Add IVF index for vec0 virtual table

Add inverted file (IVF) index type: partitions vectors into clusters via
k-means, quantizes to int8, and scans only the nearest nprobe partitions at
query time. Includes shadow table management, insert/delete, KNN integration,
compile flag (SQLITE_VEC_ENABLE_IVF), fuzz targets, and tests. Removes
superseded ivf-benchmarks/ directory.

tests/fuzz/ivf-rescore.c

@@ -0,0 +1,182 @@
+/**
+ * Fuzz target: IVF oversample + rescore path.
+ *
+ * Specifically targets the code path where quantizer != none AND
+ * oversample > 1, which triggers:
+ * 1. Quantized KNN scan to collect oversample*k candidates
+ * 2. Full-precision vector lookup from _ivf_vectors table
+ * 3. Re-scoring with float32 distances
+ * 4. Re-sort and truncation
+ *
+ * This path has the most complex memory management in the KNN query:
+ * - Two separate distance computations (quantized + float)
+ * - Cross-table lookups (cells + vectors KV store)
+ * - Candidate array resizing
+ * - qsort over partially re-scored arrays
+ *
+ * Also tests the int8 + binary quantization round-trip fidelity
+ * under adversarial float inputs.
+ */
+#include <stdint.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <math.h>
+#include "sqlite-vec.h"
+#include "sqlite3.h"
+#include <assert.h>
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  if (size < 12) return 0;
+
+  int rc;
+  sqlite3 *db;
+
+  rc = sqlite3_open(":memory:", &db);
+  assert(rc == SQLITE_OK);
+  rc = sqlite3_vec_init(db, NULL, NULL);
+  assert(rc == SQLITE_OK);
+
+  // Header
+  int quantizer_type = (data[0] % 2) + 1;  // 1=int8, 2=binary (never none)
+  int dim = (data[1] % 32) + 8;  // 8..39
+  int nlist = (data[2] % 8) + 1;  // 1..8
+  int oversample = (data[3] % 4) + 2;  // 2..5 (always > 1)
+  int num_vecs = (data[4] % 60) + 8;  // 8..67
+  int k_limit = (data[5] % 15) + 1;  // 1..15
+
+  const uint8_t *payload = data + 6;
+  size_t payload_size = size - 6;
+
+  // Binary quantizer needs D multiple of 8
+  if (quantizer_type == 2) {
+    dim = ((dim + 7) / 8) * 8;
+  }
+
+  const char *qname = (quantizer_type == 1) ? "int8" : "binary";
+
+  char sql[512];
+  snprintf(sql, sizeof(sql),
+    "CREATE VIRTUAL TABLE v USING vec0("
+    "emb float[%d] indexed by ivf(nlist=%d, nprobe=%d, quantizer=%s, oversample=%d))",
+    dim, nlist, nlist, qname, oversample);
+
+  rc = sqlite3_exec(db, sql, NULL, NULL, NULL);
+  if (rc != SQLITE_OK) { sqlite3_close(db); return 0; }
+
+  // Insert vectors with diverse values
+  sqlite3_stmt *stmtInsert = NULL;
+  sqlite3_prepare_v2(db,
+    "INSERT INTO v(rowid, emb) VALUES (?, ?)", -1, &stmtInsert, NULL);
+  if (!stmtInsert) { sqlite3_close(db); return 0; }
+
+  size_t offset = 0;
+  for (int i = 0; i < num_vecs; i++) {
+    float *vec = sqlite3_malloc(dim * sizeof(float));
+    if (!vec) break;
+    for (int d = 0; d < dim; d++) {
+      if (offset + 4 <= payload_size) {
+        // Use raw bytes as float for adversarial values
+        memcpy(&vec[d], payload + offset, sizeof(float));
+        offset += 4;
+        // Sanitize: replace NaN/Inf with bounded values to avoid
+        // poisoning the entire computation. We want edge values,
+        // not complete nonsense.
+        if (isnan(vec[d]) || isinf(vec[d])) {
+          vec[d] = (vec[d] > 0) ? 1e6f : -1e6f;
+          if (isnan(vec[d])) vec[d] = 0.0f;
+        }
+      } else if (offset < payload_size) {
+        vec[d] = ((float)(int8_t)payload[offset++]) / 30.0f;
+      } else {
+        vec[d] = (float)(i * dim + d) * 0.001f;
+      }
+    }
+    sqlite3_reset(stmtInsert);
+    sqlite3_bind_int64(stmtInsert, 1, (int64_t)(i + 1));
+    sqlite3_bind_blob(stmtInsert, 2, vec, dim * sizeof(float), SQLITE_TRANSIENT);
+    sqlite3_step(stmtInsert);
+    sqlite3_free(vec);
+  }
+  sqlite3_finalize(stmtInsert);
+
+  // Train
+  sqlite3_exec(db,
+    "INSERT INTO v(rowid) VALUES ('compute-centroids')",
+    NULL, NULL, NULL);
+
+  // Multiple KNN queries to exercise rescore path
+  for (int q = 0; q < 4; q++) {
+    float *qvec = sqlite3_malloc(dim * sizeof(float));
+    if (!qvec) break;
+    for (int d = 0; d < dim; d++) {
+      if (offset < payload_size) {
+        qvec[d] = ((float)(int8_t)payload[offset++]) / 10.0f;
+      } else {
+        qvec[d] = (q == 0) ? 1.0f : (q == 1) ? -1.0f : 0.0f;
+      }
+    }
+
+    sqlite3_stmt *sk = NULL;
+    snprintf(sql, sizeof(sql),
+      "SELECT rowid, distance FROM v WHERE emb MATCH ? LIMIT %d", k_limit);
+    sqlite3_prepare_v2(db, sql, -1, &sk, NULL);
+    if (sk) {
+      sqlite3_bind_blob(sk, 1, qvec, dim * sizeof(float), SQLITE_TRANSIENT);
+      while (sqlite3_step(sk) == SQLITE_ROW) {}
+      sqlite3_finalize(sk);
+    }
+    sqlite3_free(qvec);
+  }
+
+  // Delete some vectors, then query again (rescore with missing _ivf_vectors rows)
+  for (int i = 1; i <= num_vecs / 3; i++) {
+    char delsql[64];
+    snprintf(delsql, sizeof(delsql), "DELETE FROM v WHERE rowid = %d", i);
+    sqlite3_exec(db, delsql, NULL, NULL, NULL);
+  }
+
+  {
+    float *qvec = sqlite3_malloc(dim * sizeof(float));
+    if (qvec) {
+      for (int d = 0; d < dim; d++) qvec[d] = 0.5f;
+      sqlite3_stmt *sk = NULL;
+      snprintf(sql, sizeof(sql),
+        "SELECT rowid, distance FROM v WHERE emb MATCH ? LIMIT %d", k_limit);
+      sqlite3_prepare_v2(db, sql, -1, &sk, NULL);
+      if (sk) {
+        sqlite3_bind_blob(sk, 1, qvec, dim * sizeof(float), SQLITE_TRANSIENT);
+        while (sqlite3_step(sk) == SQLITE_ROW) {}
+        sqlite3_finalize(sk);
+      }
+      sqlite3_free(qvec);
+    }
+  }
+
+  // Retrain after deletions
+  sqlite3_exec(db,
+    "INSERT INTO v(rowid) VALUES ('compute-centroids')",
+    NULL, NULL, NULL);
+
+  // Query after retrain
+  {
+    float *qvec = sqlite3_malloc(dim * sizeof(float));
+    if (qvec) {
+      for (int d = 0; d < dim; d++) qvec[d] = -0.3f;
+      sqlite3_stmt *sk = NULL;
+      snprintf(sql, sizeof(sql),
+        "SELECT rowid, distance FROM v WHERE emb MATCH ? LIMIT %d", k_limit);
+      sqlite3_prepare_v2(db, sql, -1, &sk, NULL);
+      if (sk) {
+        sqlite3_bind_blob(sk, 1, qvec, dim * sizeof(float), SQLITE_TRANSIENT);
+        while (sqlite3_step(sk) == SQLITE_ROW) {}
+        sqlite3_finalize(sk);
+      }
+      sqlite3_free(qvec);
+    }
+  }
+
+  sqlite3_close(db);
+  return 0;
+}