From fe8c11dc9471085f6baec7c6a1770181c04dcfae Mon Sep 17 00:00:00 2001
From: Jason Volk <jason@zemos.net>
Date: Tue, 12 Jul 2022 11:35:52 -0700
Subject: [PATCH] ircd::util: Add a constexpr bitset.

---
 include/ircd/util/bitset.h | 143 +++++++++++++++++++++++++++++++++++++
 include/ircd/util/util.h   |   1 +
 2 files changed, 144 insertions(+)
 create mode 100644 include/ircd/util/bitset.h

diff --git a/include/ircd/util/bitset.h b/include/ircd/util/bitset.h
new file mode 100644
index 000000000..019be346e
--- /dev/null
+++ b/include/ircd/util/bitset.h
@@ -0,0 +1,143 @@
+// Matrix Construct
+//
+// Copyright (C) Matrix Construct Developers, Authors & Contributors
+// Copyright (C) 2016-2022 Jason Volk <jason@zemos.net>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice is present in all copies. The
+// full license for this software is available in the LICENSE file.
+
+#pragma once
+#define HAVE_IRCD_UTIL_BITSET_H
+
+namespace ircd {
+inline namespace util
+{
+	template<size_t>
+	struct bitset;
+}}
+
+template<size_t N>
+struct ircd::util::bitset
+{
+	using word = uint8_t;
+
+	static constexpr size_t word_bits
+	{
+		sizeof(word) * 8
+	};
+
+	static constexpr size_t words
+	{
+		N / word_bits?: 1
+	};
+
+  private:
+	word buf[words] {0};
+
+  public:
+	constexpr size_t size() const
+	{
+		return N;
+	}
+
+	constexpr bool test(const size_t pos) const;
+	constexpr size_t count() const;
+
+	constexpr void reset(const size_t pos);
+	constexpr void reset();
+	constexpr void set(const size_t pos, const bool val = true);
+	constexpr void set();
+	constexpr void flip(const size_t pos);
+	constexpr void flip();
+
+	constexpr bitset(const uint128_t val);
+	constexpr bitset() = default;
+};
+
+template<size_t N>
+constexpr
+ircd::util::bitset<N>::bitset(const uint128_t val)
+{
+	const auto bits
+	{
+		std::min(sizeof(buf) * 8, sizeof(val) * 8)
+	};
+
+	size_t i{0};
+	for(; i < bits; ++i)
+	{
+		const decltype(val) one{1};
+		set(i, val & (one << i));
+	}
+
+	for(; i < size(); ++i)
+		set(i, false);
+}
+
+template<size_t N>
+constexpr void
+ircd::util::bitset<N>::flip()
+{
+	for(size_t i(0); i < words; ++i)
+		buf[i] =~ buf[i];
+}
+
+template<size_t N>
+constexpr void
+ircd::util::bitset<N>::flip(const size_t pos)
+{
+	buf[pos / 8] ^= word(1) << (pos % 8);
+}
+
+template<size_t N>
+constexpr void
+ircd::util::bitset<N>::set()
+{
+	for(size_t i(0); i < words; ++i)
+		buf[i] = ~word{0};
+}
+
+template<size_t N>
+constexpr void
+ircd::util::bitset<N>::set(const size_t pos,
+                           const bool val)
+{
+	buf[pos / 8] |= word(val) << (pos % 8);
+}
+
+template<size_t N>
+constexpr void
+ircd::util::bitset<N>::reset()
+{
+	for(size_t i(0); i < words; ++i)
+		buf[i] = 0;
+}
+
+template<size_t N>
+constexpr void
+ircd::util::bitset<N>::reset(const size_t pos)
+{
+	buf[pos / 8] &= ~(word(1) << (pos % 8));
+}
+
+template<size_t N>
+constexpr size_t
+ircd::util::bitset<N>::count()
+const
+{
+	size_t ret(0);
+	for(size_t i(0); i < words; ++i)
+		ret += std::popcount(buf[i]);
+
+	return ret;
+}
+
+template<size_t N>
+constexpr bool
+ircd::util::bitset<N>::test(const size_t pos)
+const
+{
+	return buf[pos / 8] & (word(1) << (pos % 8));
+}
diff --git a/include/ircd/util/util.h b/include/ircd/util/util.h
index 7a64dab26..53d477958 100644
--- a/include/ircd/util/util.h
+++ b/include/ircd/util/util.h
@@ -65,6 +65,7 @@ namespace ircd
 #include "maybe.h"
 #include "all.h"
 #include "compare_exchange.h"
+#include "bitset.h"
 
 // Unsorted section
 namespace ircd {