Add Base64::EncodedSize to tidy up allocations (flutter/engine#46624)

As a follow-up to #46543, this adds a dedicated function to compute the
size of the buffer needed to encode data using Base64 instead of calling
the encode function with nullptr.

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responsibilities.
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Objective-C, Java style guides].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
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- [x] All existing and new tests are passing.

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engine/src/flutter/flow/layers/performance_overlay_layer_unittests.cc

@@ -111,10 +111,7 @@ static void TestPerformanceOverlayLayerGold(int refresh_rate) {
     wstream.write(snapshot_data->data(), snapshot_data->size());
     wstream.flush();
 
-    // TODO(kjlubick) We shouldn't need to call Encode once to pre-flight the
-    // encode length. It should be ceil(4/3 * sksl.value->size()).
-    size_t b64_size =
-        Base64::Encode(snapshot_data->data(), snapshot_data->size(), nullptr);
+    size_t b64_size = Base64::EncodedSize(snapshot_data->size());
     sk_sp<SkData> b64_data = SkData::MakeUninitialized(b64_size + 1);
     char* b64_char = static_cast<char*>(b64_data->writable_data());
     Base64::Encode(snapshot_data->data(), snapshot_data->size(), b64_char);

engine/src/flutter/shell/common/base64.cc

@@ -116,42 +116,41 @@ Base64::Error Base64::Decode(const void* srcv,
 }
 
 size_t Base64::Encode(const void* srcv, size_t length, void* dstv) {
+  FML_DCHECK(dstv);
   const unsigned char* src = static_cast<const unsigned char*>(srcv);
   unsigned char* dst = static_cast<unsigned char*>(dstv);
 
   const char* encode = kDefaultEncode;
-  if (dst) {
-    size_t remainder = length % 3;
-    char unsigned const* const end = &src[length - remainder];
-    while (src < end) {
-      unsigned a = *src++;
-      unsigned b = *src++;
-      unsigned c = *src++;
-      int d = c & 0x3F;
-      c = (c >> 6 | b << 2) & 0x3F;
-      b = (b >> 4 | a << 4) & 0x3F;
-      a = a >> 2;
-      *dst++ = encode[a];
-      *dst++ = encode[b];
-      *dst++ = encode[c];
-      *dst++ = encode[d];
-    }
-    if (remainder > 0) {
-      int k1 = 0;
-      int k2 = EncodePad;
-      int a = (uint8_t)*src++;
-      if (remainder == 2) {
-        int b = *src++;
-        k1 = b >> 4;
-        k2 = (b << 2) & 0x3F;
-      }
-      *dst++ = encode[a >> 2];
-      *dst++ = encode[(k1 | a << 4) & 0x3F];
-      *dst++ = encode[k2];
-      *dst++ = encode[EncodePad];
-    }
+  size_t remainder = length % 3;
+  char unsigned const* const end = &src[length - remainder];
+  while (src < end) {
+    unsigned a = *src++;
+    unsigned b = *src++;
+    unsigned c = *src++;
+    int d = c & 0x3F;
+    c = (c >> 6 | b << 2) & 0x3F;
+    b = (b >> 4 | a << 4) & 0x3F;
+    a = a >> 2;
+    *dst++ = encode[a];
+    *dst++ = encode[b];
+    *dst++ = encode[c];
+    *dst++ = encode[d];
   }
-  return (length + 2) / 3 * 4;
+  if (remainder > 0) {
+    int k1 = 0;
+    int k2 = EncodePad;
+    int a = (uint8_t)*src++;
+    if (remainder == 2) {
+      int b = *src++;
+      k1 = b >> 4;
+      k2 = (b << 2) & 0x3F;
+    }
+    *dst++ = encode[a >> 2];
+    *dst++ = encode[(k1 | a << 4) & 0x3F];
+    *dst++ = encode[k2];
+    *dst++ = encode[EncodePad];
+  }
+  return EncodedSize(length);
 }
 
 }  // namespace flutter

engine/src/flutter/shell/common/base64.h

@@ -20,21 +20,27 @@ struct Base64 {
   /**
      Base64 encodes src into dst.
 
-     Normally this is called once with 'dst' nullptr to get the required size,
-     then again with an allocated 'dst' pointer to do the actual encoding.
-
-     @param dst nullptr or a pointer to a buffer large enough to receive the
-     result
+     @param dst a pointer to a buffer large enough to receive the result.
 
      @return the required length of dst for encoding.
   */
   static size_t Encode(const void* src, size_t length, void* dst);
 
+  /**
+     Returns the length of the buffer that needs to be allocated to encode
+     srcDataLength bytes.
+  */
+  static size_t EncodedSize(size_t srcDataLength) {
+    // Take the floor of division by 3 to find the number of groups that need to
+    // be encoded. Each group takes 4 bytes to be represented in base64.
+    return ((srcDataLength + 2) / 3) * 4;
+  }
+
   /**
      Base64 decodes src into dst.
 
-     Normally this is called once with 'dst' nullptr to get the required size,
-     then again with an allocated 'dst' pointer to do the actual encoding.
+     This can be called once with 'dst' nullptr to get the required size,
+     then again with an allocated 'dst' pointer to do the actual decoding.
 
      @param dst nullptr or a pointer to a buffer large enough to receive the
      result

engine/src/flutter/shell/common/base64_unittests.cc

@@ -17,6 +17,7 @@ TEST(Base64, EncodeStrings) {
     char buffer[256];
     size_t len = Base64::Encode(input.c_str(), input.length(), &buffer);
     FML_CHECK(len <= 256);
+    ASSERT_EQ(len, Base64::EncodedSize(input.length()));
     std::string actual(buffer, len);
     ASSERT_STREQ(actual.c_str(), output.c_str());
   };
@@ -34,6 +35,7 @@ TEST(Base64, EncodeBytes) {
     char buffer[512];
     size_t len = Base64::Encode(input, num, &buffer);
     FML_CHECK(len <= 512);
+    ASSERT_EQ(len, Base64::EncodedSize(num));
     std::string actual(buffer, len);
     ASSERT_STREQ(actual.c_str(), output.c_str());
   };

engine/src/flutter/shell/common/rasterizer.cc

@@ -910,9 +910,7 @@ Rasterizer::Screenshot Rasterizer::ScreenshotLastLayerTree(
   }
 
   if (base64_encode) {
-    // TODO(kjlubick) We shouldn't need to call Encode once to pre-flight the
-    // encode length. It should be ceil(4/3 * sksl.value->size()).
-    size_t b64_size = Base64::Encode(data->data(), data->size(), nullptr);
+    size_t b64_size = Base64::EncodedSize(data->size());
     auto b64_data = SkData::MakeUninitialized(b64_size);
     Base64::Encode(data->data(), data->size(), b64_data->writable_data());
     return Rasterizer::Screenshot{b64_data, layer_tree->frame_size(), format};

engine/src/flutter/shell/common/shell.cc

@@ -1840,10 +1840,7 @@ bool Shell::OnServiceProtocolGetSkSLs(
   PersistentCache* persistent_cache = PersistentCache::GetCacheForProcess();
   std::vector<PersistentCache::SkSLCache> sksls = persistent_cache->LoadSkSLs();
   for (const auto& sksl : sksls) {
-    // TODO(kjlubick) We shouldn't need to call Encode once to pre-flight the
-    // encode length. It should be ceil(4/3 * sksl.value->size()).
-    size_t b64_size =
-        Base64::Encode(sksl.value->data(), sksl.value->size(), nullptr);
+    size_t b64_size = Base64::EncodedSize(sksl.value->size());
     sk_sp<SkData> b64_data = SkData::MakeUninitialized(b64_size + 1);
     char* b64_char = static_cast<char*>(b64_data->writable_data());
     Base64::Encode(sksl.value->data(), sksl.value->size(), b64_char);