/* The nth term of the sequence of triangle numbers is given by, tn = ½n(n+1); so the first ten triangle numbers are:
 *
 * 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, ...
 *
 * By converting each letter in a word to a number corresponding to its alphabetical position and adding these values we form a word value.
 * For example, the word value for SKY is 19 + 11 + 25 = 55 = t10. If the word value is a triangle number then we shall call the word a triangle word.
 *
 * Using words.txt (right click and 'Save Link/Target As...'), a 16K text file containing nearly two-thousand common English words,
 * how many are triangle words?*/

#define _POSIX_C_SOURCE 199309L

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

int is_triang(int n);

int main(int argc, char **argv)
{
    int i, j, n, len, value, count = 0;
    char *line, **words;
    double elapsed;
    FILE *fp;
    struct timespec start, end;

    clock_gettime(CLOCK_MONOTONIC, &start);

    if((fp = fopen("words.txt", "r")) == NULL)
    {
        fprintf(stderr, "Error while opening file %s\n", "words.txt");
        return 1;
    }

    fscanf(fp, "%ms", &line);

    fclose(fp);

    n = 1;
    len = strlen(line);

    /* Count the words.*/
    for(i = 0; i < len; i++)
    {
        if(line[i] == ',')
        {
            n++;
        }
    }

    if((words = (char **)malloc(n*sizeof(char *))) == NULL)
    {
        fprintf(stderr, "Error while allocating memory\n");
        return 1;
    }

    /* Save the words in an array of strings.*/
    words[0] = strtok(line, ",\"");

    for(i = 1; i < n; i++)
    {
        words[i] = strtok(NULL, ",\"");
    }

    /* For each word, calculate its value and check if it's a triangle number.*/
    for(i = 0; i < n; i++)
    {
        value = 0;
        len = strlen(words[i]);

        for(j = 0; j < len; j++)
        {
            value += (words[i][j] - 'A' + 1);
        }

        if(is_triang(value))
        {
            count++;
        }
    }

    free(line);
    free(words);

    clock_gettime(CLOCK_MONOTONIC, &end);

    elapsed = (end.tv_sec-start.tv_sec) + (double)(end.tv_nsec - start.tv_nsec) / 1000000000;

    printf("Project Euler, Problem 42\n");
    printf("Answer: %d\n", count);

    printf("Elapsed time: %.9lf seconds\n", elapsed);

    return 0;
}

int is_triang(int n)
{
    int i, j;

    for(i = 1, j = 1; j <= n; i++, j += i)
    {
        if(n == j)
        {
            return 1;
        }
    }

    return 0;
}