A semi-optimized java class to work with varints.

Buffers.java

package com.tk2217.util;

import io.netty.buffer.ByteBuf;

public class VarInts {
    /* Bazel varint reading routines
    * https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/util/VarInt.java
    * Licensed under the Apache License 2.0.
    * Slightly modified to use a `ByteBuf`.
    */

    /**
     * Reads a varint from the current position of the given ByteBuf and returns the decoded value
     * as 32 bit integer.
     *
     * <p>The position of the buffer is advanced to the first byte after the decoded varint.
     *
     * @param src the ByteBuf to readByte the var int from
     * @return The integer value of the decoded varint
     */
    public static int readVarInt(ByteBuf src) {
        int tmp;
        if ((tmp = src.readByte()) >= 0) {
            return tmp;
        }
        int result = tmp & 0x7f;
        if ((tmp = src.readByte()) >= 0) {
            result |= tmp << 7;
        } else {
            result |= (tmp & 0x7f) << 7;
            if ((tmp = src.readByte()) >= 0) {
                result |= tmp << 14;
            } else {
                result |= (tmp & 0x7f) << 14;
                if ((tmp = src.readByte()) >= 0) {
                    result |= tmp << 21;
                } else {
                    result |= (tmp & 0x7f) << 21;
                    result |= (tmp = src.readByte()) << 28;
                    while (tmp < 0) {
                        // We readByte into this loop only in the case of overflow.
                        // By doing this, we can call readByteVarInt() instead of
                        // readByteVarLong() when we only need an int.
                        tmp = src.readByte();
                    }
                }
            }
        }
        return result;
    }

    /**
     * Reads an up to 64 bit long varint from the current position of the
     * given ByteBuf and returns the decoded value as long.
     *
     * <p>The position of the buffer is advanced to the first byte after the
     * decoded varint.
     *
     * @param src the ByteBuf to get the var int from
     * @return The integer value of the decoded long varint
     */
    public static long getVarLong(ByteBuf src) {
        long tmp;
        if ((tmp = src.readByte()) >= 0) {
            return tmp;
        }
        long result = tmp & 0x7f;
        if ((tmp = src.readByte()) >= 0) {
            result |= tmp << 7;
        } else {
            result |= (tmp & 0x7f) << 7;
            if ((tmp = src.readByte()) >= 0) {
                result |= tmp << 14;
            } else {
                result |= (tmp & 0x7f) << 14;
                if ((tmp = src.readByte()) >= 0) {
                    result |= tmp << 21;
                } else {
                    result |= (tmp & 0x7f) << 21;
                    if ((tmp = src.readByte()) >= 0) {
                        result |= tmp << 28;
                    } else {
                        result |= (tmp & 0x7f) << 28;
                        if ((tmp = src.readByte()) >= 0) {
                            result |= tmp << 35;
                        } else {
                            result |= (tmp & 0x7f) << 35;
                            if ((tmp = src.readByte()) >= 0) {
                                result |= tmp << 42;
                            } else {
                                result |= (tmp & 0x7f) << 42;
                                if ((tmp = src.readByte()) >= 0) {
                                    result |= tmp << 49;
                                } else {
                                    result |= (tmp & 0x7f) << 49;
                                    if ((tmp = src.readByte()) >= 0) {
                                        result |= tmp << 56;
                                    } else {
                                        result |= (tmp & 0x7f) << 56;
                                        result |= ((long) src.readByte()) << 63;
                                    }
                                }
                            }
                        }
                    }
                }
            }
        }
        return result;
    }

    /* astei's fast varint writing routines
     * https://steinborn.me/posts/performance/how-fast-can-you-write-a-varint/
     * Licensed under the MIT License.
     */

    /**
    * Encodes a varint to a ByteBuf.
    * @param buf the ByteBuf to write to
    * @param value the value to encode
    */
    public static void writeVarInt(ByteBuf buf, int value) {
        if ((value & (0xFFFFFFFF << 7)) == 0) {
            buf.writeByte(value);
        } else if ((value & (0xFFFFFFFF << 14)) == 0) {
            int w = (value & 0x7F | 0x80) << 8 | (value >>> 7);
            buf.writeShort(w);
        } else if ((value & (0xFFFFFFFF << 21)) == 0) {
            int w = (value & 0x7F | 0x80) << 16 | ((value >>> 7) & 0x7F | 0x80) << 8 | (value >>> 14);
            buf.writeMedium(w);
        } else if ((value & (0xFFFFFFFF << 28)) == 0) {
            int w = (value & 0x7F | 0x80) << 24 | (((value >>> 7) & 0x7F | 0x80) << 16)
                    | ((value >>> 14) & 0x7F | 0x80) << 8 | (value >>> 21);
            buf.writeInt(w);
        } else {
            int w = (value & 0x7F | 0x80) << 24 | ((value >>> 7) & 0x7F | 0x80) << 16 | ((value >>> 14) & 0x7F | 0x80) << 8
                    | ((value >>> 21) & 0x7F | 0x80);
            buf.writeInt(w);
            buf.writeByte(value >>> 28);
        }
    }

    /* Bazel varlong writing routine ~ I can't be bothered to transfer over the previous routine and this isn't used nearly as much.
     * https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/util/VarInt.java
     * Licensed under the Apache License 2.0.
     * Slightly modified to use a `ByteBuf` and reorder arguments.
     */

    /**
     * Encodes a long integer in a variable-length encoding, 7 bits per byte, to a
     * ByteBuf.
     * @param buf the ByteBuf to add the encoded value
     * @param value the value to encode
     */
    public static void writeVarLong(ByteBuf buf, long value) {
        while (true) {
            int bits = ((int) value) & 0x7f;
            value >>>= 7;
            if (value == 0) {
                buf.writeByte((byte) bits);
                return;
            }
            buf.writeByte((byte) (bits | 0x80));
        }
    }

}