// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package web

import (
	"io/fs"
	"path"
	"strings"
	"unicode/utf8"
)

// isText reports whether a significant prefix of s looks like correct UTF-8;
// that is, if it is likely that s is human-readable text.
func isText(s []byte) bool {
	const max = 1024 // at least utf8.UTFMax
	if len(s) > max {
		s = s[0:max]
	}
	for i, c := range string(s) {
		if i+utf8.UTFMax > len(s) {
			// last char may be incomplete - ignore
			break
		}
		if c == 0xFFFD || c < ' ' && c != '\n' && c != '\t' && c != '\f' {
			// decoding error or control character - not a text file
			return false
		}
	}
	return true
}

// isTextFile reports whether the file has a known extension indicating
// a text file, or if a significant chunk of the specified file looks like
// correct UTF-8; that is, if it is likely that the file contains human-
// readable text.
func isTextFile(fsys fs.FS, filename string) bool {
	// Various special cases must be served raw, not converted to nice HTML.
	if filename == "robots.txt" || strings.HasPrefix(filename, "doc/play/") {
		return false
	}
	switch path.Ext(filename) {
	case ".css", ".js", ".svg", ".ts":
		return false
	}

	// the extension is not known; read an initial chunk
	// of the file and check if it looks like text
	f, err := fsys.Open(filename)
	if err != nil {
		return false
	}
	defer f.Close()

	var buf [1024]byte
	n, err := f.Read(buf[0:])
	if err != nil {
		return false
	}

	return isText(buf[0:n])
}