base64.cpp

//
// Created by jksun on 2021/4/15.
//

#include "base64.h"
#include <stdio.h>
#include <stdlib.h>


/**
 *  转解码过程
 *  3 * 8 = 4 * 6; 3字节占24位, 4*6=24
 *  先将要编码的转成对应的ASCII值
 *  如编码: s 1 3
 *  对应ASCII值为: 115 49 51
 *  对应二进制为: 01110011 00110001 00110011
 *  将其6个分组分4组: 011100 110011 000100 110011
 *  而计算机是以8bit存储, 所以在每组的高位补两个0如下:
 *  00011100 00110011 00000100 00110011对应:28 51 4 51
 *  查找base64 转换表 对应 c z E z
 *
 *  解码
 *  c z E z
 *  对应ASCII值为 99 122 69 122
 *  对应表base64_suffix_map的值为 28 51 4 51
 *  对应二进制值为 00011100 00110011 00000100 00110011
 *  依次去除每组的前两位, 再拼接成3字节
 *  即: 01110011 00110001 00110011
 *  对应的就是s 1 3
 */

#define BASE64_PAD '='
#define BASE64DE_FIRST '+'
#define BASE64DE_LAST 'z'

/* BASE 64 encode table */
static const char base64en[] = {
        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
        'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
        'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
        'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
        'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
        'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
        'w', 'x', 'y', 'z', '0', '1', '2', '3',
        '4', '5', '6', '7', '8', '9', '+', '/',
};

/* ASCII order for BASE 64 decode, 255 in unused character */
static const unsigned char base64de[] = {
        /* nul, soh, stx, etx, eot, enq, ack, bel, */
        255, 255, 255, 255, 255, 255, 255, 255,

        /*  bs,  ht,  nl,  vt,  np,  cr,  so,  si, */
        255, 255, 255, 255, 255, 255, 255, 255,

        /* dle, dc1, dc2, dc3, dc4, nak, syn, etb, */
        255, 255, 255, 255, 255, 255, 255, 255,

        /* can,  em, sub, esc,  fs,  gs,  rs,  us, */
        255, 255, 255, 255, 255, 255, 255, 255,

        /*  sp, '!', '"', '#', '$', '%', '&', ''', */
        255, 255, 255, 255, 255, 255, 255, 255,

        /* '(', ')', '*', '+', ',', '-', '.', '/', */
        255, 255, 255,  62, 255, 255, 255,  63,

        /* '0', '1', '2', '3', '4', '5', '6', '7', */
        52,  53,  54,  55,  56,  57,  58,  59,

        /* '8', '9', ':', ';', '<', '=', '>', '?', */
        60,  61, 255, 255, 255, 255, 255, 255,

        /* '@', 'A', 'B', 'C', 'D', 'E', 'F', 'G', */
        255,   0,   1,  2,   3,   4,   5,    6,

        /* 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', */
        7,   8,   9,  10,  11,  12,  13,  14,

        /* 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', */
        15,  16,  17,  18,  19,  20,  21,  22,

        /* 'X', 'Y', 'Z', '[', '\', ']', '^', '_', */
        23,  24,  25, 255, 255, 255, 255, 255,

        /* '`', 'a', 'b', 'c', 'd', 'e', 'f', 'g', */
        255,  26,  27,  28,  29,  30,  31,  32,

        /* 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', */
        33,  34,  35,  36,  37,  38,  39,  40,

        /* 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', */
        41,  42,  43,  44,  45,  46,  47,  48,

        /* 'x', 'y', 'z', '{', '|', '}', '~', del, */
        49,  50,  51, 255, 255, 255, 255, 255
};

unsigned int
base64_encode(const unsigned char *in, unsigned int inlen, char *out)
{
    int s;
    unsigned int i;
    unsigned int j;
    unsigned char c;
    unsigned char l;

    s = 0;
    l = 0;
    for (i = j = 0; i < inlen; i++) {
        c = in[i];

        switch (s) {
            case 0:
                s = 1;
                out[j++] = base64en[(c >> 2) & 0x3F];
                break;
            case 1:
                s = 2;
                out[j++] = base64en[((l & 0x3) << 4) | ((c >> 4) & 0xF)];
                break;
            case 2:
                s = 0;
                out[j++] = base64en[((l & 0xF) << 2) | ((c >> 6) & 0x3)];
                out[j++] = base64en[c & 0x3F];
                break;
        }
        l = c;
    }

    switch (s) {
        case 1:
            out[j++] = base64en[(l & 0x3) << 4];
            out[j++] = BASE64_PAD;
            out[j++] = BASE64_PAD;
            break;
        case 2:
            out[j++] = base64en[(l & 0xF) << 2];
            out[j++] = BASE64_PAD;
            break;
    }

    out[j] = 0;

    return j;
}

unsigned int
base64_decode(const char *in, unsigned int inlen, unsigned char *out)
{
    unsigned int i;
    unsigned int j;
    unsigned char c;

    if (inlen & 0x3) {
        return 0;
    }

    for (i = j = 0; i < inlen; i++) {
        if (in[i] == BASE64_PAD) {
            break;
        }
        if (in[i] < BASE64DE_FIRST || in[i] > BASE64DE_LAST) {
            return 0;
        }

        c = base64de[(unsigned char)in[i]];
        if (c == 255) {
            return 0;
        }

        switch (i & 0x3) {
            case 0:
                out[j] = (c << 2) & 0xFF;
                break;
            case 1:
                out[j++] |= (c >> 4) & 0x3;
                out[j] = (c & 0xF) << 4;
                break;
            case 2:
                out[j++] |= (c >> 2) & 0xF;
                out[j] = (c & 0x3) << 6;
                break;
            case 3:
                out[j++] |= c;
                break;
        }
    }

    return j;
}