brute-force method - O(n^2)

project_euler/Problem 08/sol1.c

@@ -0,0 +1,100 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+int64_t get_product(FILE *fp, long start_pos, int num_digits)
+{
+    char ch = ' ';      /* temporary variable to store character read from file */
+    uint8_t num = 0;    /* temporary variable to store digit read */
+    int64_t prod = 1;   /* product accumulator */
+    int count = 0;      /* we use this variable to count number of bytes of file read */
+
+    /* accumulate product for num_digits */
+    for(int i = 0; i < num_digits; i++, count++)
+    {
+        /* get character from file */
+        ch = getc(fp);
+
+        /* the ASCII codes of digits is between 0x30 and 0x39. 
+         * any character not in this range implies an invalid character
+         */
+        if (ch < 0x30 || ch > 0x39)
+        {
+            if (ch == EOF)
+                return 0;
+            i--;
+            continue;
+        }
+        
+        num = ch - 0x30; /* convert character digit to number */
+        if (num == 0)
+        {
+            /* If number is zero, we can skip the next 'num_digits' 
+             * because this '0' will repeat in the next 'num_digit' multiplications.
+             * Hence, we also do not update the file position */
+            /* NOTE: this is not needed but helps get results faster :) */
+            return 0;
+        }
+
+        prod *= num;     /* accumulate product */
+    }
+
+    /* set file position to the next starting character + 1 */
+    fseek(fp, -count + 1, SEEK_CUR);
+    
+    return prod;
+}
+
+
+int main(int argc, char* argv[])
+{
+    int position = 0;
+    int num_digits = 4;
+    int64_t prod, max_prod = 0;
+
+    /* if second command-line argument is ge=iven,
+     * use it as the number of digits to compute 
+     * successive product for
+     */
+    if (argc == 2)
+        num_digits = atoi(argv[1]);
+
+    /* open file to read digits from */
+    FILE *fp = fopen("digits.txt", "rt");
+    if (!fp)
+    {
+        perror("Unable to open file");
+        return -1;
+    }
+
+    /* loop through all digits in the file */
+    do 
+    {
+        /* get product of 'num_digits' from current position in file */
+        prod = get_product(fp, ftell(fp), num_digits);
+        
+        if (prod > max_prod)
+        {
+            max_prod = prod;
+            position = ftell(fp) - 1;
+        }
+    } while(!feof(fp));      /* loop till end of file is reached */
+    
+    printf("Maximum product: %lld\t Location: %d^th position\n\t", max_prod, position);
+    fseek(fp, position, SEEK_SET);  /* move cursor to identified position in file */
+    /* loop through all digits */
+    for (; num_digits > 0; num_digits--)
+    {
+        char ch = getc(fp);     /* get character */
+        /* skip invalid character */
+        if (ch < 0x30 || ch > 0x39)
+            continue;
+        if (num_digits > 1)
+            printf("%c x ", ch);
+        else
+            printf("%c = %lld\n", ch, max_prod);
+    }
+
+    fclose(fp);     /* close file */
+
+    return 0;
+}