aoc2025/day6.c

#include "lib.h"

#define DBG 1

static
ch input[] = {
#embed "day6_input.txt"
};

static
ch test[] = {
#embed "day6_test.txt"
};


static
num do_part1(size_t file_len, ch file[file_len]) {
    ch *s = file;
    num result = 0;

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

    while (*s != 10) s++;
    num stride = s - file + 1;

    s = file;
    while (*s != '+' && *s != '*') s += stride;
    num heightb = (s - file);
    num height  = (s - file) / stride;
    ch *op = s;

#if DBG
    print("STRIDE\t"); printd(stride); print("\n");
    print("HEIGHT\t"); printd(height); print("\n");
#endif

    // constructing a pointer more than 1 past the end of an array
    // is UB, sadly, so we'll use offsets
    num op_off = op - file;
    s = file;
    do {
        num ledge_off = op_off - heightb;
        if (s[op_off] == '+') {
            num a = 0;
            num rem = height;
            do {
                ch *t = s + ledge_off;
                while (*t < '0') t++;
                num b = grabnum(t, NULL);
#if DBG
                if (rem < height) { print("\t+\t"); }
                printd(b);
#endif
                a += b;
                ledge_off += stride;
                rem--;
            } while (rem);
            result += a;
#if DBG
            print("\t= "); printd(a); print("\n");
#endif
        } else if (s[op_off] == '*') {
            num a = 1;
            num rem = height;
            do {
                ch *t = s + ledge_off;
                while (*t < '0') t++;
                num b = grabnum(t, NULL);
#if DBG
                if (rem < height) { print("\t*\t"); }
                printd(b);
#endif
                a *= b;
                ledge_off += stride;
                rem--;
            } while (rem);
            result += a;
#if DBG
            print("\t= "); printd(a); print("\n");
#endif
        }
#if DBG
        else {
            print("INVALID OP\n");
            exit_group(-1);
        }
#endif
        op_off++;
        while (op_off < file_len) {
            if (s[op_off] == '+' || s[op_off] == '*')
                break;
            op_off++;
        }
    } while (op_off < file_len);

    return result;
}

static
num do_part2(size_t file_len, unsigned char file[file_len]) {
    ch *s = file;
    num result = 0;

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

    while (*s != 10) s++;
    num stride = s - file + 1;

    s = file;
    while (*s != '+' && *s != '*') s += stride;
    num heightb = (s - file);
    num height  = (s - file) / stride;
    ch *op = s;

#if DBG
    print("STRIDE\t"); printd(stride); print("\n");
    print("HEIGHT\t"); printd(height); print("\n");
#endif

    // constructing a pointer more than 1 past the end of an array
    // is UB, sadly, so we'll use offsets
    num fop_off = op - file;
    num prev_op_off = fop_off + stride - 1;
    s = file;
    do {
        // we don't need to do these in the same order as the example
        // because both these operations are commutative.
        // but you know what? we can anyway. for fun. and pretty output
        num op_off = prev_op_off - 1;
        while (s[op_off] != '+' && s[op_off] != '*' && s[op_off] != '\n')
            op_off--;
        num ledge_off = op_off - heightb;
        num redge_off = (s[prev_op_off] == '\n' ? -1 : -2) + prev_op_off - heightb;
        if (s[op_off] == '+') {
            num a = 0;
            num rem = redge_off - ledge_off + 1;
            num frem = rem;
            do {
                num b = 0;
                ch *t = s + redge_off;
                for (num i = height; i; i--,t+=stride) {
                    if (*t >= '0') {
                        b *= 10;
                        b += *t - '0';
                    }
                }
#if DBG
                if (rem < frem) { print("\t+\t"); }
                printd(b);
#endif
                a += b;
                redge_off--;
                rem--;
            } while (rem);
            result += a;
#if DBG
            print("\t=\t"); printd(a); print("\n");
#endif
        } else if (s[op_off] == '*') {
            num a = 1;
            num rem = redge_off - ledge_off + 1;
            num frem = rem;
            do {
                num b = 0;
                ch *t = s + redge_off;
                for (num i = height; i; i--,t+=stride) {
                    if (*t >= '0') {
                        b *= 10;
                        b += *t - '0';
                    }
                }
#if DBG
                if (rem < frem) { print("\t*\t"); }
                printd(b);
#endif
                a *= b;
                redge_off--;
                rem--;
            } while (rem);
            result += a;
#if DBG
            print("\t=\t"); printd(a); print("\n");
#endif
        }
#if DBG
        else {
            print("INVALID OP\n");
            exit_group(-1);
        }
#endif
        prev_op_off = op_off;
    } while (prev_op_off != fop_off);

    return result;
}

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

#define TEST1_EXPECT 4277556
#define TEST2_EXPECT 3263827

void run() {
#if RUN_TEST1
    print("PART 1 TEST: ");
    if (num 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 (num 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);
}