# Shifted Seasons Handling

This file defines the behavioral contract for mapping source season and episode
numbering to target season and episode numbering through stored shifted-season
rules.

Primary sources:
- `requirements/project.md`
- `requirements/architecture.md`
- actual tool code in `src/ffx/`

Secondary source:
- `SCRATCHPAD.md`, used only to clarify current hardening gaps and not as the
  primary contract source.

## Scope

- Persisting shifted-season rules in SQLite.
- Allowing shifted-season rules to be attached either to a show or to a
  specific pattern.
- Selecting at most one active shifted-season rule for one concrete source
  season and episode tuple.
- Applying additive season and episode offsets to produce target numbering.
- Using shifted target numbering during `convert` for TMDB episode lookup and
  generated season and episode filename tokens.
- Managing show-level default mappings and pattern-level override mappings from
  the Textual editing workflows.

## Out Of Scope

- General filename parsing rules for detecting season and episode values.
- Standalone `rename` command behavior, which currently uses explicit rename
  inputs rather than stored shifted-season rules.
- Stream or track mapping behavior unrelated to season and episode numbering.

## Terms

- `shifted-season rule`: one persisted row describing how one source-numbering
  range maps to target numbering through additive offsets.
- `show-level shifted-season rule`: a rule attached directly to a show and used
  as the fallback mapping layer for that show.
- `pattern-level shifted-season rule`: a rule attached directly to a pattern and
  used as the override mapping layer for that pattern.
- `source numbering`: the season and episode values detected from the current
  source file or supplied as source-side conversion inputs before shifting.
- `target numbering`: the season and episode values after one active
  shifted-season rule has been applied.
- `original season`: the source-domain season number a shifted-season rule is
  eligible to match.
- `episode range`: the optional source-domain episode interval covered by one
  shifted-season rule.
- `open bound`: an unbounded start or end of the episode range. Current storage
  uses `-1` as the internal sentinel for an open bound.
- `active shifted-season rule`: the single rule selected for one concrete input
  after precedence resolution.
- `identity mapping`: the default `1:1` outcome where source numbering is used
  unchanged.

## Rules

- `SHIFTED_SEASONS_HANDLING-0001`: The domain model shall allow a
  shifted-season rule to be owned by exactly one of:
  - one show
  - one pattern
- `SHIFTED_SEASONS_HANDLING-0002`: A single shifted-season rule shall not
  belong to both a show and a pattern at the same time.
- `SHIFTED_SEASONS_HANDLING-0003`: A shifted-season rule shall carry these
  fields: `original_season`, `first_episode`, `last_episode`,
  `season_offset`, and `episode_offset`.
- `SHIFTED_SEASONS_HANDLING-0004`: `season_offset` and `episode_offset` shall
  be additive signed integers applied to matched source numbering to produce
  target numbering.
- `SHIFTED_SEASONS_HANDLING-0005`: A shifted-season rule shall match a source
  tuple only when:
  - the source season equals `original_season`
  - the source episode is greater than or equal to `first_episode` when the
    lower bound is closed
  - the source episode is less than or equal to `last_episode` when the upper
    bound is closed
- `SHIFTED_SEASONS_HANDLING-0006`: An open lower or upper episode bound shall
  represent an unbounded side of the covered source episode range.
- `SHIFTED_SEASONS_HANDLING-0007`: If one shifted-season rule matches, target
  numbering shall be:
  - `target season = source season + season_offset`
  - `target episode = source episode + episode_offset`
- `SHIFTED_SEASONS_HANDLING-0008`: If no shifted-season rule matches, source
  numbering shall pass through unchanged.
- `SHIFTED_SEASONS_HANDLING-0009`: Shifted-season handling shall operate in a
  source-to-target numbering model. Stored rules map detected source numbering
  to the target numbering used by conversion-facing metadata and output naming.
- `SHIFTED_SEASONS_HANDLING-0010`: Pattern matching identifies the owning show
  and optionally a more specific owning pattern. Resolution of the active
  shifted-season rule shall use this precedence order:
  - matching pattern-level rule
  - matching show-level rule
  - identity mapping
- `SHIFTED_SEASONS_HANDLING-0011`: At most one shifted-season rule may be
  active for one concrete source season and episode tuple. Shifted-season rules
  shall never stack or compose.
- `SHIFTED_SEASONS_HANDLING-0012`: Within one owner scope, shifted-season rules
  shall not overlap in their effective episode coverage for the same
  `original_season`.
- `SHIFTED_SEASONS_HANDLING-0013`: If a shifted-season rule uses two closed
  episode bounds, `last_episode` shall be greater than or equal to
  `first_episode`.
- `SHIFTED_SEASONS_HANDLING-0014`: Shifted-season rule evaluation shall be
  deterministic. Released behavior shall not depend on arbitrary database row
  order when invalid overlapping rules exist.
- `SHIFTED_SEASONS_HANDLING-0015`: A pattern-level rule is permitted to map to
  zero offsets. Such a rule is a valid explicit override that beats show-level
  fallback and produces identity mapping for its covered source range.
- `SHIFTED_SEASONS_HANDLING-0016`: During `convert`, when show, season, and
  episode values are available and stored shifting is active, the shifted target
  numbering shall drive:
  - TMDB episode lookup
  - season and episode filename tokens such as `S01E02`
  - generated episode basenames that include season and episode numbering
- `SHIFTED_SEASONS_HANDLING-0017`: When conversion is supplied explicit
  target-domain season or episode values for TMDB naming, the system shall not
  apply stored shifting on top of those already-targeted values.
- `SHIFTED_SEASONS_HANDLING-0018`: Operator-facing editing shall expose
  shifted-season rule management in both of these places:
  - show editing for show-level default mappings
  - pattern editing for pattern-level override mappings
- `SHIFTED_SEASONS_HANDLING-0019`: User-facing shifted-season editing should
  present open episode bounds as a natural empty-state input rather than forcing
  operators to type the internal sentinel directly.

## Acceptance

- A show can exist with zero or more show-level shifted-season rules.
- A pattern can exist with zero or more pattern-level shifted-season rules.
- A shifted-season rule is stored against exactly one owner scope.
- A source tuple matching a pattern-level rule yields target numbering from that
  rule even when a matching show-level rule also exists.
- A source tuple matching no pattern-level rule but matching a show-level rule
  yields target numbering from the show-level rule.
- A source tuple matching neither scope yields identity mapping.
- A pattern-level zero-offset rule can explicitly override a nonzero show-level
  rule for the same covered source range.
- Two shifted-season rules for the same owner scope and original season cannot
  both be valid if they cover overlapping episode ranges.
- During `convert`, shifted numbering is what TMDB episode lookup and generated
  season and episode tokens see when stored shifting is active.
- The TUI can display and maintain shifted-season rules from both the show and
  pattern editing flows.

## Current Code Fit

- `src/ffx/model/show.py` and `src/ffx/model/pattern.py` now both expose
  shifted-season relationships, and `src/ffx/model/shifted_season.py` stores
  each rule against exactly one owner scope through `show_id` or `pattern_id`.
- `src/ffx/shifted_season_controller.py` now resolves mappings with
  pattern-over-show precedence and applies at most one active rule for a source
  tuple.
- `src/ffx/show_details_screen.py`,
  `src/ffx/shifted_season_details_screen.py`, and
  `src/ffx/shifted_season_delete_screen.py` provide reusable shifted-season
  editing dialogs, and `src/ffx/pattern_details_screen.py` now exposes the
  pattern-level override flow.
- `src/ffx/cli.py` now resolves shifted numbering during `convert` from:
  pattern-level match, then show-level match, then identity mapping.
- `src/ffx/database.py` now migrates version-2 databases to version 3 by
  preserving existing show-level rows and extending the schema for pattern-level
  ownership.

## Risks

- The current CLI groups `--show`, `--season`, and `--episode` under one
  override bucket used for TMDB-related behavior. Source-domain versus
  target-domain semantics of each override must stay documented clearly so
  stored shifting is neither skipped nor double-applied unexpectedly.
- Existing version-2 databases only contain show-owned shifted-season rows, so a
  version-3 migration must preserve those rows as the show-level fallback layer.
- Current modern automated test coverage for shifted-season behavior is light,
  so precedence, migration, and convert-time numbering behavior need focused
  tests.
