move: magic from internal

magic/detect.go

@@ -0,0 +1,72 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package magic
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+)
+
+// Detect returns the files that have a valid header (file signature).
+// A valid header is determined by the format.
+func Detect(format string, filenames []string) (valids []string, err error) {
+	header := headerOf(format)
+	if header == "unknown" {
+		err = fmt.Errorf("unknown format: %s", format)
+		return
+	}
+
+	buf := make([]byte, len(header))
+
+	for _, filename := range filenames {
+		if read(filename, buf) != nil {
+			continue
+		}
+
+		if bytes.Equal([]byte(header), buf) {
+			valids = append(valids, filename)
+		}
+	}
+	return
+}
+
+func headerOf(format string) string {
+	switch format {
+	case "png":
+		return "\x89PNG\r\n\x1a\n"
+	case "jpg":
+		return "\xff\xd8\xff"
+	}
+	// this should never occur
+	// panic("unknown format: " + format)
+	return "unknown"
+}
+
+// read reads len(buf) bytes to buf from a file
+func read(filename string, buf []byte) error {
+	file, err := os.Open(filename)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	fi, err := file.Stat()
+	if err != nil {
+		return err
+	}
+
+	if fi.Size() <= int64(len(buf)) {
+		return errors.New("file size < len(buf)")
+	}
+
+	_, err = io.ReadFull(file, buf)
+	return err
+}