Shortcut polynomial evaluation for x = 0

src/blst_util.c

@@ -17,6 +17,14 @@
 #include "blst_util.h"
 #include "debug_util.h"
 
+// TODO - find a better way to do this
+bool fr_is_zero(const blst_fr *p) {
+    uint64_t a[4];
+    blst_uint64_from_fr(a, p);
+    return a[0] == 0 && a[1] == 0 && a[2] == 0 && a[3] == 0;
+}
+
+// TODO - find a better way to do this
 bool fr_is_one(const blst_fr *p) {
     uint64_t a[4];
     blst_uint64_from_fr(a, p);

src/blst_util.h

@@ -16,13 +16,16 @@
 
 #include "c_kzg.h"
 
+static const blst_fr fr_zero = {0L, 0L, 0L, 0L};
+
 // This is 1 in Blst's `blst_fr` limb representation. Crazy but true.
-static const blst_fr one =
+static const blst_fr fr_one =
     {0x00000001fffffffeL, 0x5884b7fa00034802L, 0x998c4fefecbc4ff5L, 0x1824b159acc5056fL};
 
 // The G1 identity/infinity in affine representation
-static const blst_p1_affine identity_g1_affine = {{0,0,0,0,0,0},{0,0,0,0,0,0}};
+static const blst_p1_affine identity_g1_affine = {{0L,0L,0L,0L,0L,0L},{0L,0L,0L,0L,0L,0L}};
 
+bool fr_is_zero(const blst_fr *p);
 bool fr_is_one(const blst_fr *p);
 void fr_from_uint64(blst_fr *a, const uint64_t n);
 bool fr_equal(const blst_fr *aa, const blst_fr *bb);

src/blst_util_test.c

@@ -23,14 +23,20 @@ uint64_t m1[] = {0xffffffff00000000L, 0x53bda402fffe5bfeL, 0x3339d80809a1d805L,
 
 void title(void) {;}
 
+void fr_is_zero_works(void) {
+    blst_fr zero;
+    fr_from_uint64(&zero, 0);
+    TEST_CHECK(fr_is_zero(&zero));
+}
+
 void fr_is_one_works(void) {
-    TEST_CHECK(true == fr_is_one(&one));
+    TEST_CHECK(fr_is_one(&fr_one));
 }
 
 void fr_from_uint64_works(void) {
     blst_fr a;
     fr_from_uint64(&a, 1);
-    TEST_CHECK(true == fr_is_one(&a));
+    TEST_CHECK(fr_is_one(&a));
 }
 
 void fr_equal_works(void) {
@@ -156,6 +162,7 @@ void pairings_work(void) {
 TEST_LIST =
     {
      {"BLST_UTIL_TEST", title},
+     {"fr_is_zero_works", fr_is_zero_works },
      {"fr_is_one_works", fr_is_one_works },
      {"fr_from_uint64_works", fr_from_uint64_works},
      {"fr_equal_works", fr_equal_works},

src/fft_common.c

@@ -25,7 +25,7 @@ bool is_power_of_two(const uint64_t n) {
 // Create an array of powers of the root of unity
 // The `out` array must be of size `width + 1`
 C_KZG_RET expand_root_of_unity(blst_fr *roots, const blst_fr *root_of_unity, const uint64_t width) {
-    roots[0] = one;
+    roots[0] = fr_one;
     roots[1] = *root_of_unity;
 
     for (int i = 2; !fr_is_one(&roots[i - 1]); i++) {

src/fft_common_test.c

@@ -46,9 +46,9 @@ void reverse_works(void) {
     blst_fr diff;
 
     // Initialise - increasing values
-    arr[0] = one;
+    arr[0] = fr_one;
     for (int i = 1; i <= n; i++) {
-        blst_fr_add(arr + i, arr + i - 1, &one);
+        blst_fr_add(arr + i, arr + i - 1, &fr_one);
     }
 
     // Reverse

src/kzg_single_proofs.c

@@ -32,7 +32,7 @@ C_KZG_RET compute_proof_single(blst_p1 *out, const KZGSettings *ks, poly *p, con
     poly_init(&divisor, 2);
     fr_from_uint64(&tmp, x0);
     fr_negate(&divisor.coeffs[0],&tmp);
-    divisor.coeffs[1] = one;
+    divisor.coeffs[1] = fr_one;
 
     // Calculate q = p / (x - x0)
     // Discard the return codes since we already checked above that all should be fine.

src/kzg_single_proofs_test.c

@@ -21,7 +21,7 @@
 void generate_setup(blst_p1 *s1, blst_p2 *s2, const blst_scalar *secret, const uint64_t n) {
     blst_fr s_pow, s;
     blst_fr_from_scalar(&s, secret);
-    s_pow = one;
+    s_pow = fr_one;
     for (uint64_t i = 0; i < n; i++) {
         p1_mul(&s1[i], blst_p1_generator(), &s_pow);
         p2_mul(&s2[i], blst_p2_generator(), &s_pow);

src/poly.c

@@ -36,7 +36,9 @@ void poly_eval(blst_fr *out, const poly *p, const blst_fr *x) {
     if (p->length == 0) {
         fr_from_uint64(out, 0);
     }
-    // TODO x = 0 case
+    if (fr_is_zero(x)) {
+        *out = p->coeffs[0];
+    }
 
     // Horner's method
     *out = p->coeffs[p->length - 1];

src/poly_test.c

@@ -121,7 +121,22 @@ void poly_eval_check(void) {
     }
     fr_from_uint64(&expected, n * (n + 1) / 2);
 
-    poly_eval(&res, &p, &one);
+    poly_eval(&res, &p, &fr_one);
+
+    TEST_CHECK(fr_equal(&expected, &res));
+}
+
+void poly_eval_0_check(void) {
+    uint64_t n = 7, a = 597;
+    blst_fr res, expected;
+    poly p;
+    poly_init(&p, n);
+    for (uint64_t i = 0; i < n; i++) {
+        fr_from_uint64(&p.coeffs[i], i + a);
+    }
+    fr_from_uint64(&expected, a);
+
+    poly_eval(&res, &p, &fr_zero);
 
     TEST_CHECK(fr_equal(&expected, &res));
 }
@@ -135,5 +150,6 @@ TEST_LIST =
      {"poly_div_1", poly_div_1},
      {"poly_wrong_size", poly_wrong_size},
      {"poly_eval_check", poly_eval_check},
+     {"poly_eval_0_check", poly_eval_0_check},
      { NULL, NULL }     /* zero record marks the end of the list */
     };