google-billboard-challenge/google-billboard.c

#include <inttypes.h>
#include <math.h>
#include <stdio.h>

/*
 * Function Prototypes
 */
int isPrime(uint64_t);
int isNumeric(char);
int isFullLength(char[]);
uint64_t arrToNum(char[], int);
void modBuff(char*, int);

/*
 * Main Method
 */
int main(int argc, char **argv) {
    /* Open the site as a file for reading the source */
    FILE *fptr = popen("wget --quiet -O - http://www-history.mcs.st-andrews.ac.uk/HistTopics/e_10000.html", "r");
    
    /* Variables --------------------------------------------------------- */
    int ctr = 1, itr; /* Two counter variables for the for loop and count  */
    char buf[10];     /* A character buffer to store the individual digits */
    char cptr;        /* A character to hold the next item in the stream   */
    uint64_t num;     /* An unsigned 64-bit integer to hold the number     */
    /* ------------------------------------------------------------------- */
    
    /* Move the pointer to the numbers past the decimal point */
    while ((cptr = fgetc(fptr)) != '.');
    
    /* Put the first 10-digit number in the buffer */
    for (itr = 0; itr < 10; itr++) {
        /* Grab the next character */
        cptr = fgetc(fptr);
        
        /* Check to see if the character is numeric */
        if (isNumeric(cptr)) {
            /* Store it in the buffer if it is */
            buf[itr] = cptr;
        } else {
            /* Peel off any non-numeric characters */
            while (!isNumeric(cptr)) cptr = fgetc(fptr);
            
            /* Assign the character to the buffer if it's numeric */
            if (isNumeric(cptr)) buf[itr] = cptr;
        }
    }
    
    /* Iterate across the data and test the numbers */
    do {
        /* Check that the 10-digit number doesn't actually start with a 0,
         * and skip this iteration if it does. */
        if (isFullLength(buf)) {
            /* Convert the buffer into an integer */
            num = arrToNum(buf, 10);
            
            /* Print the number and increment the counter */
            printf("%02d.) %" PRIu64 "", ctr++, num);
            
            /* Check for primality of the number to exit the loop */
            if (isPrime(num)) {
                printf (" (1st 10-digit prime)\n");
                break;
            }
            
            /* Separate the numbers */
            printf("\n");
        }
        
        /* Grab the next character */
        cptr = fgetc(fptr);
        
        /* Peel off any non-numeric characters due to pre-formatted text */
        while (!isNumeric(cptr) && cptr != '<') cptr = fgetc(fptr);
        
        /* Check to make sure we're not at the end of the list */
        if (cptr == '<') break;
        
        /* Slide the unused numbers in the buffer down one step */
        modBuff(buf, 10);
        
        /* Put the next character at the end of the buffer */
        buf[9] = cptr;
    } while (isNumeric(cptr));
    
    /* Close the website pointer */
    pclose(fptr);
    
    /* Exit */
    return 0;
}

/*
 * isPrime(uint64_t)
 * Checks for primality of a number
 */
int isPrime(uint64_t num) {
    if (num < 2) return 0;
    if (num == 2 || num == 3) return 1;
    if (num % 2 == 0) return 0;
    uint64_t sr = ceil(pow(num, 0.5));
    uint64_t ctr;
    for (ctr = 3; ctr <= sr; ctr += 2) if (num % ctr == 0) return 0;
    return 1;
}

/*
 * isNumeric(char)
 * Checks to make sure the character passed is a number
 */
int isNumeric(char num) {
    return (num - '0' >= 0 && num - '0' <= 9);
}

/*
 * isFullLength(char[])
 * Simply checks to see that an n-digit number is, in fact, n-digits long
 */
int isFullLength(char arr[]) {
    return (arr[0] - '0');
}

/*
 * arrToNum(char[], int)
 * Converts a character array to an integer
 */
uint64_t arrToNum(char arr[], int len) {
    uint64_t res = 0;
    int ctr;
    for (ctr = 0; ctr < len; ctr++) res = (res * 10) + (arr[ctr] - '0');
    return res;
}

/*
 * modBuff(*char, int)
 * Slides the elements in the buffer down one slot
 */
void modBuff(char *arr, int len) {
    int ctr;
    for (ctr = 0; ctr < (len - 1); ctr++) arr[ctr] = arr[ctr + 1];
    return;
}