Multiply strings

Signed-off-by: begeekmyfriend <[email protected]>

043_multiply_strings/Makefile

@@ -0,0 +1,2 @@
+all:
+	gcc -O2 -o test multiply_strings.c

043_multiply_strings/multiply_strings.c

@@ -0,0 +1,67 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+static void reverse(char *s, int len)
+{
+    int low = 0;
+    int high = len - 1;
+    while (low < high) {
+        char c = s[low];
+        s[low] = s[high];
+        s[high] = c;
+        low++;
+        high--;
+    }
+}
+
+static char* multiply(char* num1, char* num2) {
+    if (*num1 == '\0') {
+        return num1;
+    }
+    if (*num2 == '\0') {
+        return num2;
+    }
+
+    int i, j;
+    char *result = malloc(110 + 110);
+    memset(result, '0', 220);
+    int len1 = strlen(num1);
+    int len2 = strlen(num2);
+    reverse(num1, len1);
+    reverse(num2, len2);
+    for (i = 0; i < len1; i++) {
+        int carry = 0;
+        for (j = 0; j < len2; j++) {
+            carry += (num1[i] - '0') * (num2[j] - '0') + (result[i + j] - '0');
+            result[i + j] = carry % 10 + '0';
+            carry /= 10;
+        }
+        if (carry != 0) {
+            result[len2 + i] = carry + '0';
+        }
+    }
+    int len = 220;
+    while (--len >= 0) {
+        if (result[len] > '0') {
+            result[++len] = '\0';
+            break;
+        }
+    }
+    if (len == -1) {
+        len = 1;
+        result[len] = '\0';
+    }
+    reverse(result, len);
+    return result;
+}
+
+int main(int argc, char **argv)
+{
+    if (argc != 3) {
+        fprintf(stderr, "Usage: ./test m1 m2\n");
+        exit(-1);
+    }
+    printf("%s\n", multiply(argv[1], argv[2]));
+    return 0;
+}