From dd8509fb3fb8f5df6f86d99df354c29c013d2e25 Mon Sep 17 00:00:00 2001
From: Rose <83477269+AtariDreams@users.noreply.github.com>
Date: Wed, 27 Sep 2023 16:32:34 -0400
Subject: [PATCH] Simplify calculations of the nearest power of 2

Use compiler intrinsics to find the nearest power of 2 as quickly as possible.

In addition, handle the 0 case, so that the program does not become stuck in an infinite loop if iNum is 0.
---
 src/CalcManager/CEngine/sciset.cpp        | 31 ++++++-----------------
 src/CalcManager/Header Files/CalcEngine.h |  4 +--
 2 files changed, 10 insertions(+), 25 deletions(-)

diff --git a/src/CalcManager/CEngine/sciset.cpp b/src/CalcManager/CEngine/sciset.cpp
index 57aeda3bd..7d7e5cc3a 100644
--- a/src/CalcManager/CEngine/sciset.cpp
+++ b/src/CalcManager/CEngine/sciset.cpp
@@ -50,7 +50,7 @@ void CCalcEngine::SetRadixTypeAndNumWidth(RadixType radixtype, NUM_WIDTH numwidt
     DisplayNum();
 }
 
-int32_t CCalcEngine::DwWordBitWidthFromNumWidth(NUM_WIDTH numwidth)
+uint32_t CCalcEngine::DwWordBitWidthFromNumWidth(NUM_WIDTH numwidth)
 {
     switch (numwidth)
     {
@@ -103,30 +103,15 @@ bool CCalcEngine::TryToggleBit(CalcEngine::Rational& rat, uint32_t wbitno)
 }
 
 // Returns the nearest power of two
-int CCalcEngine::QuickLog2(int iNum)
+unsigned int CCalcEngine::QuickLog2(unsigned int iNum)
 {
-    int iRes = 0;
+    // Nearest power of 2 to 0 is 1
+    if (!iNum)
+        return 1;
 
-    // while first digit is a zero
-    while (!(iNum & 1))
-    {
-        iRes++;
-        iNum >>= 1;
-    }
-
-    // if our number isn't a perfect square
-    iNum = iNum >> 1;
-    if (iNum)
-    {
-        // find the largest digit
-        for (iNum = iNum >> 1; iNum; iNum = iNum >> 1)
-            ++iRes;
-
-        // and then add two
-        iRes += 2;
-    }
-
-    return iRes;
+    // Count the number of leading 0's of iNum - 1,
+    // and then shift 1 by that amount
+    return 1U << (32 - __builtin_clz(iNum - 1));
 }
 
 ////////////////////////////////////////////////////////////////////////
diff --git a/src/CalcManager/Header Files/CalcEngine.h b/src/CalcManager/Header Files/CalcEngine.h
index f387c020a..9c012be45 100644
--- a/src/CalcManager/Header Files/CalcEngine.h	
+++ b/src/CalcManager/Header Files/CalcEngine.h	
@@ -179,7 +179,7 @@ class CCalcEngine
     CalcEngine::Rational SciCalcFunctions(CalcEngine::Rational const& rat, uint32_t op);
     CalcEngine::Rational DoOperation(int operation, CalcEngine::Rational const& lhs, CalcEngine::Rational const& rhs);
     void SetRadixTypeAndNumWidth(RadixType radixtype, NUM_WIDTH numwidth);
-    int32_t DwWordBitWidthFromNumWidth(NUM_WIDTH numwidth);
+    uint32_t DwWordBitWidthFromNumWidth(NUM_WIDTH numwidth);
     uint32_t NRadixFromRadixType(RadixType radixtype);
     double GenerateRandomNumber();
 
@@ -199,7 +199,7 @@ class CCalcEngine
     static std::vector<uint32_t> DigitGroupingStringToGroupingVector(std::wstring_view groupingString);
     std::wstring GroupDigits(std::wstring_view delimiter, std::vector<uint32_t> const& grouping, std::wstring_view displayString, bool isNumNegative = false);
 
-    static int QuickLog2(int iNum);
+    static unsigned int QuickLog2(unsigned int iNum);
     static void ChangeBaseConstants(uint32_t radix, int maxIntDigits, int32_t precision);
     void BaseOrPrecisionChanged();