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#include <sys/stat.h>
#include <err.h>
#include <errno.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define new(a, T) ((T *)alloc(a, sizeof(T), _Alignof(T), 1))
#define BETWEEN(lo, n, hi) ((lo) <= (n) && (n) <= (hi))
typedef struct {
ptrdiff_t x, y;
} vec2_t;
typedef enum {
U,
R,
D,
L,
} dir_t;
#if PART2
typedef struct {
char *beg, *end;
} arena_t;
typedef struct {
dir_t d;
vec2_t p;
} dpos_t;
typedef struct gposset {
struct gposset *child[4];
dpos_t key;
} gposset_t;
#endif
static char *readinput(FILE *stream, vec2_t *start, vec2_t *size);
static vec2_t move(vec2_t pos, dir_t d);
#if PART1
static void simulate(char *buf, vec2_t pos, vec2_t size);
#else /* PART 2 */
static size_t loopcnt(char *buf, vec2_t pos, vec2_t size);
static bool loops(char *buf, vec2_t pos, vec2_t size);
static uint64_t hash(dpos_t dp);
static bool ismember(gposset_t **m, dpos_t key, arena_t *perm);
static void *alloc(arena_t *a, ptrdiff_t size, ptrdiff_t align,
ptrdiff_t count);
static char MEM[1024 * 1024]; /* 1 MiB */
#endif
int
main(void)
{
FILE *fp = fopen("input", "r");
if (fp == NULL)
err(EXIT_FAILURE, "fopen: input");
vec2_t pos, size;
char *buf = readinput(fp, &pos, &size);
#if PART1
size_t n = 0;
simulate(buf, pos, size);
for (const char *p = buf; *p != 0; p++) {
if (*p == 'X')
n++;
}
#else
size_t n = loopcnt(buf, pos, size);
#endif
printf("%zu\n", n);
fclose(fp);
return EXIT_SUCCESS;
}
char *
readinput(FILE *stream, vec2_t *start, vec2_t *size)
{
struct stat sb;
if (fstat(fileno_unlocked(stream), &sb) == -1)
err(EXIT_FAILURE, "fstat: input");
char *buf = malloc(sb.st_size + 1);
if (buf == NULL)
err(EXIT_FAILURE, "malloc");
buf[sb.st_size] = 0;
if (fread_unlocked(buf, 1, sb.st_size, stream) != (size_t)sb.st_size)
err(EXIT_FAILURE, "fread: input");
start->x = start->y = 0;
for (const char *p = buf; *p != '^'; p++) {
if (*p == '\n') {
start->x = 0;
start->y++;
} else
start->x++;
}
size->x = strchr(buf, '\n') - buf;
size->y = (sb.st_size + 1) / (size->x + 1); /* Last line lacks newline */
return buf;
}
#if PART1
void
simulate(char *buf, vec2_t pos, vec2_t size)
{
#define I(p) (p.y*(size.x + 1) + p.x)
dir_t d = U;
do {
buf[I(pos)] = 'X';
vec2_t np = move(pos, d);
if (buf[I(np)] == '#') {
d = (d + 1) % 4;
pos = move(pos, d);
} else
pos = np;
} while (BETWEEN(0, pos.x, size.x - 1)
&& BETWEEN(0, pos.y, size.y - 1));
#undef I
}
#else /* PART 2 */
size_t
loopcnt(char *buf, vec2_t pos, vec2_t size)
{
size_t n = 0;
for (char *p = buf; *p != 0; p++) {
if (*p == '.') {
*p = '#';
n += loops(buf, pos, size);
*p = '.';
}
}
return n;
}
bool
loops(char *buf, vec2_t pos, vec2_t size)
{
dir_t d = U;
gposset_t *set = NULL;
arena_t perm = {MEM, MEM + sizeof(MEM)};
do {
if (ismember(&set, (dpos_t){d, pos}, &perm))
return true;
int i;
for (i = 0; i < 4; i++) {
vec2_t np = move(pos, d);
if (buf[np.y*(size.x + 1) + np.x] != '#') {
pos = np;
break;
}
d = (d + 1) % 4;
}
if (i == 4)
return true;
} while (BETWEEN(0, pos.x, size.x - 1)
&& BETWEEN(0, pos.y, size.y - 1));
return false;
}
void *
alloc(arena_t *a, ptrdiff_t size, ptrdiff_t align, ptrdiff_t count)
{
ptrdiff_t padding = -(uintptr_t)a->beg & (align - 1);
ptrdiff_t available = a->end - a->beg - padding;
if (available < 0 || count > available/size) {
errno = ENOMEM;
err(EXIT_FAILURE, "%s", __func__);
}
void *p = a->beg + padding;
a->beg += padding + count*size;
return memset(p, 0, count * size);
}
bool
ismember(gposset_t **m, dpos_t key, arena_t *perm)
{
for (uint64_t h = hash(key); *m; h <<= 2) {
dpos_t l = key, r = (*m)->key;
if (l.d == r.d && l.p.x == r.p.x && l.p.y == r.p.y)
return true;
m = &(*m)->child[h >> 62];
}
*m = new(perm, gposset_t);
(*m)->key = key;
return false;
}
static uint64_t
_hash(uint64_t x)
{
x = ((x >> 16) ^ x) * 0x45D9F3B;
x = ((x >> 16) ^ x) * 0x45D9F3B;
x = ((x >> 16) ^ x);
return x;
}
uint64_t
hash(dpos_t dp)
{
return _hash(dp.d) ^ _hash(dp.p.x) ^ _hash(dp.p.y);
}
#endif
vec2_t
move(vec2_t pos, dir_t d)
{
static const vec2_t dirs[] = {
[U] = {0, -1},
[D] = {0, +1},
[L] = {-1, 0},
[R] = {+1, 0},
};
vec2_t np = dirs[d];
return (vec2_t){
.x = pos.x + np.x,
.y = pos.y + np.y,
};
}
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