closest X on the target Y line.

### 8.7 Correctness Guarantees

| Scenario | How it works |
|---|---|
| **Word wrap** | Shaper's `WrapHeuristically` produces different Y values at wrap points. Layout captures them exactly. |
| **Screen resize** | Next frame → new `wrapWidth` → shaper produces new layout → main loop sends new layout → cursor follows text. |
| **Editing** | Buffer changes → layout re-computed on next frame → `ByteOffsets` reflect new positions. |
| **Scroll** | Y values are computed minus `scrollOffset` → cursor Y tracks naturally. |
| **LastLineY** | Derived from `layout.Y[len-1]` — no separate tracking needed. |
| **Non-fixed-width fonts** | `Advance` comes from actual glyph metrics, not approximation. |
| **Empty buffer** | Layout is empty (`len == 0`). Cursor rendering handles this as the "after last glyph" case. |

### 8.8 Summary of Changes

| File | Change |
|---|---|
| `internal/ui/unit.go` | Add `GlyphLayout` struct |
| `internal/ui/render.go` | `drawWrappedText` returns `GlyphLayout`; derive `lastLineY` from it; expose via `r.GlyphLayout()` |
| `internal/ui/element.go` | `TextField.Draw` stores returned layout on renderer |
| `internal/editor/logic.go` | Add `layoutChan`; handle it in `Run()` loop; remove `lastLineYChan` |
| `internal/editor/state.go` | `EditorState.GlyphLayout` field; `EditorLayout` uses it for cursor positioning; `HandleCursorMove` uses it for navigation |
| `cmd/pad/main.go` | Main loop sends `r.GlyphLayout()` on `layoutChan` after draw; remove `lastLineYChan` usage |