Move field element division to blst_util

src/blst_util.c

@@ -62,6 +62,11 @@ void fr_pow(blst_fr *out, const blst_fr *a, uint64_t n) {
     }
 }
 
+void fr_div(blst_fr *out, const blst_fr *a, const blst_fr *b) {
+    blst_fr_eucl_inverse(out, b);
+    blst_fr_mul(out, out, a);
+}
+
 void p1_mul(blst_p1 *out, const blst_p1 *a, const blst_fr *b) {
     blst_scalar s;
     blst_scalar_from_fr(&s, b);

src/blst_util.h

@@ -32,6 +32,7 @@ void fr_from_uint64(blst_fr *a, const uint64_t n);
 bool fr_equal(const blst_fr *aa, const blst_fr *bb);
 void fr_negate(blst_fr *out, const blst_fr *in);
 void fr_pow(blst_fr *out, const blst_fr *a, const uint64_t n);
+void fr_div(blst_fr *out, const blst_fr *a, const blst_fr *b);
 void p1_mul(blst_p1 *out, const blst_p1 *a, const blst_fr *b);
 void p1_sub(blst_p1 *out, const blst_p1 *a, const blst_p1 *b);
 void p2_mul(blst_p2 *out, const blst_p2 *a, const blst_fr *b);

src/blst_util_test.c

@@ -74,6 +74,18 @@ void fr_pow_works(void) {
     TEST_CHECK(fr_equal(&expected, &actual));
 }
 
+void fr_div_works(void) {
+    blst_fr a, b, tmp, actual;
+
+    fr_from_uint64(&a, 197);
+    fr_from_uint64(&b, 123456);
+
+    fr_div(&tmp, &a, &b);
+    blst_fr_mul(&actual, &tmp, &b);
+
+    TEST_CHECK(fr_equal(&a, &actual));
+}
+
 void p1_mul_works(void) {
     blst_fr minus1;
     blst_p1 g1_gen, g1_gen_neg, res;
@@ -184,6 +196,7 @@ TEST_LIST = {
     {"fr_equal_works", fr_equal_works},
     {"fr_negate_works", fr_negate_works},
     {"fr_pow_works", fr_pow_works},
+    {"fr_div_works", fr_div_works},
     {"p1_mul_works", p1_mul_works},
     {"p1_sub_works", p1_sub_works},
     {"p2_mul_works", p2_mul_works},

src/poly.c

@@ -20,30 +20,6 @@
 #include "c_kzg_util.h"
 #include "poly.h"
 
-static void poly_factor_div(blst_fr *out, const blst_fr *a, const blst_fr *b) {
-    blst_fr_eucl_inverse(out, b);
-    blst_fr_mul(out, out, a);
-}
-
-C_KZG_RET init_poly(poly *out, const uint64_t length) {
-    out->length = length;
-    return c_kzg_malloc((void **)&out->coeffs, length * sizeof *out->coeffs);
-}
-
-C_KZG_RET init_poly_with_coeffs(poly *out, const uint64_t *coeffs, const uint64_t length) {
-    ASSERT(init_poly(out, length) == C_KZG_OK, C_KZG_MALLOC);
-    for (uint64_t i = 0; i < length; i++) {
-        fr_from_uint64(&out->coeffs[i], coeffs[i]);
-    }
-    return C_KZG_OK;
-}
-
-void free_poly(poly *p) {
-    if (p->coeffs != NULL) {
-        free(p->coeffs);
-    }
-}
-
 void eval_poly(blst_fr *out, const poly *p, const blst_fr *x) {
     blst_fr tmp;
     uint64_t i;
@@ -96,7 +72,7 @@ C_KZG_RET poly_long_div(poly *out, const poly *dividend, const poly *divisor) {
     }
 
     while (diff > 0) {
-        poly_factor_div(&out->coeffs[diff], &a[a_pos], &divisor->coeffs[b_pos]);
+        fr_div(&out->coeffs[diff], &a[a_pos], &divisor->coeffs[b_pos]);
         for (uint64_t i = 0; i <= b_pos; i++) {
             blst_fr tmp;
             // a[diff + i] -= b[i] * quot
@@ -106,7 +82,26 @@ C_KZG_RET poly_long_div(poly *out, const poly *dividend, const poly *divisor) {
         --diff;
         --a_pos;
     }
-    poly_factor_div(&out->coeffs[0], &a[a_pos], &divisor->coeffs[b_pos]);
+    fr_div(&out->coeffs[0], &a[a_pos], &divisor->coeffs[b_pos]);
 
     return C_KZG_OK;
 }
+
+C_KZG_RET init_poly_with_coeffs(poly *out, const uint64_t *coeffs, const uint64_t length) {
+    ASSERT(init_poly(out, length) == C_KZG_OK, C_KZG_MALLOC);
+    for (uint64_t i = 0; i < length; i++) {
+        fr_from_uint64(&out->coeffs[i], coeffs[i]);
+    }
+    return C_KZG_OK;
+}
+
+C_KZG_RET init_poly(poly *out, const uint64_t length) {
+    out->length = length;
+    return c_kzg_malloc((void **)&out->coeffs, length * sizeof *out->coeffs);
+}
+
+void free_poly(poly *p) {
+    if (p->coeffs != NULL) {
+        free(p->coeffs);
+    }
+}

src/poly.h

@@ -19,14 +19,19 @@
 #include "c_kzg.h"
 #include "blst_util.h"
 
+/**
+ * Defines a polynomial whose coefficients are members of the finite field F_r.
+ *
+ * Initialise the storage with #init_poly. After use, free the storage with #free_poly.
+ */
 typedef struct {
-    blst_fr *coeffs; // coeffs[i] is the x^i term
+    blst_fr *coeffs; /**< `coeffs[i]` is the coefficient of the `x^i` term of the polynomial. */
-    uint64_t length; // one more than the polynomial's degree
+    uint64_t length; /**< One more than the polynomial's degree */
 } poly;
 
-C_KZG_RET init_poly(poly *out, const uint64_t length);
-C_KZG_RET init_poly_with_coeffs(poly *out, const uint64_t *coeffs, const uint64_t length);
-void free_poly(poly *p);
 void eval_poly(blst_fr *out, const poly *p, const blst_fr *x);
 uint64_t poly_quotient_length(const poly *dividend, const poly *divisor);
 C_KZG_RET poly_long_div(poly *out, const poly *dividend, const poly *divisor);
+C_KZG_RET init_poly_with_coeffs(poly *out, const uint64_t *coeffs, const uint64_t length);
+C_KZG_RET init_poly(poly *out, const uint64_t length);
+void free_poly(poly *p);