From 30ee75c0cf1f62f5379f372d6e3f86f26d76ecb7 Mon Sep 17 00:00:00 2001
From: Dmitriy Shilin <dshil@fastmail.com>
Date: Wed, 28 Jan 2026 15:24:00 +0100
Subject: [PATCH] Allow to reserve space for byte writer

---
 components/ocs_core/byte_writer.cpp           |  7 ++++++
 components/ocs_core/byte_writer.h             |  6 +++++
 components/ocs_core/test/test_byte_writer.cpp | 23 +++++++++++++++++++
 3 files changed, 36 insertions(+)

diff --git a/components/ocs_core/byte_writer.cpp b/components/ocs_core/byte_writer.cpp
index e731d912..0532ca43 100644
--- a/components/ocs_core/byte_writer.cpp
+++ b/components/ocs_core/byte_writer.cpp
@@ -39,6 +39,13 @@ size_t ByteWriter::write(const uint8_t* data, size_t size) {
     return ret;
 }
 
+size_t ByteWriter::reserve(size_t size) {
+    const size_t ret = std::min(size, left_());
+    offset_ += ret;
+
+    return ret;
+}
+
 size_t ByteWriter::left_() const {
     return size_ - offset_;
 }
diff --git a/components/ocs_core/byte_writer.h b/components/ocs_core/byte_writer.h
index b39451e4..d49abf5a 100644
--- a/components/ocs_core/byte_writer.h
+++ b/components/ocs_core/byte_writer.h
@@ -33,6 +33,12 @@ class ByteWriter : private core::NonCopyable<> {
     //! Write @p size bytes of @p data.
     size_t write(const uint8_t* data, size_t size);
 
+    //! Reserve space for @p size bytes.
+    //!
+    //! @return
+    //!  Number of reserved bytes.
+    size_t reserve(size_t size);
+
     //! Write any integer value.
     template <typename T> bool write(const T& t) {
         static_assert(std::is_integral<T>::value, "require integral type");
diff --git a/components/ocs_core/test/test_byte_writer.cpp b/components/ocs_core/test/test_byte_writer.cpp
index 465c744c..9d81d6da 100644
--- a/components/ocs_core/test/test_byte_writer.cpp
+++ b/components/ocs_core/test/test_byte_writer.cpp
@@ -67,5 +67,28 @@ TEST_CASE("Byte writer: write array: by parts", "[ocs_core], [byte_writer]") {
     TEST_ASSERT_TRUE(memcmp(want_buf, writer.get_data(), sizeof(want_buf)) == 0);
 }
 
+TEST_CASE("Byte writer: reserve space: in range", "[ocs_core], [byte_writer]") {
+    uint8_t write_buf[2];
+    memset(write_buf, 7, sizeof(write_buf));
+
+    ByteWriter writer(write_buf, sizeof(write_buf));
+
+    TEST_ASSERT_EQUAL(1, writer.reserve(1));
+    TEST_ASSERT_TRUE(writer.write(static_cast<uint8_t>(2)));
+    TEST_ASSERT_FALSE(writer.write(static_cast<uint8_t>(2)));
+    TEST_ASSERT_EQUAL(7, write_buf[0]);
+    TEST_ASSERT_EQUAL(2, write_buf[1]);
+}
+
+TEST_CASE("Byte writer: reserve space: no space left", "[ocs_core], [byte_writer]") {
+    uint8_t write_buf[2];
+    memset(write_buf, 7, sizeof(write_buf));
+
+    ByteWriter writer(write_buf, sizeof(write_buf));
+
+    TEST_ASSERT_EQUAL(writer.get_cap(), writer.reserve(writer.get_cap() + 1));
+    TEST_ASSERT_EQUAL(0, writer.reserve(1));
+}
+
 } // namespace core
 } // namespace ocs