#ifndef GC_H_6b8a27c99f2eb5eb5437e045fa4af1c3
#define GC_H_6b8a27c99f2eb5eb5437e045fa4af1c3

#include <assert.h>
#include <inttypes.h>
#include <stdbool.h>
#include <stdio.h>

#ifndef NDEBUG
#  define debug(printf_args) ((void)printf printf_args)
#else
#  define debug(printf_args) ((void)0)
#endif

typedef uintptr_t value_t;

/* NIL:    00000000 00000000 00000000 00000000                       */
/* Object: aaaaaaaa aaaaaaaa aaaaaaaa aaaaaa00 (where aa... >= 1024) */
/* Pair:   aaaaaaaa aaaaaaaa aaaaaaaa aaaaaa10                       */
/* Fixnum: snnnnnnn nnnnnnnn nnnnnnnn nnnnnnn1                       */

#define NIL              ((value_t)0)

/* Special values (0 <= n < 1024) */
/* These correspond to objects within the first page of memory */
#define SPECIAL_VALUE(n) ((value_t)(4*n+2))
#define MAX_SPECIAL      SPECIAL_VALUE(1023)

#define BROKEN_HEART     SPECIAL_VALUE(0)
#define TYPE_TAG_BOX     SPECIAL_VALUE(1)
#define TYPE_TAG_VECTOR  SPECIAL_VALUE(2)
#define TYPE_TAG_BYTESTR SPECIAL_VALUE(3)

typedef struct pair
{
  value_t car;
  value_t cdr;
} pair_t;

typedef struct object
{
  value_t tag;
  union {
    value_t values[0];
    char bytes[0];
  } payload;
} object_t;

typedef struct vector
{
  value_t tag;
  size_t  size;
  value_t elements[0];
} vector_t;

typedef struct byte_string
{
  value_t tag;
  size_t  size;
  uint8_t bytes[0];
} byte_string_t;

typedef struct gc_root
{
  value_t value;
  struct gc_root *prev;
  struct gc_root *next;
} gc_root_t;

static inline bool is_pair(value_t v)
{
  return ((v & 0x3) == 2);
}

static inline bool is_object(value_t v)
{
  /* Neither pairs nor objects can exist below (void*)4096. */
  return ((v & 0x1) == 0) && (v > MAX_SPECIAL);
}

static inline bool is_fixnum(value_t v)
{
  return (v & 1) != 0;
}

static inline value_t to_fixnum(intptr_t n)
{
  return (value_t)(n << 1) | 1;
}

static inline intptr_t from_fixnum(value_t n)
{
  return ((intptr_t)n) >> 1;
}

static inline value_t pair_value(pair_t *p)
{
  assert((uintptr_t)p >= 4096);
  assert(((uintptr_t)p & 3) == 0);
  return (value_t)p + 2;
}

static inline value_t object_value(void *obj)
{
  assert((uintptr_t)obj >= 4096);
  assert(((uintptr_t)obj & 3) == 0);
  return (value_t)obj;
}

object_t *get_object(value_t v);
pair_t *get_pair(value_t pair);
value_t get_cdr(value_t pair);
value_t cons(value_t car, value_t cdr);

void gc_init(size_t min_size, size_t max_size);
void register_gc_root(gc_root_t *root, value_t v);
void unregister_gc_root(gc_root_t *root);
void *gc_alloc(size_t nbytes);
void collect_garbage(size_t min_free);

/* To be provided by the main application */
void out_of_memory(void) __attribute__ ((noreturn));

#endif

/* vim:set sw=2 expandtab: */