mirror of
https://github.com/TheAlgorithms/C
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129 lines
3.6 KiB
C
129 lines
3.6 KiB
C
/**
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* \file
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* \brief [Problem 8](https://projecteuler.net/problem=8) solution
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* \author [Krishna Vedala](https://github.com/kvedala)
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h> /* for memmove */
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/** Main function */
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int main(int argc, char *argv[])
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{
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int position = 0, num_bad_chars = 0;
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int num_digits = 4;
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char ch;
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unsigned char num, num_prev;
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unsigned char *buffer = NULL;
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long long int prod = 1, max_prod = 0;
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/* if second command-line argument is given,
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* use it as the number of digits to compute
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* successive product for
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*/
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if (argc == 2)
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num_digits = atoi(argv[1]);
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/* allocate memory to store past values */
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buffer = calloc(num_digits, sizeof(unsigned char));
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if (!buffer)
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{
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perror("Unable to allocate memory for buffer");
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return -1;
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}
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/* open file to read digits from */
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FILE *fp = fopen("digits.txt", "rt");
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if (!fp)
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{
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perror("Unable to open file");
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free(buffer); /* free allocated memory */
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return -1;
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}
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/* loop through all digits in the file */
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do
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{
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/* get character from file */
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ch = getc(fp);
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/* the ASCII codes of digits is between 0x30 and 0x39.
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* any character not in this range implies an invalid character
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*/
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if (ch < 0x30 || ch > 0x39)
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{
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num_bad_chars++; /* this is used to get the bad characters in the
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sequence of 13 characters */
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continue;
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}
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else if (num_bad_chars > 0)
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num_bad_chars--;
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num = ch - 0x30; /* convert character digit to number */
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num_prev = buffer[0]; /* previous n^th digit */
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/* left shift the buffer -
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* using a for loop or a faster memory move
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*/
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memmove(buffer, buffer + 1, num_digits - 1);
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/*
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for (int i = 1; i < num_digits; i++)
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buffer[i-1] = buffer[i];
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*/
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buffer[num_digits - 1] = num; /* save the latest number in buffer */
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if (num_prev != 0)
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{
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/* since product is accumulated, the new product can be obtained by
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* simply multiplying the new digit and dividing with the oldest
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* digit
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*/
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prod /= num_prev; /* divide first to avoid over-flows */
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prod *= num;
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}
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else
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{
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prod = 1;
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for (int i = 0; i < num_digits; i++)
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{
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if (buffer[i] == 0)
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{
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prod = 0;
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break; /* break innermost for-loop */
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}
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prod *= buffer[i];
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}
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}
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/* check if a new maxima was found */
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if (prod > max_prod)
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{
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max_prod = prod;
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position = ftell(fp) - num_bad_chars - num_digits - 1;
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}
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} while (!feof(fp)); /* loop till end of file is reached */
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printf("Maximum product: %lld\t Location: %d^th position\n\t", max_prod,
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position);
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fseek(fp, position,
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SEEK_SET); /* move cursor to identified position in file */
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/* loop through all digits */
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for (; num_digits > 0; num_digits--)
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{
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char ch = getc(fp); /* get character */
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/* skip invalid character */
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if (ch < 0x30 || ch > 0x39)
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continue;
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if (num_digits > 1)
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printf("%c x ", ch);
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else
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printf("%c = %lld\n", ch, max_prod);
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}
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fclose(fp); /* close file */
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free(buffer); /* free allocated memory */
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return 0;
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}
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