# `StatifierBlocks.Runtime.Selection`
[🔗](https://github.com/riddler/statifier_blocks/blob/v0.44.1/lib/statifier_blocks/runtime/selection.ex#L1)

Where a run is being watched from: the scrubber's four moves, and a direct
pick of one macrostep.

`StatifierBlocks.Runtime.Marks` reads a run's selection and resolves it to
blocks. Something has to *move* that selection when an author presses Prev
or clicks a step, and this is it. It is a separate module from `Marks` for
the reason `Marks` is separate from the editor: the editor's handler should
be one line, and what that line does is worth testing without LiveView.

## One of the two moves needs statifier-ui and the other does not

Picking a macrostep is a field write. `selection` is a public field of
`StatifierUI.Live.State` and its `{:macrostep, n}` shape is published in
the same typespec `Marks` already reads `messages` and
`initial_configuration` from, so `select/2` writes it directly and costs no
dependency at all.

Scrubbing is not, because *where* Prev lands is a decision about the
stream: which macrosteps are points, whether the one below the current is
the oldest, and what "next" means at the tip. statifier-ui decides that in
`StatifierUI.Inspector.step/3` and this package must not answer it a second
time - a scrubber that disagreed with the note beside it about which point
is on screen is the drift the whole seam is arranged to prevent. So the two
reads are dispatched dynamically out of `:statifier_blocks,
:trace_inspector_module`, the same key `Marks` resolves and defaulting to
the same module, checked with `function_exported?/3`. Without a resolvable
inspector a scrub does nothing, which is what a scrubber over a stream
nothing can read should do.

# `scrub`

```elixir
@spec scrub(map(), atom()) :: map()
```

`state` with its selection moved one scrubber step.

`move` is one of `:first`, `:prev`, `:next`, `:live`. The state is returned
unchanged for anything else, for a value carrying no `messages`, and for a
tree where the reads do not resolve.

# `select`

```elixir
@spec select(map(), non_neg_integer()) :: map()
```

`state` selecting macrostep `n` - what clicking an event-log entry does.

A macrostep the stream holds no bucket for is not an error here any more
than it is in statifier-ui: it resolves against whatever sits below it when
something comes to read it.

---

*Consult [api-reference.md](api-reference.md) for complete listing*