#include <assert.h>
#include <inttypes.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

#include "gc.h"

extern int gc_counter;
extern int gc_ticks;

/* Alignment must ensure each object has enough room to hold a pair (BH . new_addr) */
#define GC_ALIGNMENT  ((size_t)(sizeof(pair_t)))

/* Pairs are a type of object, but the value representation is different */
object_t *get_object(value_t v)
{
  if (is_object(v))
    return (object_t*)(v & ~(value_t)3);
  else
    abort();
}

pair_t *get_pair(value_t v)
{
  if (is_pair(v))
    return (pair_t*)(v - 2);
  else
    abort();
}

value_t cons(value_t car, value_t cdr)
{
  gc_root_t car_root, cdr_root;
  pair_t *p;

  register_gc_root(&car_root, car);
  register_gc_root(&cdr_root, cdr);

  p = gc_alloc(sizeof(pair_t));
  p->car = car_root.value;
  p->cdr = cdr_root.value;

  unregister_gc_root(&car_root);
  unregister_gc_root(&cdr_root);

  return pair_value(p);
}

static char      *gc_ranges[2];
static size_t     gc_min_size;
static size_t     gc_max_size;
static size_t     gc_soft_limit;

static int        gc_current_range;
static size_t     gc_free_space;
static char      *gc_free_ptr;

static gc_root_t  gc_root_list = {
  .value = NIL,
  .prev  = &gc_root_list,
  .next  = &gc_root_list
};

static inline size_t gc_align(size_t nbytes) __attribute__ ((const));
static int gc_range_of(void *object) __attribute__ ((const));
static void transfer_object(value_t *value);
static size_t transfer_children(object_t *object);

static inline size_t gc_align(size_t nbytes)
{
  return ((nbytes + GC_ALIGNMENT - 1) & ~(GC_ALIGNMENT - 1));
}

static int gc_range_of(void *object)
{
  if (((uintptr_t)object >= (uintptr_t)gc_ranges[0]) &&
      ((uintptr_t)object <  (uintptr_t)gc_ranges[1]))
    return 0;
  if (((uintptr_t)object >= (uintptr_t)gc_ranges[1]) &&
      ((uintptr_t)object <  (uintptr_t)gc_ranges[2]))
    return 1;
  return -1;
}

void gc_init(size_t min_size, size_t max_size)
{
  assert(min_size <= max_size);

  gc_ranges[0] = (char*)malloc(max_size);
  gc_ranges[1] = (char*)malloc(max_size);

  assert(gc_ranges[0] && gc_ranges[1]);

  gc_current_range = 0;
  gc_free_ptr = gc_ranges[gc_current_range];

  gc_min_size   = min_size;
  gc_max_size   = max_size;
  gc_soft_limit = gc_min_size;
  gc_free_space = gc_soft_limit;
}

void register_gc_root(gc_root_t *root, value_t v)
{
  root->value = v;
  root->prev  = &gc_root_list;
  root->next  = gc_root_list.next;
  root->next->prev  = root;
  gc_root_list.next = root;
}

void unregister_gc_root(gc_root_t *root)
{
  assert(root && root->prev && root->next);             /* Uninitialized */
  assert((root->prev != root) && (root->next != root)); /* Already removed */

  /* Cut the given root out of the list */
  root->prev->next = root->next;
  root->next->prev = root->prev;

  /* Remove dead references to root list; protects against double-removal */
  root->prev = root->next = root;
}

void *gc_alloc(size_t nbytes)
{
  nbytes = gc_align(nbytes);

  if (nbytes > gc_free_space)
    collect_garbage(nbytes);

  void *p = gc_free_ptr;
  gc_free_ptr   += nbytes;
  gc_free_space -= nbytes;
  //debug(("Found %d bytes at %#.8p.\n", nbytes, p));
  return p;
}

static void transfer_object(value_t *value)
{
  if (is_object(*value))
  {
    object_t *obj = get_object(*value);
    value_t new_value;

    assert(gc_range_of(obj) != gc_current_range);

    if (obj->tag == BROKEN_HEART)
    {
      /* Object has already been moved; just update the reference */
      new_value = obj->payload.values[0];
    }
    else
    {
      switch (obj->tag)
      {
      case TYPE_TAG_VECTOR:
        {
          const vector_t *vec = (const vector_t*)obj;
          const size_t nbytes = sizeof(vector_t) + sizeof(value_t) * vec->size;
          vector_t *newvec = (vector_t*)gc_alloc(nbytes);
          memcpy(newvec, vec, nbytes);
          new_value = object_value(newvec);
        }
        break;
      case TYPE_TAG_BYTESTR:
        {
          const byte_string_t *str = (const byte_string_t*)obj;
          const size_t nbytes = sizeof(byte_string_t) + str->size;
          byte_string_t *newstr = (byte_string_t*)gc_alloc(nbytes);
          memcpy(newstr, str, nbytes);
          new_value = object_value(newstr);
        }
        break;
      case TYPE_TAG_BOX:
      default: /* pair */
        {
          pair_t *p = (pair_t*)gc_alloc(sizeof(pair_t));
          const pair_t *q = (const pair_t*)obj;
          *p = *q;

          //debug(("Moved pair from 0x%0.8X to 0x%0.8X.\n", q, p));

          /* Keep the original tag bits (pair or object) */
          new_value = pair_value(p) | (*value & 2);
        }
        break;
      }

      obj->tag = BROKEN_HEART;
      obj->payload.values[0] = new_value;
    }

#if 0
#ifndef NDEBUG
    if (is_pair(new_value))
    {
      if (gc_range_of(get_pair(new_value)) != gc_current_range)
      {
        debug(("Invalid address after transfer: 0x%0.8X. Current GC: %d.\n",
              get_pair(new_value), gc_current_range));
        abort();
      }
    }
#endif
#endif

    *value = new_value;
  }
}

static inline size_t transfer_children(object_t *obj)
{
  switch (obj->tag)
  {
  case TYPE_TAG_VECTOR:
    {
      vector_t *vec = (vector_t*)obj;
      const intptr_t nelem = from_fixnum(vec->size);

      for (intptr_t i = 0; i < nelem; ++i)
      {
        transfer_object(&vec->elements[i]);
      }

      return sizeof(vector_t) + (nelem * sizeof(value_t));
    }
  case TYPE_TAG_BYTESTR:
    {
      const byte_string_t *str = (const byte_string_t*)obj;
      return sizeof(byte_string_t) + from_fixnum(str->size);
    }
  case TYPE_TAG_BOX:
  default: /* pair */
    {
      pair_t *p = (pair_t*)obj;
      transfer_object(&p->car);
      transfer_object(&p->cdr);
      return sizeof(pair_t);
    }
  }
}

void collect_garbage(size_t min_free)
{
  static bool collecting = false;

  gc_root_t *root;
  char *object_ptr;

  //debug(("Collecting garbage...\n"));

  /* Recursive calls to collector should never occur */
  if (collecting)
  {
    debug(("Ran out of memory while collecting garbage!\n"));
    abort();
  }
  else
    collecting = true;

  ++gc_counter;
  gc_ticks -= (int)clock();

  /* Swap ranges; new "current" range is initially empty, old one is full */
  gc_current_range = 1 - gc_current_range;
  gc_free_ptr      = (char*)&gc_ranges[gc_current_range][0];
  gc_free_space    = gc_soft_limit;
  object_ptr       = gc_free_ptr;

  /* Transfer GC roots (if necessary) */
  root = gc_root_list.next;
  
  while (root != &gc_root_list)
  {
    transfer_object(&root->value);
    root = root->next;
  }

  /* Keep transferring until no more objects in the new range refer to the old one */
  while (object_ptr < gc_free_ptr)
  {
    object_ptr += gc_align(transfer_children((object_t*)object_ptr));
  }
  
  //debug(("Finished collection with %d bytes to spare (out of %d bytes).\n", gc_free_space, gc_soft_limit));

  {
    size_t bytes_used = gc_soft_limit - gc_free_space;
    size_t min_limit = bytes_used + min_free;
    size_t new_limit = (3 * min_limit) / 2;

    if (new_limit > gc_max_size)
      new_limit = gc_max_size;
    else if (new_limit < gc_min_size)
      new_limit = gc_min_size;

    gc_free_space = (gc_free_space + new_limit) - gc_soft_limit;
    gc_soft_limit = new_limit;
  }

  if (gc_free_space < min_free)
  {
    out_of_memory();
  }

  /* Done collecting. */
  collecting = false;

  gc_ticks += (int)clock();
}

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