From 50c2791b091cd116f54866c7efe1d025a8d5825b Mon Sep 17 00:00:00 2001
From: John Children <john.children@cambridgequantum.com>
Date: Wed, 27 Nov 2024 15:53:47 +0000
Subject: [PATCH] perf: Avoid rehashing in index_map

The IndexMap data structure is an unordered_map which means that if the
hash table is too small it can result in having to rehash all members of
the table as seen in https://github.com/CQCL/tket/issues/1696 .

This commit attempts to reserve an exact size of hash table up front
such that rehashing is unlikely as an additional optimization for large
circuits.
---
 tket/src/Circuit/Circuit.cpp | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tket/src/Circuit/Circuit.cpp b/tket/src/Circuit/Circuit.cpp
index 115880b746..b5826dc75c 100644
--- a/tket/src/Circuit/Circuit.cpp
+++ b/tket/src/Circuit/Circuit.cpp
@@ -132,6 +132,9 @@ VertexVec Circuit::vertices_in_order() /*const*/ {
 
 IndexMap Circuit::index_map() const {
   IndexMap im;
+  // Reserve space for every edge to avoid rehashing.
+  im.reserve(boost::concepts::EdgeListGraph<DAG>::num_edges(dag));
+
   std::size_t i = 0;
   BGL_FORALL_VERTICES(v, dag, DAG) { im[v] = i++; }
   return im;