Add 3-Day Outlook generation

internal/forecast/derive.go

@@ -100,6 +100,62 @@ func BuildDailySummary(bundle *Bundle, date time.Time, location *time.Location,
 	return summary, nil
 }
 
+func BuildPeriodDailySummaries(bundle *Bundle, period timeutil.Period, location *time.Location, dayparts []DaypartDefinition) ([]DailySummary, error) {
+	if !period.IsValid() {
+		return nil, fmt.Errorf("valid forecast period is required")
+	}
+	if location == nil {
+		location = time.UTC
+	}
+	var summaries []DailySummary
+	for day := timeutil.CivilDay(period.Start, location); day.Start.Before(period.End); day = timeutil.CivilDay(day.Start.AddDate(0, 0, 1), location) {
+		overlap, ok := day.Intersection(period)
+		if !ok {
+			continue
+		}
+		summary, err := buildDailySummaryForPeriod(bundle, overlap, location, dayparts)
+		if err != nil {
+			return nil, err
+		}
+		summaries = append(summaries, *summary)
+	}
+	return summaries, nil
+}
+
+func buildDailySummaryForPeriod(bundle *Bundle, period timeutil.Period, location *time.Location, dayparts []DaypartDefinition) (*DailySummary, error) {
+	if bundle == nil {
+		return nil, fmt.Errorf("forecast bundle is required")
+	}
+	if bundle.Hourly == nil || len(bundle.Hourly.Periods) == 0 {
+		return nil, fmt.Errorf("hourly forecast data is required")
+	}
+	windows, err := ResolveDayparts(period.Start, location, dayparts)
+	if err != nil {
+		return nil, err
+	}
+	alerts := AlertOverlaps(bundle.Alerts, period)
+	summary := &DailySummary{
+		Date:             period.Start.In(location).Format(timeutil.DateLayout),
+		Period:           period,
+		NarrativePeriods: SelectNarrativePeriods(bundle, period),
+		AlertOverlaps:    alerts,
+		Discussion:       SelectDiscussion(bundle),
+		SourceWarnings:   bundle.Warnings,
+		SourceProvenance: bundle.Sources,
+	}
+	for _, window := range windows {
+		clipped, ok := window.Period.Intersection(period)
+		if !ok {
+			continue
+		}
+		periods := SelectHourlyPeriods(bundle.Hourly, clipped)
+		daypartSummary := SummarizeDaypart(window.Name, clipped, periods)
+		daypartSummary.AlertOverlaps = overlapsWithin(alerts, clipped)
+		summary.Dayparts = append(summary.Dayparts, daypartSummary)
+	}
+	return summary, nil
+}
+
 func SelectHourlyPeriods(run *ForecastRun, period timeutil.Period) []ForecastPeriod {
 	if run == nil {
 		return nil

internal/forecast/derive_test.go

@@ -153,6 +153,41 @@ func TestBuildDailySummaryRequiresHourlyData(t *testing.T) {
 	}
 }
 
+func TestBuildPeriodDailySummariesClipsPartialDays(t *testing.T) {
+	location := time.FixedZone("Test", -5*60*60)
+	bundle := &Bundle{Hourly: &ForecastRun{Periods: []ForecastPeriod{
+		hour(location, "2026-05-29T05:00:00-05:00", "2026-05-29T06:00:00-05:00", "Before", 50, nil, nil, nil, nil),
+		hour(location, "2026-05-29T08:00:00-05:00", "2026-05-29T09:00:00-05:00", "Showers", 60, nil, ptr(60), nil, nil),
+		hour(location, "2026-05-30T14:00:00-05:00", "2026-05-30T15:00:00-05:00", "Hot", 95, nil, nil, nil, nil),
+		hour(location, "2026-05-31T20:00:00-05:00", "2026-05-31T21:00:00-05:00", "Wind", 70, nil, nil, nil, ptr(35)),
+	}}}
+	period := timeutil.Period{
+		Start: mustParse("2026-05-29T07:00:00-05:00").In(location),
+		End:   mustParse("2026-06-01T00:00:00-05:00").In(location),
+	}
+
+	summaries, err := BuildPeriodDailySummaries(bundle, period, location, []DaypartDefinition{
+		{Name: "morning", Start: "06:00", End: "12:00"},
+		{Name: "afternoon", Start: "12:00", End: "18:00"},
+		{Name: "evening", Start: "18:00", End: "24:00"},
+	})
+	if err != nil {
+		t.Fatalf("BuildPeriodDailySummaries() error = %v", err)
+	}
+	if len(summaries) != 3 {
+		t.Fatalf("summaries length = %d, want 3", len(summaries))
+	}
+	if summaries[0].Period.Start.Format(time.RFC3339) != "2026-05-29T07:00:00-05:00" {
+		t.Fatalf("first period start = %s, want clipped start", summaries[0].Period.Start.Format(time.RFC3339))
+	}
+	if len(summaries[0].Dayparts[0].HourlyPeriods) != 1 || summaries[0].Dayparts[0].HourlyPeriods[0].TextDescription != "Showers" {
+		t.Fatalf("first morning periods = %#v, want only post-start hour", summaries[0].Dayparts[0].HourlyPeriods)
+	}
+	if summaries[2].Dayparts[2].PeakWindGust == nil || summaries[2].Dayparts[2].PeakWindGust.Value != 35 {
+		t.Fatalf("third evening gust = %#v, want 35", summaries[2].Dayparts[2].PeakWindGust)
+	}
+}
+
 func TestAlertOverlap(t *testing.T) {
 	location := time.FixedZone("Test", -5*60*60)
 	raw := json.RawMessage(`{"event":"Flood Watch","headline":"Flooding possible","severity":"Moderate","effective":"2026-05-29T07:00:00-05:00","expires":"2026-05-29T10:00:00-05:00"}`)