// Copyright 2018 Google LLC. All Rights Reserved. This file and proprietary
// source code may only be used and distributed under the Widevine License
// Agreement.
//
// simple base64 decoder

#include "base64decode.h"

#include <iostream>
#include <memory>
#include <string>

namespace base64 {

using namespace std;

string Base64::Decode(const string &encoded) {
  string s = encoded;

  while (s.size() % 4 != 0) {
    s.append("=");
  }

  size_t n = s.size();

  size_t padding = 0;
  if (n >= 1 && s[n - 1] == '=') {
    padding = 1;

    if (n >= 2 && s[n - 2] == '=') {
      padding = 2;

      if (n >= 3 && s[n - 3] == '=') {
        padding = 3;
      }
    }
  }

  // We divide first to avoid overflow. It's OK to do this because we
  // already made sure that n % 4 == 0.
  size_t outLen = (n / 4) * 3 - padding;

  std::unique_ptr<uint8_t> buffer(new uint8_t[outLen]);
  if (!buffer) {
    return "";
  }
  uint8_t *out = buffer.get();
  size_t j = 0;
  uint32_t accum = 0;
  for (size_t i = 0; i < n; ++i) {
    char c = s[i];
    unsigned value;
    if (c >= 'A' && c <= 'Z') {
      value = c - 'A';
    } else if (c >= 'a' && c <= 'z') {
      value = 26 + c - 'a';
    } else if (c >= '0' && c <= '9') {
      value = 52 + c - '0';
    } else if (c == '+' || c == '-') {
      value = 62;
    } else if (c == '/' || c == '_') {
      value = 63;
    } else {
      if (c != '=') {
        std::cerr << "unexpected character in base64 decode:" << c << std::endl;
      }
      value = 0;
    }

    accum = (accum << 6) | value;

    if (((i + 1) % 4) == 0) {
      if (j < outLen) {
        out[j++] = (accum >> 16);
      }
      if (j < outLen) {
        out[j++] = (accum >> 8) & 0xff;
      }
      if (j < outLen) {
        out[j++] = accum & 0xff;
      }

      accum = 0;
    }
  }
  return string((char *)buffer.get(), outLen);
}

}  // namespace base64