# Pad — Text Editor Specification

## 1. Overview

Pad is a minimal, high-performance plain text editor for Android. It is configured with a single directory of text files and provides instant file access, automatic persistence, and seamless recovery from process death. The editor is designed for a directory synced across devices, with reactive awareness of external file modifications.

**Design philosophy:**

- **No practical limits** — files and directories of arbitrary size must work without degradation
- **Minimal surface area** — plain text only, no formatting, no file management UI
- **Automatic everything** — save, state persistence, sync awareness happen without user intervention
- **Survive anything** — process kill, battery death, device restart: the editor restores exactly where the user left off

## 2. Core Requirements

| Requirement | Detail |
|---|---|
| Instant open | Files open immediately regardless of size; content loads on demand |
| Auto-save | Every edit is persisted to disk with a short debounce; no save button exists |
| Unlimited scale | No caps on file size, line count, or directory entry count |
| External change detection | The editor monitors the configured directory for modifications from sync tools |
| Cross-session undo | Full undo history survives process death, file close, and device restart |
| State restoration | On relaunch, the editor restores the exact file, cursor position, and scroll state |
| Per-file cursor memory | Reopening any previously visited file restores the last cursor position |
| Plain text only | No formatting, no syntax highlighting, no markdown rendering |

## 3. Architecture

```
cmd/
  pad/                  # Mobile entry point

internal/
  browser/              # Directory listing, pagination, search, alphabetical index
  editor/               # Virtual scroll renderer, chunked edit buffer, input handling
  filesystem/           # Chunked file I/O, debounced writes, atomic operations
  watcher/              # File system monitoring, change detection, notification
  conflict/             # External change reconciliation, merge strategies
  state/                # App state persistence, per-file cursor map, restore logic
  undo/                 # Undo operation log, context anchoring, rebasing
```

## 4. File Browser

The browser presents the configured directory's contents. It must handle directories with hundreds of thousands of entries.

### Behavior

- **Lazy loading** — directory entries are loaded in pages, not all at once
- **Alphabetical index** — a tap-on-letter sidebar jumps to the corresponding section
- **Search** — incremental text filter over entry names
- **Single tap to open** — no long-press menus, no file management actions

### Performance

- First paint: directory root visible before all entries are read
- Scroll: smooth at 60 fps with virtualized list (only visible rows rendered)
- Search: results filter in real time, no debounce on keystrokes

## 5. Text Editor

The editor renders plain text with virtual scrolling and chunked loading. The entire file is never loaded into memory.

### Virtual Scrolling

Only lines visible on screen (plus a small prefetch buffer) are rendered. Scrolling triggers on-demand loading of adjacent chunks.

### Chunked Buffer

- Files are divided into fixed-size chunks (configurable, e.g., 64 KB)
- Only chunks near the cursor or visible region reside in memory
- Chunks are evicted under memory pressure, reloaded from disk on demand
- Edits at chunk boundaries are handled by splitting/merging chunks

### Line Index

On first open, the editor builds a line-offset index (mapping line number → byte offset). This index is:

- Built in a single pass over the file (streaming, O(n) time, O(1) memory per line)
- Cached on disk for subsequent opens
- Invalidated when the file is edited (rebuilt on next open, or incrementally updated)

The index enables O(1) or O(log n) jumps to any line number.

### Input

- Standard text input: insert characters, backspace, delete
- Selection: tap-and-drag or tap-hold to select a range
- Paste: clipboard content inserted at cursor or replacing selection
- No keyboard shortcuts beyond what the OS input method provides

## 6. Undo System

The undo system records reversible edit operations with context anchoring, persisted to disk for cross-session survival.

### 6.1 Operation Model

Edits are recorded as a **stack of chained operations**. A chain is a contiguous sequence of operations sharing a common anchor (position and context). A new chain begins whenever the cursor moves or a new selection is made.

### 6.2 Entry Formats

**Insert chain** (typing at a fixed cursor position):

```
Head:   {pos: 1024, before: "<N bytes before>", after: "<M bytes after>", inserted: "h"}
Tail:   {inserted: "e"}
Tail:   {inserted: "l"}
Tail:   {inserted: "l"}
Tail:   {inserted: "o"}
```

Entries without `pos`, `before`, or `after` inherit from the head. Position advances by the length of the previous entry's `inserted` text.

**Delete chain** (backspacing without hitting original text):

```
Head:   {pos: 1023, after: "<M bytes after>", deleted: "o"}
Tail:   {deleted: "l"}
Tail:   {deleted: "l"}
```

Entries without `pos` or `after` inherit from the head. Position for entry `i` is `head.pos + i`. The `before` field is omitted because it shifts with each deletion; `after` is the stable anchor.

**Selection replacement** (select text, type to replace):

```
Head:   {pos: 5000, before: "<N>", after: "<M>", deleted: "<selected text>", inserted: "x"}
Tail:   {inserted: "y"}
Tail:   {inserted: "z"}
```

The head records both the deleted selection and the first inserted character. Continuation typing appends to the chain.

**Field semantics:**

| Field | Present on | Meaning |
|---|---|---|
| `pos` | Head only | Byte offset of the first operation in the chain |
| `before` | Head of insert/replace chains | N bytes immediately before `pos`, used for re-anchoring |
| `after` | Head of all chains | M bytes immediately after the affected range, used for re-anchoring |
| `deleted` | Head of delete chains, selection replace | Text that was removed |
| `inserted` | Every entry | Text that was added (empty string for pure deletion entries) |

Fields absent from an entry are inherited from the chain head.

### 6.3 Backspace Semantics

Backspace interacts with the undo stack:

1. **If the last stack entry is an `inserted` entry** and the character immediately before the cursor matches that entry's `inserted` text: **pop** that entry from the stack. No new operation is recorded. The character is removed from the file.

2. **Otherwise** (stack empty, or last entry is a delete): record a **new chain head** with `deleted` set to the backspaced character, `pos` set to the byte offset of that character, and `after` set to the context after the cursor.

3. **Subsequent backspaces** (after entering delete mode): each appends a `{deleted: "<char>"}` entry to the current chain, inheriting `pos` and `after` from the head.

This means backspacing over text the user just typed is "free" — it removes operations from the stack rather than adding new ones. Undo after backspace naturally restores the popped character.

### 6.4 Undo Replay

To undo, pop the top chain from the stack and replay in reverse:

1. **Re-anchor**: search the file for the head's context (`before` + `after` for insert chains, `after` for delete chains). If found, update `pos` to the new location. If not found, skip the entire chain.

2. **Apply in reverse**: for each entry from last to first:
   - **Insert entry**: delete `inserted` text at the current position
   - **Delete entry**: insert `deleted` text at the current position
   - Advance/retreat position by the length of the text affected

3. For delete chains, replay right-to-left (last entry first) so that each insert shifts subsequent text right without affecting earlier positions.

### 6.5 Stack Limits

| Parameter | Value | Rationale |
|---|---|---|
| Max operations | 1000 | Sufficient for ~1000 typing sessions or individual keystrokes |
| Context window | 50 bytes each side | Unique enough for prose/code, small enough for persistence |
| `deleted` field | **No cap** | Large deletions are the most critical undo case; the philosophy of the editor is no practical limits |
| Redo | Not supported | Can be added later; keeps initial implementation minimal |

### 6.6 Persistence

The undo stack is serialized into the state file (see §8) and persisted on every edit, debounced separately from auto-save. On restore, each operation's context is verified; operations that cannot be anchored are silently dropped from the stack.

The stack survives:
- Process kill by the OS
- Device restart
- File close and reopen
- App restart

The stack does **not** survive:
- Explicit user action to clear it (if such a feature is added)
- External file changes that make anchoring impossible (text context no longer exists)

## 7. Auto-save

Every edit to a file is written to disk automatically.

### Behavior

- **Debounce**: 500–1000 ms after the last keystroke (configurable)
- **Atomic write**: write to a temporary file, then rename over the target (prevents corruption if killed mid-write)
- **No UI**: there is no save button, no unsaved indicator, no "do you want to save?" dialog

### Interaction with Undo

Auto-save writes the current file state. The undo stack is persisted separately. Undo works transparently across saves — saving does not "commit" or "clear" undo history.

## 8. State Management

The editor continuously persists its state to a small file (`.pad/state.json`) inside the configured directory.

### Persisted State

```json
{
  "active_file": "path/to/open/file.txt",
  "cursor_pos": 1024,
  "scroll_offset": 500,
  "file_cursors": {
    "path/to/file1.txt": 2048,
    "path/to/file2.txt": 512
  },
  "browser_scroll": 1200,
  "undo_stack": [ ... ]
}
```

| Field | Description |
|---|---|
| `active_file` | Path of the currently open file (relative to configured directory) |
| `cursor_pos` | Byte offset of the cursor in the active file |
| `scroll_offset` | Vertical scroll position (line or pixel offset) |
| `file_cursors` | Map of file path → last cursor position, for all visited files |
| `browser_scroll` | Scroll position in the directory browser |
| `undo_stack` | Serialized undo operations (see §6) |

### Write Strategy

- **Continuous**: state is written on every navigation, edit, or cursor movement
- **Debounced**: writes are batched with a short debounce (separate from auto-save debounce)
- **Atomic**: write to temp file, then rename (same as auto-save)
- **Bounded**: `file_cursors` evicts oldest entries to stay under a size limit (e.g., 1000 entries). `undo_stack` is capped at 1000 operations.

### Restore Flow

1. App launches → read state file
2. If `active_file` exists on disk → open it, load chunk around `cursor_pos`, place cursor
3. If `active_file` was deleted → show browser at `browser_scroll` position
4. If no state file exists → show browser at root
5. Undo stack is loaded and verified; unanchorable operations are dropped

## 9. File System Monitor

The editor watches the configured directory for external modifications (e.g., from a sync tool on another device).

### Detection

- Uses the platform's file system watch API (inotify on Android/Linux equivalent)
- Detects: file content changes, file creation, file deletion, file rename
- Watches the entire directory tree recursively

### Response

| Event | Action |
|---|---|
| File modified externally (editor not open) | Update browser listing (timestamp, size) |
| File modified externally (editor is open) | Trigger conflict resolution (§10) |
| File created | Appear in browser on next scroll/search |
| File deleted | Remove from browser; if open, close and show browser |
| File renamed | Update browser listing |

## 10. Conflict Resolution

When the file system monitor detects an external modification to the currently open file:

### Detection

Compare the file's last-modified timestamp and size against the editor's known state. If either differs, the file has changed externally.

### Strategies

| Strategy | Behavior |
|---|---|
| **Discard local** | Replace buffer with disk content, preserve cursor position if possible |
| **Keep local** | Ignore external change, continue editing (next sync may overwrite) |
| **Manual** | Show a brief prompt asking the user to choose |

The default strategy is configurable. The prompt (if shown) is the only modal UI in the application.

### Interaction with Undo

When an external change is applied to the buffer:

- The undo stack is **preserved** (not flushed)
- Operations are re-anchored at undo time using context search (§6.4)
- Operations that cannot be anchored are silently skipped
- New operations recorded after the external change use positions in the new coordinate space

## 11. Performance Guarantees

| Operation | Target | Mechanism |
|---|---|---|
| Open any file | < 100 ms | Chunked load: only the chunk around the cursor is read |
| Scroll | 60 fps | Virtual rendering: only visible lines + prefetch buffer |
| Type | < 16 ms per keystroke | In-memory chunk edit, async chunk flush |
| Search in directory | < 100 ms for 100k files | Lazy index built on first browse, cached on disk |
| Undo | < 100 ms | Context search is O(n) in file size but bounded by context window |
| State restore | < 500 ms | Read state file, open file, load chunk, place cursor |
| Auto-save | < 100 ms | Debounced atomic write |

### Memory

| Component | Budget |
|---|---|
| Chunk cache | Configurable, e.g., 16–64 MB |
| Undo stack | ~200 KB for 1000 ops with 50B context (excluding large `deleted` fields) |
| State file | ~200 KB typical, bounded by largest deletion in undo window |
| Line index | ~4 bytes per line (stored on disk, paged into memory) |

## 12. Out of Scope

- Syntax highlighting, themes, or formatting
- Multiple tabs or split views
- Find and replace (may be added later)
- File management (create, delete, rename, move) — handled by external tools
- Cloud sync — handled by external tools (Syncthing, Dropbox, etc.)
- Collaborative editing
- Redo (may be added later)
- Keyboard shortcut configuration
- Plugins or extensions