ecmult: add function to compute optimal scratch space size

bitcoin-core · Jan 29, 2022 · 0d574b4 · 0d574b4
1 parent 2d7a903
commit 0d574b4
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Showing 2 changed files with 28 additions and 8 deletions.
diff --git a/src/ecmult_impl.h b/src/ecmult_impl.h
@@ -986,4 +986,20 @@ static int secp256k1_ecmult_multi_var(const secp256k1_callback* error_callback,
  return 1;
 }
 
+/**
+ * Returns the optimal scratch space size for a given number of points
+ * excluding base point G.
+ */
+static size_t secp256k1_ecmult_multi_scratch_size(size_t n_points) {
+ if (n_points > ECMULT_MAX_POINTS_PER_BATCH) {
+ n_points = ECMULT_MAX_POINTS_PER_BATCH;
+ }
+ if (n_points >= ECMULT_PIPPENGER_THRESHOLD) {
+ int bucket_window = secp256k1_pippenger_bucket_window(n_points);
+ return secp256k1_pippenger_scratch_size(n_points, bucket_window);
+ } else {
+ return secp256k1_strauss_scratch_size(n_points);
+ }
+}
+
 #endif /* SECP256K1_ECMULT_IMPL_H */
diff --git a/src/tests.c b/src/tests.c
@@ -4509,14 +4509,8 @@ void test_ecmult_multi_batching(void) {
  secp256k1_scratch_destroy(&ctx->error_callback, scratch);
 
  for(i = 1; i <= n_points; i++) {
- if (i >= ECMULT_PIPPENGER_THRESHOLD) {
- int bucket_window = secp256k1_pippenger_bucket_window(i);
- size_t scratch_size = secp256k1_pippenger_scratch_size(i, bucket_window);
- scratch = secp256k1_scratch_create(&ctx->error_callback, scratch_size);
- } else {
- size_t scratch_size = secp256k1_strauss_scratch_size(i);
- scratch = secp256k1_scratch_create(&ctx->error_callback, scratch_size);
- }
+ size_t scratch_size = secp256k1_ecmult_multi_scratch_size(i);
+ scratch = secp256k1_scratch_create(&ctx->error_callback, scratch_size);
  CHECK(secp256k1_ecmult_multi_var(&ctx->error_callback, scratch, &r, &scG, ecmult_multi_callback, &data, n_points));
  secp256k1_gej_add_var(&r, &r, &r2, NULL);
  CHECK(secp256k1_gej_is_infinity(&r));
@@ -4526,6 +4520,15 @@ void test_ecmult_multi_batching(void) {
  free(pt);
 }
 
+void test_ecmult_multi_scratch_size(void) {
+ /* test ECMULT_MAX_POINTS_PER_BATCH limit */
+ size_t n_points = ECMULT_MAX_POINTS_PER_BATCH + 1;
+ size_t scratch_size = secp256k1_ecmult_multi_scratch_size(n_points);
+ int expected_bucket_window = secp256k1_pippenger_bucket_window(n_points - 1);
+ size_t expected_scratch_size = secp256k1_pippenger_scratch_size(n_points - 1, expected_bucket_window);
+ CHECK(expected_scratch_size == scratch_size);
+}
+
 void run_ecmult_multi_tests(void) {
  secp256k1_scratch *scratch;
  int64_t todo = (int64_t)320 * count;
@@ -4553,6 +4556,7 @@ void run_ecmult_multi_tests(void) {
 
  test_ecmult_multi_batch_size_helper();
  test_ecmult_multi_batching();
+ test_ecmult_multi_scratch_size();
 }
 
 void test_wnaf(const secp256k1_scalar *number, int w) {