commit 97d869db6c32c51bfdd3dee087b5ce1a58c29135 Author: Greg Pomerantz Date: Thu May 7 12:24:15 2026 -0400 Initial commit: project specification diff --git a/doc/SPEC.md b/doc/SPEC.md new file mode 100644 index 0000000..2498317 --- /dev/null +++ b/doc/SPEC.md @@ -0,0 +1,330 @@ +# 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: "", 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: "", 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: "", after: "", deleted: "", 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: ""}` 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