Add tutorial on PQFastScan for cpp (#3468)

Summary:
Pull Request resolved: #3468

This commit includes the tutorial for PQFastScan in the cpp environment.

Reviewed By: junjieqi

Differential Revision: D57631441

Author: Xiao Fu

tutorial/cpp/7-PQFastScan.cpp

@@ -0,0 +1,72 @@
+/**
+ * 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>
+
+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[d * nb];
+    float* xq = new float[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);
+    printf("Index is trained? %s\n", index.is_trained ? "true" : "false");
+    index.train(nb, xb);
+    printf("Index is trained? %s\n", index.is_trained ? "true" : "false");
+    index.add(nb, xb);
+
+    int k = 4;
+
+    { // search xq
+        idx_t* I = new idx_t[k * nq];
+        float* D = new float[k * nq];
+
+        index.search(nq, xq, k, D, I);
+
+        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[] xb;
+    delete[] xq;
+
+    return 0;
+} // namespace facebook::detail

tutorial/cpp/CMakeLists.txt

@@ -21,3 +21,6 @@ target_link_libraries(5-Multiple-GPUs PRIVATE faiss)
 
 add_executable(6-HNSW EXCLUDE_FROM_ALL 6-HNSW.cpp)
 target_link_libraries(6-HNSW PRIVATE faiss)
+
+add_executable(7-PQFastScan EXCLUDE_FROM_ALL 7-PQFastScan.cpp)
+target_link_libraries(7-PQFastScan PRIVATE faiss)