package handlers

import (
	"embed"
	"fmt"
	"html/template"
	"math"
	"net/http"
	"strings"
	"time"

	"sneak.berlin/go/dnswatcher/internal/notify"
	"sneak.berlin/go/dnswatcher/internal/state"
)

//go:embed templates/dashboard.html
var dashboardFS embed.FS

// Time unit constants for relative time calculations.
const (
	secondsPerMinute = 60
	minutesPerHour   = 60
	hoursPerDay      = 24
)

// newDashboardTemplate parses the embedded dashboard HTML
// template with helper functions.
func newDashboardTemplate() *template.Template {
	funcs := template.FuncMap{
		"relTime":       relTime,
		"joinStrings":   joinStrings,
		"formatRecords": formatRecords,
		"expiryDays":    expiryDays,
	}

	return template.Must(
		template.New("dashboard.html").
			Funcs(funcs).
			ParseFS(dashboardFS, "templates/dashboard.html"),
	)
}

// dashboardData is the data passed to the dashboard template.
type dashboardData struct {
	Snapshot    state.Snapshot
	Alerts      []notify.AlertEntry
	StateAge    string
	GeneratedAt string
}

// HandleDashboard returns the dashboard page handler.
func (h *Handlers) HandleDashboard() http.HandlerFunc {
	tmpl := newDashboardTemplate()

	return func(
		writer http.ResponseWriter,
		_ *http.Request,
	) {
		snap := h.state.GetSnapshot()
		alerts := h.notifyHistory.Recent()

		data := dashboardData{
			Snapshot:    snap,
			Alerts:      alerts,
			StateAge:    relTime(snap.LastUpdated),
			GeneratedAt: time.Now().UTC().Format("2006-01-02 15:04:05"),
		}

		writer.Header().Set(
			"Content-Type", "text/html; charset=utf-8",
		)

		err := tmpl.Execute(writer, data)
		if err != nil {
			h.log.Error(
				"dashboard template error",
				"error", err,
			)
		}
	}
}

// relTime returns a human-readable relative time string such
// as "2 minutes ago" or "never" for zero times.
func relTime(t time.Time) string {
	if t.IsZero() {
		return "never"
	}

	d := time.Since(t)
	if d < 0 {
		return "just now"
	}

	seconds := int(math.Round(d.Seconds()))
	if seconds < secondsPerMinute {
		return fmt.Sprintf("%ds ago", seconds)
	}

	minutes := seconds / secondsPerMinute
	if minutes < minutesPerHour {
		return fmt.Sprintf("%dm ago", minutes)
	}

	hours := minutes / minutesPerHour
	if hours < hoursPerDay {
		return fmt.Sprintf(
			"%dh %dm ago", hours, minutes%minutesPerHour,
		)
	}

	days := hours / hoursPerDay

	return fmt.Sprintf(
		"%dd %dh ago", days, hours%hoursPerDay,
	)
}

// joinStrings joins a string slice with a separator.
func joinStrings(items []string, sep string) string {
	return strings.Join(items, sep)
}

// formatRecords formats a map of record type → values into a
// compact display string.
func formatRecords(records map[string][]string) string {
	if len(records) == 0 {
		return "-"
	}

	var parts []string

	for rtype, values := range records {
		for _, v := range values {
			parts = append(parts, rtype+": "+v)
		}
	}

	return strings.Join(parts, ", ")
}

// expiryDays returns the number of days until the given time,
// rounded down. Returns 0 if already expired.
func expiryDays(t time.Time) int {
	d := time.Until(t).Hours() / hoursPerDay
	if d < 0 {
		return 0
	}

	return int(d)
}