// Copyright 2020 Google LLC. All Rights Reserved.

#include "base/base64.h"

#include <stddef.h>
#include <stdint.h>

namespace base {

namespace {

static const char kBase64Codes[] =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

// Calculates a/b using round-up division (only works for numbers
// greater than 0).
constexpr size_t CeilDivide(size_t a, size_t b) {
  return ((a - 1) / b) + 1;
}

}  // namespace

void Base64Encode(const std::string& input, std::string* output) {
  if (input.empty()) {
    output->clear();
    return;
  }

  // |temp| stores a 24-bit block that is treated as an array where insertions
  // occur from high to low.
  uint32_t temp = 0;
  size_t out_index = 0;
  const size_t out_size = CeilDivide(input.size(), 3) * 4;
  std::string result(out_size, '\0');
  for (size_t i = 0; i < input.size(); i++) {
    // "insert" 8-bits of data
    temp = (temp << 8) | (input[i] & 0x0FF);

    if (i % 3 == 2) {
      result[out_index++] = kBase64Codes[(temp >> 18) & 0x3f];
      result[out_index++] = kBase64Codes[(temp >> 12) & 0x3f];
      result[out_index++] = kBase64Codes[(temp >> 6) & 0x3f];
      result[out_index++] = kBase64Codes[temp & 0x3f];
      temp = 0;
    }
  }

  if (input.size() % 3 == 1) {
    result[out_index++] = kBase64Codes[(temp >> 18) & 0x3f];
    result[out_index++] = kBase64Codes[(temp >> 12) & 0x3f];
    result[out_index++] = '=';
    result[out_index++] = '=';
  } else if (input.size() % 3 == 2) {
    result[out_index++] = kBase64Codes[(temp >> 18) & 0x3f];
    result[out_index++] = kBase64Codes[(temp >> 12) & 0x3f];
    result[out_index++] = kBase64Codes[(temp >> 6) & 0x3f];
    result[out_index++] = '=';
  }

  output->swap(result);
}

}  // namespace base