2022-day6-fast.c

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

#define R 14

static int fullcheck(const char *roll)
{
	int i, j;

	for(i = R-2; i >= 0; i--)
		for(j = R-1; j >= i+1; j--)
			if(roll[i] == roll[j])
				return i+1;

	return 0;
}

static int fastcheck(const char *roll)
{
	int i;


	for(i = R-2; i >= 0; i--)
		if(roll[R-1] == roll[i])
			return i+1;

	return 0;
}

int main(void)
{
	char line[4100];
	FILE *f;
	int pos;

	f = fopen("day6.txt", "r");

	while(1)
	{
		char roll[R];
		int i, prevsize;

		if(!fgets(line, 4100, f))
			break;

		memcpy(roll, line, R);

		pos = R;
		prevsize = 0;

		while(1)
		{
			int r;

			if(prevsize != R-1)
				r = fullcheck(roll);
			else
				r = fastcheck(roll);

			if(!r)
				break;

			prevsize = R-r;

			memmove(roll, roll+r, R-r);

			for(i = 0; i < r; i++)
			{
				if(!line[pos] || line[pos] == '\n')
				{
					pos++;
					goto out;
				}

				pos++;

				roll[R-r+i] = line[pos];
			}
		}

out:
		for(i = 0; i < R; i++)
			putchar(roll[i]);
		putchar('\n');

		printf("pos: %d\n", pos+1);
	}

	return 0;
}