Day 22: Monkey Market

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FAQ

  • sjmulder@lemmy.sdf.org
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    4 days ago

    C

    Really proud of this one! Started with with an O(n^atoms in the universe) scan which took 44s even after adding a dedup check.

    But iterating on a trick to encode the deltas for the dedup check, using it to build a mapping table here, a lookup there etc brought it down to a very fast, fairly low memory, linear complexity solution!

    Code
    #include "common.h"
    
    #define STEPS	2000
    #define NCODES	(19*19*19*19)
    
    int
    main(int argc, char **argv)
    {
    	static int8_t prices[STEPS];
    	static int8_t by_deltas[NCODES];
    	static int sums[NCODES];
    
    	uint64_t p1=0, secret;
    	int p2=0, i;
    
    	if (argc > 1)
    		DISCARD(freopen(argv[1], "r", stdin));
    	
    	while (scanf(" %"SCNu64, &secret) == 1) {
    		memset(by_deltas, 0, sizeof(by_deltas));
    
    		for (i=0; i<STEPS; i++) {
    			secret = (secret ^ secret << 6)  & 0xFFFFFF;
    			secret = (secret ^ secret >> 5);
    			secret = (secret ^ secret << 11) & 0xFFFFFF;
    
    			prices[i] = secret % 10;
    		}
    
    		/*
    		 * Build a deltas->price map for the buyer. Deltas are
    		 * encoded as an integer for easy indexing. Iterating
    		 * backwards ensures the stored price is the _earliest_
    		 * occurence of that sequence.
    		 */
    		for (i=STEPS-1; i>=4; i--)
    			by_deltas[
    			    (prices[i-3] - prices[i-4] +9) *19*19*19 +
    			    (prices[i-2] - prices[i-3] +9) *19*19 +
    			    (prices[i-1] - prices[i-2] +9) *19 +
    			    (prices[i]   - prices[i-1] +9)
    			] = prices[i];
    
    		for (i=0; i<NCODES; i++)
    			sums[i] += by_deltas[i];
    
    		p1 += secret;
    	}
    
    	for (i=0; i<NCODES; i++)
    		p2 = MAX(p2, sums[i]);
    
    	printf("22: %"PRIu64" %d\n", p1, p2);
    	return 0;
    }
    

    day22 0m00.04s real

    https://codeberg.org/sjmulder/aoc/src/branch/master/2024/c/day22.c