optimize collision detection (#235)

collider: improve performance

Reduces the number of atomic instructions needed and only perform
allocation once. This improves the performance for complicated models by
~10%.

src/collider/src/collider.cpp

@@ -157,26 +157,28 @@ struct CreateRadixTree {
   }
 };
 
-template <typename T>
+template <typename T, const bool allocateOnly>
 struct FindCollisions {
   thrust::pair<int*, int*> querryTri_;
-  int* numOverlaps_;
-  const int maxOverlaps_;
+  int* counts;
   const Box* nodeBBox_;
   const thrust::pair<int, int>* internalChildren_;
 
   __host__ __device__ int RecordCollision(int node,
-                                          const thrust::tuple<T, int>& query) {
+                                          thrust::tuple<T, int>& query) {
     const T& queryObj = thrust::get<0>(query);
     const int queryIdx = thrust::get<1>(query);
+    int& count = counts[queryIdx];
 
     bool overlaps = nodeBBox_[node].DoesOverlap(queryObj);
     if (overlaps && IsLeaf(node)) {
-      int pos = AtomicAdd(*numOverlaps_, 1);
-      if (pos >= maxOverlaps_)
-        return -1;  // Didn't allocate enough memory; bail out
-      querryTri_.first[pos] = queryIdx;
-      querryTri_.second[pos] = Node2Leaf(node);
+      if (allocateOnly) {
+        count++;
+      } else {
+        int pos = count++;
+        querryTri_.first[pos] = queryIdx;
+        querryTri_.second[pos] = Node2Leaf(node);
+      }
     }
     return overlaps && IsInternal(node);  // Should traverse into node
   }
@@ -188,15 +190,16 @@ struct FindCollisions {
     int top = -1;
     // Depth-first search
     int node = kRoot;
+    const int queryIdx = thrust::get<1>(query);
+    // same implies that this query do not have any collision
+    if (!allocateOnly && counts[queryIdx] == counts[queryIdx + 1]) return;
     while (1) {
       int internal = Node2Internal(node);
       int child1 = internalChildren_[internal].first;
       int child2 = internalChildren_[internal].second;
 
       int traverse1 = RecordCollision(child1, query);
-      if (traverse1 < 0) return;
       int traverse2 = RecordCollision(child2, query);
-      if (traverse2 < 0) return;
 
       if (!traverse1 && !traverse2) {
         if (top < 0) break;   // done
@@ -268,33 +271,24 @@ Collider::Collider(const VecDH<Box>& leafBB,
  */
 template <typename T>
 SparseIndices Collider::Collisions(const VecDH<T>& querriesIn) const {
-  int maxOverlaps = querriesIn.size() * 4;
-  SparseIndices querryTri(maxOverlaps);
-  int nOverlaps = 0;
-  while (1) {
-    // scalar number of overlaps found
-    VecDH<int> nOverlapsD(1, 0);
-    // calculate Bounding Box overlaps
-    for_each_n(
-        autoPolicy(querriesIn.size()), zip(querriesIn.cbegin(), countAt(0)),
-        querriesIn.size(),
-        FindCollisions<T>({querryTri.ptrDpq(), nOverlapsD.ptrD(), maxOverlaps,
-                           nodeBBox_.ptrD(), internalChildren_.ptrD()}));
-    nOverlaps = nOverlapsD[0];
-    if (nOverlaps <= maxOverlaps)
-      break;
-    else {  // if not enough memory was allocated, guess how much will be needed
-      int lastQuery = querryTri.Get(0).back();
-      float ratio = static_cast<float>(querriesIn.size()) / lastQuery;
-      if (ratio > 1000)  // do not trust the ratio if it is too large
-        maxOverlaps *= 2;
-      else
-        maxOverlaps *= 2 * ratio;
-      querryTri.Resize(maxOverlaps);
-    }
-  }
-  // remove unused part of array
-  querryTri.Resize(nOverlaps);
+  // note that the length is 1 larger than the number of queries so the last
+  // element can store the sum when using exclusive scan
+  VecDH<int> counts(querriesIn.size() + 1, 0);
+  auto policy = autoPolicy(querriesIn.size());
+  // compute the number of collisions to determine the size for allocation and
+  // offset, this avoids the need for atomic
+  for_each_n(policy, zip(querriesIn.cbegin(), countAt(0)), querriesIn.size(),
+             FindCollisions<T, true>(
+                 {thrust::pair<int*, int*>(nullptr, nullptr), counts.ptrD(),
+                  nodeBBox_.ptrD(), internalChildren_.ptrD()}));
+  // compute start index for each query and total count
+  exclusive_scan(policy, counts.begin(), counts.end(), counts.begin());
+  SparseIndices querryTri(counts.back());
+  // actually recording collisions
+  for_each_n(
+      policy, zip(querriesIn.cbegin(), countAt(0)), querriesIn.size(),
+      FindCollisions<T, false>({querryTri.ptrDpq(), counts.ptrD(),
+                                nodeBBox_.ptrD(), internalChildren_.ptrD()}));
   return querryTri;
 }