# Stage: prepare

## Purpose
Materialize canonical current-session input state and provenance before downstream stages run.

Prepare owns:
- local input file materialization (`inputs/**`);
- audio input materialization (`audio/**`);
- previous-session cache hydration (`previous/**`) for canonical previous-session artifact sources.

## Inputs and outputs
Inputs:
- resolved config/session (`pipeline.yml`, `session.yml`);
- session-local input files (`speakers`, `autocorrect`, `glossary`);
- audio source:
  - local: `session.inputs.audio_dir` or `session.inputs.audio_files`;
  - S3: `session.inputs.audio_s3.prefix`;
- configured enabled Scriptorium artifact inputs (for previous-session requirement scanning);
- remote previous-session current archive state when previous hydration is required.

Outputs:
- `inputs/session.yml`;
- `inputs/pipeline.resolved.yml`;
- `inputs/speakers.yml`;
- `inputs/autocorrect.yml`;
- `inputs/glossary.yml`;
- `audio/*.flac` in canonical session `audio/`;
- optional `previous/manifest.json`;
- optional `previous/artifacts/**`;
- deterministic `manifest.Inputs` records with checksums and provenance metadata.

## Boundaries
Owns:
- input path resolution and materialization;
- S3 audio list/download/copy flow;
- previous-session artifact requirement collection from enabled configured artifacts;
- previous cache lifecycle when requirements exist (clear and rehydrate managed `previous/` state).

Does not own:
- transcript or artifact generation;
- analyze-stage source resolution;
- archive commit behavior.

## Config fields used
- `session.session_id`
- `session.previous_session_id`
- `session.campaign`
- `session.inputs.speakers_file`
- `session.inputs.autocorrect_file`
- `session.inputs.glossary_file`
- `session.inputs.audio_dir`
- `session.inputs.audio_files`
- `session.inputs.audio_s3.prefix`
- `pipeline.workspace.root`
- `pipeline.spool.root`
- `pipeline.storage.s3.bucket`
- `pipeline.storage.s3.root_prefix`
- `pipeline.scriptorium.artifacts.<name>.enabled`
- `pipeline.scriptorium.artifacts.<name>.inputs.<key>.source`
- `pipeline.scriptorium.artifacts.<name>.inputs.<key>.required`

## External adapters used
- `storage.ObjectStore` for:
  - S3 audio listing/downloads;
  - previous-session current pointer/manifest/artifact object checks and downloads.

## State and manifest behavior
- Ensures workspace layout exists.
- Materializes canonical input files and audio files.
- Scans enabled configured artifact inputs for canonical sources:
  - `narratio.previous_session.artifact.<artifact_key>`
- If one or more canonical previous-session requirements exist:
  - clears managed `previous/` state;
  - hydrates required/optional previous artifacts from the configured previous session’s committed archive current state;
  - writes `previous/manifest.json` and hydrated `previous/artifacts/**`;
  - records hydrated previous inputs in `manifest.Inputs` with source `previous_session_archive.current`.
- If no canonical previous-session requirements exist, prepare does not manage `previous/`.
- `manifest.Inputs` is sorted deterministically by `(kind, path)`.

## Required and optional previous-session behavior
- `previous_session_id` unset:
  - if any referenced previous artifact is required: fail;
  - if all referenced previous artifacts are optional: continue and omit them.
- Previous session archive current pointer or manifest missing:
  - if any referenced previous artifact is required: fail;
  - if all referenced previous artifacts are optional: continue and omit missing ones.
- Missing required previous artifact object: fail.
- Missing optional previous artifact object: omit.
- Downloaded previous artifacts must validate as non-empty files.

## Skip and resume behavior
- Runner-level skip remains authoritative:
  - if `prepare` already succeeded and run is not forced, `prepare` does not run and no hydration/download occurs.
- If `prepare` runs (including with `--force`), it owns managed `previous/` state for canonical previous-session inputs.

## Failure behavior
- Fails on missing required input files, invalid audio-source combinations, empty/duplicate audio inputs, missing object store for S3 modes, and remote access/download/validation errors.
- For required canonical previous-session inputs, analyze-time missing-input guidance is to rerun:
  - `narratio run-stage --force prepare`

## Tests to inspect before changing
- `internal/stage/prepare_test.go`
- `internal/stage/prepare_previous_test.go`
- `internal/artifacts/previous_requirements_test.go`
- `internal/app/runner_test.go`

## Architectural invariants
- `audio_dir`/`audio_files` and `audio_s3` are mutually exclusive.
- Storage keys are computed by callers using archive/path helpers; storage adapter receives explicit keys.
- `prepare` is the only stage that hydrates canonical previous-session cache state.