package main

import (
	"bufio"
	"errors"
	"io"
	"regexp"
	"strings"
)

var (
	tvgIDRe  = regexp.MustCompile(`tvg-id="([^"]*)"`)
	urlTvgRe = regexp.MustCompile(`url-tvg="([^"]*)"`)
)

// processPlaylist streams in→out, rewriting the #EXTM3U header so url-tvg
// points at epgURL. Returns the playlist's tvg-ids (in order, with duplicates)
// and the URL the playlist already declared in url-tvg (empty if absent).
//
// If epgURL is empty the header is passed through unchanged. The playlist
// must start with #EXTM3U after any leading blank lines, otherwise an error
// is returned without writing the header.
func processPlaylist(in io.Reader, out io.Writer, epgURL string) (tvgIDs []string, originalEPG string, err error) {
	r := bufio.NewReader(in)
	headerDone := false

	for {
		line, readErr := r.ReadString('\n')

		if line != "" {
			switch {
			case !headerDone && strings.TrimSpace(line) == "":
				// leading blank line — write through, keep looking for header

			case !headerDone:
				if !strings.HasPrefix(strings.TrimSpace(line), "#EXTM3U") {
					return nil, "", errors.New("playlist: missing #EXTM3U header")
				}
				line, originalEPG = rewriteHeader(line, epgURL)
				headerDone = true

			default:
				if strings.HasPrefix(line, "#EXTINF:") {
					if m := tvgIDRe.FindStringSubmatch(line); m != nil {
						if id := strings.TrimSpace(m[1]); id != "" {
							tvgIDs = append(tvgIDs, id)
						}
					}
				}
			}

			if _, werr := out.Write([]byte(line)); werr != nil {
				return nil, "", werr
			}
		}

		if readErr == io.EOF {
			if !headerDone {
				return nil, "", errors.New("playlist: missing #EXTM3U header")
			}
			return tvgIDs, originalEPG, nil
		}
		if readErr != nil {
			return nil, "", readErr
		}
	}
}

func rewriteHeader(line, epgURL string) (rewritten, original string) {
	if m := urlTvgRe.FindStringSubmatch(line); m != nil {
		original = m[1]
		if epgURL != "" {
			line = urlTvgRe.ReplaceAllLiteralString(line, `url-tvg="`+epgURL+`"`)
		}
		return line, original
	}
	if epgURL == "" {
		return line, ""
	}
	body, eol := splitEOL(line)
	return body + ` url-tvg="` + epgURL + `"` + eol, ""
}

func splitEOL(s string) (body, eol string) {
	n := len(s)
	for n > 0 && (s[n-1] == '\n' || s[n-1] == '\r') {
		n--
	}
	return s[:n], s[n:]
}