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Merged
TysonRayJones merged 10 commits into from
Feb 28, 2026
Merged

Implement Pauli sorting #708

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dc2a35f
feat(utilities): add invertPermutation
vaferreiQMT Jan 20, 2026
975d788
feat(paulilogic): implement permutation and sorting
vaferreiQMT Jan 20, 2026
ca70b40
feat(api): add paulistrsum lexicographic and magnitude sorting
vaferreiQMT Jan 20, 2026
1b834a0
tests(paulis): add sorting tests
vaferreiQMT Jan 20, 2026
9f00655
renaming SETTERS to SORTING
TysonRayJones Feb 28, 2026
f7a2e47
renamed util_invertPermutation to util_getInversePermutation
TysonRayJones Feb 28, 2026
89a0594
added unguarded alloc cautions
TysonRayJones Feb 28, 2026
691ecff
clarified names
TysonRayJones Feb 28, 2026
00f91ba
added missing includes
TysonRayJones Feb 28, 2026
54064a5
add missing include
TysonRayJones Feb 28, 2026
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88 changes: 88 additions & 0 deletions quest/include/paulis.h
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Expand Up@@ -99,6 +99,9 @@ typedef struct {
*
* @defgroup paulis_reporters Reporters
* @brief Functions for printing Pauli data structures.
*
* @defgroup paulis_setters Setters
* @brief Functions for overwriting the elements of Pauli data structures.
*/


Expand DownExpand Up@@ -420,6 +423,91 @@ extern "C" {



/*
* SETTERS
*/

// enable invocation by both C and C++ binaries
#ifdef __cplusplus
extern "C" {
#endif


/** @ingroup paulis_setters
*
* Reorders the terms within a @p sum of weighted Pauli strings to sort Pauli
* strings into lexicographic (dictionary) ordering.
*
* @formulae
* Let @f$ H = @f$ @p sum, which can be represented as
* @f[
H = \sum\limits_j c_j \, \hat{\sigma}_j
* @f]
* where @f$ c_j @f$ is the coefficient of the @f$ j @f$-th PauliStr @f$ \hat{\sigma}_j @f$.
*
* This function constructs and applies the permutation @f$ \pi @f$ to @f$ H @f$
* @f[
H = \sum\limits_j c_{\pi(j)} \, \hat{\sigma}_{\pi(j)}
* @f]
* such that
* @f[
* \hat{\sigma}_{\pi(i)} <_{lex} \hat{\sigma}_{\pi(j)} \ \forall \ \pi(i) < \pi(j).
* @f]
*
*
* @param[in,out] sum a weighted sum of Pauli strings to reorder.
*
* @throws @validationerror
* - if @p sum is not initialised.
*
* @see
* - sortPauliStrSumMagnitude()
* @author Vasco Ferreira
*/
void sortPauliStrSumLexicographic(PauliStrSum sum);


/** @ingroup paulis_setters
*
* Reorders the terms within a @p sum of weighted Pauli strings to sort Pauli
* strings into decreasing magnitude weights.
*
* @formulae
* Let @f$ H = @f$ @p sum, represented as the weighted sum
* @f[
H = \sum\limits_j c_j \, \hat{\sigma}_j
* @f]
* where @f$ c_j @f$ is the coefficient of the @f$ j @f$-th PauliStr @f$ \hat{\sigma}_j @f$.
*
* This function constructs and applies the permutation @f$ \pi @f$ to @f$ H @f$
* @f[
H = \sum\limits_j c_{\pi(j)} \, \hat{\sigma}_{\pi(j)}
* @f]
* such that
* @f[
* |c_{\pi(i)}| > |c_{\pi(j)}| \, \forall \, \pi(i) < \pi(j).
* @f]
*
* @param[in,out] sum a weighted sum of Pauli strings to reorder.
*
* @throws @validationerror
* - if @p sum is not initialised.
*
* @see
* - sortPauliStrSumLexicographic()
*
* @author Vasco Ferreira
*/
void sortPauliStrSumMagnitude(PauliStrSum sum);


// end de-mangler
#ifdef __cplusplus
}
#endif



#endif // PAULIS_H

/** @} */ // (end file-wide doxygen defgroup)
30 changes: 30 additions & 0 deletions quest/src/api/paulis.cpp
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Expand Up@@ -291,3 +291,33 @@ extern "C" void reportPauliStrSum(PauliStrSum sum) {
// exclude mandatory newline above
print_oneFewerNewlines();
}



/*
* SORTING
*/


extern "C" void sortPauliStrSumLexicographic(PauliStrSum sum) {
validate_pauliStrSumFields(sum, __func__);

auto lexSort = [&](qindex i, qindex j) {
PauliStr strI = sum.strings[i];
PauliStr strJ = sum.strings[j];
return std::tie(strI.highPaulis, strI.lowPaulis) < std::tie(strJ.highPaulis, strJ.lowPaulis);
};

paulis_sortTermsViaComparator(sum, lexSort);
}


extern "C" void sortPauliStrSumMagnitude(PauliStrSum sum) {
validate_pauliStrSumFields(sum, __func__);

auto magSort = [&](qindex i, qindex j) {
return std::norm(sum.coeffs[i]) > std::norm(sum.coeffs[j]);
};

paulis_sortTermsViaComparator(sum, magSort);
}
34 changes: 34 additions & 0 deletions quest/src/core/paulilogic.cpp
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Expand Up@@ -13,9 +13,12 @@
#include "quest/src/core/bitwise.hpp"
#include "quest/src/core/errors.hpp"

#include <numeric>
#include <utility>
#include <vector>
#include <array>
#include <functional>
#include <algorithm>

using std::vector;

Expand DownExpand Up@@ -307,6 +310,37 @@ qindex paulis_getTargetBitMask(PauliStrSum sum) {
}


void paulis_applyPermutationToTerms(PauliStrSum sum, vector<qindex> scatterPermutation) {
// permutation passed by value since we modify it

// scatterPermutation[i] = destination index for element originally at i
for (qindex i = 0; i < sum.numTerms; i++) {
while (scatterPermutation[i] != i) {
qindex j = scatterPermutation[i];
std::swap(sum.strings[i], sum.strings[j]);
std::swap(sum.coeffs[i], sum.coeffs[j]);
std::swap(scatterPermutation[i], scatterPermutation[j]);
}
}
}


void paulis_sortTermsViaComparator(PauliStrSum sum, std::function<bool(qindex, qindex)> comparator) {

// TODO: below is an unguarded vector alloc, forgiven since a subsequent
// change (giving PauliStrSum an 'ordering' list) supersedes it

// gatherPermutation[j] = source index of element placed at j
vector<qindex> gatherPermutation(sum.numTerms);
std::iota(gatherPermutation.begin(), gatherPermutation.end(), 0);
std::stable_sort(gatherPermutation.begin(), gatherPermutation.end(), comparator);

// invert permutation and apply
vector<qindex> scatterPermutation = util_getInversePermutation(gatherPermutation);
paulis_applyPermutationToTerms(sum, scatterPermutation);
}


void paulis_setPauliStrSumToScaledTensorProdOfConjWithSelf(PauliStrSum out, qreal factor, PauliStrSum in, int numQubits) {

// sets out = factor * conj(in) (x) in, where in has dim of numQubits
Expand Down
7 changes: 6 additions & 1 deletion quest/src/core/paulilogic.hpp
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Expand Up@@ -15,6 +15,7 @@
#include <utility>
#include <vector>
#include <array>
#include <functional>

using std::vector;

Expand DownExpand Up@@ -76,6 +77,10 @@ int paulis_getIndOfLefmostNonIdentityPauli(PauliStrSum sum);

qindex paulis_getTargetBitMask(PauliStrSum sum);

void paulis_applyPermutationToTerms(PauliStrSum sum, vector<qindex> permutation);

void paulis_sortTermsViaComparator(PauliStrSum sum, std::function<bool(qindex, qindex)> comparator);


// below are used exclusively by Trotterisation

Expand All@@ -88,4 +93,4 @@ void paulis_setPauliStrSumToScaledProdOfAdjointWithSelf(PauliStrSum out, qreal f
void paulis_setPauliStrSumToShiftedConj(PauliStrSum out, PauliStrSum in, int numQubits);


#endif // PAULILOGIC_HPP
#endif // PAULILOGIC_HPP
16 changes: 15 additions & 1 deletion quest/src/core/utilities.cpp
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Expand Up@@ -368,7 +368,7 @@ bool util_willSumOverflow(vector<qindex> terms) {


/*
* VECTOR REDUCTION
* LIST PROCESSING
*/

qreal util_getSum(vector<qreal> list) {
Expand All@@ -387,6 +387,20 @@ qreal util_getSum(vector<qreal> list) {
return sum;
}

vector<qindex> util_getInversePermutation(vector<qindex> permutation) {

// TODO: below is an unguarded vector alloc, forgiven since a subsequent
// change (giving PauliStrSum an 'ordering' list) supersedes it

qindex numTerms = permutation.size();
vector<qindex> out(numTerms);

for (qindex i = 0; i < numTerms; i++)
out[permutation[i]] = i;

return out;
}



/*
Expand Down
6 changes: 4 additions & 2 deletions quest/src/core/utilities.hpp
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Expand Up@@ -240,11 +240,13 @@ qcomp* util_getGpuMemPtr(T matr) {


/*
* VECTOR REDUCTION
* LIST PROCESSING
*/

qreal util_getSum(vector<qreal> list);

vector<qindex> util_getInversePermutation(vector<qindex> permutation);



/*
Expand DownExpand Up@@ -430,4 +432,4 @@ void util_tryAllocMatrix(vector<vector<qcomp>> &vec, qindex numRows, qindex numC



#endif // UTILITIES_HPP
#endif // UTILITIES_HPP
55 changes: 55 additions & 0 deletions tests/unit/paulis.cpp
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Expand Up@@ -587,6 +587,61 @@ TEST_CASE( "destroyPauliStrSum", TEST_CATEGORY ) {
}
}

TEST_CASE( "sortPauliStrSumLexicographic", TEST_CATEGORY ) {

SECTION( LABEL_CORRECTNESS ) {

vector<qcomp> coeffs = {0.1_i, 2+1_i, 5, 3+4_i};
vector<PauliStr> strings = {
getPauliStr("XY", {31,32}),
getPauliStr("YX", {0,1}),
getPauliStr("II", {0,1}),
getPauliStr("YY", {31,32})
};

PauliStrSum sum = createPauliStrSum(strings, coeffs);
sortPauliStrSumLexicographic(sum);

REQUIRE(sum.coeffs[0] == 5+0_i);
REQUIRE(sum.coeffs[1] == 2+1_i);
REQUIRE(sum.coeffs[3] == 3+4_i);

REQUIRE(sum.strings[0].lowPaulis == 0);
REQUIRE(sum.strings[1].lowPaulis == 2 + 1*4);
REQUIRE(sum.strings[3].highPaulis == 2);
REQUIRE(sum.strings[3].lowPaulis == 2*std::pow(4, 31));

destroyPauliStrSum(sum);
}
}

TEST_CASE( "sortPauliStrSumMagnitude", TEST_CATEGORY ) {

SECTION( LABEL_CORRECTNESS ) {

vector<qcomp> coeffs = {0.1_i, 2+1_i, 5, 3+4_i};
vector<PauliStr> strings = {
getPauliStr("XY", {0,1}),
getPauliStr("ZX", {0,1}),
getPauliStr("II", {0,1}),
getPauliStr("YZ", {0,1})
};

PauliStrSum sum = createPauliStrSum(strings, coeffs);
sortPauliStrSumMagnitude(sum);

REQUIRE(sum.coeffs[0] == 5+0_i);
REQUIRE(sum.coeffs[1] == 3+4_i);
REQUIRE(sum.coeffs[3] == 0+0.1_i);

REQUIRE(sum.strings[0].lowPaulis == 0);
REQUIRE(sum.strings[1].lowPaulis == 2 + 3*4);
REQUIRE(sum.strings[3].lowPaulis == 1 + 2*4);

destroyPauliStrSum(sum);
}
}


/** @} (end defgroup) */

Expand Down
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