Adds a base32 component.

components/base32/base32.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "components/base32/base32.h"
+
+#include <stddef.h>
+#include <algorithm>
+#include <limits>
+
+#include "base/logging.h"
+
+namespace base32 {
+
+namespace {
+constexpr char kEncoding[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
+}  // namespace
+
+std::string Base32Encode(base::StringPiece input, Base32EncodePolicy policy) {
+  if (input.empty())
+    return std::string();
+
+  if (input.size() > std::numeric_limits<size_t>::max() / 8) {
+    NOTREACHED()
+        << "Input is too large and would overflow encoded size computation.";
+    return std::string();
+  }
+
+  // Per RFC4648, the output is formed of 8 characters per 40 bits of input and
+  // another 8 characters for the last group of [1,39] bits in the input.
+  // That is: ceil(input.size() * 8.0 / 40.0) * 8 ==
+  //          ceil(input.size() / 5.0) * 8 ==
+  //          ((input.size() + 4) / 5) * 8.
+  const size_t padded_length = ((input.size() + 4) / 5) * 8;
+
+  // When no padding is used, the output is exactly 1 character per 5 bits of
+  // input and one more for the last [1,4] bits.
+  // That is: ceil(input.size() * 8.0 / 5.0) ==
+  //          (input.size() * 8 + 4) / 5.
+  const size_t unpadded_length = (input.size() * 8 + 4) / 5;
+
+  std::string output;
+  const size_t encoded_length = policy == Base32EncodePolicy::INCLUDE_PADDING
+                                    ? padded_length
+                                    : unpadded_length;
+  output.reserve(encoded_length);
+
+  // A bit stream which will be read from the left and appended to from the
+  // right as it's emptied.
+  uint16_t bit_stream = (static_cast<uint8_t>(input[0]) << 8);
+  size_t next_byte_index = 1;
+  int free_bits = 8;
+  while (free_bits < 16) {
+    // Extract the 5 leftmost bits in the stream
+    output.push_back(kEncoding[(bit_stream & 0xf800) >> 11]);
+    bit_stream <<= 5;
+    free_bits += 5;
+
+    // If there is enough room in the bit stream, inject another byte (if there
+    // are any left...).
+    if (free_bits >= 8 && next_byte_index < input.size()) {
+      free_bits -= 8;
+      bit_stream += static_cast<uint8_t>(input[next_byte_index++]) << free_bits;
+    }
+  }
+
+  if (policy == Base32EncodePolicy::INCLUDE_PADDING) {
+    output.append(padded_length - unpadded_length, '=');
+  }
+
+  DCHECK_EQ(encoded_length, output.size());
+  return output;
+}
+
+}  // namespace base32