Add paint renderer compositor tests

src/paint_renderer/compositor.cpp

@@ -0,0 +1,65 @@
+#include "paint_renderer/compositor.h"
+
+#include <limits>
+
+namespace pp::paint_renderer {
+
+namespace {
+
+[[nodiscard]] pp::foundation::Result<std::size_t> expected_pixel_count(pp::renderer::Extent2D extent) noexcept
+{
+    const auto extent_status = pp::renderer::validate_extent(extent);
+    if (!extent_status.ok()) {
+        return pp::foundation::Result<std::size_t>::failure(extent_status);
+    }
+
+    const auto width = static_cast<std::uint64_t>(extent.width);
+    const auto height = static_cast<std::uint64_t>(extent.height);
+    if (width > std::numeric_limits<std::uint64_t>::max() / height) {
+        return pp::foundation::Result<std::size_t>::failure(
+            pp::foundation::Status::out_of_range("pixel count overflows uint64"));
+    }
+
+    const auto count = width * height;
+    if (count > static_cast<std::uint64_t>(std::numeric_limits<std::size_t>::max())) {
+        return pp::foundation::Result<std::size_t>::failure(
+            pp::foundation::Status::out_of_range("pixel count exceeds addressable memory"));
+    }
+
+    return pp::foundation::Result<std::size_t>::success(static_cast<std::size_t>(count));
+}
+
+}
+
+pp::foundation::Status composite_layer(
+    std::span<pp::paint::Rgba> destination,
+    pp::renderer::Extent2D extent,
+    LayerCompositeView layer) noexcept
+{
+    const auto pixel_count = expected_pixel_count(extent);
+    if (!pixel_count) {
+        return pixel_count.status();
+    }
+
+    if (destination.size() != pixel_count.value() || layer.pixels.size() != pixel_count.value()) {
+        return pp::foundation::Status::invalid_argument("composite buffers must match the render extent");
+    }
+
+    if (layer.opacity < 0.0F || layer.opacity > 1.0F) {
+        return pp::foundation::Status::out_of_range("layer opacity must be between 0 and 1");
+    }
+
+    if (!layer.visible || layer.opacity == 0.0F) {
+        return pp::foundation::Status::success();
+    }
+
+    for (std::size_t i = 0; i < destination.size(); ++i) {
+        auto stroke = layer.pixels[i];
+        stroke.a *= layer.opacity;
+        destination[i] = pp::paint::blend_pixels(destination[i], stroke, layer.blend_mode);
+    }
+
+    return pp::foundation::Status::success();
+}
+
+}