ᛜ Added some core util functions ᛜ

util/bits.go

@@ -0,0 +1,6 @@
+package util
+
+// Tells if bit is set in given byte
+func IsSet(n byte, bitN int) bool {
+	return n&byte(1<<bitN-1) != 0
+}

util/bits_test.go

@@ -0,0 +1,14 @@
+package util
+
+import "testing"
+
+func TestIsSet(t *testing.T) {
+	testBytes := []byte{1 << 0, 1 << 1, 1 << 2, 1 << 3}
+
+	for index, testByte := range testBytes {
+		if !IsSet(testByte, index+1) {
+			t.Errorf("IsSet failed: expected %dth bit of %d to be %v; got %v",
+				index+1, testByte, true, IsSet(testByte, index+1))
+		}
+	}
+}

util/conversion.go

@@ -7,8 +7,7 @@ import (
 )
 
 // got the logic from: https://github.com/bogem/id3v2 , thank you very much.
-
+const first7BitsMask = uint32(254 << 24) // shifting 11111110 to the end of uint32
-const first7BitsMask = uint32(254) << 24 // shifting 11111110 to the end of uint32
 
 // Converts given bytes into integer
 func BytesToInt(gBytes []byte) uint32 {
@@ -32,19 +31,30 @@ func BytesToIntSynchsafe(gBytes []byte) uint32 {
 	return integer
 }
 
-// The exact opposite of what `BytesToIntIgnoreFirstBit` does
+// The exact opposite of what `BytesToIntSynchsafe` does
-func SynchsafeIntToBytes(gInt uint32) []byte {
+// Finally understood with the help of: https://github.com/bogem/id3v2/blob/master/size.go ,
-	bytes := make([]byte, 32)
+// thank you very much !
+func IntToBytesSynchsafe(gInt uint32) []byte {
+	synchsafeIBytes := make([]byte, 4)
 
-	// looping 4 times (32 bits / 8 bits (4 bytes in int32))
+	// skip 4 0`ed bits
-	for i := 0; i < 32; i += 8 {
+	gInt = gInt << 4
-		gIntCopy := gInt                    //11010101 11001011 00100000 10111111
+
-		first7 := gIntCopy & first7BitsMask //11010100 00000000 00000000 00000000
+	// int32 == 4 bytes
-		shifted := first7 >> 25             //00000000 00000000 00000000 01101010
+	for i := 0; i < 32/8; i++ {
-		bytes = append(bytes, byte(shifted))
+		// get first 7 bits
-	}
+		first7Bits := gInt & first7BitsMask
 
-	return bytes
+		// shift captured bits to the beginning
+		first7Bits = first7Bits >> (3*8 + 1)
+
+		b := byte(first7Bits)
+		synchsafeIBytes = append(synchsafeIBytes, b)
+
+		// prepare next 7 bits for the next iteration
+		gInt = gInt << 7
+	}
+	return synchsafeIBytes
 }
 
 // Converts given bytes into string, ignoring the first 31 non-printable ASCII characters.

util/conversion_test.go

@@ -25,13 +25,23 @@ func TestDecodeText(t *testing.T) {
 	}
 }
 
-// func TestIntToBytesFirstBitZeroed(t *testing.T) {
+func TestIntToBytesSynchsafe(t *testing.T) {
-// 	var testint uint32 = 123456
+	testInts := []uint32{
+		1234,
+		12,
+		1,
+		0,
+		99999,
+		87654321,
+	}
+
+	for _, testInt := range testInts {
+		synchSafeBytes := IntToBytesSynchsafe(testInt)
 
-// 	intbytes := IntToBytesFirstBitZeroed(testint)
+		synchsafeInt := BytesToIntSynchsafe(synchSafeBytes)
 
-// 	if BytesToIntIgnoreFirstBit(intbytes) != testint {
+		if synchsafeInt != testInt {
-// 		t.Errorf("IntToBytesFirstBitZeroed failed: expected to get %v; got %v",
+			t.Errorf("BytesToIntSynchsafe failed: expected to get %d; got %d", testInt, synchsafeInt)
-// 			testint, BytesToIntIgnoreFirstBit(intbytes))
+		}
-// 	}
+	}
-// }
+}