From 9daf5549287b9e3addcac00e688f5ebc2decf0d0 Mon Sep 17 00:00:00 2001
From: Felix Morgner <felix.morgner@ost.ch>
Date: Wed, 30 Sep 2026 17:29:59 +0200
Subject: kstd: ring_buffer: implement n-values ctor.

---
 libs/kstd/kstd/ring_buffer.hpp       | 39 ++++++++++++++++++++++-
 libs/kstd/kstd/ring_buffer.tests.cpp | 61 ++++++++++++++++++++++++++++++++++++
 2 files changed, 99 insertions(+), 1 deletion(-)

(limited to 'libs/kstd')

diff --git a/libs/kstd/kstd/ring_buffer.hpp b/libs/kstd/kstd/ring_buffer.hpp
index 6a152731..61fbf360 100644
--- a/libs/kstd/kstd/ring_buffer.hpp
+++ b/libs/kstd/kstd/ring_buffer.hpp
@@ -3,8 +3,12 @@
 
 #include <kstd/os/error.hpp>
 
+#include <algorithm>
 #include <array>
 #include <cstddef>
+#include <memory>
+#include <ranges>
+#include <variant>
 
 namespace kstd
 {
@@ -23,6 +27,27 @@ namespace kstd
     using const_reference = value_type const &;
     using size_type = std::size_t;
 
+    //! @name Special Member Functions
+    //! @{
+
+    //! Construct an empty ring buffer.
+    constexpr ring_buffer() noexcept = default;
+
+    //! Construct a ring buffer with a given number of copies of a given value.
+    constexpr ring_buffer(size_type count, ValueType const & value)
+    {
+      if (count > capacity())
+      {
+        os::panic("[KSTD] Tried to construct a ring buffer with more elements than it can support.");
+      }
+
+      std::ranges::for_each(std::views::iota(0uz, count),
+                            [&](auto i) { std::construct_at(&m_storage[i].value, value); });
+      m_write_index = count % capacity();
+    }
+
+    //! @}
+
     //! @name Element Access
     //! @{
 
@@ -94,6 +119,18 @@ namespace kstd
     //! @}
 
   private:
+    //! A helper type for value storage to enable constexpr construction in C++26.
+    union value_storage
+    {
+      std::monostate empty;
+      value_type value;
+
+      constexpr value_storage() noexcept
+          : empty{}
+      {}
+      constexpr ~value_storage() noexcept {}
+    };
+
     //! Get a pointer to the element at the given read-index relative position.
     //!
     //! @param position The logical (read-index relative) index of the element.
@@ -124,7 +161,7 @@ namespace kstd
     }
 
     //! The underlying storage for the elements of this ring buffer.
-    alignas(ValueType) std::array<std::byte, sizeof(value_type) * Capacity> m_storage{};
+    alignas(ValueType) std::array<value_storage, Capacity> m_storage{};
     //! The index of the next element to be written.
     size_type m_write_index{};
     //! The index of the first element to be read.
diff --git a/libs/kstd/kstd/ring_buffer.tests.cpp b/libs/kstd/kstd/ring_buffer.tests.cpp
index 86ff4b30..d2515289 100644
--- a/libs/kstd/kstd/ring_buffer.tests.cpp
+++ b/libs/kstd/kstd/ring_buffer.tests.cpp
@@ -34,6 +34,31 @@ SCENARIO("Ring Buffer initialization and construction", "[ring_buffer]")
         REQUIRE(buffer.max_size() == buffer.capacity());
       }
     }
+
+    WHEN("constructing by n-copies-of-value constructor using a capacity of 5")
+    {
+      auto buffer = kstd::ring_buffer<char, 5>{3, 'a'};
+
+      THEN("the capacity is 5")
+      {
+        REQUIRE(buffer.capacity() == 5);
+      }
+
+      THEN("the buffer is not empty")
+      {
+        REQUIRE_FALSE(buffer.empty());
+      }
+
+      THEN("the size is equal to the number of copies")
+      {
+        REQUIRE(buffer.size() == 3);
+      }
+
+      THEN("the maximum size is equal to the capacity")
+      {
+        REQUIRE(buffer.max_size() == buffer.capacity());
+      }
+    }
   }
 }
 
@@ -60,4 +85,40 @@ SCENARIO("Ring Buffer element access", "[kstd][ring_buffer]")
       }
     }
   }
+
+  GIVEN("A ring buffer with a capacity of 5 containing 3 copies of the letter 'a'")
+  {
+    auto buffer = kstd::ring_buffer<char, 5>{3, 'a'};
+
+    THEN("accessing the first 3 element return 'a'")
+    {
+      REQUIRE(buffer.at(0) == 'a');
+      REQUIRE(buffer.at(1) == 'a');
+      REQUIRE(buffer.at(2) == 'a');
+    }
+
+    THEN("accessing the fourth element panics")
+    {
+      REQUIRE_THROWS_MATCHES(buffer.at(3), kstd::tests::os_panic,
+                             Catch::Matchers::Message("[KSTD] Index out-of-bounds in ring_buffer element access!"));
+    }
+
+    WHEN("working through a const reference")
+    {
+      auto const & ref = buffer;
+
+      THEN("accessing the first 3 element return 'a'")
+      {
+        REQUIRE(ref.at(0) == 'a');
+        REQUIRE(ref.at(1) == 'a');
+        REQUIRE(ref.at(2) == 'a');
+      }
+
+      THEN("accessing the fourth element panics")
+      {
+        REQUIRE_THROWS_MATCHES(ref.at(3), kstd::tests::os_panic,
+                               Catch::Matchers::Message("[KSTD] Index out-of-bounds in ring_buffer element access!"));
+      }
+    }
+  }
 }
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