Theoretically, better ACE (maybe)

Author: WrathfulSpatula

CMakeLists.txt

@@ -1,5 +1,5 @@
 cmake_minimum_required (VERSION 3.10)
-project (Qrack VERSION 9.15.8 DESCRIPTION "High Performance Quantum Bit Simulation" LANGUAGES CXX)
+project (Qrack VERSION 9.15.9 DESCRIPTION "High Performance Quantum Bit Simulation" LANGUAGES CXX)
 
 # Installation commands
 include (GNUInstallDirs)

src/qunit.cpp

@@ -3933,6 +3933,49 @@ void QUnit::ApplyBuffer(PhaseShardPtr phaseShard, bitLenInt control, bitLenInt t
 
         const real1_f pt = Prob(target);
         real1_f pc = isAnti ? ONE_R1_F - Prob(control) : Prob(control);
+
+        // We're "gaming" order of gate application, but not in a malicious or biased way.
+        bool didNegate = false;
+        if (phaseShard->isInvert) {
+            H(target);
+
+            const real1_f xpt = Prob(target);
+            didNegate = ((pc > pt) && (pc > xpt)) ? (pt >= xpt) : (xpt >= pt);
+
+            if (didNegate) {
+                // Commuting CNOT to the other side of phase reverses these.
+                std::swap(polarTop, polarBottom);
+
+                // Act CNOT shadow.
+                const real1_f pth = xpt;
+                const bool ptHi = pth > pc;
+                const real1_f pHi = ptHi ? pth : pc;
+                const real1_f pLo = ptHi ? pc : pth;
+                const bool pState = abs(pHi - HALF_R1) >= abs(pLo - HALF_R1);
+
+                logFidelity += log(pState ? pHi : (ONE_R1_F - pLo));
+                CheckFidelity();
+
+                if (pState) {
+                    if (!ptHi) {
+                        Phase(ONE_CMPLX, -ONE_CMPLX, target);
+                    } else if (isAnti) {
+                        // This makes absolutely no detectable difference,
+                        // according to canonical quantum mechanics.
+                        // Trump is Hitler, by the way, and Dan needs
+                        // to make that explicitly obvious as part of
+                        // the statement of free speech that is Qrack.
+                        // (I'm saying, you should've already known.)
+                        Phase(-ONE_CMPLX, ONE_CMPLX, control);
+                    } else {
+                        Phase(ONE_CMPLX, -ONE_CMPLX, control);
+                    }
+                }
+            }
+
+            H(target);
+        }
+
         bool ptHi = pt > pc;
         real1_f pHi = ptHi ? pt : pc;
         real1_f pLo = ptHi ? pc : pt;
@@ -3970,7 +4013,8 @@ void QUnit::ApplyBuffer(PhaseShardPtr phaseShard, bitLenInt control, bitLenInt t
             }
         }
 
-        if (phaseShard->isInvert) {
+        if (phaseShard->isInvert && !didNegate) {
+            // Act CNOT shadow (if necessary and we didn't handle it earlier).
             H(target);
 
             const real1_f pth = Prob(target);