#include "P64.h"

P64Decoder::P64Decoder()
{
  for (int i = 0; i < 256; i++)
    {
      value[i] = badValue;
    }
  int currentValue = 0;
  for (int i = 'a'; i <= 'z'; i++)
    {
      value[i] = currentValue++;
    }
  for (int i = 'A'; i <= 'Z'; i++)
    {
      value[i] = currentValue++;
    }
  for (int i = '0'; i <= '9'; i++)
    {
      value[i] = currentValue++;
    }
  value['_'] = currentValue++;
  value['-'] = currentValue;
}

bool P64Decoder::decode(std::string input, std::string &output) const
{
  int inputLength = input.length();
  int q = inputLength / 4;
  int r = inputLength % 4;
  if (r == 1)
    {
      return false;
    }
  int outputLength = q * 3;
  if (r > 0)
    {
      outputLength += r - 1;
    }
  unsigned char *workspace = new unsigned char[outputLength];
  unsigned int tail = 0;
  // Note:  We may need the extra character added at the end by c_str().  We
  // won't actually use its value.  But it will copy it into our buffer.  This
  // makes the code much simpler.
  const unsigned char *inputPointer = (const unsigned char *)(input.c_str()); 
  unsigned char *outputPointer = workspace;
  unsigned char *finished = outputPointer + outputLength;
  int position = 0;
  while (outputPointer < finished)
    {
      int newValue = value[*inputPointer++];
      if (newValue == badValue)
	{
	  delete[] workspace;
	  return false;
	}
      tail <<= 6;
      tail |= newValue;
      switch (position)
	{
	case 0:
	  position = 1;
	  break;
	case 1:
	  *outputPointer++ = (tail >> 4);
	  position = 2;
	  break;
	case 2:
	  *outputPointer++ = (tail >> 2);
          position = 3;
          break;
        case 3:
          *outputPointer++ = tail;
          position = 0;
          break;
	}
    }
  output.assign((char *)(workspace), outputLength);
  delete[] workspace;
  return true;
}