Implement Base32Decode (flutter#12253)

For flutter#32170

This is to enable reading back SkSL persistent cache filenames
and decode them as SkData.

Author: liyuqian

fml/base32.cc

@@ -24,22 +24,52 @@ std::pair<bool, std::string> Base32Encode(std::string_view input) {
   const size_t encoded_length = (input.size() * 8 + 4) / 5;
   output.reserve(encoded_length);
 
-  uint16_t bit_stream = (static_cast<uint8_t>(input[0]) << 8);
+  Base32EncodeConverter converter;
+  converter.Append(input[0]);
   size_t next_byte_index = 1;
-  int free_bits = 8;
 
-  while (free_bits < 16) {
-    output.push_back(kEncoding[(bit_stream & 0xf800) >> 11]);
-    bit_stream <<= 5;
-    free_bits += 5;
-
-    if (free_bits >= 8 && next_byte_index < input.size()) {
-      free_bits -= 8;
-      bit_stream += static_cast<uint8_t>(input[next_byte_index++]) << free_bits;
+  while (converter.CanExtract()) {
+    output.push_back(kEncoding[converter.Extract()]);
+    if (converter.CanAppend() && next_byte_index < input.size()) {
+      converter.Append(static_cast<uint8_t>(input[next_byte_index++]));
     }
   }
 
+  if (converter.BitsAvailable() > 0) {
+    output.push_back(kEncoding[converter.Peek()]);
+  }
+
   return {true, output};
 }
 
+static constexpr signed char kDecodeMap[] = {
+    // starting from ASCII 50 '2'
+    26, 27, 28, 29, 30, 31, -1, -1, -1, -1, -1, -1, -1, -1,
+    -1, 0,  1,  2,  3,  4,  5,  6,  7,  8,  9,  10, 11, 12,
+    13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25};
+
+static constexpr int kDecodeMapSize =
+    sizeof(kDecodeMap) / sizeof(kDecodeMap[0]);
+
+std::pair<bool, std::string> Base32Decode(const std::string& input) {
+  std::string result;
+  Base32DecodeConverter converter;
+  for (char c : input) {
+    int map_index = c - '2';
+    if (map_index < 0 || map_index >= kDecodeMapSize ||
+        kDecodeMap[map_index] == -1) {
+      return {false, result};
+    }
+    converter.Append(kDecodeMap[map_index]);
+    if (converter.CanExtract()) {
+      result.push_back(converter.Extract());
+    }
+  }
+  if (converter.Peek() != 0) {
+    // The padding should always be zero. Return false if not.
+    return {false, result};
+  }
+  return {true, result};
+}
+
 }  // namespace fml

fml/base32.h

@@ -8,9 +8,49 @@
 #include <string_view>
 #include <utility>
 
+#include "flutter/fml/logging.h"
+
 namespace fml {
 
+template <int from_length, int to_length, int buffer_length>
+class BitConverter {
+ public:
+  void Append(int bits) {
+    FML_DCHECK(bits < (1 << from_length));
+    FML_DCHECK(CanAppend());
+    lower_free_bits_ -= from_length;
+    buffer_ |= (bits << lower_free_bits_);
+  }
+
+  int Extract() {
+    FML_DCHECK(CanExtract());
+    int result = Peek();
+    buffer_ = (buffer_ << to_length) & mask_;
+    lower_free_bits_ += to_length;
+    return result;
+  }
+
+  int Peek() const { return (buffer_ >> (buffer_length - to_length)); }
+  int BitsAvailable() const { return buffer_length - lower_free_bits_; }
+  bool CanAppend() const { return lower_free_bits_ >= from_length; }
+  bool CanExtract() const { return BitsAvailable() >= to_length; }
+
+ private:
+  static_assert(buffer_length >= 2 * from_length);
+  static_assert(buffer_length >= 2 * to_length);
+  static_assert(buffer_length < sizeof(int) * 8);
+
+  static constexpr int mask_ = (1 << buffer_length) - 1;
+
+  int buffer_ = 0;
+  int lower_free_bits_ = buffer_length;
+};
+
+using Base32DecodeConverter = BitConverter<5, 8, 16>;
+using Base32EncodeConverter = BitConverter<8, 5, 16>;
+
 std::pair<bool, std::string> Base32Encode(std::string_view input);
+std::pair<bool, std::string> Base32Decode(const std::string& input);
 
 }  // namespace fml
 

fml/base32_unittest.cc

@@ -5,6 +5,8 @@
 #include "flutter/fml/base32.h"
 #include "gtest/gtest.h"
 
+#include <iostream>
+
 TEST(Base32Test, CanEncode) {
   {
     auto result = fml::Base32Encode("hello");
@@ -36,3 +38,49 @@ TEST(Base32Test, CanEncode) {
     ASSERT_EQ(result.second, "NBSWYTDP");
   }
 }
+
+TEST(Base32Test, CanEncodeDecodeStrings) {
+  std::vector<std::string> strings = {"hello", "helLo", "", "1", "\0"};
+  for (size_t i = 0; i < strings.size(); i += 1) {
+    auto encode_result = fml::Base32Encode(strings[i]);
+    ASSERT_TRUE(encode_result.first);
+    auto decode_result = fml::Base32Decode(encode_result.second);
+    ASSERT_TRUE(decode_result.first);
+    const std::string& decoded = decode_result.second;
+    std::string decoded_string(decoded.data(), decoded.size());
+    ASSERT_EQ(strings[i], decoded_string);
+  }
+}
+
+TEST(Base32Test, DecodeReturnsFalseForInvalideInput) {
+  // "B" is invalid because it has a non-zero padding.
+  std::vector<std::string> invalid_inputs = {"a", "1", "9", "B"};
+  for (const std::string& input : invalid_inputs) {
+    auto decode_result = fml::Base32Decode(input);
+    if (decode_result.first) {
+      std::cout << "Base32Decode should return false on " << input << std::endl;
+    }
+    ASSERT_FALSE(decode_result.first);
+  }
+}
+
+TEST(Base32Test, CanDecodeSkSLKeys) {
+  std::vector<std::string> inputs = {
+      "CAZAAAACAAAAADQAAAABKAAAAAJQAAIA7777777777776EYAAEAP777777777777AAAAAABA"
+      "ABTAAAAAAAAAAAAAAABAAAAAGQAGGAA",
+      "CAZAAAICAAAAAAAAAAADOAAAAAJQAAIA777777Y4AAKAAEYAAEAP777777777777EAAGMAAA"
+      "AAAAAAAAAAACQACNAAAAAAAAAAAAAAACAAAAAPAAMMAA",
+      "CAZACAACAAAABAYACAAAAAAAAAJQAAIADQABIAH777777777777RQAAOAAAAAAAAAAAAAABE"
+      "AANQAAAAAAAAAAAAAAYAAJYAAAAAAAANAAAQAAAAAAAEAAAHAAAAAAAAAAAAAAANAAAQAAAA"
+      "AAAFIADKAAAAAAAAAAAAAAACAAAAAZAAMMAA"};
+  for (const std::string& input : inputs) {
+    auto decode_result = fml::Base32Decode(input);
+    if (!decode_result.first) {
+      std::cout << "Base32Decode should return true on " << input << std::endl;
+    }
+    ASSERT_TRUE(decode_result.first);
+    auto encode_result = fml::Base32Encode(decode_result.second);
+    ASSERT_TRUE(encode_result.first);
+    ASSERT_EQ(encode_result.second, input);
+  }
+}