Improve Big Number implementation

Author: blueskyson

lib/unsigned_bignum.h

@@ -7,9 +7,30 @@ typedef struct BigN {
     unsigned long long cell[BIGNSIZE];
 } ubig;
 
-#define init_ubig(x) for (int i = 0; i < BIGNSIZE; x.cell[i++] = 0)
+static inline void init_ubig(ubig *x)
+{
+    for (int i = 0; i < BIGNSIZE; x->cell[i++] = 0)
+        ;
+}
+
+/**
+ * ubig_add() - Add two large unsigned integers.
+ * @dest: Pointer to the destination ubig where the result will be stored.
+ * @a: Pointer to the first ubig.
+ * @b: Pointer to the second ubig.
+ *
+ * This function performs addition of two large unsigned integers represented by
+ * the `ubig` structures pointed to by @a and @b. The result of the addition is
+ * stored in the `ubig` structure pointed to by @dest.
+ */
+static inline void ubig_add(ubig *dest, ubig *a, ubig *b)
+{
+    for (int i = 0; i < BIGNSIZE; i++)
+        dest->cell[i] = a->cell[i] + b->cell[i];
 
-#define P10_UINT64 10000000000000000000ULL
+    for (int i = 0; i < BIGNSIZE - 1; i++)
+        dest->cell[i + 1] += (dest->cell[i] < a->cell[i]);
+}
 
 /**
  * fib_sequence() - Calculate the k-th Fibonacci number.
@@ -21,28 +42,18 @@ static ubig fib_sequence(long long k)
 {
     if (k <= 1LL) {
         ubig result;
-        init_ubig(result);
+        init_ubig(&result);
         result.cell[0] = (unsigned long long) k;
         return result;
     }
 
     ubig a, b, c;
-    init_ubig(a);
-    init_ubig(b);
-    b.cell[0] = 1;
+    init_ubig(&a);
+    init_ubig(&b);
+    b.cell[0] = 1ULL;
 
     for (int i = 2; i <= k; i++) {
-        for (int j = 0; j < BIGNSIZE; j++)
-            c.cell[j] = a.cell[j] + b.cell[j];
-
-        for (int j = 0; j < BIGNSIZE - 1; j++) {
-            // if lower overflows or lower >= 10^9, add a carry to upper
-            if ((c.cell[j] < a.cell[j]) || (c.cell[j] >= P10_UINT64)) {
-                c.cell[j] -= P10_UINT64;
-                c.cell[j + 1] += 1;
-            }
-        }
-
+        ubig_add(&c, &a, &b);
         a = b;
         b = c;
     }
@@ -64,22 +75,30 @@ int fib_to_string(char *buf, int buf_sz, void *fib, int fib_sz)
 {
     const ubig *f = (ubig *) fib;
 
-    memset(buf, '\0', buf_sz);
+    memset(buf, '0', buf_sz);
+    buf[buf_sz - 1] = '\0';
     int index = BIGNSIZE - 1;
     while (index >= 0 && !f->cell[index])
         index--;
     if (index == -1) {
-        buf[0] = '0';
-        buf[1] = '\0';
-        return 0;
+        return buf_sz - 2;
     }
 
-    char buf2[22];
-    snprintf(buf2, 22, "%llu", f->cell[index--]);
-    strcat(buf, buf2);
-    while (index >= 0) {
-        snprintf(buf2, 22, "%019llu", f->cell[index--]);
-        strcat(buf, buf2);
+    for (int i = index; i >= 0; i--) {
+        for (unsigned long long mask = 0x8000000000000000ULL; mask;
+             mask >>= 1) {
+            int carry = ((f->cell[i] & mask) != 0);
+            for (int j = buf_sz - 2; j >= 0; j--) {
+                buf[j] += buf[j] - '0' + carry;
+                carry = (buf[j] > '9');
+                if (carry)
+                    buf[j] -= 10;
+            }
+        }
     }
-    return 0;
+
+    int offset = 0;
+    while (buf[offset] == '0')
+        offset++;
+    return (buf[offset] == '\0') ? (offset - 1) : offset;
 }