# Extract the `--pack` Writer into the packed Module — Implementation Plan

> **Parent spec:** none — tracker issue #5 (a behaviour-preserving
> refactor of shipped code; the issue body is the contract). The packed
> format itself is specced in
> `docs/specs/2026-05-31-packed-versioned-index-bundle.md`.
>
> **For agentic workers:** REQUIRED SUB-SKILL: use the `implement`
> skill to run this plan. Steps use `- [ ]` checkboxes for tracking.

**Goal:** Move the `index --pack` writer logic out of `main` into a
`packed::pack_from_store` library fn so the header construction is
unit-testable without spawning the binary, with zero behaviour change.

**Architecture:** A new `PackOutcome { Packed(PackedHeader), Incomplete
{ cached, total } }` and `pack_from_store(index_dir, embed_model,
alpha_id_path, entries)` in `src/packed.rs` own the completeness check, row
collection in corpus order, header derivation, and the write. The binary's
pack branch shrinks to: load entries, call the fn, match the outcome to the
same stderr messages and exit code it emits today.

**Tech Stack:** Rust; `src/packed.rs` (new fn + enum), `src/bin/alpha_id.rs`
(rewire), `tests/packed_tests.rs` (new unit tests); reuses the existing
`EmbeddingStore`, `corpus_sha256`, `write_packed`, `packed_path`.

**Scope boundary:** Behaviour-preserving refactor only. No change to the
load path, `IndexStatus`, the packed format, or `mmap`. The four existing
pack CLI tests (`pack_refuses_incomplete_store_exit_2`,
`pack_writes_file_from_complete_store`, `cli_packed_file_loads_back_as_ok_index`,
`cli_packed_rows_are_in_corpus_order`) MUST stay green unchanged — they are
the behaviour-parity guard.

---

**Files this plan creates or modifies:**

- Modify: `src/packed.rs` — add `PackOutcome` enum and `pack_from_store`;
  extend the imports to `EmbeddingStore` and `AlphaIdEntry`.
- Modify: `src/bin/alpha_id.rs:87-117` — replace the inline pack body with a
  call to `pack_from_store` and an outcome match.
- Test: `tests/packed_tests.rs` — unit tests for the complete and incomplete
  outcomes of `pack_from_store`.

---

### Task 1: Extract `pack_from_store` and rewire the binary

**Files:**
- Modify: `src/packed.rs`
- Modify: `src/bin/alpha_id.rs:87-117`
- Test: `tests/packed_tests.rs`

- [ ] **Step 1: Write the failing unit tests**

Append to `tests/packed_tests.rs`. Add these imports at the top of the file
(it currently imports `corpus_sha256, read_packed, write_packed,
PackedHeader` from `alpha_id::packed`):

```rust
use alpha_id::embed::EmbeddingStore;
use alpha_id::model::AlphaIdEntry;
use alpha_id::packed::{pack_from_store, packed_path, PackOutcome};
```

Then append:

```rust
fn mk_entry(text: &str) -> AlphaIdEntry {
    AlphaIdEntry {
        alpha_id: "A0001".to_string(),
        valid: true,
        icd_primary: "A00".to_string(),
        icd_star: String::new(),
        icd_addon: String::new(),
        icd_primary2: String::new(),
        orpha: String::new(),
        text: text.to_string(),
    }
}

#[test]
fn pack_from_store_complete_builds_header_and_writes_file() {
    let dir = tempfile::tempdir().unwrap();
    let index_dir = dir.path().join("index");
    std::fs::create_dir_all(&index_dir).unwrap();
    let corpus = dir.path().join("corpus.txt");
    std::fs::write(&corpus, b"corpus bytes").unwrap();

    let store = EmbeddingStore::open(index_dir.to_str().unwrap(), "test/model").unwrap();
    store.put("alpha", &[1.0f32, 0.0, 0.0]).unwrap();
    store.put("beta", &[0.0f32, 1.0, 0.0]).unwrap();
    let entries = vec![mk_entry("alpha"), mk_entry("beta")];

    let outcome = pack_from_store(
        index_dir.to_str().unwrap(),
        "test/model",
        corpus.to_str().unwrap(),
        &entries,
    )
    .unwrap();

    match outcome {
        PackOutcome::Packed(h) => {
            assert_eq!(h.n_rows, 2);
            assert_eq!(h.dim, 3);
            assert_eq!(h.embed_model, "test/model");
            assert_eq!(
                h.corpus_sha256,
                alpha_id::packed::corpus_sha256(corpus.to_str().unwrap()).unwrap()
            );
        }
        PackOutcome::Incomplete { .. } => panic!("a complete store must pack"),
    }
    assert!(packed_path(index_dir.to_str().unwrap(), "test/model").exists());
}

#[test]
fn pack_from_store_incomplete_reports_counts_and_writes_nothing() {
    let dir = tempfile::tempdir().unwrap();
    let index_dir = dir.path().join("index");
    std::fs::create_dir_all(&index_dir).unwrap();
    let corpus = dir.path().join("corpus.txt");
    std::fs::write(&corpus, b"corpus bytes").unwrap();

    let store = EmbeddingStore::open(index_dir.to_str().unwrap(), "test/model").unwrap();
    store.put("alpha", &[1.0f32, 0.0]).unwrap(); // only 1 of 2 entries cached
    let entries = vec![mk_entry("alpha"), mk_entry("beta")];

    let outcome = pack_from_store(
        index_dir.to_str().unwrap(),
        "test/model",
        corpus.to_str().unwrap(),
        &entries,
    )
    .unwrap();

    assert!(matches!(outcome, PackOutcome::Incomplete { cached: 1, total: 2 }));
    assert!(
        !packed_path(index_dir.to_str().unwrap(), "test/model").exists(),
        "an incomplete store must not write a packed file"
    );
}
```

- [ ] **Step 2: Run the tests to verify they fail**

Run: `cargo test --test packed_tests`
Expected: FAIL — compile error: `pack_from_store` and `PackOutcome` are not
found in `alpha_id::packed` (they do not exist yet), which reddens the test
binary.

- [ ] **Step 3: Add `PackOutcome` + `pack_from_store` in `src/packed.rs`**

Extend the model import at the top of `src/packed.rs` — change:

```rust
use crate::model::AppError;
```

to:

```rust
use crate::embed::EmbeddingStore;
use crate::model::{AlphaIdEntry, AppError};
```

Then add, after the `corpus_sha256` fn (before `write_packed`):

```rust
/// Result of a pack attempt: either the header that was written, or a report
/// that the per-file store is missing some corpus entry's embedding (in which
/// case nothing is written, and the caller refuses loudly).
pub enum PackOutcome {
    Packed(PackedHeader),
    Incomplete { cached: usize, total: usize },
}

/// Pack a complete per-file store into the single packed matrix file at
/// `packed_path(index_dir, embed_model)`, returning the header written. Rows
/// are collected in `entries` order (row `i` = `entries[i]`), so the packed
/// matrix is corpus-aligned. If any entry's embedding is not cached, returns
/// `Incomplete { cached, total }` and writes nothing.
pub fn pack_from_store(
    index_dir: &str,
    embed_model: &str,
    alpha_id_path: &str,
    entries: &[AlphaIdEntry],
) -> Result<PackOutcome, AppError> {
    let store = EmbeddingStore::open(index_dir, embed_model)?;
    let cached = entries.iter().filter(|e| store.has(&e.text)).count();
    if cached != entries.len() {
        return Ok(PackOutcome::Incomplete { cached, total: entries.len() });
    }
    let rows: Vec<Vec<f32>> = entries
        .iter()
        .map(|e| {
            store
                .get(&e.text)
                .ok_or_else(|| AppError::Io("cached vector vanished during pack".to_string()))
        })
        .collect::<Result<Vec<_>, _>>()?;
    let dim = rows.first().map(|r| r.len()).unwrap_or(0) as u32;
    let corpus_name = Path::new(alpha_id_path)
        .file_name()
        .and_then(|n| n.to_str())
        .unwrap_or(alpha_id_path)
        .to_string();
    let header = PackedHeader {
        n_rows: entries.len() as u32,
        dim,
        embed_model: embed_model.to_string(),
        corpus_sha256: corpus_sha256(alpha_id_path)?,
        corpus_name,
    };
    write_packed(&packed_path(index_dir, embed_model), &header, &rows)?;
    Ok(PackOutcome::Packed(header))
}
```

- [ ] **Step 4: Rewire the binary's pack branch**

In `src/bin/alpha_id.rs`, replace the inline pack body (`src/bin/alpha_id.rs:87-117`,
the whole `if pack { … }` block) with:

```rust
            if pack {
                use alpha_id::packed::{pack_from_store, packed_path, PackOutcome};
                let entries = alpha_id::corpus::load(&cfg.alpha_id_path).expect("corpus");
                match pack_from_store(&cfg.index_dir, &cfg.embed_model, &cfg.alpha_id_path, &entries)
                    .expect("pack")
                {
                    PackOutcome::Incomplete { cached, total } => {
                        eprintln!(
                            "error: store incomplete: {} of {} entries embedded; \
                             run `index --full --confirm` first",
                            cached, total
                        );
                        std::process::exit(2);
                    }
                    PackOutcome::Packed(header) => {
                        let path = packed_path(&cfg.index_dir, &cfg.embed_model);
                        eprintln!("packed {} rows × {} dims → {}", header.n_rows, header.dim, path.display());
                    }
                }
                return;
            }
```

(The `use alpha_id::embed::{EmbeddingStore, build_corpus_embeddings, estimate};`
line at `src/bin/alpha_id.rs:84` stays — the `sample` / `full` branches below
still use it.)

- [ ] **Step 5: Run the new unit tests to verify they pass**

Run: `cargo test --test packed_tests`
Expected: PASS — the two new tests plus the existing format tests all pass.

- [ ] **Step 6: Run the full suite to verify behaviour parity**

Run: `cargo test`
Expected: PASS — the whole suite is green. In particular the four pack CLI
tests (`pack_refuses_incomplete_store_exit_2`,
`pack_writes_file_from_complete_store`, `cli_packed_file_loads_back_as_ok_index`,
`cli_packed_rows_are_in_corpus_order`) pass unchanged, confirming the
extraction preserved behaviour.