test.c

#include "stdlib.h"
#include "stdio.h"
#include "time.h"
#include <stdint.h>
#include <string.h>

typedef unsigned int uint;
const char* chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
static char decode_table[256];

int encode_size(int size) {
  return (int)(size * 4 / 3.0) + 6;
}

int decode_size(int size) {
  return (int)(size * 3 / 4.0) + 6;
}

void init_decode_table() {
  for (int i = 0; i < 256; i++) {
    char ch = (char)i;
    char code = -1;
    if (ch >= 'A' && ch <= 'Z') code = ch - 0x41;
    if (ch >= 'a' && ch <= 'z') code = ch - 0x47;
    if (ch >= '0' && ch <= '9') code = ch + 0x04; 
    if (ch == '+' || ch == '-') code = 0x3E;
    if (ch == '/' || ch == '_') code = 0x3F;
    decode_table[i] = code;
  }
}

#define next_char(x) char x = decode_table[(unsigned char)*str++]; if (x < 0) return 1;

int decode(int size, const char* str, size_t* out_size, char* output) {
  char *out = output;
  while (size > 0 && (str[size - 1] == '\n' || str[size - 1] == '\r' || str[size - 1] == '=')) size--;
  const char* ends = str + size - 4;
  while (1) {
    if (str > ends) break;
    while (*str == '\n' || *str == '\r') str++;

    if (str > ends) break;
    next_char(a); next_char(b); next_char(c); next_char(d);

    *out++ = (char)(a << 2 | b >> 4);
    *out++ = (char)(b << 4 | c >> 2);
    *out++ = (char)(c << 6 | d >> 0);
  }

  int mod = (str - ends) % 4;
  if (mod == 2) {
    next_char(a); next_char(b);
    *out++ = (char)(a << 2 | b >> 4);
  } else if (mod == 3) {
    next_char(a); next_char(b); next_char(c);
    *out++ = (char)(a << 2 | b >> 4);
    *out++ = (char)(b << 4 | c >> 2);
  }

  *out = '\0';
  *out_size = out - output;
  return 0;
}

void encode(int size, const char* str, size_t* out_size, char* output) {
  char *out = output;
  const char* ends = str + (size - size % 3);
  uint n;
  while (str != ends) {
    uint32_t n = __builtin_bswap32(*(uint32_t*)str);
    *out++ = chars[(n >> 26) & 63];
    *out++ = chars[(n >> 20) & 63];
    *out++ = chars[(n >> 14) & 63];
    *out++ = chars[(n >> 8) & 63];
    str += 3;
  }
  int pd = size % 3;
  if  (pd == 1) {
    n = (uint)*str << 16;
    *out++ = chars[(n >> 18) & 63];
    *out++ = chars[(n >> 12) & 63];
    *out++ = '=';
    *out++ = '=';
  } else if (pd == 2) {
    n = (uint)*str++ << 16;
    n |= (uint)*str << 8;
    *out++ = chars[(n >> 18) & 63];
    *out++ = chars[(n >> 12) & 63];
    *out++ = chars[(n >> 6) & 63];
    *out++ = '=';
  }
  *out = '\0';
  *out_size = out - output;
}

int main() {
  init_decode_table();

  const int STR_SIZE = 131072;
  const int TRIES = 8192;

  char str[STR_SIZE + 1];
  memset(str, 'a', STR_SIZE);
  str[STR_SIZE] = '\0';

  size_t str2_size;
  char str2[encode_size(STR_SIZE)];
  encode(STR_SIZE, str, &str2_size, str2);
  printf("encode %s... to %s...: ", strndup(str, 4), strndup(str2, 4));

  int s = 0;
  clock_t t = clock();
  for (int i = 0; i < TRIES; i++) { 
    size_t str2_size;
    char str2[encode_size(STR_SIZE)];
    encode(STR_SIZE, str, &str2_size, str2);
    s += str2_size;
  }

  printf("%d, %.2f\n", s, (float)(clock() - t)/CLOCKS_PER_SEC);

  size_t str3_size;
  char str3[decode_size(str2_size)];
  decode(str2_size, str2, &str3_size, str3);
  printf("decode %s... to %s...: ", strndup(str2, 4), strndup(str3, 4));

  s = 0;
  t = clock();
  for (int i = 0; i < TRIES; i++) {
    size_t str3_size;
    char str3[decode_size(str2_size)];
    if (decode(str2_size, str2, &str3_size, str3) != 0) {
      printf("error when decoding");
    }
    s += str3_size;
  }
  printf("%d, %.2f\n", s, (float)(clock() - t)/CLOCKS_PER_SEC);
}