VarInt And VarLong

Variable-length format such that smaller numbers use fewer bytes. These are very similar to Protocol Buffer Varints: the 7 least significant bits are used to encode the value and the most significant bit indicates whether there's another byte after it for the next part of the number. The least significant group is written first, followed by each of the more significant groups; thus, VarInts are effectively little endian (however, groups are 7 bits, not 8).

VarInts are never longer than 5 bytes, and VarLongs are never longer than 10 bytes.

Pseudocode to read and write VarInts and VarLongs:

public int readVarInt() {
    int value = 0;
    int bitOffset = 0;
    byte currentByte;
    do {
        if (bitOffset == 35) throw new RuntimeException("VarInt is too big");

        currentByte = readByte();
        value |= (currentByte & 0b01111111) << bitOffset;

        bitOffset += 7;
    } while ((currentByte & 0b10000000) != 0);

    return value;
}

public long readVarLong() {
    long value = 0;
    int bitOffset = 0;
    byte currentByte;
    do {
        if (bitOffset == 70) throw new RuntimeException("VarLong is too big");

        currentByte = readByte();
        value |= (currentByte & 0b01111111) << bitOffset;

        bitOffset += 7;
    } while ((currentByte & 0b10000000) != 0);

    return value;
}

public static void writeVarInt(int value) {
    while (true) {
        // Only the first 7 bits are needed
        if ((value & 0b11111111111111111111111110000000) == 0) {
          writeByte(value);
          return;
        }

        writeByte(value & 0b01111111 | 0b10000000);
        // Note: >>> means that the sign bit is shifted with the rest of the number rather than being left alone
        value >>>= 7;
    }
}

public static void writeVarLong(long value) {
    while (true) {
        // Only the first 7 bits are needed
        if ((value & 0b1111111111111111111111111111111111111111111111111111111110000000) == 0) {
          writeByte(value);
          return;
        }

        writeByte(value & 0b01111111 | 0b10000000);
        // Note: >>> means that the sign bit is shifted with the rest of the number rather than being left alone
        value >>>= 7;
    }
}

Note Minecraft's VarInts are identical to LEB128 with the slight change of throwing a exception if it goes over a set amount of bytes.

Note that Minecraft's VarInts are not encoded using Protocol Buffers; it's just similar. If you try to use Protocol Buffers Varints with Minecraft's VarInts, you'll get incorrect results in some cases. The major differences:

• Minecraft's VarInts are all signed, but do not use the ZigZag encoding. Protocol buffers have 3 types of Varints: uint32 (normal encoding, unsigned), sint32 (ZigZag encoding, signed), and int32 (normal encoding, signed). Minecraft's are the int32 variety. Because Minecraft uses the normal encoding instead of ZigZag encoding, negative values always use the maximum number of bytes.
• Minecraft's VarInts are never longer than 5 bytes and its VarLongs will never be longer than 10 bytes, while Protocol Buffer Varints will always use 10 bytes when encoding negative numbers, even if it's an int32.

Sample VarInts:

Value	Hex bytes	Decimal bytes
0	0x00	0
1	0x01	1
2	0x02	2
127	0x7f	127
128	0x80 0x01	128 1
255	0xff 0x01	255 1
25565	0xdd 0xc7 0x01	221 199 1
2097151	0xff 0xff 0x7f	255 255 127
2147483647	0xff 0xff 0xff 0xff 0x07	255 255 255 255 7
-1	0xff 0xff 0xff 0xff 0x0f	255 255 255 255 15
-2147483648	0x80 0x80 0x80 0x80 0x08	128 128 128 128 8

Sample VarLongs:

Value	Hex bytes	Decimal bytes
0	0x00	0
1	0x01	1
2	0x02	2
127	0x7f	127
128	0x80 0x01	128 1
255	0xff 0x01	255 1
2147483647	0xff 0xff 0xff 0xff 0x07	255 255 255 255 7
9223372036854775807	0xff 0xff 0xff 0xff 0xff 0xff 0xff 0xff 0x7f	255 255 255 255 255 255 255 255 127
-1	0xff 0xff 0xff 0xff 0xff 0xff 0xff 0xff 0xff 0x01	255 255 255 255 255 255 255 255 255 1
-2147483648	0x80 0x80 0x80 0x80 0xf8 0xff 0xff 0xff 0xff 0x01	128 128 128 128 248 255 255 255 255 1
-9223372036854775808	0x80 0x80 0x80 0x80 0x80 0x80 0x80 0x80 0x80 0x01	128 128 128 128 128 128 128 128 128 1