package imap

import (
	"bytes"
	"mime"
	"net/mail"
	"regexp"
	"strings"
	"unicode"
)

var (
	htmlTitleRE = regexp.MustCompile(`(?is)<title[^>]*>\s*([^<]+?)\s*</title>`)
	qpHexSeqRE  = regexp.MustCompile(`=[0-9A-Fa-f]{2}`)
)

var mimeWordDecoder mime.WordDecoder

// RepairSubject decodes RFC 2047 / broken envelope subjects using headers or body fallbacks.
func RepairSubject(subject string, bodyText, bodyHTML string, raw []byte) string {
	if s := decodeMIMEHeaderValue(subject); !subjectLooksBroken(s) {
		return s
	}
	if len(raw) > 0 {
		if hdr := subjectFromRawMessage(raw); hdr != "" && !subjectLooksBroken(hdr) {
			return hdr
		}
	}
	decodedHTML := decodeBareQuotedPrintableIfNeeded(bodyHTML)
	decodedText := decodeBareQuotedPrintableIfNeeded(bodyText)
	if t := extractSubjectFromHTML(decodedHTML); t != "" {
		return t
	}
	if fallback := subjectFromBodyFallback(decodedText, decodedHTML); fallback != "" {
		return fallback
	}
	return decodeMIMEHeaderValue(subject)
}

func decodeMIMEHeaderValue(s string) string {
	s = strings.TrimSpace(s)
	if s == "" {
		return s
	}
	dec, err := mimeWordDecoder.DecodeHeader(s)
	if err != nil {
		return toValidUTF8(s)
	}
	return toValidUTF8(dec)
}

func subjectFromRawMessage(raw []byte) string {
	msg, err := mail.ReadMessage(bytes.NewReader(raw))
	if err != nil {
		return ""
	}
	return decodeMIMEHeaderValue(msg.Header.Get("Subject"))
}

func extractSubjectFromHTML(html string) string {
	html = strings.TrimSpace(html)
	if html == "" {
		return ""
	}
	if m := htmlTitleRE.FindStringSubmatch(html); len(m) > 1 {
		t := strings.TrimSpace(m[1])
		if t != "" && !subjectLooksBroken(t) {
			return t
		}
	}
	return ""
}

func subjectFromBodyFallback(text, html string) string {
	plain := strings.TrimSpace(text)
	if plain == "" {
		plain = strings.TrimSpace(stripHTMLForSnippet(html))
	}
	if plain == "" || subjectLooksBroken(plain) {
		return ""
	}
	if idx := strings.IndexAny(plain, ".\n\r"); idx >= 15 && idx <= 100 {
		return truncate(strings.TrimSpace(plain[:idx]), 120)
	}
	return truncate(plain, 120)
}

func subjectLooksBroken(s string) bool {
	s = strings.TrimSpace(s)
	if s == "" {
		return true
	}
	letters := 0
	for _, r := range s {
		if unicode.IsLetter(r) || unicode.IsNumber(r) {
			letters++
		}
	}
	return letters < 2
}