# Episode 06 --- Demo Assets

Supporting `build-a-domain.md`. The domain built live in this episode is
**todo** --- the simplest possible real domain plugin, implemented at
`muse/plugins/todo/plugin.py` in the `muse` repo (started from
`muse/plugins/scaffold`).

## Production note: this already existed before Episode 04 was recorded

If you're reading this after watching episodes out of order and wondering
why `muse/plugins/todo` (and even `muse/plugins/chess`, Episode 22's
finale plugin) already existed in the `muse` repo before Episode 04 was
recorded --- that's expected, not a mistake. Scripts for the whole season
get written up front, and demo code/plugins get built and tested well
ahead of the recording schedule, in whatever order is convenient --- not
necessarily episode order. Recording is the thing that happens
sequentially; script-writing and demo-building don't. Check this
directory's presence and the top-level `README.md` status table before
assuming something is missing or was added by mistake.

## The todo domain --- one file, one task per line, content-addressed

Model: a single tracked file, `todo.txt`. Each task is addressed by its
own content, not by line position:

```python
def _task_address(task: str) -> str:
    digest = hashlib.sha256(task.encode("utf-8")).hexdigest()[:12]
    return f"{_TASK_FILE}::{digest}"
```

That one design choice --- **identity by content, not position** --- is
what makes every other method fall out cleanly, and it's the idea every
later, more complex domain (MIDI, code symbols, chess) builds on.

## How the six-method protocol maps onto it

| Method | What it does for `todo` |
|---|---|
| **`snapshot`** | Reads `todo.txt` off disk, hashes its raw bytes as one blob. No per-task parsing here --- just "what does the file contain right now." |
| **`diff`** | Loads both snapshots' task sets (splitting `todo.txt` into lines) and computes `target - base` (added) / `base - target` (removed) as **set difference**. Each added/removed task becomes its own `AddressedInsertOp` / `AddressedDeleteOp`, nested inside one outer `PatchOp` keyed on `todo.txt` (that outer key is what `checkout` actually restores from disk). |
| **`merge`** | Three-way, but does no line-based patching: `merged = (base ∪ left ∪ right) − removed`, where `removed` is anything either side deleted relative to base. Because tasks are content-addressed set elements, not line positions, **two branches adding two different tasks can never conflict** --- there's no "line 5 vs line 5" collision to even detect. |
| **`drift`** | `muse status` calls this: re-snapshot live state, diff it against the last commit, report whether anything changed. |
| **`apply`** | A no-op --- the core engine already restores `todo.txt`'s bytes from the object store during `checkout`; there's no domain-specific reconstruction needed since the file is opaque bytes, not a structured format. |
| **`schema`** | Declares `SetSchema(kind="set", element_type="task", identity="by_content")`, `merge_mode="three_way"`, zero sub-dimensions --- the minimum viable domain: no address-keyed merge extension, no CRDT, just the required core. |

## The contrast with Git this episode is built around

Git would treat `todo.txt` as text and merge by line --- two people adding
a task on the same line number is a conflict even if the tasks are
unrelated. `todo` instead treats the file as an **unordered set of
content-addressed elements**, so "did the same *task* change" is the only
thing that can conflict --- and since there's no edit operation (only
add/remove of whole tasks), that basically never happens.

## `make-todo-episode06-demo.sh` --- fully scripted, run it directly

```bash
./make-todo-episode06-demo.sh
```

Builds a disposable `todo-episode06/` repo: one task committed on `main`,
two branches each adding a *different* task, both merged back with zero
conflicts. Uses `muse-dev` rather than plain `muse` --- the todo domain
ships on `muse`'s `dev` branch but hadn't gone out in a released tarball
as of this recording. Swap `MUSE_BIN` to `muse` once a release includes
it.