From ca276ca5471a4e7a137f68a81feb150282eae62f Mon Sep 17 00:00:00 2001
From: Dan Goodliffe <dan@randomdan.homeip.net>
Date: Wed, 7 Feb 2024 23:50:50 +0000
Subject: Add stripiter

A generic iterator wrapper returning a tuple of 3 references to the original values, as
processed in the fashion of an OpenGL triangle strip.
---
 lib/collections.h | 60 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 test/test-lib.cpp | 20 +++++++++++++++++++
 2 files changed, 80 insertions(+)

diff --git a/lib/collections.h b/lib/collections.h
index 943b986..6cee10c 100644
--- a/lib/collections.h
+++ b/lib/collections.h
@@ -2,6 +2,7 @@
 
 #include <algorithm>
 #include <array>
+#include <cstdint>
 #include <span>
 #include <utility>
 #include <vector>
@@ -160,3 +161,62 @@ template<typename T> struct pair_range {
 };
 
 template<typename T> pair_range(std::pair<T, T>) -> pair_range<T>;
+
+template<typename iter> struct stripiter {
+	[[nodiscard]] constexpr bool
+	operator!=(const stripiter & other) const
+	{
+		return current != other.current;
+	}
+
+	[[nodiscard]] constexpr bool
+	operator==(const stripiter & other) const
+	{
+		return current == other.current;
+	}
+
+	constexpr stripiter &
+	operator++()
+	{
+		++current;
+		off = 1 - off;
+		return *this;
+	}
+
+	constexpr stripiter &
+	operator--()
+	{
+		--current;
+		off = 1 - off;
+		return *this;
+	}
+
+	constexpr auto
+	operator-(const stripiter & other) const
+	{
+		return current - other.current;
+	}
+
+	constexpr auto
+	operator*() const
+	{
+		return std::tie(*(current - (2 - off)), *(current - off - 1), *current);
+	}
+
+	iter current;
+	uint8_t off {};
+};
+
+template<typename T> struct std::iterator_traits<stripiter<T>> : std::iterator_traits<T> { };
+
+constexpr auto
+strip_begin(IterableCollection auto & cont)
+{
+	return stripiter {cont.begin() + 2};
+}
+
+constexpr auto
+strip_end(IterableCollection auto & cont)
+{
+	return stripiter {cont.end()};
+}
diff --git a/test/test-lib.cpp b/test/test-lib.cpp
index 58b769a..5f0b5e5 100644
--- a/test/test-lib.cpp
+++ b/test/test-lib.cpp
@@ -4,6 +4,7 @@
 #include <boost/test/data/test_case.hpp>
 #include <boost/test/unit_test.hpp>
 
+#include <collections.h>
 #include <glArrays.h>
 #include <glad/gl.h>
 #include <set>
@@ -49,3 +50,22 @@ BOOST_AUTO_TEST_CASE(generate_move_and_delete)
 	}
 	BOOST_CHECK(active.empty());
 }
+
+constexpr std::array TRIANGLE_STRIP_IN {0, 1, 2, 3, 4, 5};
+static_assert(std::distance(strip_begin(TRIANGLE_STRIP_IN), strip_end(TRIANGLE_STRIP_IN)) == 4);
+
+BOOST_AUTO_TEST_CASE(triangle_strip_iter)
+{
+	constexpr std::array TRIANGLE_STRIP_EXPECTED {0, 1, 2, 2, 1, 3, 2, 3, 4, 4, 3, 5};
+
+	std::vector<int> out;
+	std::for_each(strip_begin(TRIANGLE_STRIP_IN), strip_end(TRIANGLE_STRIP_IN), [&out](const auto & t) {
+		const auto [a, b, c] = t;
+		out.push_back(a);
+		out.push_back(b);
+		out.push_back(c);
+	});
+	BOOST_REQUIRE_EQUAL(out.size(), (TRIANGLE_STRIP_IN.size() - 2) * 3);
+	BOOST_CHECK_EQUAL_COLLECTIONS(
+			out.begin(), out.end(), TRIANGLE_STRIP_EXPECTED.begin(), TRIANGLE_STRIP_EXPECTED.end());
+}
-- 
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