aoc2025/day4.c

#include "lib.h"

#define DBG 0

static
unsigned char input[] = {
#embed "day4_input.txt"
};

static
unsigned char test[] = {
#embed "day4_test.txt"
};

static
unsigned char buf[UINT16_MAX];

static
unsigned long do_part1(size_t file_len, unsigned char file[file_len]) {
    unsigned char *s = file, *d = buf;
    unsigned long result = 0;

#if DBG
    print("\n");
#endif

    unsigned file_stride;
    while (*s != '\n') s++;
    file_stride = s - file + 1;
    s = file;

    unsigned line_count = file_len / file_stride;

    unsigned line_stride = file_stride + 1;

    memset(d, '0', sizeof(buf));

#if DBG
    printh(file_stride); print("\n");
    printh(line_stride * (line_count + 2)); print("\n");
#endif

#if DBG
    printns(line_stride, d); print("\n");
#endif
    d += line_stride;
    do {
        memcpy(d + 1, s, file_stride - 1);
#if DBG
        printns(line_stride, d); print("\n");
#endif
        d += line_stride;
        s += file_stride;
    } while (s != &file[file_len]);
#if DBG
    printns(line_stride, d); print("\n");
#endif

    unsigned char *l = buf + line_stride + 1;
    for (unsigned i = line_count; i; i--) {
        d = l;
        for (unsigned j = line_stride - 2; j; j--) {
            if (*d == '@') {
                unsigned neighbors = 0
                    + (*(d - 1 - line_stride) == '@')
                    + (*(d - 0 - line_stride) == '@')
                    + (*(d + 1 - line_stride) == '@')
                    + (*(d - 1 - 0) == '@')
                    + (*(d + 1 - 0) == '@')
                    + (*(d - 1 + line_stride) == '@')
                    + (*(d - 0 + line_stride) == '@')
                    + (*(d + 1 + line_stride) == '@');
                if (neighbors < 4) {
                    result++;
                }
            }
            d++;
        }
        l += line_stride;
    }

    return result;
}

static
unsigned long do_part2(size_t file_len, unsigned char file[file_len]) {
    unsigned char *s = file, *d = buf;
    unsigned long result = 0;

#if DBG
    print("\n");
#endif

    unsigned file_stride;
    while (*s != '\n') s++;
    file_stride = s - file + 1;
    s = file;

    unsigned line_count = file_len / file_stride;

    unsigned line_stride = file_stride + 1;

    memset(d, '0', sizeof(buf));

#if DBG
    printh(file_stride); print("\n");
    printh(line_stride * (line_count + 2)); print("\n");
#endif

#if DBG
    printns(line_stride, d); print("\n");
#endif
    d += line_stride;
    do {
        memcpy(d + 1, s, file_stride - 1);
#if DBG
        printns(line_stride, d); print("\n");
#endif
        d += line_stride;
        s += file_stride;
    } while (s != &file[file_len]);
#if DBG
    printns(line_stride, d); print("\n");
#endif

    unsigned char *l = buf + line_stride + 1;
    for (unsigned i = line_count; i; i--) {
line_start:
        d = l;
        for (unsigned j = line_stride - 2; j; j--) {
            if (*d == '@') {
                unsigned neighbors = 0
                    + (*(d - 1 - line_stride) == '@')
                    + (*(d - 0 - line_stride) == '@')
                    + (*(d + 1 - line_stride) == '@')
                    + (*(d - 1 - 0) == '@')
                    + (*(d + 1 - 0) == '@')
                    + (*(d - 1 + line_stride) == '@')
                    + (*(d - 0 + line_stride) == '@')
                    + (*(d + 1 + line_stride) == '@');
                if (neighbors < 4) {
                    *d = '.';
                    result++;
                    if (i > line_count-1) {
                        i = line_count;
                        l = buf + line_stride + 1;
                        goto line_start;
                    } else {
                        i += 1;
                        l -= line_stride * 1;
                        goto line_start;
                    }
                }
            }
            d++;
        }
        l += line_stride;
    }

    return result;
}

#define RUN_TEST1 1
#define RUN_PART1 1
#define RUN_TEST2 1
#define RUN_PART2 1

#define TEST1_EXPECT 13
#define TEST2_EXPECT 43

void run() {
#if RUN_TEST1
    print("PART 1 TEST: ");
    if (unsigned long v = do_part1(countof(test), test); v != TEST1_EXPECT) {
        print("FAILED (got ");
        printd(v);
        print(", expected " xstr(TEST1_EXPECT) ")\n");
    } else {
        print("PASSED\n");
    }
#endif

#if RUN_PART1
    print("PART 1 RESULT: ");
    printd(do_part1(countof(input), input));
    print("\n");
#endif

#if RUN_TEST2
    print("PART 2 TEST: ");
    if (unsigned long v = do_part2(countof(test), test); v != TEST2_EXPECT) {
        print("FAILED (got ");
        printd(v);
        print(", expected " xstr(TEST2_EXPECT) ")\n");
    } else {
        print("PASSED\n");
    }
#endif

#if RUN_PART2
    print("PART 2 RESULT: ");
    printd(do_part2(countof(input), input));
    print("\n");
#endif

    exit_group(0);
}