From ae92c47cda59e279e2c552eed4f586b67b834b46 Mon Sep 17 00:00:00 2001
From: sjdilkes <silas.dilkes@cambridgequantum.com>
Date: Mon, 4 Dec 2023 20:17:29 +0000
Subject: [PATCH] clean up intersection code

---
 tket/src/Characterisation/Cycles.cpp          | 19 +++++++++----------
 .../Characterisation/FrameRandomisation.cpp   |  2 --
 2 files changed, 9 insertions(+), 12 deletions(-)

diff --git a/tket/src/Characterisation/Cycles.cpp b/tket/src/Characterisation/Cycles.cpp
index 4205663284..20ba39c420 100644
--- a/tket/src/Characterisation/Cycles.cpp
+++ b/tket/src/Characterisation/Cycles.cpp
@@ -170,16 +170,15 @@ std::pair<unsigned, std::set<unsigned>> CycleFinder::make_cycle(
         if (std::find(
                 history_uids.begin(), history_uids.end(), not_present_uid) !=
             history_uids.end()) {
-          // => the not present uid is in this cycle
-          // now we check the overlap
-          //
-          for (const UnitID& a :
-               this->cycle_history.history[candidate_cycle_key]) {
-            for (const UnitID& b : history_uids) {
-              if (a == b) {
-                not_mergeable_keys.insert(candidate_cycle_key);
-              }
-            }
+          std::vector<UnitID> intersection;
+          std::vector<UnitID> candidates =
+              cycle_history.history[candidate_cycle_key];
+          std::set_intersection(
+              candidates.begin(), candidates.end(), history_uids.begin(),
+              history_uids.end(), std::back_inserter(intersection));
+          if (!intersection.empty()) {
+            not_mergeable_keys.insert(candidate_cycle_key);
+            break;
           }
         }
       }
diff --git a/tket/src/Characterisation/FrameRandomisation.cpp b/tket/src/Characterisation/FrameRandomisation.cpp
index c2cc8fc0f2..eb899a9c16 100644
--- a/tket/src/Characterisation/FrameRandomisation.cpp
+++ b/tket/src/Characterisation/FrameRandomisation.cpp
@@ -45,8 +45,6 @@ std::string FrameRandomisation::to_string() const {
 // Wires Identity gates into each cycle edge. Identity gates then relabelled
 // with Ops from OpTypeSet to create instances of Frame Randomisation
 void add_noop_frames(std::vector<Cycle>& cycles, Circuit& circ) {
-  IndexMap imap = circ.index_map();
-
   std::map<Edge, Edge> replacement_rewiring_edges;
   for (Cycle& full_cycle : cycles) {
     std::vector<edge_pair_t> cycle = full_cycle.boundary_edges_;