#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#define DAY(n) "input/day" #n ".txt"

/* Advent of Code 2016
 * Solution for Day 1, Part 2
 */

char *get_filebuffer(const char *path)
{
    FILE *fp = fopen(path, "rb");
    fseek(fp, 0, SEEK_END);
    long len = ftell(fp);
    fseek(fp, 0, SEEK_SET);
    char *buf = (char *) malloc(sizeof(char) * len + 1);
    fread(buf, len, 1, fp);
    buf[len] = '\0';
    fclose(fp);
    return buf;
}

struct point {
    int x;
    int y;
    enum { NORTH, EAST, SOUTH, WEST } dir;
    struct point *next;
};

struct list {
    struct point *head;
    unsigned depth;
};

/* blocks, not shortest distance */
#define distance(a, b) abs(abs((b).x - (a).x) + abs((b).y - (a).y))
#define is_equal(a, b) (((b).x == (a).x) && ((b).y == (a).y))

void list_insert(struct list *ls, struct point *pt)
{
    struct point *node = malloc(sizeof(struct point));
    *node = *pt; /* struct copy */
    node->next = ls->head;
    ls->head = node;
    ls->depth++;
}

void list_free(struct list *ls)
{
    while (ls->head != NULL)
    {
        struct point *node = ls->head;
        ls->head = node->next;
        free(node);
        ls->depth--;
    }
}

int list_search(struct list *ls, struct point *pt)
{
    unsigned i, found = 0;
    if (ls->head != NULL)
    {
        struct point **arr = malloc(sizeof(struct point *) * ls->depth);
        struct point *node = ls->head;
        for (i = 0; i < ls->depth; i++) /* unroll */
            arr[i] = node, node = node->next;
        for (i = ls->depth - 1; i > 0 && !found; --i)
            if (is_equal(*arr[i], *pt))
                found = !found;
        free(arr);
    }
    return found;
}

int main(void)
{
    char *ins = get_filebuffer(DAY(01));
/*  char *ins = get_filebuffer("input/poop.txt"); */
    struct list prev = { 0 };
    struct point orig = { 0 }, pt = { 0 }, act = { 0 };
    unsigned passed = 0;
    char *tok = strtok(ins, " ,");
    while (tok != NULL)
    {
        char rot;
        int dist;
        sscanf(tok, "%c%d", &rot, &dist);
        switch (rot)
        {
            case 'L': pt.dir = (pt.dir == NORTH) ? WEST : pt.dir - 1; break;
            case 'R': pt.dir = (pt.dir == WEST) ? NORTH : pt.dir + 1; break;
        }
        switch (pt.dir)
        {
            case NORTH: pt.y += dist; break;
            case EAST: pt.x += dist; break;
            case SOUTH: pt.y -= dist; break;
            case WEST: pt.x -= dist; break;
        }
        if (!passed && list_search(&prev, &pt))
            act = pt, passed = !passed; /* passed first duplicate coord */
        list_insert(&prev, &pt);
        tok = strtok(NULL, " ,");
    }
    printf("Coords: X: %d Y: %d Blocks: %d\n", pt.x, pt.y, distance(orig, pt));
    printf("Actual: X: %d Y: %d Blocks away: %d\n", act.x, act.y, distance(pt, act));
    list_free(&prev);
    free(ins);
    return 0;
}