From 38547fe8ef1595726706ee3ecc7d33aaf352fbf7 Mon Sep 17 00:00:00 2001
From: Dan Goodliffe <dan@randomdan.homeip.net>
Date: Sat, 29 Jun 2024 17:55:32 +0100
Subject: Implement partition on InstanceVertices

---
 gfx/gl/instanceVertices.h | 23 +++++++++++++++++++++++
 test/test-instancing.cpp  | 30 ++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+)

diff --git a/gfx/gl/instanceVertices.h b/gfx/gl/instanceVertices.h
index d9d9123..ad1716d 100644
--- a/gfx/gl/instanceVertices.h
+++ b/gfx/gl/instanceVertices.h
@@ -124,6 +124,13 @@ public:
 		return base::size();
 	}
 
+	template<typename Pred>
+	glContainer<T>::iterator
+	partition(Pred pred)
+	{
+		return partition(base::begin(), base::end(), pred);
+	}
+
 protected:
 	friend InstanceProxy;
 
@@ -152,6 +159,22 @@ protected:
 		return base::data()[index[pindex]];
 	}
 
+	template<typename Pred>
+	glContainer<T>::iterator
+	partition(glContainer<T>::iterator first, glContainer<T>::iterator last, Pred pred)
+	{
+		while (first < last) {
+			first = std::find_if_not(first, last, pred);
+			last = --std::find_if(std::make_reverse_iterator(last), std::make_reverse_iterator(first), pred).base();
+			if (first < last) {
+				std::iter_swap(first, last);
+				std::iter_swap(std::find(index.begin(), index.end(), first - base::begin()),
+						std::find(index.begin(), index.end(), last - base::begin()));
+			}
+		}
+		return first;
+	}
+
 	//  Index into buffer given to nth proxy
 	std::vector<size_t> index;
 	//  List of free spaces in index
diff --git a/test/test-instancing.cpp b/test/test-instancing.cpp
index c1860a4..37f32c8 100644
--- a/test/test-instancing.cpp
+++ b/test/test-instancing.cpp
@@ -174,4 +174,34 @@ BOOST_DATA_TEST_CASE(shuffle_random, boost::unit_test::data::xrange(0, 10), x)
 	}
 }
 
+BOOST_AUTO_TEST_CASE(partition_by, *boost::unit_test::timeout(1))
+{
+	std::mt19937 gen(std::random_device {}());
+	std::uniform_int_distribution<int> dist(0, 100000);
+	static constexpr auto N = 1000;
+	reserve(N);
+	std::vector<decltype(acquire(0))> instances;
+	instances.reserve(N);
+	// At least one of each
+	instances.push_back(acquire(1));
+	instances.push_back(acquire(3));
+	while (instances.size() < N) {
+		instances.push_back(acquire(dist(gen)));
+	}
+	const std::vector<int> values(instances.begin(), instances.end());
+	BOOST_REQUIRE_EQUAL(size(), N);
+
+	const auto pred = [](auto x) {
+		return (x % 3) == 0;
+	};
+	auto matchedEnd = partition(pred);
+	// The underlying data is partitioned...
+	BOOST_REQUIRE(std::is_partitioned(mkcspan().cbegin(), mkcspan().cend(), pred));
+	// The external view of the data is unchanged...
+	BOOST_CHECK_EQUAL_COLLECTIONS(values.cbegin(), values.cend(), instances.cbegin(), instances.cend());
+	// The partition point is right...
+	BOOST_CHECK(!pred(*matchedEnd));
+	BOOST_CHECK(pred(*--matchedEnd));
+}
+
 BOOST_AUTO_TEST_SUITE_END()
-- 
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