#include "base64.h"

static int mg_base64_encode_single(int c) {
  if (c < 26) {
    return c + 'A';
  } else if (c < 52) {
    return c - 26 + 'a';
  } else if (c < 62) {
    return c - 52 + '0';
  } else {
    return c == 62 ? '+' : '/';
  }
}

static int mg_base64_decode_single(int c) {
  if (c >= 'A' && c <= 'Z') {
    return c - 'A';
  } else if (c >= 'a' && c <= 'z') {
    return c + 26 - 'a';
  } else if (c >= '0' && c <= '9') {
    return c + 52 - '0';
  } else if (c == '+') {
    return 62;
  } else if (c == '/') {
    return 63;
  } else if (c == '=') {
    return 64;
  } else {
    return -1;
  }
}

size_t mg_base64_update(unsigned char ch, char *to, size_t n) {
  unsigned long rem = (n & 3) % 3;
  if (rem == 0) {
    to[n] = (char) mg_base64_encode_single(ch >> 2);
    to[++n] = (char) ((ch & 3) << 4);
  } else if (rem == 1) {
    to[n] = (char) mg_base64_encode_single(to[n] | (ch >> 4));
    to[++n] = (char) ((ch & 15) << 2);
  } else {
    to[n] = (char) mg_base64_encode_single(to[n] | (ch >> 6));
    to[++n] = (char) mg_base64_encode_single(ch & 63);
    n++;
  }
  return n;
}

size_t mg_base64_final(char *to, size_t n) {
  size_t saved = n;
  // printf("---[%.*s]\n", n, to);
  if (n & 3) n = mg_base64_update(0, to, n);
  if ((saved & 3) == 2) n--;
  // printf("    %d[%.*s]\n", n, n, to);
  while (n & 3) to[n++] = '=';
  to[n] = '\0';
  return n;
}

size_t mg_base64_encode(const unsigned char *p, size_t n, char *to, size_t dl) {
  size_t i, len = 0;
  if (dl > 0) to[0] = '\0';
  if (dl < ((n / 3) + (n % 3 ? 1 : 0)) * 4 + 1) return 0;
  for (i = 0; i < n; i++) len = mg_base64_update(p[i], to, len);
  len = mg_base64_final(to, len);
  return len;
}

size_t mg_base64_decode(const char *src, size_t n, char *dst, size_t dl) {
  const char *end = src == NULL ? NULL : src + n;  // Cannot add to NULL
  size_t len = 0;
  if (dl < n / 4 * 3 + 1) goto fail;
  while (src != NULL && src + 3 < end) {
    int a = mg_base64_decode_single(src[0]),
        b = mg_base64_decode_single(src[1]),
        c = mg_base64_decode_single(src[2]),
        d = mg_base64_decode_single(src[3]);
    if (a == 64 || a < 0 || b == 64 || b < 0 || c < 0 || d < 0) {
      goto fail;
    }
    dst[len++] = (char) ((a << 2) | (b >> 4));
    if (src[2] != '=') {
      dst[len++] = (char) ((b << 4) | (c >> 2));
      if (src[3] != '=') dst[len++] = (char) ((c << 6) | d);
    }
    src += 4;
  }
  dst[len] = '\0';
  return len;
fail:
  if (dl > 0) dst[0] = '\0';
  return 0;
}