# Fact Contracts Internals

This document describes the fact contract boundary.

## Purpose

`internal/facts` separates normalized upstream facts collected for a report run
from conservative report-scoped facts derived from them. The package gives app
orchestration one place to build reusable facts before module execution.

## Inputs And Outputs

Inputs:

- `weatherdata.Bundle` from the Weather API adapter
- resolved report definition and valid period
- report timezone
- configured daypart definitions

Outputs:

- `facts.CollectedFacts` with normalized source facts plus separate source
  provenance and warnings
- `facts.DerivedFacts` with valid-period forecast slices, alert overlaps,
  daily summaries, daypart summaries, and Storm Report window summary

## Boundaries

- This package owns fact assembly and reusable deterministic derivation for a
  report run.
- It does not fetch upstream data, build prompt wording, compare prior
  snapshots, write workflow state, invoke Scriptorium, or define modules.

## Config Fields Used

- `dayparts[].name`
- `dayparts[].start`
- `dayparts[].end`
- `weather_api.timezone`

## External Adapters Used

None directly. Collected facts are built from `weatherdata.Bundle`.

## State Or Manifest Behavior

None. Source provenance and warnings remain data fields for downstream metadata
and inspection.

## Failure Behavior

- Invalid or missing report valid periods return an error.
- Invalid timezone names return an error.
- Missing required hourly forecast data returns the underlying forecast
  derivation error for reports that require daily summaries.
- Missing optional narrative, alert, discussion, daily, or weather story data
  produces empty or nil derived fields.

## Tests

Inspect:

- `internal/facts/facts_test.go`
- `internal/app/app_test.go`

## Invariants

- Collected facts are built once from a fetched bundle.
- Derived facts are scoped to one resolved report.
- Source provenance and warnings stay separate from ordinary fact fields.
- Prompt-specific wording and one-off presentation decisions stay outside this
  package.