From be469405df134f3b9a13020b64873acf1226453f Mon Sep 17 00:00:00 2001
From: Jonathan G Rennison <j.g.rennison@gmail.com>
Date: Sun, 28 Jan 2024 15:41:50 +0000
Subject: [PATCH] Bitmath: Add utility functions for calculating bit masks

Add tests
---
 src/core/bitmath_func.hpp  | 29 +++++++++++++++++++++++++++++
 src/tests/bitmath_func.cpp | 19 +++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/src/core/bitmath_func.hpp b/src/core/bitmath_func.hpp
index 12bd02138d..9d3b91a883 100644
--- a/src/core/bitmath_func.hpp
+++ b/src/core/bitmath_func.hpp
@@ -187,6 +187,35 @@ inline T ToggleBit(T &x, const uint8_t y)
 	return x = (T)(x ^ ((T)1U << y));
 }
 
+/**
+ * Return a bit mask of count bits starting at start.
+ *
+ * @param start The start bit
+ * @param count The number of bits
+ * @pre start + count < sizeof(T) * 8
+ * @return The bit mask
+ */
+template <typename T>
+constexpr T GetBitMaskSC(const uint8_t start, const uint8_t count)
+{
+	typename std::make_unsigned<T>::type mask = 1;
+	return (T)(((mask << count) - 1) << start);
+}
+
+/**
+ * Return a bit mask of bits from first to last (inclusive).
+ *
+ * @param first The first bit
+ * @param last The last bits (inclusive)
+ * @pre first <= last && last < sizeof(T) * 8
+ * @return The bit mask
+ */
+template <typename T>
+constexpr T GetBitMaskFL(const uint8_t first, const uint8_t last)
+{
+	return GetBitMaskSC<T>(first, 1 + last - first);
+}
+
 /**
  * Search the first set bit in a value.
  * When no bit is set, it returns 0.
diff --git a/src/tests/bitmath_func.cpp b/src/tests/bitmath_func.cpp
index 8aaf3eae09..7b2fb9c21c 100644
--- a/src/tests/bitmath_func.cpp
+++ b/src/tests/bitmath_func.cpp
@@ -68,3 +68,22 @@ TEST_CASE("SetBitIterator tests")
 	CHECK(test_case(INT32_MIN, { 31 }));
 	CHECK(test_case(INT64_MIN, { 63 }));
 }
+
+TEST_CASE("GetBitMaskSC tests")
+{
+	CHECK(GetBitMaskSC<uint>(4, 4) == 0xF0);
+	CHECK(GetBitMaskSC<uint>(28, 4) == 0xF0000000);
+	CHECK(GetBitMaskSC<uint8_t>(7, 1) == 0x80);
+	CHECK(GetBitMaskSC<uint8_t>(0, 1) == 1);
+	CHECK(GetBitMaskSC<uint8_t>(0, 0) == 0);
+	CHECK(GetBitMaskSC<uint8_t>(7, 0) == 0);
+}
+
+TEST_CASE("GetBitMaskFL tests")
+{
+	CHECK(GetBitMaskFL<uint>(4, 7) == 0xF0);
+	CHECK(GetBitMaskFL<uint>(28, 31) == 0xF0000000);
+	CHECK(GetBitMaskFL<uint8_t>(7, 7) == 0x80);
+	CHECK(GetBitMaskFL<uint8_t>(0, 0) == 1);
+	CHECK(GetBitMaskFL<uint8_t>(3, 4) == 0x18);
+}