From 9d2e7c3104d9cefda6934372e50541e069e9a0f8 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Thu, 15 Feb 2018 18:06:44 +0100
Subject: [PATCH] Optimize activity strength computation.

---
 src/visualisations.cpp | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/src/visualisations.cpp b/src/visualisations.cpp
index 2d68faf..aa72cea 100644
--- a/src/visualisations.cpp
+++ b/src/visualisations.cpp
@@ -1,3 +1,4 @@
+#include <numeric>
 #include <utility>
 #include "visualisations.hpp"
 #include "DummyLayerVisualisation.hpp"
@@ -32,7 +33,6 @@ computeActivityStrengths(const LayerData &prevState, const LayerInfo &layer)
     LOG(INFO) << "Computing top interactions for " << layer.name() << endl;
 
     typedef pair <DType, pair<size_t, size_t>> Entry;
-    vector <Entry> result;
 
     auto data = prevState.data();
 
@@ -40,19 +40,28 @@ computeActivityStrengths(const LayerData &prevState, const LayerInfo &layer)
 
     const auto shape = layer.parameters()[0]->shape();
     auto weights = layer.parameters()[0]->cpu_data();
-    const auto numEntries = accumulate(shape.begin(), shape.end(), 1u, multiplies<void>());
-    result.reserve(numEntries);
+    const auto numEntries = accumulate(shape.begin(), shape.end(), static_cast<size_t>(1), multiplies<void>());
+
+    vector<float> interactions(numEntries);
 
     for (auto i : Range(numEntries)) {
-        result.emplace_back(weights[i] * data[i % shape[0]], make_pair(i % shape[0], i / shape[0]));
+        interactions[i] = weights[i] * data[i % shape[0]];
     }
 
-    const auto desiredSize = min(INTERACTION_LIMIT, result.size());
-    partial_sort(result.begin(), result.begin() + desiredSize, result.end(), [](const Entry &a, const Entry &b) {
-        return abs(a.first) > abs(b.first);
+    // Now use a creative argsort
+    vector<size_t> idx(numEntries);
+    iota(idx.begin(), idx.end(), 0);
+
+    const auto desiredSize = min(INTERACTION_LIMIT, numEntries);
+    partial_sort(idx.begin(), idx.begin() + desiredSize, idx.end(), [&interactions](size_t a, size_t b) {
+        return abs(interactions[a]) > abs(interactions[b]);
     });
 
-    result.resize(desiredSize);
+    vector<Entry> result;
+    result.reserve(desiredSize);
+    for (auto i : Range(desiredSize)) {
+        result.emplace_back(interactions[idx[i]], make_pair(idx[i] % shape[0], idx[i] / shape[0]));
+    }
 
     return result;
 }