package notifications

import (
	"bytes"
	"embed"
	"encoding/json"
	"fmt"
	htmltemplate "html/template"
	"strings"
	texttemplate "text/template"
)

//go:embed templates/digest.txt templates/digest.html
var digestFS embed.FS

var (
	digestText = texttemplate.Must(texttemplate.ParseFS(digestFS, "templates/digest.txt"))
	digestHTML = htmltemplate.Must(htmltemplate.ParseFS(digestFS, "templates/digest.html"))
)

// digestEmail is one rendered email.
type digestEmail struct {
	Subject string
	Text    string
	HTML    string
}

type digestLine struct {
	Title string
	Body  string
	URL   string
}

type digestArtist struct {
	Name   string
	URL    string
	Titles []digestLine
}

type digestVars struct {
	Name        string
	Intro       string
	Items       []digestLine
	Artists     []digestArtist
	SettingsURL string
}

// renderDigest renders one group's email. base is the public address; with
// none set, the email still goes out, without links.
func renderDigest(group EmailGroup, name string, items []pendingItem, base string) (digestEmail, error) {
	vars := digestVars{Name: name, SettingsURL: joinURL(base, "/settings#notifications")}
	var subject string
	if group == EmailSummary {
		vars.Artists = summaryArtists(items, base)
		vars.Intro = "New in your library since the last summary:"
		subject = "Minstrel: new music in your library"
	} else {
		for _, it := range items {
			r := Render(it.Kind, it.Payload)
			vars.Items = append(vars.Items, digestLine{Title: r.Title, Body: r.Body, URL: joinURL(base, r.Link)})
		}
		vars.Intro = "Here's what happened on Minstrel:"
		subject = fmt.Sprintf("Minstrel: %d updates", len(items))
		if len(items) == 1 {
			subject = "Minstrel: " + vars.Items[0].Title
		}
	}

	var text, html bytes.Buffer
	if err := digestText.Execute(&text, vars); err != nil {
		return digestEmail{}, fmt.Errorf("render text: %w", err)
	}
	if err := digestHTML.Execute(&html, vars); err != nil {
		return digestEmail{}, fmt.Errorf("render html: %w", err)
	}
	return digestEmail{Subject: subject, Text: text.String(), HTML: html.String()}, nil
}

// summaryArtists groups new-music arrivals by artist, in the order each
// artist first arrived, with each artist's albums and tracks under them. An
// artist request has no title of its own; the artist links to its page.
func summaryArtists(items []pendingItem, base string) []digestArtist {
	var out []digestArtist
	index := map[string]int{}
	for _, it := range items {
		var p Payload
		_ = json.Unmarshal(it.Payload, &p)
		artist, title := p.Artist, p.Title
		if artist == "" {
			// Rows from before Artist was recorded carry only "Artist – Title".
			artist, title, _ = strings.Cut(p.Name, " – ")
		}
		if artist == "" {
			artist = "Something you asked for"
		}
		i, ok := index[artist]
		if !ok {
			i = len(out)
			index[artist] = i
			out = append(out, digestArtist{Name: artist})
		}
		link := Render(it.Kind, it.Payload).Link
		if title == "" {
			if p.ArtistID != "" {
				out[i].URL = joinURL(base, link)
			}
			continue
		}
		out[i].Titles = append(out[i].Titles, digestLine{Title: title, URL: joinURL(base, link)})
	}
	return out
}

// joinURL is base followed by path, or "" when no public address is set.
func joinURL(base, path string) string {
	if base == "" || path == "" {
		return ""
	}
	return strings.TrimRight(base, "/") + path
}