#include <stdlib.h>
#include <stdio.h>

#include <glib.h>

#define REGS 6

typedef int reg_t;

enum instr_id {
    INSTR_ADDR,
    INSTR_ADDI,
    INSTR_MULR,
    INSTR_MULI,
    INSTR_BANR,
    INSTR_BANI,
    INSTR_BORR,
    INSTR_BORI,
    INSTR_SETR,
    INSTR_SETI,
    INSTR_GTIR,
    INSTR_GTRI,
    INSTR_GTRR,
    INSTR_EQIR,
    INSTR_EQRI,
    INSTR_EQRR,
    INSTR_ID_COUNT
};

struct instr {
    enum instr_id i;
    int a;
    int b;
    int c;
};

const char *instr_str[] = {
    "addr",
    "addi",
    "mulr",
    "muli",
    "banr",
    "bani",
    "borr",
    "bori",
    "setr",
    "seti",
    "gtir",
    "gtri",
    "gtrr",
    "eqir",
    "eqri",
    "eqrr"
};

void instr_interpret(reg_t *r, struct instr *i)
{
    switch (i->i) {
    case INSTR_ADDR:
    case INSTR_MULR:
    case INSTR_BANR:
    case INSTR_BORR:
    case INSTR_GTRR:
    case INSTR_EQRR:
        if (i->b >= REGS) return;
        // fallthrough
    case INSTR_ADDI:
    case INSTR_MULI:
    case INSTR_BANI:
    case INSTR_BORI:
    case INSTR_SETR:
    case INSTR_GTRI:
    case INSTR_EQRI:
        if (i->a >= REGS) return;
        // fallthrough
    case INSTR_SETI:
        if (i->c >= REGS) return;
        break;

    case INSTR_GTIR:
    case INSTR_EQIR:
        if (i->b >= REGS) return;
        if (i->c >= REGS) return;
        break;

    default:
        break;
    }

    switch (i->i) {
    case INSTR_ADDR: r[i->c] = r[i->a] + r[i->b]; break;
    case INSTR_ADDI: r[i->c] = r[i->a] + i->b; break;
    case INSTR_MULR: r[i->c] = r[i->a] * r[i->b]; break;
    case INSTR_MULI: r[i->c] = r[i->a] * i->b; break;
    case INSTR_BANR: r[i->c] = r[i->a] & r[i->b]; break;
    case INSTR_BANI: r[i->c] = r[i->a] & i->b; break;
    case INSTR_BORR: r[i->c] = r[i->a] | r[i->b]; break;
    case INSTR_BORI: r[i->c] = r[i->a] | i->b; break;
    case INSTR_SETR: r[i->c] = r[i->a]; break;
    case INSTR_SETI: r[i->c] = i->a; break;
    case INSTR_GTIR: r[i->c] = (i->a > r[i->b]) ? 1 : 0; break;
    case INSTR_GTRI: r[i->c] = (r[i->a] > i->b) ? 1 : 0; break;
    case INSTR_GTRR: r[i->c] = (r[i->a] > r[i->b]) ? 1 : 0; break;
    case INSTR_EQIR: r[i->c] = (i->a == r[i->b]) ? 1 : 0; break;
    case INSTR_EQRI: r[i->c] = (r[i->a] == i->b) ? 1 : 0; break;
    case INSTR_EQRR: r[i->c] = (r[i->a] == r[i->b]) ? 1 : 0; break;
    default: break;
    }
}

struct instr *parse_instructions(const char *fname, int *len, int *ip)
{
    FILE *f = fopen(fname, "r");
    if (!f) {
        perror(fname);
        return NULL;
    }

    GArray *array = g_array_new(FALSE, FALSE, sizeof(struct instr));

    char *line = NULL;
    size_t line_len = 0;
    while (getline(&line, &line_len, f) != -1) {
        char *args = strchr(line, ' ');
        if (!args) continue;
        *args++ = '\0';

        if (strcmp(line, "#ip") == 0) {
            *ip = strtol(args, NULL, 0);
            continue;
        }

        struct instr instr = {0};

        for (int i = 0; i < INSTR_ID_COUNT; i++) {
            if (strcmp(line, instr_str[i]) == 0) {
                instr.i = i;
                break;
            }
        }

        sscanf(args, "%d %d %d", &instr.a, &instr.b, &instr.c);
        g_array_append_val(array, instr);
    }

    free(line);
    fclose(f);

    *len = array->len;
    return (struct instr *)g_array_free(array, FALSE);
}

void print_regs(reg_t *regs, FILE *f)
{
    putc('[', f);
    for (int i = 0; i < REGS - 1; i++) {
        fprintf(f, "%d, ", regs[i]);
    }
    fprintf(f, "%d]", regs[REGS - 1]);
}

int main(int argc, char *argv[])
{
    int debug = 0;
    if (argc > 1 && strcmp(argv[1], "-debug") == 0) debug = 1;

    int ip = 0;
    int instructions_len = 0;
    struct instr *instructions = parse_instructions("input", &instructions_len, &ip);
    if (!instructions) return 1;
    if (ip >= REGS) ip = 0;

    GHashTable *seen = g_hash_table_new(NULL, NULL);
    gpointer last = 0;

    reg_t *regs = g_new0(reg_t, REGS);
    while (regs[ip] >= 0 && regs[ip] < instructions_len) {
        struct instr *instr = &instructions[regs[ip]];

        // Day 21 part 2 optimizations...
        if (regs[ip] == 17) {
            regs[5] >>= 8;
            regs[ip] = 8;
            continue;
        }

        if (debug) {
            fprintf(stderr, "ip=%d ", regs[ip]);
            print_regs(regs, stderr);
            fprintf(stderr, " %s %d %d %d ",
                    instr_str[instr->i], instr->a, instr->b, instr->c);
        }

        instr_interpret(regs, instr);

        if (debug) {
            print_regs(regs, stderr);
            putc('\n', stderr);
        }

        // Day 21 part 2 solution...
        if (instr->i == INSTR_EQRR && (instr->a == 0 || instr->b == 0)) {
            int reg = instr->a;
            if (reg == 0) reg = instr->b;
            if (reg != 0 && reg < REGS) {
                gpointer val = GINT_TO_POINTER(regs[reg]);
                if (g_hash_table_contains(seen, val)) {
                    printf("Last value: %p\n", last);
                    break;
                }
                last = val;
                g_hash_table_add(seen, val);
            }
        }

        regs[ip]++;
    }

    print_regs(regs, stdout);
    putchar('\n');

    g_hash_table_destroy(seen);
    g_free(regs);
    g_free(instructions);
    return 0;
}