package extract

import (
	"bytes"
	"strings"
	"unicode/utf8"

	"golang.org/x/net/html/charset"
	"golang.org/x/text/encoding/charmap"
	"golang.org/x/text/encoding/unicode"
	"golang.org/x/text/unicode/norm"
)

func hasUTF16BOM(b []byte) bool {
	return len(b) >= 2 && ((b[0] == 0xFF && b[1] == 0xFE) || (b[0] == 0xFE && b[1] == 0xFF))
}

// decodeText turns a text file into UTF-8, NFC.
//
// UTF-8 is taken as it is (its BOM dropped); UTF-16 by its BOM, which is how
// Windows' Notepad writes "Unicode". Anything else is a single-byte code
// page, and which one cannot be known from the bytes; Windows-1254 is
// assumed, since it is what a Turkish Windows writes and it agrees with
// Windows-1252 on every letter but six (ğĞıİşŞ in place of ðÐýÝþÞ) — which
// are rare in the western languages 1252 serves.
func decodeText(b []byte) string {
	var s string
	switch {
	case bytes.HasPrefix(b, []byte{0xEF, 0xBB, 0xBF}):
		s = string(b[3:])
	case hasUTF16BOM(b):
		out, err := unicode.UTF16(unicode.LittleEndian, unicode.ExpectBOM).NewDecoder().Bytes(b)
		if err != nil {
			return ""
		}
		s = string(out)
	case utf8.Valid(b):
		s = string(b)
	default:
		out, err := charmap.Windows1254.NewDecoder().Bytes(b)
		if err != nil {
			return ""
		}
		s = string(out)
	}
	return norm.NFC.String(strings.ReplaceAll(s, "\r\n", "\n"))
}

// decodeCharset decodes bytes declared to be in a charset, as mail and web
// pages declare theirs, by any of the WHATWG labels; an unknown or missing
// one is treated as decodeText would.
func decodeCharset(b []byte, cs string) string {
	cs = strings.ToLower(strings.TrimSpace(cs))
	if cs == "" || cs == "utf-8" || cs == "utf8" || cs == "us-ascii" {
		return decodeText(b)
	}
	enc, _ := charset.Lookup(cs)
	if enc == nil {
		return decodeText(b)
	}
	out, err := enc.NewDecoder().Bytes(b)
	if err != nil {
		return decodeText(b)
	}
	return norm.NFC.String(strings.ReplaceAll(string(out), "\r\n", "\n"))
}

// codePage is the decoder for an RTF \ansicpgN or a Windows code page number.
func codePage(n int) *charmap.Charmap {
	switch n {
	case 1250:
		return charmap.Windows1250
	case 1251:
		return charmap.Windows1251
	case 1252:
		return charmap.Windows1252
	case 1253:
		return charmap.Windows1253
	case 1254:
		return charmap.Windows1254
	case 1255:
		return charmap.Windows1255
	case 1256:
		return charmap.Windows1256
	case 1257:
		return charmap.Windows1257
	case 1258:
		return charmap.Windows1258
	case 850:
		return charmap.CodePage850
	case 437:
		return charmap.CodePage437
	}
	return charmap.Windows1252
}