Add oshash support (#667)

pkg/utils/oshash.go

@@ -0,0 +1,82 @@
+package utils
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"os"
+)
+
+// OSHashFromFilePath calculates the hash using the same algorithm that
+// OpenSubtitles.org uses.
+//
+// Calculation is as follows:
+// size + 64 bit checksum of the first and last 64k bytes of the file.
+func OSHashFromFilePath(filePath string) (string, error) {
+	f, err := os.Open(filePath)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+
+	fi, err := f.Stat()
+	if err != nil {
+		return "", err
+	}
+
+	fileSize := int64(fi.Size())
+
+	if fileSize == 0 {
+		return "", nil
+	}
+
+	const chunkSize = 64 * 1024
+	fileChunkSize := int64(chunkSize)
+	if fileSize < fileChunkSize {
+		fileChunkSize = fileSize
+	}
+
+	head := make([]byte, fileChunkSize)
+	tail := make([]byte, fileChunkSize)
+
+	// read the head of the file into the start of the buffer
+	_, err = f.Read(head)
+	if err != nil {
+		return "", err
+	}
+
+	// seek to the end of the file - the chunk size
+	_, err = f.Seek(-fileChunkSize, 2)
+	if err != nil {
+		return "", err
+	}
+
+	// read the tail of the file
+	_, err = f.Read(tail)
+	if err != nil {
+		return "", err
+	}
+
+	// put the head and tail together
+	buf := append(head, tail...)
+
+	// convert bytes into uint64
+	ints := make([]uint64, len(buf)/8)
+	reader := bytes.NewReader(buf)
+	err = binary.Read(reader, binary.LittleEndian, &ints)
+	if err != nil {
+		return "", err
+	}
+
+	// sum the integers
+	var sum uint64
+	for _, v := range ints {
+		sum += v
+	}
+
+	// add the filesize
+	sum += uint64(fileSize)
+
+	// output as hex
+	return fmt.Sprintf("%016x", sum), nil
+}