Day 8: Resonant Collinearity

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FAQ

  • sjmulder@lemmy.sdf.org
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    4 days ago

    C

    Not hard but a little fiddly.

    Code
    #include "common.h"
    
    #define GZ 52
    
    static char g[GZ][GZ];
    #define ANTI_P1 1
    #define ANTI_P2 2
    static uint8_t anti[GZ][GZ];
    static int w,h;
    
    int
    main(int argc, char **argv)
    {
    	int p1=0,p2=0, x,y, x1,y1, ax,ay, i;
    	char *lf;
    
    	if (argc > 1)
    		DISCARD(freopen(argv[1], "r", stdin));
    	for (h=0; h<GZ && fgets(g[h], GZ, stdin); h++)
    		;
    
    	assert(feof(stdin));
    	lf = strchr(g[0], '\n');
    	assert(lf);
    	w = lf - g[0];
    
    	/*
    	 * Find antenna pairs, then project backwards from the first,
    	 * forwards from the second. Don't like the repetition but it
    	 * makes for easy code.
    	 */
    	for (y=0; y<h; y++)
    	for (x=0; x<w; x++) {
    		if (!isalnum(g[y][x]))
    			continue;
    
    		for (y1=y; y1<h; y1++)
    		for (x1=(y==y1?x+1:0); x1<w; x1++) {
    			if (g[y][x] != g[y1][x1])
    				continue;
    
    			for (i=0; ; i++) {
    				if ((ax = x-(x1-x)*i) <0 || ax>w ||
    				    (ay = y-(y1-y)*i) <0 || ay>h)
    					break;
    				anti[ay][ax] |= ANTI_P1 * i==1;
    				anti[ay][ax] |= ANTI_P2;
    			}
    
    			for (i=0; ; i++) {
    				if ((ax = x1+(x1-x)*i) <0 || ax>w ||
    				    (ay = y1+(y1-y)*i) <0 || ay>h)
    					break;
    				anti[ay][ax] |= ANTI_P1 * i==1;
    				anti[ay][ax] |= ANTI_P2;
    			}
    		}
    	}
    
    	for (y=0; y<h; y++)
    	for (x=0; x<w; x++) {
    		p1 += !!(anti[y][x] & ANTI_P1);
    		p2 += !!(anti[y][x] & ANTI_P2);
    	}
    
    	printf("08: %d %d\n", p1, p2);
    	return 0;
    }
    

    https://github.com/sjmulder/aoc/blob/master/2024/c/day08.c