Initial commit: project specification

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+# 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