Fix #3379: Add tutorial for HNSW index (#3381)

Summary:
Fixes https://github.com/facebookresearch/faiss/issues/3379

Pull Request resolved: https://github.com/facebookresearch/faiss/pull/3381

Reviewed By: junjieqi

Differential Revision: D56570120

Pulled By: kuarora

fbshipit-source-id: 758ea4ab866609d6dd5621e6e6ffda583ba52503

tutorial/cpp/6-HNSW.cpp

@@ -0,0 +1,76 @@
+/**
+ * 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/IndexHNSW.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 nlist = 100;
+    int k = 4;
+
+    faiss::IndexHNSWFlat index(d, 32);
+    index.add(nb, xb);
+
+    { // 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");
+        }
+
+        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;
+}

tutorial/cpp/CMakeLists.txt

@@ -18,3 +18,6 @@ target_link_libraries(4-GPU PRIVATE faiss)
 
 add_executable(5-Multiple-GPUs EXCLUDE_FROM_ALL 5-Multiple-GPUs.cpp)
 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)