[crispy] base64: provide encoding via state machine based API.

Author: christianparpart

src/crispy/base64.h

@@ -42,55 +42,99 @@ namespace detail
     };
 }
 
-template <typename Iterator, typename Alphabet>
-std::string encode(Iterator begin, Iterator end, Alphabet alphabet)
+struct EncoderState
 {
-    int const inputLength = static_cast<int>(std::distance(begin, end));
-    int const outputLength = ((inputLength + 2) / 3 * 4) + 1;
+    int modulo = 0;
+    uint8_t pending[3];
+};
 
-    std::string output;
-    output.resize(static_cast<unsigned>(outputLength));
+template <typename Alphabet, typename Sink>
+constexpr void encode(uint8_t _byte, Alphabet const& alphabet, EncoderState& _state, Sink&& _sink)
+{
+    _state.pending[_state.modulo] = _byte;
+    if (++_state.modulo != 3)
+        return;
+
+    _state.modulo = 0;
+    uint8_t const* input = _state.pending;
+    char const out[4] = {
+        alphabet[(input[0] >> 2) & 0x3F],
+        alphabet[((input[0] & 0x03) << 4) | ((uint8_t)(input[1] & 0xF0) >> 4)],
+        alphabet[((input[1] & 0x0F) << 2) | ((uint8_t)(input[2] & 0xC0) >> 6)],
+        alphabet[input[2] & 0x3F]
+    };
+    _sink(std::string_view(out, 4));
+}
 
-    auto i = 0;
-    auto const e = inputLength - 2;
-    auto const input = begin;
-    auto out = output.begin();
+namespace detail
+{
 
-    while (i < e)
-    {
-        *out++ = alphabet[(input[i] >> 2) & 0x3F];
+template <typename Alphabet, typename Sink>
+constexpr void finish(Alphabet const& alphabet, EncoderState& _state, Sink&& _sink)
+{
+    if (_state.modulo == 0)
+        return;
 
-        *out++ = alphabet[((input[i] & 0x03) << 4) |
-                           ((uint8_t)(input[i + 1] & 0xF0) >> 4)];
+    auto const* input = _state.pending;
 
-        *out++ = alphabet[((input[i + 1] & 0x0F) << 2) |
-                          ((uint8_t)(input[i + 2] & 0xC0) >> 6)];
+    switch (_state.modulo)
+    {
+        case 2:
+            {
+                char const out[4] = {
+                    alphabet[(input[0] >> 2) & 0x3F],
+                    alphabet[((input[0] & 0x03) << 4) | ((uint8_t)(input[1] & 0xF0) >> 4)],
+                    alphabet[((input[1] & 0x0F) << 2)],
+                    '='
+                };
+                _sink(std::string_view{out});
+            }
+            break;
+        case 1:
+            {
+                char const out[4] = {
+                    alphabet[(input[0] >> 2) & 0x3F],
+                    alphabet[((input[0] & 0x03) << 4)],
+                    '=',
+                    '='
+                };
+                _sink(std::string_view{out});
+            }
+            break;
+        case 0:
+            break;
+    }
+}
 
-        *out++ = alphabet[input[i + 2] & 0x3F];
+}
 
-        i += 3;
-    }
+template <typename Sink>
+constexpr void encode(uint8_t _byte, EncoderState& _state, Sink _sink)
+{
+    constexpr char alphabet[] =
+        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+        "abcdefghijklmnopqrstuvwxyz"
+        "0123456789+/";
+    return encode(_byte, alphabet, _state, std::forward<Sink>(_sink));
+    //return encode(_byte, detail::indexmap, _state, std::forward<Sink>(_sink));
+}
 
-    if (i < inputLength)
-    {
-        *out++ = alphabet[(input[i] >> 2) & 0x3F];
+template <typename Sink>
+constexpr void finish(EncoderState& _state, Sink&& _sink)
+{
+    finish(detail::indexmap, _state, std::forward<Sink>(_sink));
+}
 
-        if (i == (inputLength - 1))
-        {
-            *out++ = alphabet[((input[i] & 0x03) << 4)];
-            *out++ = '=';
-        }
-        else
-        {
-            *out++ = alphabet[((input[i] & 0x03) << 4) |
-                              ((uint8_t)(input[i + 1] & 0xF0) >> 4)];
-            *out++ = alphabet[((input[i + 1] & 0x0F) << 2)];
-        }
-        *out++ = '=';
-    }
+template <typename Iterator, typename Alphabet>
+std::string encode(Iterator begin, Iterator end, Alphabet const& alphabet)
+{
+    std::string output;
+    output.reserve(((std::distance(begin, end) + 2) / 3 * 4) + 1);
 
-    auto const outlen = std::distance(output.begin(), out);
-    output.resize(static_cast<unsigned>(outlen));
+    EncoderState state{};
+    for (auto i = begin; i != end; ++i)
+        encode(*i, alphabet, state, [&](std::string_view _data) { output += _data; });
+    detail::finish(alphabet, state, [&](std::string_view _data) { output += _data; });
 
     return output;
 }
@@ -106,9 +150,9 @@ std::string encode(Iterator begin, Iterator end)
 }
 
 
-inline std::string encode(const std::string_view& value)
+inline std::string encode(std::string_view _value)
 {
-    return encode(value.begin(), value.end());
+    return encode(_value.begin(), _value.end());
 }
 
 template <typename Iterator, typename IndexTable>
@@ -195,12 +239,12 @@ size_t decode(Iterator begin, Iterator end, Output output)
 }
 
 template <typename Output>
-size_t decode(std::string_view const& input, Output output)
+size_t decode(std::string_view input, Output output)
 {
     return decode(input.begin(), input.end(), output);
 }
 
-inline std::string decode(const std::string_view& input)
+inline std::string decode(std::string_view input)
 {
     std::string output;
     output.resize(decodeLength(input));