From 8faaedeff9445b78841c145dc8cef24b5d30e2f7 Mon Sep 17 00:00:00 2001
From: Rubidium <rubidium@openttd.org>
Date: Thu, 18 Jan 2024 18:40:52 +0100
Subject: [PATCH] Codechange: replace hand written function to find first/last
 bit with C++ variant

---
 src/core/bitmath_func.cpp | 56 ---------------------------------------
 src/core/bitmath_func.hpp | 31 ++++++++++++++++++++--
 src/graph_gui.cpp         |  2 +-
 src/signal.cpp            |  2 +-
 4 files changed, 31 insertions(+), 60 deletions(-)

diff --git a/src/core/bitmath_func.cpp b/src/core/bitmath_func.cpp
index 09f2a9cb43..e108d96794 100644
--- a/src/core/bitmath_func.cpp
+++ b/src/core/bitmath_func.cpp
@@ -22,59 +22,3 @@ const uint8_t _ffb_64[64] = {
 	4,  0,  1,  0,  2,  0,  1,  0,
 	3,  0,  1,  0,  2,  0,  1,  0,
 };
-
-/**
- * Search the first set bit in a 64 bit variable.
- *
- * This algorithm is a static implementation of a log
- * congruence search algorithm. It checks the first half
- * if there is a bit set search there further. And this
- * way further. If no bit is set return 0.
- *
- * @param x The value to search
- * @return The position of the first bit set
- */
-uint8_t FindFirstBit(uint64_t x)
-{
-	if (x == 0) return 0;
-	/* The macro FIND_FIRST_BIT is better to use when your x is
-	  not more than 128. */
-
-	uint8_t pos = 0;
-
-	if ((x & 0xffffffffULL) == 0) { x >>= 32; pos += 32; }
-	if ((x & 0x0000ffffULL) == 0) { x >>= 16; pos += 16; }
-	if ((x & 0x000000ffULL) == 0) { x >>= 8;  pos += 8;  }
-	if ((x & 0x0000000fULL) == 0) { x >>= 4;  pos += 4;  }
-	if ((x & 0x00000003ULL) == 0) { x >>= 2;  pos += 2;  }
-	if ((x & 0x00000001ULL) == 0) { pos += 1; }
-
-	return pos;
-}
-
-/**
- * Search the last set bit in a 64 bit variable.
- *
- * This algorithm is a static implementation of a log
- * congruence search algorithm. It checks the second half
- * if there is a bit set search there further. And this
- * way further. If no bit is set return 0.
- *
- * @param x The value to search
- * @return The position of the last bit set
- */
-uint8_t FindLastBit(uint64_t x)
-{
-	if (x == 0) return 0;
-
-	uint8_t pos = 0;
-
-	if ((x & 0xffffffff00000000ULL) != 0) { x >>= 32; pos += 32; }
-	if ((x & 0x00000000ffff0000ULL) != 0) { x >>= 16; pos += 16; }
-	if ((x & 0x000000000000ff00ULL) != 0) { x >>= 8;  pos += 8;  }
-	if ((x & 0x00000000000000f0ULL) != 0) { x >>= 4;  pos += 4;  }
-	if ((x & 0x000000000000000cULL) != 0) { x >>= 2;  pos += 2;  }
-	if ((x & 0x0000000000000002ULL) != 0) { pos += 1; }
-
-	return pos;
-}
diff --git a/src/core/bitmath_func.hpp b/src/core/bitmath_func.hpp
index fdd3dfa437..31eda23845 100644
--- a/src/core/bitmath_func.hpp
+++ b/src/core/bitmath_func.hpp
@@ -222,8 +222,35 @@ inline uint8_t FindFirstBit2x64(const int value)
 	}
 }
 
-uint8_t FindFirstBit(uint64_t x);
-uint8_t FindLastBit(uint64_t x);
+/**
+ * Search the first set bit in a value.
+ * When no bit is set, it returns 0.
+ *
+ * @param x The value to search.
+ * @return The position of the first bit set.
+ */
+template <typename T>
+constexpr uint8_t FindFirstBit(T x)
+{
+	if (x == 0) return 0;
+
+	return std::countr_zero(x);
+}
+
+/**
+ * Search the last set bit in a value.
+ * When no bit is set, it returns 0.
+ *
+ * @param x The value to search.
+ * @return The position of the last bit set.
+ */
+template <typename T>
+constexpr uint8_t FindLastBit(T x)
+{
+	if (x == 0) return 0;
+
+	return std::numeric_limits<T>::digits - std::countl_zero(x) - 1;
+}
 
 /**
  * Clear the first bit in an integer.
diff --git a/src/graph_gui.cpp b/src/graph_gui.cpp
index 934522208e..070ad8754c 100644
--- a/src/graph_gui.cpp
+++ b/src/graph_gui.cpp
@@ -440,7 +440,7 @@ protected:
 						 * integer, so at about 31 bits because of the sign bit, the
 						 * least significant bits are removed.
 						 */
-						int mult_range = FindLastBit(x_axis_offset) + FindLastBit(abs(datapoint));
+						int mult_range = FindLastBit<uint32_t>(x_axis_offset) + FindLastBit<uint64_t>(abs(datapoint));
 						int reduce_range = std::max(mult_range - 31, 0);
 
 						/* Handle negative values differently (don't shift sign) */
diff --git a/src/signal.cpp b/src/signal.cpp
index 329b1b05d9..2326ad78fb 100644
--- a/src/signal.cpp
+++ b/src/signal.cpp
@@ -309,7 +309,7 @@ static SigFlags ExploreSegment(Owner owner)
 					Track track = TrackBitsToTrack(tracks_masked); // mask TRACK_BIT_X and Y too
 					if (HasSignalOnTrack(tile, track)) { // now check whole track, not trackdir
 						SignalType sig = GetSignalType(tile, track);
-						Trackdir trackdir = (Trackdir)FindFirstBit((tracks * 0x101) & _enterdir_to_trackdirbits[enterdir]);
+						Trackdir trackdir = (Trackdir)FindFirstBit((tracks * 0x101U) & _enterdir_to_trackdirbits[enterdir]);
 						Trackdir reversedir = ReverseTrackdir(trackdir);
 						/* add (tile, reversetrackdir) to 'to-be-updated' set when there is
 						 * ANY conventional signal in REVERSE direction