From 85bf771ea6d8d584f1c735cc963e17571ca0a0d1 Mon Sep 17 00:00:00 2001
From: Dan Goodliffe <dan@randomdan.homeip.net>
Date: Mon, 17 Apr 2023 11:54:07 +0100
Subject: First cut of instance vertices and proxy

Untested outside unit test, allows the use of a glBuffer as a storage container. To be combined
with a vertex array and/or mesh etc for massing drawing with glDrawElementsInstanced
---
 gfx/gl/instanceVertices.h | 147 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 147 insertions(+)
 create mode 100644 gfx/gl/instanceVertices.h

(limited to 'gfx/gl/instanceVertices.h')

diff --git a/gfx/gl/instanceVertices.h b/gfx/gl/instanceVertices.h
new file mode 100644
index 0000000..cf3b75b
--- /dev/null
+++ b/gfx/gl/instanceVertices.h
@@ -0,0 +1,147 @@
+#pragma once
+
+#include "glArrays.h"
+#include <iterator>
+#include <span>
+#include <special_members.hpp>
+#include <utility>
+#include <vector>
+
+template<typename T> class InstanceVertices {
+public:
+	InstanceVertices(size_t initialSize = 16) : data {allocBuffer(buffer, initialSize)}, next {} { }
+
+	~InstanceVertices()
+	{
+		glUnmapNamedBuffer(buffer);
+	}
+
+	class InstanceProxy {
+	public:
+		InstanceProxy(InstanceVertices * iv, std::size_t idx) : instances {iv}, index {idx} { }
+		InstanceProxy(InstanceProxy && other) : instances {std::exchange(other.instances, nullptr)}, index {other.index}
+		{
+		}
+		NO_COPY(InstanceProxy);
+
+		~InstanceProxy()
+		{
+			if (instances) {
+				instances->release(*this);
+			}
+		}
+
+		InstanceProxy &
+		operator=(InstanceProxy && other)
+		{
+			instances = std::exchange(other.instances, nullptr);
+			index = other.index;
+		}
+
+		operator T &()
+		{
+			return *get();
+		}
+		operator const T &() const
+		{
+			return *get();
+		}
+		template<typename U>
+		T &
+		operator=(U && v)
+		{
+			return instances->data[index] = std::forward<U>(v);
+		}
+
+		T *
+		get()
+		{
+			return &instances->data[index];
+		}
+		const T *
+		get() const
+		{
+			return &instances->data[index];
+		}
+		T *
+		operator->()
+		{
+			return get();
+		}
+		const T *
+		operator->() const
+		{
+			return get();
+		}
+		T &
+		operator*()
+		{
+			return *get();
+		}
+		const T &
+		operator*() const
+		{
+			return *get();
+		}
+
+	private:
+		friend InstanceVertices;
+
+		InstanceVertices<T> * instances;
+		std::size_t index;
+	};
+
+	template<typename... Params>
+	InstanceProxy
+	acquire(Params &&... params)
+	{
+		if (!unused.empty()) {
+			auto idx = unused.back();
+			unused.pop_back();
+			new (&data[idx]) T(std::forward<Params>(params)...);
+			return InstanceProxy {this, idx};
+		}
+		if (next >= data.size()) {
+			resize(data.size() * 2);
+		}
+		new (&data[next]) T(std::forward<Params>(params)...);
+		return InstanceProxy {this, next++};
+	}
+
+	void
+	release(const InstanceProxy & p)
+	{
+		data[p.index].~T();
+		unused.push_back(p.index);
+	}
+
+protected:
+	friend InstanceProxy;
+
+	static std::span<T>
+	allocBuffer(const glBuffer & buffer, std::size_t count)
+	{
+		glBindBuffer(GL_ARRAY_BUFFER, buffer);
+		glBufferData(GL_ARRAY_BUFFER, static_cast<GLsizeiptr>(sizeof(T) * count), nullptr, GL_DYNAMIC_DRAW);
+		auto data = static_cast<T *>(glMapNamedBuffer(buffer, GL_READ_WRITE));
+		glBindBuffer(GL_ARRAY_BUFFER, 0);
+		return {data, count};
+	}
+
+	void
+	resize(size_t newCapacity)
+	{
+		const auto maintain = std::min(newCapacity, data.size());
+		const auto maintaind = static_cast<typename decltype(data)::difference_type>(maintain);
+		std::vector<T> existing;
+		existing.reserve(maintain);
+		std::move(data.begin(), data.begin() + maintaind, std::back_inserter(existing));
+		data = allocBuffer(buffer, newCapacity);
+		std::move(existing.begin(), existing.begin() + maintaind, data.begin());
+	}
+
+	glBuffer buffer;
+	std::span<T> data;
+	std::size_t next;
+	std::vector<size_t> unused;
+};
-- 
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