Dijkstra arbitrary weights (#269)

Author: EliasLF

include/Architecture.hpp

@@ -364,21 +364,18 @@ class Architecture {
   void createDistanceTable();
   void createFidelityTable();
 
-  static double costHeuristicBidirectional(const Dijkstra::Node& node) {
-    auto length = node.cost - 1;
+  static double dijkstraNodeToCost(const Dijkstra::Node& node) {
+    // Dijkstra determines the minimal path cost from one physical qubit to
+    // another.  In the non-fidelity case we are only interested in swapping 2
+    // logical qubits next to each other. Therefore the last swap cost has to
+    // be ignored. That cost is stored in the field `node.prevCost` of each
+    // node.
     if (node.containsCorrectEdge) {
-      return length * COST_BIDIRECTIONAL_SWAP;
+      return node.prevCost;
     }
-    throw QMAPException("In a bidrectional architecture it should not happen "
-                        "that a node does not contain the right edge.");
-  }
-
-  static double costHeuristicUnidirectional(const Dijkstra::Node& node) {
-    auto length = node.cost - 1;
-    if (node.containsCorrectEdge) {
-      return length * COST_UNIDIRECTIONAL_SWAP;
-    }
-    return length * COST_UNIDIRECTIONAL_SWAP + COST_DIRECTION_REVERSE;
+    // in case the last edge is a back-edge, we will need to reverse the CNOT,
+    // executed on that edge
+    return node.prevCost + COST_DIRECTION_REVERSE;
   }
 
   // added for teleportation

include/utils.hpp

@@ -58,15 +58,16 @@ class Dijkstra {
     bool                         visited             = false;
     std::optional<std::uint16_t> pos                 = std::nullopt;
     double                       cost                = -1.;
+    double                       prevCost            = -1.;
   };
 
   static void buildTable(std::uint16_t n, const CouplingMap& couplingMap,
-                         Matrix& distanceTable,
+                         Matrix& distanceTable, const Matrix& edgeWeights,
                          const std::function<double(const Node&)>& cost);
 
 protected:
   static void dijkstra(const CouplingMap& couplingMap, std::vector<Node>& nodes,
-                       std::uint16_t start);
+                       std::uint16_t start, const Matrix& edgeWeights);
 
   struct NodeComparator {
     bool operator()(const Node* x, const Node* y) {

src/Architecture.cpp

@@ -185,20 +185,21 @@ Architecture::Architecture(const std::uint16_t nQ, const CouplingMap& cm,
 }
 
 void Architecture::createDistanceTable() {
+  isBidirectional = true;
+  Matrix edgeWeights(nqubits, std::vector<double>(
+                                  nqubits, std::numeric_limits<double>::max()));
   for (const auto& edge : couplingMap) {
     if (couplingMap.find({edge.second, edge.first}) == couplingMap.end()) {
-      isBidirectional = false;
-      break;
+      isBidirectional                            = false;
+      edgeWeights.at(edge.second).at(edge.first) = COST_UNIDIRECTIONAL_SWAP;
+      edgeWeights.at(edge.first).at(edge.second) = COST_UNIDIRECTIONAL_SWAP;
+    } else {
+      edgeWeights.at(edge.first).at(edge.second) = COST_BIDIRECTIONAL_SWAP;
     }
   }
 
-  if (isBidirectional) {
-    Dijkstra::buildTable(nqubits, couplingMap, distanceTable,
-                         Architecture::costHeuristicBidirectional);
-  } else {
-    Dijkstra::buildTable(nqubits, couplingMap, distanceTable,
-                         Architecture::costHeuristicUnidirectional);
-  }
+  Dijkstra::buildTable(nqubits, couplingMap, distanceTable, edgeWeights,
+                       Architecture::dijkstraNodeToCost);
 }
 
 void Architecture::createFidelityTable() {

src/utils.cpp

@@ -8,7 +8,7 @@
 #include <cassert>
 
 void Dijkstra::buildTable(const std::uint16_t n, const CouplingMap& couplingMap,
-                          Matrix& distanceTable,
+                          Matrix& distanceTable, const Matrix& edgeWeights,
                           const std::function<double(const Node&)>& cost) {
   distanceTable.clear();
   distanceTable.resize(n, std::vector<double>(n, -1.));
@@ -20,11 +20,13 @@ void Dijkstra::buildTable(const std::uint16_t n, const CouplingMap& couplingMap,
       nodes.at(j).visited             = false;
       nodes.at(j).pos                 = j;
       nodes.at(j).cost                = -1.;
+      nodes.at(j).prevCost            = -1.;
     }
 
-    nodes.at(i).cost = 0.;
+    nodes.at(i).cost     = 0.;
+    nodes.at(i).prevCost = 0.;
 
-    dijkstra(couplingMap, nodes, i);
+    dijkstra(couplingMap, nodes, i, edgeWeights);
 
     for (std::uint16_t j = 0; j < n; ++j) {
       if (i == j) {
@@ -37,7 +39,8 @@ void Dijkstra::buildTable(const std::uint16_t n, const CouplingMap& couplingMap,
 }
 
 void Dijkstra::dijkstra(const CouplingMap& couplingMap,
-                        std::vector<Node>& nodes, std::uint16_t start) {
+                        std::vector<Node>& nodes, std::uint16_t start,
+                        const Matrix& edgeWeights) {
   std::priority_queue<Node*, std::vector<Node*>, NodeComparator> queue{};
   queue.push(&nodes.at(start));
   while (!queue.empty()) {
@@ -61,8 +64,9 @@ void Dijkstra::dijkstra(const CouplingMap& couplingMap,
         }
 
         Node newNode;
-        newNode.cost                = current->cost + 1.0;
-        newNode.pos                 = to;
+        newNode.cost     = current->cost + edgeWeights.at(*pos).at(*to);
+        newNode.prevCost = current->cost;
+        newNode.pos      = to;
         newNode.containsCorrectEdge = correctEdge;
         if (nodes.at(*to).cost < 0 || newNode < nodes.at(*to)) {
           nodes.at(*to) = newNode;

test/test_general.cpp

@@ -4,6 +4,7 @@
 //
 
 #include "Architecture.hpp"
+#include "utils.hpp"
 
 #include "gtest/gtest.h"
 #include <iostream>
@@ -52,3 +53,33 @@ TEST(General, TestLineParsing) {
   EXPECT_EQ(data[1], "Entry2");
   EXPECT_EQ(data[2], "EscapedEntry1;EscapedEntry2");
 }
+
+TEST(General, Dijkstra) {
+  /*
+            (6)
+        .----------.
+       \/          \/
+  0 <-> 1 --> 2 <-> 3
+    (1)   (2)   (3)
+  */
+
+  const CouplingMap cm = {{0, 1}, {1, 0}, {1, 2}, {2, 3},
+                          {3, 2}, {1, 3}, {3, 1}};
+
+  const Matrix edgeWeights = {
+      {0, 1, 0, 0}, {1, 0, 2, 6}, {0, 15, 0, 3}, {0, 6, 3, 0}};
+
+  const Matrix targetTable1 = {
+      {0, 1, 3, 6}, {1, 0, 2, 5}, {10, 9, 0, 3}, {7, 6, 3, 0}};
+  Matrix distanceTable{};
+  Dijkstra::buildTable(4, cm, distanceTable, edgeWeights,
+                       [](const Dijkstra::Node& n) { return n.cost; });
+  EXPECT_EQ(distanceTable, targetTable1);
+
+  const Matrix targetTable2 = {
+      {0, 0, 1, 3}, {0, 0, 0, 2}, {9, 3, 0, 0}, {6, 0, 0, 0}};
+  distanceTable = {};
+  Dijkstra::buildTable(4, cm, distanceTable, edgeWeights,
+                       [](const Dijkstra::Node& n) { return n.prevCost; });
+  EXPECT_EQ(distanceTable, targetTable2);
+}