# 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` is extended to track active editing session data:

```go
type EditorState struct {
    Buffer         string        // Document content (plain string for now)
    CursorPosition int           // Byte offset
    SelectionStart int           // -1 if no selection (field exists, unused)
    SelectionEnd   int           // -1 if no selection (field exists, unused)
    Dirty          bool          // Needs save
    CursorVisible  bool          // Blink state (field exists, no timer yet)
    // UndoStack []EditCommand // Planned for Phase 4
}
```

**Current state** (as of June 2026): `EditorState` is defined in `internal/editor/state.go` with the fields above. `UndoStack` is deferred. `SelectionStart`/`SelectionEnd` are present but not yet wired into any UI or input handler.

## 3. Implementation Phases

### Phase 1: Basic Buffer Management & Cursor
- [x] Define `EditorState` (Cursor, Selection, Dirty flag). — **DONE**
- [x] Implement `GlyphLayout` capture in `drawWrappedText` and feedback via `layoutChan`. — **DONE**
- [x] Implement cursor navigation (Arrow keys: Left, Right, Up, Down). — **DONE** (Left/Right: byte-based ±1; Up/Down: GlyphLayout-based)
- [x] Implement basic buffer updates (Insert character, Delete/Backspace). — **DONE**
- [x] Implement cursor display. — **DONE** (Rendered inline in `Renderer.drawWrappedText` using `GlyphLayout`)
- [ ] Implement cursor blink animation. — **NOT STARTED** (`CursorVisible` field exists but no timer drives it)
- [x] Tap-to-position cursor from GlyphLayout. — **DONE** (`SetCursorFromPoint` groups glyphs by Y, finds closest X)

### Phase 2: File IO Integration
- [ ] Implement `SaveFile` handler: Dispatch `WriteFileTask` to worker pool. — **NOT STARTED**
- [x] Implement `LoadFile` handler: Dispatch `ReadFileTask` to worker pool. — **DONE**
- [ ] Status bar integration: Show "Saving..." indicator, "Modified" status. — **NOT STARTED** (currently status bar shows static labels + byte-based approximation for line/col)

### Phase 3: Text Editing Operations
- [ ] Implement Cut/Copy/Paste interactions. — **NOT STARTED** (icon elements exist but handlers are `nil`)
- [ ] Implement multi-line text navigation (Home/End, PageUp/PageDown). — **NOT STARTED**
- [ ] Implement text selection display and mouse interaction. — **NOT STARTED** (`SelectionStart`/`SelectionEnd` fields exist in `EditorState` but are unused)
- [x] Implement soft-wrap toggle. — **DONE** (`ToggleWordWrap` in `state.go`, wired to bottom bar "Wrap" label tap)

### Phase 4: Polish & Advanced Features
- [ ] Undo/Redo stack implementation. — **NOT STARTED** (`UndoStack` field is commented out in `EditorState`)
- [ ] Performance testing with large files (>1MB). — **NOT STARTED**

---

## 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

**Implemented.** `drawWrappedText` in `render.go` captures full per-glyph layout data during the shaping loop:

```go
func (r *Renderer) drawWrappedText(gtx layout.Context, str string, reg Region, wrapWidth Dp, scrollOffset Dp, cursorPos int) {
    // ... shaping setup with WrapHeuristically ...
    r.shp.LayoutString(params, str)

    var layout GlyphLayout
    byteOffset := 0
    for g, ok := r.shp.NextGlyph(); ok; g, ok = r.shp.NextGlyph() {
        layout.ByteOffsets = append(layout.ByteOffsets, byteOffset)
        layout.X = append(layout.X, Dp(float32(g.X>>6)/r.scale.Scale()))
        layout.Y = append(layout.Y, Dp(float32(g.Y)/r.scale.Scale()))
        layout.Advance = append(layout.Advance, Dp(float32(g.Advance>>6)/r.scale.Scale()))

        for i := uint16(0); i < g.Runes; i++ {
            _, sz := utf8.DecodeRuneInString(str[byteOffset:])
            byteOffset += sz
        }
        // ... draw logic ...
    }
    // Store captured layout; derive lastLineY from it.
    r.glyphLayout = layout
    if len(layout.Y) > 0 {
        r.lastLineY = layout.Y[len(layout.Y)-1]
    }
}
```

The layout is stored on the renderer as `r.glyphLayout`. `lastLineY` is derived from `layout.Y[len-1]`. Exposed via `Renderer.GlyphLayout()`.

### 8.5 Feedback Path

**Implemented.** A `layoutChan` carries the full `GlyphLayout` from renderer to logic. `LastLineY` is derived from `GlyphLayout.Y[len-1]`.

```go
// main.go — after each FrameEvent
glyphLayout := renderer.GlyphLayout()
logic.LayoutChan() <- glyphLayout

// logic.go
case layout := <-l.layoutChan:
    l.state.Editor.GlyphLayout = layout
    var derivedLastLineY ui.Dp
    if len(layout.Y) > 0 {
        derivedLastLineY = layout.Y[len(layout.Y)-1]
    }
    if derivedLastLineY != l.state.LastLineY {
        l.state.LastLineY = derivedLastLineY
        l.frameChan <- l.state.layout(l.browserManager)
    }
```

### 8.6 Editor State Consumption

**Implemented.** `EditorState` contains a `GlyphLayout` field. All cursor operations are GlyphLayout-based.

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

**Cursor rendering** — handled inline in `drawWrappedText` (`render.go`). After shaping, the renderer binary-searches `ByteOffsets` for `cursorPos` and draws a 2dp vertical bar at the computed position. No separate cursor element is needed in the element tree.

**Cursor movement** (`HandleCursorMove`, `state.go`) — byte-based ±1 with bounds clamping. For proportional fonts this is a reasonable approximation; a glyph-index-based version could be added later.

**Vertical cursor movement** (`HandleVerticalCursorMove`, `state.go`) — fully GlyphLayout-based. Finds the current glyph's Y via binary search, scans for the target line's Y, then picks the glyph with the closest X on that line.

**Tap-to-position** (`SetCursorFromPoint`, `state.go`) — GlyphLayout-based. Groups glyphs by Y-baseline, identifies the target visual line from the tap Y, then finds the closest glyph by X distance on that line. Handles edge case of tapping past the rightmost glyph.

### 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 | Status | Notes |
|---|---|---|
| `internal/ui/unit.go` | **Done** | `GlyphLayout` struct defined with `ByteOffsets`, `X`, `Y`, `Advance` |
| `internal/ui/render.go` | **Done** | `drawWrappedText` captures full `GlyphLayout`; derives `lastLineY` from it; renders cursor inline |
| `internal/ui/element.go` | **Done** | `TextField.Draw` passes `CursorPosition` to `drawWrappedText` |
| `internal/editor/logic.go` | **Done** | Uses `layoutChan` (full `GlyphLayout`) instead of `lastLineYChan` |
| `internal/editor/state.go` | **Done** | `GlyphLayout` stored on `EditorState`; cursor rendering handled by `Renderer` |
| `cmd/pad/main.go` | **Done** | Sends `renderer.GlyphLayout()` on `LayoutChan()` after each frame |

---

## 9. Current Implementation Status (as of June 2026)

This section documents what has been implemented beyond the original plan, and what gaps remain.

### 9.1 Implemented Components

| Component | Status | Notes |
|---|---|---|
| `EditorState` struct | **Done** | `Buffer`, `CursorPosition`, `GlyphLayout`, `SelectionStart/End` (unused), `Dirty`, `CursorVisible` |
| `GlyphLayout` capture | **Done** | Full per-glyph capture in `drawWrappedText` (ByteOffsets, X, Y, Advance) |
| `layoutChan` feedback | **Done** | Main loop sends `GlyphLayout` after each frame; logic derives `LastLineY` from it |
| `HandleInsert` | **Done** | String concatenation at cursor position, advances cursor, sets `Dirty` |
| `HandleBackspace` | **Done** | Removes byte before cursor, decrements cursor, sets `Dirty` |
| `HandleDelete` | **Done** | Removes byte after cursor, sets `Dirty` |
| `HandleCursorMove` | **Done** | Byte-based ±1 movement, bounded by buffer length |
| `HandleVerticalCursorMove` | **Done** | GlyphLayout-based: binary search for current Y, scan for target Y, find closest X |
| `SetCursorFromPoint` | **Done** | GlyphLayout-based tap-to-position: groups glyphs by Y, finds closest X on target line |
| `HandleKeyDown` | **Done** | Dispatches `key.Name` (arrows, delete, return) and `key.EditEvent` (text input) |
| `EditorLayout` | **Done** | Full layout: status bar, text field, bottom bar |
| Cursor rendering | **Done** | Inline in `Renderer.drawWrappedText` — 2dp vertical bar positioned from `GlyphLayout` |
| File open flow | **Done** | `OpenFile` → `openFileChan` → `ReadFileTask` → worker pool → `handleWorkerResult` → `Editor.Buffer` |
| Soft-wrap toggle | **Done** | `ToggleWordWrap` wired to bottom bar label |
| Page navigation | **Done** | Browser ↔ Editor page switching via `GoToBrowser`/`GoToEditor` |
| Keyboard focus | **Done** | `FocusedElementID` + `key.FocusCmd` + `key.Filter`/`key.FocusFilter` routing |
| Scroll handling | **Done** | `HandleScroll` clamped to `[0, MaxScroll]` derived from `LastLineY` |
| Mock filesystem | **Done** | `populateMockFileSystem` with realistic directory structure |
| Worker pool | **Done** | 4-goroutine pool with `ReadFileTask`, `BuildIndexTask`, etc. |

### 9.2 Test Coverage

| Test File | Coverage |
|---|---|
| `buffer_test.go` | `TestInsertChar` — verifies insert at end of buffer |
| `buffer_test.go` | `TestBackspace` — verifies backspace at end of buffer |
| `cursor_test.go` | `TestCursorPositioning_Basic` — verifies GlyphLayout-based tap-to-position |
| `cursor_test.go` | `TestCursorPositioning_EmptyDocument` — verifies empty document handling |
| `cursor_test.go` | `TestCursorPositioning_IncompleteLayout` — verifies graceful handling of partial layout |
| `cursor_test.go` | `TestHandleCursorMove_Bounds` — verifies cursor clamping at buffer start/end |
| `integration_test.go` | `TestOpenFileIntegration` — verifies full file open flow through worker pool |

### 9.3 Known Gaps

| Gap | Description |
|---|---|
| Cursor blink | `CursorVisible` field exists but no blink timer is driven |
| Home/End/PageUp/PageDown | No handlers for these keys |
| Cut/Copy/Paste | Icon elements exist with `nil` handlers |
| Text selection | `SelectionStart`/`SelectionEnd` fields present but unused |
| Save file | No `SaveFile` handler or `WriteFileTask` integration |
| Undo/Redo | Commented out in `EditorState`, no implementation |
| Status bar dynamics | Filename, line/col, byte count are static strings |
| `Dirty` flag UI | `Dirty` is set on edits but not displayed in status bar |