package imap

import (
	"mime"
	"strings"
	"unicode/utf8"

	"golang.org/x/text/encoding"
	"golang.org/x/text/encoding/charmap"
	"golang.org/x/text/encoding/htmlindex"
	"golang.org/x/text/transform"
)

func charsetFromContentType(contentType string) string {
	if contentType == "" {
		return ""
	}
	_, params, err := mime.ParseMediaType(contentType)
	if err != nil {
		return ""
	}
	return strings.Trim(strings.TrimSpace(params["charset"]), `"`)
}

func isUTF8Charset(charset string) bool {
	switch strings.ToLower(strings.TrimSpace(charset)) {
	case "", "utf-8", "utf8", "unicode-1-1-utf-8":
		return true
	default:
		return false
	}
}

// decodeBodyBytesToUTF8 converts a MIME part payload to UTF-8 using Content-Type charset.
func decodeBodyBytesToUTF8(data []byte, contentType string) string {
	if len(data) == 0 {
		return ""
	}
	charset := charsetFromContentType(contentType)
	if charset != "" && !isUTF8Charset(charset) {
		if decoded := decodeBytesWithCharset(data, charset); decoded != "" {
			return decoded
		}
	}
	if utf8.Valid(data) {
		return string(data)
	}
	return repairRawBytesToUTF8(data)
}

func decodeBytesWithCharset(data []byte, charset string) string {
	enc, err := htmlindex.Get(charset)
	if err != nil || enc == nil {
		return ""
	}
	decoded, err := enc.NewDecoder().Bytes(data)
	if err != nil || !utf8.Valid(decoded) {
		return ""
	}
	return string(decoded)
}

// repairRawBytesToUTF8 fixes bodies stored without charset conversion (Latin-1 / Windows-1252).
func repairRawBytesToUTF8(data []byte) string {
	if len(data) == 0 {
		return ""
	}
	if utf8.Valid(data) {
		return string(data)
	}
	for _, enc := range []encoding.Encoding{charmap.Windows1252, charmap.ISO8859_1} {
		decoded, _, err := transform.Bytes(enc.NewDecoder(), data)
		if err == nil && utf8.Valid(decoded) && isMostlyReadableText(decoded) {
			return string(decoded)
		}
	}
	return strings.ToValidUTF8(string(data), "")
}

// repairLegacyCharsetString fixes text already loaded as a Go string with invalid UTF-8 bytes.
func repairLegacyCharsetString(s string) string {
	if s == "" || utf8.ValidString(s) {
		return s
	}
	return repairRawBytesToUTF8([]byte(s))
}