weatherreporter/internal/changes/daily.go

// Package changes compares structured briefing snapshots.
package changes

import (
	"fmt"
	"math"
	"sort"
	"time"

	"gitea.maximumdirect.net/eric/weatherreporter/internal/briefing"
	"gitea.maximumdirect.net/eric/weatherreporter/internal/forecast"
)

type Thresholds struct {
	TemperatureDegrees       float64
	PrecipProbabilityPoints  int
	WindGustMilesPerHour     int
	PrecipTimingShiftMinutes int
}

type Change struct {
	Type     string `json:"type"`
	Message  string `json:"message"`
	Previous string `json:"previous,omitempty"`
	Current  string `json:"current,omitempty"`
}

func CompareDaily(previous briefing.Package, current briefing.Package, thresholds Thresholds) ([]Change, error) {
	if previous.Daily == nil {
		return nil, fmt.Errorf("previous daily briefing is required")
	}
	if current.Daily == nil {
		return nil, fmt.Errorf("current daily briefing is required")
	}
	var changes []Change
	changes = append(changes, compareTemperature(previous.Daily.BottomLine.Temperature, current.Daily.BottomLine.Temperature, thresholds.TemperatureDegrees)...)
	changes = append(changes, comparePrecipitation(previous.Daily.BottomLine.MaxPrecipProbability, current.Daily.BottomLine.MaxPrecipProbability, thresholds)...)
	changes = append(changes, compareWind(previous.Daily.BottomLine.PeakWindGust, current.Daily.BottomLine.PeakWindGust, float64(thresholds.WindGustMilesPerHour))...)
	changes = append(changes, compareAlerts(previous.Daily.RelevantAlerts, current.Daily.RelevantAlerts)...)
	changes = append(changes, compareIndicators(aggregateIndicators(previous.Daily.Dayparts), aggregateIndicators(current.Daily.Dayparts))...)
	return changes, nil
}

func compareTemperature(previous forecast.Range, current forecast.Range, threshold float64) []Change {
	var changes []Change
	if previous.Min != nil && current.Min != nil && differenceAtLeast(*previous.Min, *current.Min, threshold) {
		changes = append(changes, Change{
			Type:     "temperature_shift",
			Message:  fmt.Sprintf("Low temperature changed from %.0f to %.0f.", *previous.Min, *current.Min),
			Previous: fmt.Sprintf("%.0f", *previous.Min),
			Current:  fmt.Sprintf("%.0f", *current.Min),
		})
	}
	if previous.Max != nil && current.Max != nil && differenceAtLeast(*previous.Max, *current.Max, threshold) {
		changes = append(changes, Change{
			Type:     "temperature_shift",
			Message:  fmt.Sprintf("High temperature changed from %.0f to %.0f.", *previous.Max, *current.Max),
			Previous: fmt.Sprintf("%.0f", *previous.Max),
			Current:  fmt.Sprintf("%.0f", *current.Max),
		})
	}
	return changes
}

func comparePrecipitation(previous *forecast.TimedValue, current *forecast.TimedValue, thresholds Thresholds) []Change {
	if previous == nil || current == nil {
		return nil
	}
	var changes []Change
	previousCategory := precipitationCategory(previous.Value)
	currentCategory := precipitationCategory(current.Value)
	if previousCategory != currentCategory || differenceAtLeast(previous.Value, current.Value, float64(thresholds.PrecipProbabilityPoints)) {
		changes = append(changes, Change{
			Type:     "precip_probability_change",
			Message:  fmt.Sprintf("Peak precipitation chance changed from %.0f%% (%s) to %.0f%% (%s).", previous.Value, previousCategory, current.Value, currentCategory),
			Previous: fmt.Sprintf("%.0f%% %s", previous.Value, previousCategory),
			Current:  fmt.Sprintf("%.0f%% %s", current.Value, currentCategory),
		})
	}
	shiftMinutes := int(math.Abs(current.Time.Sub(previous.Time).Minutes()))
	if thresholds.PrecipTimingShiftMinutes > 0 && shiftMinutes >= thresholds.PrecipTimingShiftMinutes {
		changes = append(changes, Change{
			Type:     "precip_timing_shift",
			Message:  fmt.Sprintf("Peak precipitation timing shifted from %s to %s.", clock(previous.Time), clock(current.Time)),
			Previous: clock(previous.Time),
			Current:  clock(current.Time),
		})
	}
	return changes
}

func compareWind(previous *forecast.TimedValue, current *forecast.TimedValue, threshold float64) []Change {
	if previous == nil || current == nil || !differenceAtLeast(previous.Value, current.Value, threshold) {
		return nil
	}
	return []Change{{
		Type:     "wind_gust_change",
		Message:  fmt.Sprintf("Peak wind gust changed from %.0f mph to %.0f mph.", previous.Value, current.Value),
		Previous: fmt.Sprintf("%.0f mph", previous.Value),
		Current:  fmt.Sprintf("%.0f mph", current.Value),
	}}
}

func compareAlerts(previous []forecast.AlertOverlap, current []forecast.AlertOverlap) []Change {
	previousSet := alertSet(previous)
	currentSet := alertSet(current)
	var changes []Change
	for event := range currentSet {
		if _, ok := previousSet[event]; !ok {
			changes = append(changes, Change{Type: "alert_added", Message: fmt.Sprintf("Alert added: %s.", event), Current: event})
		}
	}
	for event := range previousSet {
		if _, ok := currentSet[event]; !ok {
			changes = append(changes, Change{Type: "alert_removed", Message: fmt.Sprintf("Alert removed: %s.", event), Previous: event})
		}
	}
	sortChanges(changes)
	return changes
}

func compareIndicators(previous forecast.Indicators, current forecast.Indicators) []Change {
	var changes []Change
	for _, item := range []struct {
		name     string
		previous bool
		current  bool
	}{
		{name: "thunder", previous: previous.Thunder, current: current.Thunder},
		{name: "snow", previous: previous.Snow, current: current.Snow},
		{name: "ice", previous: previous.Ice, current: current.Ice},
	} {
		if item.previous == item.current {
			continue
		}
		changeType := item.name + "_risk_change"
		if item.current {
			changes = append(changes, Change{Type: changeType, Message: fmt.Sprintf("%s risk is now present.", item.name), Current: "present"})
		} else {
			changes = append(changes, Change{Type: changeType, Message: fmt.Sprintf("%s risk is no longer present.", item.name), Previous: "present"})
		}
	}
	return changes
}

func aggregateIndicators(dayparts []forecast.DaypartSummary) forecast.Indicators {
	out := forecast.Indicators{}
	for _, daypart := range dayparts {
		out.Thunder = out.Thunder || daypart.Indicators.Thunder
		out.Snow = out.Snow || daypart.Indicators.Snow
		out.Ice = out.Ice || daypart.Indicators.Ice
	}
	return out
}

func alertSet(alerts []forecast.AlertOverlap) map[string]struct{} {
	out := map[string]struct{}{}
	for _, alert := range alerts {
		event := alert.Event
		if event == "" {
			event = alert.Headline
		}
		if event != "" {
			out[event] = struct{}{}
		}
	}
	return out
}

func precipitationCategory(value float64) string {
	switch {
	case value >= 70:
		return "high"
	case value >= 50:
		return "likely"
	case value >= 20:
		return "possible"
	default:
		return "low"
	}
}

func differenceAtLeast(previous float64, current float64, threshold float64) bool {
	if threshold <= 0 {
		return previous != current
	}
	return math.Abs(current-previous) >= threshold
}

func clock(t time.Time) string {
	return t.Format("15:04")
}

func sortChanges(items []Change) {
	sort.SliceStable(items, func(i, j int) bool {
		if items[i].Type == items[j].Type {
			return items[i].Message < items[j].Message
		}
		return items[i].Type < items[j].Type
	})
}