Add functions that check if a file is text file

internal/localcheck/file.go

@@ -0,0 +1,47 @@
+package localcheck
+
+import (
+	"os"
+	"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 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(filename string) bool {
+	// read an initial chunk of the file
+	// and check if it looks like text
+	f, err := os.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])
+}