# Changes Internals

This document describes structured Recent Changes comparison.

## Purpose

`internal/changes` compares current and prior briefing packages and emits
compact change records for prompt input data packages.

## Inputs And Outputs

Inputs:

- prior briefing package
- current briefing package
- comparison thresholds from configuration

Outputs:

- ordered `changes.Change` items with type, message, previous value, and current
  value where useful

## Boundaries

- This package compares structured briefing data only.
- It does not read filesystem state, find prior snapshots, render Markdown,
  invoke Scriptorium, or compare generated report text.

## Config Fields Used

The app maps these fields into comparison thresholds:

- `recent_change.temperature_degrees`
- `recent_change.precip_probability_points`
- `recent_change.wind_gust_miles_per_hour`
- `recent_change.precip_timing_shift_minutes`

## External Adapters Used

None.

## State Or Manifest Behavior

None directly. The app loads prior briefing snapshots through `internal/state`
before calling comparison functions.

## Skip And Resume Behavior

No resume behavior. When the app has no prior comparable snapshot, it sends an
empty Recent Changes list without calling a comparison function.

## Failure Behavior

- Daily comparison requires both inputs to contain Daily briefing content.
- 3-Day comparison requires both inputs to contain 3-Day briefing content.
- Weekend comparison requires both inputs to contain Weekend briefing content.
- Storm Report currently has no comparison implementation, so the app leaves
  Recent Changes empty for Storm reports.

## Tests

Inspect:

- `internal/changes/daily_test.go`
- `internal/changes/three_day_test.go`
- `internal/changes/weekend_test.go`
- `internal/app/app_test.go`

## Invariants

- Recent Changes are based on structured snapshots, not Markdown report text.
- Report compatibility is determined outside this package by report definitions
  and state lookup.
- Output stays compact enough for prompt input.