Optimize numeric array conversion and serialize directly into Python bytes

capnp/helpers/serialize.h

@@ -0,0 +1,34 @@
+// Copyright (c) 2026 IQ.Lvbs. All rights reserved.
+#pragma once
+
+#include <Python.h>
+#include <capnp/message.h>
+#include <capnp/serialize.h>
+#include <kj/debug.h>
+#include <cstring>
+
+inline PyObject* messageToPythonBytes(capnp::MessageBuilder& builder) {
+  auto segments = builder.getSegmentsForOutput();
+  auto words = capnp::computeSerializedSizeInWords(segments);
+  KJ_REQUIRE(words <= static_cast<size_t>(PY_SSIZE_T_MAX) / sizeof(capnp::word));
+  PyObject* result = PyBytes_FromStringAndSize(nullptr, words * sizeof(capnp::word));
+  if (result == nullptr) return nullptr;
+  char* data = PyBytes_AS_STRING(result);
+  auto write32 = [data](size_t index, uint32_t value) {
+    for (unsigned int byte = 0; byte < 4; ++byte) {
+      data[index * 4 + byte] = static_cast<char>(value >> (byte * 8));
+    }
+  };
+  write32(0, segments.size() - 1);
+  for (size_t index = 0; index < segments.size(); ++index) {
+    write32(index + 1, segments[index].size());
+  }
+  if (segments.size() % 2 == 0) write32(segments.size() + 1, 0);
+  char* output = data + (segments.size() / 2 + 1) * sizeof(capnp::word);
+  for (auto segment : segments) {
+    auto size = segment.size() * sizeof(capnp::word);
+    std::memcpy(output, segment.begin(), size);
+    output += size;
+  }
+  return result;
+}