From 489af1b943f357d918c0ca79ea760b3f9b179b3a Mon Sep 17 00:00:00 2001
From: TysonRayJones <tyson.jones.input@gmail.com>
Date: Thu, 14 Mar 2024 19:25:59 +0100
Subject: [PATCH] patched numeric Pauli string functions

which previously did not accept symbolically-expressed but ultimately numerical Pauli-string coefficients, like "Rational[1,10]", "Sqrt[2]" or "Pi". Instead, they catastrophically crashed the QuEST process! This is despite passing validation because they were recognised as numeric, although they were not instantiated as floating-point types necessary for passing to the C process.

Now, they are correctly explicitly cast to floats before being dispatched to the backend.

The affected functions are all which numerically dispatch Pauli strings to the backend:
- CalcExpecPauliStringDerivs
- SetQuregToPauliString
- CalcExpecPauliString
- ApplyPauliString
- CalcPauliStringMatrix
- SampleExpecPauliString

What a doozie!
---
 Link/QuESTlink.m | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Link/QuESTlink.m b/Link/QuESTlink.m
index 708fce19..76da22f8 100644
--- a/Link/QuESTlink.m
+++ b/Link/QuESTlink.m
@@ -781,13 +781,13 @@ The probability of the forced measurement outcome (as if it were hypothetically
             {{1}, {getOpCode@op}, {q}, {1}}
         (* .1 X1 *)
         getEncodedNumericPauliString[ Verbatim[Times][c:_?NumericQ, p:pauliOpPatt.. ] ] := 
-            {{c}, getOpCode /@ {p}[[All,1]], {p}[[All,2]], {Length@{p}[[All,2]]}}
+            {{N@c}, getOpCode /@ {p}[[All,1]], {p}[[All,2]], {Length@{p}[[All,2]]}}
         (* X1 X2 *)
         getEncodedNumericPauliString[ Verbatim[Times][p:pauliOpPatt.. ] ] :=
             {{1}, getOpCode /@ {p}[[All,1]], {p}[[All,2]], {Length@{p}[[All,2]]}}
         (* .1 X1 X2 *)
         getEncodedNumericPauliString[ p:numericCoeffPauliProdPatt ] :=
-            {p[[1]], getOpCode /@ Rest[List@@p][[All,1]], Rest[List@@p][[All,2]], Length[p]-1}
+            {N@p[[1]], getOpCode /@ Rest[List@@p][[All,1]], Rest[List@@p][[All,2]], Length[p]-1}
         (* .5 X1 X2 + X1 X2 + X1 + .5 X1 *)
         getEncodedNumericPauliString[ s_Plus ] /; AllTrue[List@@s, MatchQ[numericCoeffPauliProdPatt]] :=
             Join @@@ Transpose[getEncodedNumericPauliString /@ (List @@ s)]