mirror of
https://github.com/TheAlgorithms/C
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117 lines
3.3 KiB
C
117 lines
3.3 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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/** Compute the product of two numbers in a file
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*
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* \param[in] fp pointer to file that is already open
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* \param[in] start_pos line number of the first numer
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* \param[in] num_digits number of digits on the line to multiply
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* \returns expected product
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*/
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long long int get_product(FILE *fp, long start_pos, int num_digits)
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{
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char ch = ' '; /* temporary variable to store character read from file */
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unsigned char num = 0; /* temporary variable to store digit read */
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long long int prod = 1; /* product accumulator */
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int count =
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0; /* we use this variable to count number of bytes of file read */
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/* accumulate product for num_digits */
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for (int i = 0; i < num_digits; i++, count++)
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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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if (ch == EOF)
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return 0;
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i--;
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continue;
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}
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num = ch - 0x30; /* convert character digit to number */
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if (num == 0)
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{
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/* If number is zero, we can skip the next 'num_digits'
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* because this '0' will repeat in the next 'num_digit'
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* multiplications. Hence, we also do not update the file position
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*/
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/* NOTE: this is not needed but helps get results faster :) */
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return 0;
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}
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prod *= num; /* accumulate product */
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}
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/* set file position to the next starting character + 1 */
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fseek(fp, -count + 1, SEEK_CUR);
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return prod;
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}
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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;
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int num_digits = 4;
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long long int prod, max_prod = 0;
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/* if second command-line argument is ge=iven,
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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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/* 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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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 product of 'num_digits' from current position in file */
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prod = get_product(fp, ftell(fp), num_digits);
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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) - 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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return 0;
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}
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