Fast multiplication

Makefile

@@ -10,6 +10,7 @@ all:
 	@$(CC) $(CFLAGS) bn.c ./tests/factorial.c   -o ./build/test_factorial
 	@$(CC) $(CFLAGS) bn.c ./tests/randomized.c  -o ./build/test_random
 	@#$(CC) $(CFLAGS) bn.c ./tests/rsa.c         -o ./build/test_rsa
+	@$(CC) $(CFLAGS) bn.c ./tests/mul_test.c         -o ./build/test_mul
 
 
 test:

bn.c

@@ -273,6 +273,45 @@ void bignum_mul(struct bn* a, struct bn* b, struct bn* c)
 }
 
 
+void bignum_mul_alt(struct bn *a, struct bn *b, struct bn *c)
+{
+  require(a, "a is null");
+  require(b, "b is null");
+  require(c, "c is null");
+
+  bignum_init(c);
+  DTYPE_TMP tmp = 0;
+  DTYPE tmp_to_add = 0;
+
+  int usable_len = BN_ARRAY_SIZE;
+
+  /*  this section speads up algorithm by "cutting" len of bignum*/
+  for (int i = BN_ARRAY_SIZE - 1; i >= 0; --i)
+  {
+    if (a->array[i] != 0 || b->array[i] != 0)
+    {
+        usable_len = 2 * (i + 1);
+        break;
+    }
+  }
+
+  usable_len = usable_len > BN_ARRAY_SIZE ? BN_ARRAY_SIZE : usable_len;
+  //
+
+  for (int i = 0; i < usable_len; ++i)
+  {
+    c->array[i] += tmp_to_add;
+    tmp_to_add = tmp = 0;
+    for (int j = 0, k = i; j < i + 1 && j < usable_len; ++j, --k)
+    {
+      tmp = (DTYPE_TMP)a->array[j] * (DTYPE_TMP)b->array[k];
+      tmp_to_add += tmp >> 32;
+      c->array[i] += tmp;
+    }
+  }
+}
+
+
 void bignum_div(struct bn* a, struct bn* b, struct bn* c)
 {
   require(a, "a is null");

bn.h

@@ -29,8 +29,9 @@ There may well be room for performance-optimizations and improvements.
   #define WORD_SIZE 4
 #endif
 
-/* Size of big-numbers in bytes */
-#define BN_ARRAY_SIZE    (128 / WORD_SIZE)
+/* Size of big-numbers in WORDS */ // because you dividing by WORD size
+// #define BN_ARRAY_SIZE    (128 / WORD_SIZE)
+#define BN_ARRAY_SIZE    1024
 
 
 /* Here comes the compile-time specialization for how large the underlying array size should be. */
@@ -97,6 +98,7 @@ void bignum_to_string(struct bn* n, char* str, int maxsize);
 void bignum_add(struct bn* a, struct bn* b, struct bn* c); /* c = a + b */
 void bignum_sub(struct bn* a, struct bn* b, struct bn* c); /* c = a - b */
 void bignum_mul(struct bn* a, struct bn* b, struct bn* c); /* c = a * b */
+void bignum_mul_alt(struct bn* a, struct bn* b, struct bn* c); /* c = a * b alternative, much faster method*/
 void bignum_div(struct bn* a, struct bn* b, struct bn* c); /* c = a / b */
 void bignum_mod(struct bn* a, struct bn* b, struct bn* c); /* c = a % b */
 void bignum_divmod(struct bn* a, struct bn* b, struct bn* c, struct bn* d); /* c = a/b, d = a%b */

tests/mul_test.c

@@ -0,0 +1,107 @@
+#include <stdio.h>
+#include <time.h>
+#include <stdlib.h>
+
+#include "bn.h"
+
+int mul_get_clocks(struct bn *a, struct bn *b, struct bn *c)
+{
+    int clocks = clock();
+    bignum_mul(a, b, c);
+    return clock() - clocks;
+}
+
+int mul_alt_get_clocks(struct bn *a, struct bn *b, struct bn *c)
+{
+    int clocks = clock();
+    bignum_mul_alt(a, b, c);
+    return clock() - clocks;
+}
+
+int int_test()
+{
+    struct bn a, b, c;
+    int clocks = 0,
+        clocks_alt = 0,
+        num1 = 0,
+        num2 = 0;
+    char res[17000] = {0},
+         res_alt[17000] = {0};
+    bignum_init(&a);
+    bignum_init(&b);
+    bignum_init(&c);
+    printf("method\tnum1\tnum2\tresult\tclocks\n");
+
+    for (int i = 0; i < 10; ++i)
+    {
+        num1 = rand();
+        num2 = rand();
+
+        bignum_from_int(&a, num1);
+        bignum_from_int(&b, num2);
+
+        clocks = mul_get_clocks(&a, &b, &c);
+        bignum_to_string(&c, res, 17000);
+
+        clocks_alt = mul_alt_get_clocks(&a, &b, &c);
+        bignum_to_string(&c, res_alt, 17000);
+
+        printf("normal\t%d\t%d\t%s\t%d\n", num1, num2, res, clocks);
+        printf("alter\t%d\t%d\t%s\t%d\n", num1, num2, res_alt, clocks_alt);
+        printf("\n");
+    }
+
+    return 0;
+}
+
+int bigger_then_int_test()
+{
+    struct bn a, b, c;
+    int clocks = 0,
+        clocks_alt = 0;
+    char res[17000] = {0},
+         res_alt[17000] = {0},
+         num1[17000] = {0},
+         num2[17000] = {0};
+
+    bignum_init(&a);
+    bignum_init(&b);
+    bignum_init(&c);
+    printf("method\tnum1\tnum2\tresult\tclocks\n");
+
+    for (int i = 0; i < 10; ++i)
+    {
+        a.array[0] = rand();
+        a.array[1] = rand();
+        a.array[2] = rand();
+        a.array[3] = rand();
+
+        b.array[0] = rand();
+        b.array[1] = rand();
+        b.array[2] = rand();
+        b.array[3] = rand();
+
+        bignum_to_string(&a, num1, 17000);
+        bignum_to_string(&b, num2, 17000);
+
+        clocks = mul_get_clocks(&a, &b, &c);
+        bignum_to_string(&c, res, 17000);
+
+        clocks_alt = mul_alt_get_clocks(&a, &b, &c);
+        bignum_to_string(&c, res_alt, 17000);
+
+        printf("normal\t%s\t%s\t%s\t%d\n", num1, num2, res, clocks);
+        printf("alter\t%s\t%s\t%s\t%d\n", num1, num2, res_alt, clocks_alt);
+        printf("\n");
+    }
+
+    return 0;
+}
+
+int main()
+{
+    srand(time(NULL));
+    int_test();
+    bigger_then_int_test();
+    return 0;
+}