adventofcode

day1_2.c (1433B)

// SPDX-FileCopyrightText: 2025 Haelwenn (lanodan) Monnier
// SPDX-License-Identifier: BSD-3-Clause
#define _POSIX_C_SOURCE 202405L
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
int
main(void)
{
	int pos = 50; // The dial starts by pointing at 50
	int pass = 0;
	char *line = NULL;
	size_t linez = 0;
	while(1)
	{
		errno = 0;
		ssize_t nread = getline(&line, &linez, stdin);
		if(nread < 0)
		{
			if(errno != 0)
			{
				perror("getline");
				free(line);
				return 1;
			}
			break;
		}
		if(line[nread-1] == '\n')
			line[nread-1] = '\0';
		if(line[0] != 'L' && line[0] != 'R')
		{
			fprintf(stderr, "Got %c instead of [LR]\n", line[0]);
			free(line);
			return 1;
		}
		if(line[1] < '0' || line[1] > '9')
		{
			fprintf(stderr, "Got %c instead of [0-9] as first digit\n", line[0]);
			free(line);
			return 1;
		}
		int rot = line[1] - '0';
		for(int i = 2; i < nread; i++)
		{
			if(line[i] < '0' || line[i] > '9') break;
			rot = (rot*10) + (line[i] - '0');
		}
		/* brute-force it rather than math it out */
		switch(line[0])
		{
		case 'L':
			for(int a = rot; a > 0; a--)
			{
				pos = pos == 0 ? 99 : pos-1;
				if(pos == 0) pass++;
			}
			break;
		case 'R':
			for(int a = rot; a > 0; a--)
			{
				pos = pos == 99 ? 0 : pos+1;
				if(pos == 0) pass++;
			}
			break;
		}
		printf("%-4s rot:%-4d pos:%-2d pass:%d\n", line, rot, pos, pass);
	}
	free(line);
	printf("Result: %d\n", pass);
}