# Changes Internals

This document describes the implemented structured change comparison boundary.

## Purpose

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

## Inputs and Outputs

Inputs:

- prior Daily briefing package
- current Daily briefing package
- configured Recent Changes thresholds

Output:

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

## Boundaries

- This package compares structured briefing data only.
- It does not read state directly, render Markdown, invoke `scriptorium`, or
  compare generated report text.

## Config Fields Used

The app maps these config 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`

## Behavior

Daily comparison currently detects:

- temperature changes crossing configured thresholds
- precipitation probability and timing changes
- alert additions and removals
- peak wind gust changes
- snow, ice, and thunder risk changes

When no prior comparable snapshot exists, the app sends an empty Recent Changes
section in the data package.

## Failure Behavior

Daily comparison requires both inputs to contain Daily briefing content.

## Tests

Inspect:

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

## Invariants

- Recent Changes are based on structured snapshots, not Markdown report text.
- Comparison thresholds come from configuration.
- The comparison output remains compact enough for prompt input.