perf: Optimize mutable mesh partition lookup and bulk vertex copying

ink/geometry/mutable_mesh.h

@@ -301,6 +301,9 @@ class MutableMesh {
   // Returns the raw data of the mesh's vertices.
   absl::Span<const std::byte> RawVertexData() const { return vertex_data_; }
 
+  // Returns the mutable raw data of the mesh's vertices.
+  absl::Span<std::byte> MutableRawVertexData() { return absl::MakeSpan(vertex_data_); }
+
   // Returns the number of bytes used to represent a vertex in this mesh. This
   // is equivalent to:
   //   mesh.Format().UnpackedVertexStride();

ink/strokes/internal/mutable_multi_mesh.cc

@@ -1,8 +1,11 @@
 #include "ink/strokes/internal/mutable_multi_mesh.h"
 
+#include <algorithm>
 #include <array>
 #include <cstddef>
 #include <cstdint>
+#include <cstring>
+#include <iterator>
 #include <limits>
 #include <vector>
 
@@ -247,17 +250,18 @@ void MutableMultiMesh::AddNewPartition() {
 
 TriangleIndexPair MutableMultiMesh::GetPartitionTriangle(
     uint32_t triangle_index) const {
-  // TODO: b/295166196 - Consider using a binary search here.
-  for (int partition_index = partitions_.size() - 1; partition_index >= 0;
-       --partition_index) {
-    uint32_t previous_triangle_count =
-        partitions_[partition_index].previous_triangle_count;
-    if (triangle_index >= previous_triangle_count) {
-      return TriangleIndexPair{
-          static_cast<uint16_t>(partition_index),
-          static_cast<uint16_t>(triangle_index - previous_triangle_count),
-      };
-    }
+  auto it = std::upper_bound(
+      partitions_.begin(), partitions_.end(), triangle_index,
+      [](uint32_t val, const Partition& p) {
+        return val < p.previous_triangle_count;
+      });
+  if (it != partitions_.begin()) {
+    --it;
+    uint32_t partition_index = std::distance(partitions_.begin(), it);
+    return TriangleIndexPair{
+        static_cast<uint16_t>(partition_index),
+        static_cast<uint16_t>(triangle_index - it->previous_triangle_count),
+    };
   }
   ABSL_CHECK(false) << "triangle_index out of bounds";
 }
@@ -291,13 +295,11 @@ uint16_t MutableMultiMesh::CopyVertexIntoPartition(uint32_t vertex_index,
       VertexIndexPair{partition_index, mesh_vertex_index});
   partitions_[partition_index].vertex_indices.push_back(vertex_index);
   mesh.AppendVertex(other_mesh.VertexPosition(other_mesh_vertex_index));
-  // TODO: b/306149329 - Investigate memcpy-ing the vertex data instead of
-  // repeatedly calling `SetFloatVertexAttribute()`.
-  for (size_t i = 0; i < format_.Attributes().size(); ++i) {
-    mesh.SetFloatVertexAttribute(
-        mesh_vertex_index, i,
-        other_mesh.FloatVertexAttribute(other_mesh_vertex_index, i));
-  }
+  std::memcpy(
+      mesh.MutableRawVertexData().data() + mesh_vertex_index * mesh.VertexStride(),
+      other_mesh.RawVertexData().data() +
+          other_mesh_vertex_index * other_mesh.VertexStride(),
+      mesh.VertexStride());
   return mesh_vertex_index;
 }