Add tutorial for FastScan with refinement for cpp (#3474)

Summary:

This commit focus on the cpp version of PQfastscan tutorial with index refinement by defining the k factor.

Differential Revision: D57680905

tutorial/cpp/8-PQFastScanRefine.cpp

@@ -0,0 +1,84 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <cassert>
+#include <cstdio>
+#include <cstdlib>
+#include <random>
+
+#include <faiss/IndexPQFastScan.h>
+#include <faiss/IndexRefine.h>
+
+using idx_t = faiss::idx_t;
+
+int main() {
+    int d = 64;      // dimension
+    int nb = 100000; // database size
+    int nq = 10000;  // nb of queries
+
+    std::mt19937 rng;
+    std::uniform_real_distribution<> distrib;
+
+    float* xb = new float[(int)(d * nb)];
+    float* xq = new float[(int)(d * nq)];
+
+    for (int i = 0; i < nb; i++) {
+        for (int j = 0; j < d; j++) {
+            xb[d * i + j] = distrib(rng);
+        }
+        xb[d * i] += i / 1000.;
+    }
+
+    for (int i = 0; i < nq; i++) {
+        for (int j = 0; j < d; j++) {
+            xq[d * i + j] = distrib(rng);
+        }
+        xq[d * i] += i / 1000.;
+    }
+
+    int m = 8;
+    int n_bit = 4;
+
+    faiss::IndexPQFastScan index(d, m, n_bit);
+    faiss::IndexRefineFlat index_refine(&index);
+    // refine index after PQFastScan
+
+    printf("Index is trained? %s\n",
+           index_refine.is_trained ? "true" : "false");
+    index_refine.train(nb, xb);
+    printf("Index is trained? %s\n",
+           index_refine.is_trained ? "true" : "false");
+    index_refine.add(nb, xb);
+
+    int k = 4;
+    { // search xq
+        idx_t* I = new idx_t[(int)(k * nq)];
+        float* D = new float[(int)(k * nq)];
+        float k_factor = 3;
+        faiss::IndexRefineSearchParameters* params =
+                new faiss::IndexRefineSearchParameters();
+        params->k_factor = k_factor;
+        index_refine.search(nq, xq, k, D, I, params);
+
+        printf("I=\n");
+        for (int i = nq - 5; i < nq; i++) {
+            for (int j = 0; j < k; j++) {
+                printf("%5zd ", I[i * k + j]);
+            }
+            printf("\n");
+        }
+
+        delete[] I;
+        delete[] D;
+        delete params;
+    }
+
+    delete[] xb;
+    delete[] xq;
+
+    return 0;
+}

tutorial/cpp/CMakeLists.txt

@@ -24,3 +24,6 @@ target_link_libraries(6-HNSW PRIVATE faiss)
 
 add_executable(7-PQFastScan EXCLUDE_FROM_ALL 7-PQFastScan.cpp)
 target_link_libraries(7-PQFastScan PRIVATE faiss)
+
+add_executable(8-PQFastScanRefine EXCLUDE_FROM_ALL 8-PQFastScanRefine.cpp)
+target_link_libraries(8-PQFastScanRefine PRIVATE faiss)