package gemtext

import (
	"bytes"
	"html/template"
	"io"
	"net/url"
	"regexp"
	"strconv"
	"strings"
	"time"
)

// GmisubToAtom converts a gemini document to Atom format.
//
// It identifies feed fields and entries according to the specification at
// gemini://gemini.circumlunar.space/docs/companion/subscription.gmi
func GmisubToAtom(doc Document, location url.URL, out io.Writer) error {
	if _, err := out.Write([]byte(`<?xml version="1.0" encoding="utf-8"?>`)); err != nil {
		return err
	}
	if _, err := out.Write([]byte{'\n'}); err != nil {
		return err
	}
	if err := atomTmpl.Execute(out, parseGemSub(doc, &location)); err != nil {
		return err
	}
	return nil
}

type gmiSub struct {
	ID       template.URL
	Title    string
	Subtitle string
	Updated  string

	Entries []gmiSubEntry
}

type gmiSubEntry struct {
	ID      template.URL
	Updated string
	Title   string
}

var linkElemRE = regexp.MustCompile(`(\d{4})-([0-1]\d)-([0-3]\d)`)

func parseGemSub(doc Document, location *url.URL) *gmiSub {
	sub := &gmiSub{ID: template.URL(location.String())}
	updated := time.Time{}

	for i, line := range doc {
		switch line.Type() {
		case LineTypeHeading1:
			if sub.Title != "" {
				continue
			}

			sub.Title = line.(HeadingLine).Body()

			for { // skip any empty lines
				i += 1
				if i >= len(doc) || strings.TrimPrefix(doc[i].String(), "\r") != "\n" {
					break
				}
			}
			if i < len(doc) && doc[i].Type() == LineTypeHeading2 {
				sub.Subtitle = doc[i].(HeadingLine).Body()
			}
		case LineTypeLink:
			label := line.(LinkLine).Label()
			if len(label) < 10 {
				continue
			}
			match := linkElemRE.FindStringSubmatch(label[:10])
			if match == nil {
				continue
			}

			year, err := strconv.Atoi(match[1])
			if err != nil {
				continue
			}
			month, err := strconv.Atoi(match[2])
			if err != nil || month > 12 {
				continue
			}
			day, err := strconv.Atoi(match[3])
			if err != nil || day > 31 {
				continue
			}

			entryUpdated := time.Date(year, time.Month(month), day, 12, 0, 0, 0, time.UTC)
			entryTitle := strings.TrimLeft(strings.TrimPrefix(strings.TrimLeft(label[10:], " \t"), "-"), " \t")

			sub.Entries = append(sub.Entries, gmiSubEntry{
				ID:      template.URL(line.(LinkLine).URL()),
				Updated: entryUpdated.Format(time.RFC3339),
				Title:   entryTitle,
			})

			if entryUpdated.After(updated) {
				updated = entryUpdated
				sub.Updated = updated.Format(time.RFC3339)
			}
		}
	}

	return sub
}

var atomTmpl = template.Must(template.New("atom").Parse(`
<feed xmlns="http://www.w3.org/2005/Atom">
  <id>{{.ID}}</id>
  <link href="{{.ID}}"/>
  <title>{{.Title}}</title>
  {{- if .Subtitle }}
  <subtitle>{{.Subtitle}}</subtitle>
  {{- end }}
  <updated>{{.Updated}}</updated>
{{- range .Entries }}
  <entry>
    <id>{{.ID}}</id>
    <link rel="alternate" href="{{.ID}}"/>
    <title>{{.Title}}</title>
    <updated>{{.Updated}}</updated>
  </entry>
{{- end }}
</feed>
`[1:]))