186 lines
5.2 KiB
C
186 lines
5.2 KiB
C
/* aoc-c.c: Advent of Code 2021, day 24 parts 1 & 2
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*
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* Copyright (C) 2022 Bruno Raoult ("br")
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* Licensed under the GNU General Public License v3.0 or later.
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* Some rights reserved. See COPYING.
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*
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* You should have received a copy of the GNU General Public License along with this
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* program. If not, see <https://www.gnu.org/licenses/gpl-3.0-standalone.html>.
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*
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* SPDX-License-Identifier: GPL-3.0-or-later <https://spdx.org/licenses/GPL-3.0-or-later.html>
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <errno.h>
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#include "debug.h"
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#include "bits.h"
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/*
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* All 14 blocks have the same code, with a1, b1, c1 differing :
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* inp w
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* mul x 0
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* add x z # x = z (0)
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* mod x 26 # x %= 26 (0)
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* # = z % 26
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* div z 1 a1 # z = z / a1 (0)
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* add x 13 b1 # x += b1 (13)
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* # = (z % 26) + b1
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* eql x w # x = (x == w) (0)
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* eql x 0 # x = !!x (1)
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* # if (w == (z % 26 + b1)
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* # x = 0
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* # else
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* # x = 1
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* mul y 0
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* add y 25 # y = 25 (25)
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* mul y x # y *= x (25)
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* # if (w == (z % 26 + b1)
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* # y = 0
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*
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* add y 1 # y++ (26)
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* # if (w == (z % 26 + b1)
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* # y = 1
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* # else
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* # y = 26
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*
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* mul z y # z *= y (0)
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* # if (w != (z % 26 + b1)
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* # z *= 26
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* mul y 0
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* add y w # y = w
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* add y 10 c1 # y += c1 (w + 10)
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* # = w + c1
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* mul y x # y *= x (w + 10) * 1
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* # if (w == (z % 26 + b1)
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* # y = 0
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* add z y # z += y (w + 10) * 1
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* # if (w != (z % 26 + b1))
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* # z = z / a1 * 26 + w + c1
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* # else
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* # z = z / a1
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*
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* So we end up with :
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* if (w == (z % 26 + b))
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* z = z / a
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* else
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* z = z / a * 26 + w + c
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*
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* a1 is 1 or 26. it means we can have an upper limit MAXZ at each of
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* the 14 steps : if z > MAXZ, final will not be able to end at zero.
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* MAXZ = 26^n, n being the number of A=26 from current step to 14th.
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*/
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struct var {
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int a, b, c;
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s64 maxz;
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} var[14];
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static inline s64 step(int n, int w, s64 z)
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{
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return z % 26 + var[n].b - w ? z / var[n].a * 26 + w + var[n].c : z / var[n].a;
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}
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static void read_input()
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{
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size_t alloc = 0;
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ssize_t buflen;
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char *buf = NULL;
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int line = 0;
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u64 maxz = 1;
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while ((buflen = getline(&buf, &alloc, stdin)) > 0) {
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buf[--buflen] = 0;
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/* each block is 18 lines */
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switch (line % 18) {
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case 4: /* a */
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sscanf(buf, "%*s %*s %d", &(var[line / 18].a));
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break;
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case 5: /* b */
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sscanf(buf, "%*s %*s %d", &(var[line / 18].b));
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break;
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case 15: /* c */
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sscanf(buf, "%*s %*s %d", &(var[line / 18].c));
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break;
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}
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line++;
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}
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/* adjust maxz */
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for (int i = 13; i >= 0; --i) {
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maxz *= var[i].a;
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var[i].maxz = maxz;
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}
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free(buf);
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}
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static char *part1(int n, s64 z, char *s)
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{
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if (n == 14)
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return z? NULL: s;
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else if (z > var[n].maxz)
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return NULL;
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for(int i = 9; i >= 1; --i) {
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if (part1(n + 1, step(n, i, z), s)) {
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s[n] = '0' + i;
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return s;
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}
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}
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return NULL;
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}
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static char *part2(int n, s64 z, char *s)
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{
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if (n == 14)
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return z? NULL: s;
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else if (z > var[n].maxz)
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return NULL;
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for (int i = 1; i <= 9; ++i) {
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if (part2(n + 1, step(n, i, z), s)) {
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s[n] = '0' + i;
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return s;
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}
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}
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return NULL;
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}
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static int usage(char *prg)
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{
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fprintf(stderr, "Usage: %s [-d debug_level] [-p part]\n", prg);
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return 1;
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}
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int main(int ac, char **av)
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{
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int opt, part = 1;
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static char res[16];
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while ((opt = getopt(ac, av, "d:p:")) != -1) {
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switch (opt) {
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case 'd':
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debug_level_set(atoi(optarg));
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break;
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case 'p': /* 1 or 2 */
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part = atoi(optarg);
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if (part < 1 || part > 2)
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return usage(*av);
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break;
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default:
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return usage(*av);
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}
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}
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if (optind < ac)
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return usage(*av);
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read_input();
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printf("%s : res=%s\n", *av, part == 1? part1(0, 0, res): part2(0, 0, res));
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exit(0);
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}
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