Here you can find the source of encode3to4(byte[] source, int srcOffset, int numSigBytes, byte[] destination, int destOffset, byte[] alphabet)
| Parameter | Description |
|---|---|
| source | the array to convert |
| srcOffset | the index where conversion begins |
| numSigBytes | the number of significant bytes in your array |
| destination | the array to hold the conversion |
| destOffset | the index where output will be put |
| alphabet | is the encoding alphabet |
private static byte[] encode3to4(byte[] source, int srcOffset, int numSigBytes, byte[] destination, int destOffset, byte[] alphabet)
//package com.java2s; // Licensed under the Apache License, Version 2.0 (the "License"); public class Main { /** The equals sign (=) as a byte. */ private final static byte EQUALS_SIGN = (byte) '='; /**/*from www .j a va 2 s .co m*/ * Encodes up to three bytes of the array <var>source</var> * and writes the resulting four Base64 bytes to <var>destination</var>. * The source and destination arrays can be manipulated * anywhere along their length by specifying * <var>srcOffset</var> and <var>destOffset</var>. * This method does not check to make sure your arrays * are large enough to accommodate <var>srcOffset</var> + 3 for * the <var>source</var> array or <var>destOffset</var> + 4 for * the <var>destination</var> array. * The actual number of significant bytes in your array is * given by <var>numSigBytes</var>. * * @param source the array to convert * @param srcOffset the index where conversion begins * @param numSigBytes the number of significant bytes in your array * @param destination the array to hold the conversion * @param destOffset the index where output will be put * @param alphabet is the encoding alphabet * @return the <var>destination</var> array * @since 1.3 */ private static byte[] encode3to4(byte[] source, int srcOffset, int numSigBytes, byte[] destination, int destOffset, byte[] alphabet) { // 1 2 3 // 01234567890123456789012345678901 Bit position // --------000000001111111122222222 Array position from threeBytes // --------| || || || | Six bit groups to index alphabet // >>18 >>12 >> 6 >> 0 Right shift necessary // 0x3f 0x3f 0x3f Additional AND // Create buffer with zero-padding if there are only one or two // significant bytes passed in the array. // We have to shift left 24 in order to flush out the 1's that appear // when Java treats a value as negative that is cast from a byte to an int. int inBuff = (numSigBytes > 0 ? ((source[srcOffset] << 24) >>> 8) : 0) | (numSigBytes > 1 ? ((source[srcOffset + 1] << 24) >>> 16) : 0) | (numSigBytes > 2 ? ((source[srcOffset + 2] << 24) >>> 24) : 0); switch (numSigBytes) { case 3: destination[destOffset] = alphabet[(inBuff >>> 18)]; destination[destOffset + 1] = alphabet[(inBuff >>> 12) & 0x3f]; destination[destOffset + 2] = alphabet[(inBuff >>> 6) & 0x3f]; destination[destOffset + 3] = alphabet[(inBuff) & 0x3f]; return destination; case 2: destination[destOffset] = alphabet[(inBuff >>> 18)]; destination[destOffset + 1] = alphabet[(inBuff >>> 12) & 0x3f]; destination[destOffset + 2] = alphabet[(inBuff >>> 6) & 0x3f]; destination[destOffset + 3] = EQUALS_SIGN; return destination; case 1: destination[destOffset] = alphabet[(inBuff >>> 18)]; destination[destOffset + 1] = alphabet[(inBuff >>> 12) & 0x3f]; destination[destOffset + 2] = EQUALS_SIGN; destination[destOffset + 3] = EQUALS_SIGN; return destination; default: return destination; } // end switch } }