Head

GCC Code Coverage Report
Directory:	../src/		Exec	Total	Coverage
File:	/home/joels/Current/lispbm/src/heap.c	Lines:	687	824	83.4 %
Date:	2024-12-05 14:36:58	Branches:	247	351	70.4 %

/*
    Copyright 2018, 2020, 2022 - 2024 Joel Svensson  svenssonjoel@yahoo.se
                          2022        Benjamin Vedder

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdarg.h>
#include <inttypes.h>
#include <lbm_memory.h>
#include <lbm_custom_type.h>
#include <lbm_defrag_mem.h>

#include "heap.h"
#include "symrepr.h"
#include "stack.h"
#include "lbm_channel.h"
#include "platform_mutex.h"
#include "eval_cps.h"
#ifdef VISUALIZE_HEAP
#include "heap_vis.h"
#endif


static inline lbm_value lbm_set_gc_mark(lbm_value x) {
  return x | LBM_GC_MARKED;
}

static inline lbm_value lbm_clr_gc_mark(lbm_value x) {
  return x & ~LBM_GC_MASK;
}

static inline bool lbm_get_gc_mark(lbm_value x) {
  return x & LBM_GC_MASK;
}

// flag is the same bit as mark, but in car
static inline bool lbm_get_gc_flag(lbm_value x) {
  return x & LBM_GC_MARKED;
}

static inline lbm_value lbm_set_gc_flag(lbm_value x) {
  return x | LBM_GC_MARKED;
}

static inline lbm_value lbm_clr_gc_flag(lbm_value x) {
  return x & ~LBM_GC_MASK;
}


lbm_heap_state_t lbm_heap_state;

lbm_const_heap_t *lbm_const_heap_state;

lbm_cons_t *lbm_heaps[2] = {NULL, NULL};

static mutex_t lbm_const_heap_mutex;
static bool    lbm_const_heap_mutex_initialized = false;

static mutex_t lbm_mark_mutex;
static bool    lbm_mark_mutex_initialized = false;

#ifdef USE_GC_PTR_REV
void lbm_gc_lock(void) {
  mutex_lock(&lbm_mark_mutex);
}
void lbm_gc_unlock(void) {
  mutex_unlock(&lbm_mark_mutex);
}
#else
void lbm_gc_lock(void) {
}
void lbm_gc_unlock(void) {
}
#endif

/****************************************************/
/* ENCODERS DECODERS                                */

lbm_value lbm_enc_i32(int32_t x) {
#ifndef LBM64
  lbm_value i = lbm_cons((lbm_uint)x, ENC_SYM_RAW_I_TYPE);
  if (lbm_type_of(i) == LBM_TYPE_SYMBOL) return i;
  return lbm_set_ptr_type(i, LBM_TYPE_I32);
#else
  return (((lbm_uint)x) << LBM_VAL_SHIFT) | LBM_TYPE_I32;
#endif
}

lbm_value lbm_enc_u32(uint32_t x) {
#ifndef LBM64
  lbm_value u = lbm_cons(x, ENC_SYM_RAW_U_TYPE);
  if (lbm_type_of(u) == LBM_TYPE_SYMBOL) return u;
  return lbm_set_ptr_type(u, LBM_TYPE_U32);
#else
  return (((lbm_uint)x) << LBM_VAL_SHIFT) | LBM_TYPE_U32;
#endif
}

lbm_value lbm_enc_float(float x) {
#ifndef LBM64
  lbm_uint t;
  memcpy(&t, &x, sizeof(lbm_float));
  lbm_value f = lbm_cons(t, ENC_SYM_RAW_F_TYPE);
  if (lbm_type_of(f) == LBM_TYPE_SYMBOL) return f;
  return lbm_set_ptr_type(f, LBM_TYPE_FLOAT);
#else
  lbm_uint t = 0;
  memcpy(&t, &x, sizeof(float));
  return (((lbm_uint)t) << LBM_VAL_SHIFT) | LBM_TYPE_FLOAT;
#endif
}

#ifndef LBM64
static lbm_value enc_64_on_32(uint8_t *source, lbm_uint type_qual, lbm_uint type) {
  lbm_value res = lbm_cons(ENC_SYM_NIL,ENC_SYM_NIL);
  if (lbm_type_of(res) != LBM_TYPE_SYMBOL) {
    uint8_t* storage = lbm_malloc(sizeof(uint64_t));
    if (storage) {
      memcpy(storage,source, sizeof(uint64_t));
      lbm_set_car_and_cdr(res, (lbm_uint)storage,  type_qual);
      res = lbm_set_ptr_type(res, type);
    } else {
      res = ENC_SYM_MERROR;
    }
  }
  return res;
}
#endif

lbm_value lbm_enc_i64(int64_t x) {
#ifndef LBM64
  return enc_64_on_32((uint8_t *)&x, ENC_SYM_IND_I_TYPE, LBM_TYPE_I64);
#else
  lbm_value u = lbm_cons((uint64_t)x, ENC_SYM_RAW_I_TYPE);
  if (lbm_type_of(u) == LBM_TYPE_SYMBOL) return u;
  return lbm_set_ptr_type(u, LBM_TYPE_I64);
#endif
}

lbm_value lbm_enc_u64(uint64_t x) {
#ifndef LBM64
  return enc_64_on_32((uint8_t *)&x, ENC_SYM_IND_U_TYPE, LBM_TYPE_U64);
#else
  lbm_value u = lbm_cons(x, ENC_SYM_RAW_U_TYPE);
  if (lbm_type_of(u) == LBM_TYPE_SYMBOL) return u;
  return lbm_set_ptr_type(u, LBM_TYPE_U64);
#endif
}

lbm_value lbm_enc_double(double x) {
#ifndef LBM64
  return enc_64_on_32((uint8_t *)&x, ENC_SYM_IND_F_TYPE, LBM_TYPE_DOUBLE);
#else
  lbm_uint t;
  memcpy(&t, &x, sizeof(double));
  lbm_value f = lbm_cons(t, ENC_SYM_RAW_F_TYPE);
  if (lbm_type_of(f) == LBM_TYPE_SYMBOL) return f;
  return lbm_set_ptr_type(f, LBM_TYPE_DOUBLE);
#endif
}

// Type specific (as opposed to the dec_as_X) functions
// should only be run on values KNOWN to represent a value of the type
// that the decoder decodes.

float lbm_dec_float(lbm_value x) {
#ifndef LBM64
  float f_tmp;
  lbm_uint tmp = lbm_car(x);
  memcpy(&f_tmp, &tmp, sizeof(float));
  return f_tmp;
#else
  uint32_t tmp = (uint32_t)(x >> LBM_VAL_SHIFT);
  float f_tmp;
  memcpy(&f_tmp, &tmp, sizeof(float));
  return f_tmp;
#endif
}

double lbm_dec_double(lbm_value x) {
#ifndef LBM64
  double d;
  uint32_t *data = (uint32_t*)lbm_car(x);
  memcpy(&d, data, sizeof(double));
  return d;
#else
  double f_tmp;
  lbm_uint tmp = lbm_car(x);
  memcpy(&f_tmp, &tmp, sizeof(double));
  return f_tmp;
#endif
}

uint64_t lbm_dec_u64(lbm_value x) {
#ifndef LBM64
  uint64_t u;
  uint32_t *data = (uint32_t*)lbm_car(x);
  memcpy(&u, data, 8);
  return u;
#else
  return (uint64_t)lbm_car(x);
#endif
}

int64_t lbm_dec_i64(lbm_value x) {
#ifndef LBM64
  int64_t i;
  uint32_t *data = (uint32_t*)lbm_car(x);
  memcpy(&i, data, 8);
  return i;
#else
  return (int64_t)lbm_car(x);
#endif
}

char *lbm_dec_str(lbm_value val) {
  char *res = 0;
  // If val is an array, car of val will be non-null.
  if (lbm_is_array_r(val)) {
    lbm_array_header_t *array = (lbm_array_header_t *)lbm_car(val);
    res = (char *)array->data;
  }
  return res;
}

lbm_char_channel_t *lbm_dec_channel(lbm_value val) {
  lbm_char_channel_t *res = NULL;

  if (lbm_type_of(val) == LBM_TYPE_CHANNEL) {
    res = (lbm_char_channel_t *)lbm_car(val);
  }
  return res;
}

lbm_uint lbm_dec_custom(lbm_value val) {
  lbm_uint res = 0;
  if (lbm_type_of(val) == LBM_TYPE_CUSTOM) {
    res = (lbm_uint)lbm_car(val);
  }
  return res;
}

uint8_t lbm_dec_as_char(lbm_value a) {
  uint8_t r = 0;
  switch (lbm_type_of_functional(a)) {
  case LBM_TYPE_CHAR:
    r = (uint8_t)lbm_dec_char(a); break;
  case LBM_TYPE_I:
    r = (uint8_t)lbm_dec_i(a); break;
  case LBM_TYPE_U:
    r = (uint8_t)lbm_dec_u(a); break;
  case LBM_TYPE_I32:
    r = (uint8_t)lbm_dec_i32(a); break;
  case LBM_TYPE_U32:
    r = (uint8_t)lbm_dec_u32(a); break;
  case LBM_TYPE_FLOAT:
    r = (uint8_t)lbm_dec_float(a); break;
  case LBM_TYPE_I64:
    r = (uint8_t)lbm_dec_i64(a); break;
  case LBM_TYPE_U64:
    r = (uint8_t)lbm_dec_u64(a); break;
  case LBM_TYPE_DOUBLE:
    r = (uint8_t) lbm_dec_double(a); break;
  }
  return r;
}

uint32_t lbm_dec_as_u32(lbm_value a) {
  uint32_t r = 0;
  switch (lbm_type_of_functional(a)) {
  case LBM_TYPE_CHAR:
    r = (uint32_t)lbm_dec_char(a); break;
  case LBM_TYPE_I:
    r = (uint32_t)lbm_dec_i(a); break;
  case LBM_TYPE_U:
    r = (uint32_t)lbm_dec_u(a); break;
  case LBM_TYPE_I32: /* fall through */
  case LBM_TYPE_U32:
    r = (uint32_t)lbm_dec_u32(a); break;
  case LBM_TYPE_FLOAT:
    r = (uint32_t)lbm_dec_float(a); break;
  case LBM_TYPE_I64:
    r = (uint32_t)lbm_dec_i64(a); break;
  case LBM_TYPE_U64:
    r = (uint32_t)lbm_dec_u64(a); break;
  case LBM_TYPE_DOUBLE:
    r = (uint32_t)lbm_dec_double(a); break;
  }
  return r;
}

int32_t lbm_dec_as_i32(lbm_value a) {
  int32_t r = 0;
  switch (lbm_type_of_functional(a)) {
  case LBM_TYPE_CHAR:
    r = (int32_t)lbm_dec_char(a); break;
  case LBM_TYPE_I:
    r = (int32_t)lbm_dec_i(a); break;
  case LBM_TYPE_U:
    r = (int32_t)lbm_dec_u(a); break;
  case LBM_TYPE_I32:
    r = (int32_t)lbm_dec_i32(a); break;
  case LBM_TYPE_U32:
    r = (int32_t)lbm_dec_u32(a); break;
  case LBM_TYPE_FLOAT:
    r = (int32_t)lbm_dec_float(a); break;
  case LBM_TYPE_I64:
    r = (int32_t)lbm_dec_i64(a); break;
  case LBM_TYPE_U64:
    r = (int32_t)lbm_dec_u64(a); break;
  case LBM_TYPE_DOUBLE:
    r = (int32_t) lbm_dec_double(a); break;
  }
  return r;
}

int64_t lbm_dec_as_i64(lbm_value a) {
  int64_t r = 0;
  switch (lbm_type_of_functional(a)) {
  case LBM_TYPE_CHAR:
    r = (int64_t)lbm_dec_char(a); break;
  case LBM_TYPE_I:
    r = (int64_t)lbm_dec_i(a); break;
  case LBM_TYPE_U:
    r = (int64_t)lbm_dec_u(a); break;
  case LBM_TYPE_I32:
    r = (int64_t)lbm_dec_i32(a); break;
  case LBM_TYPE_U32:
    r = (int64_t)lbm_dec_u32(a); break;
  case LBM_TYPE_FLOAT:
    r = (int64_t)lbm_dec_float(a); break;
  case LBM_TYPE_I64:
    r = (int64_t)lbm_dec_i64(a); break;
  case LBM_TYPE_U64:
    r = (int64_t)lbm_dec_u64(a); break;
  case LBM_TYPE_DOUBLE:
    r = (int64_t) lbm_dec_double(a); break;
  }
  return r;
}

uint64_t lbm_dec_as_u64(lbm_value a) {
  uint64_t r = 0;
  switch (lbm_type_of_functional(a)) {
  case LBM_TYPE_CHAR:
    r = (uint64_t)lbm_dec_char(a); break;
  case LBM_TYPE_I:
    r = (uint64_t)lbm_dec_i(a); break;
  case LBM_TYPE_U:
    r = (uint64_t)lbm_dec_u(a); break;
  case LBM_TYPE_I32:
    r = (uint64_t)lbm_dec_i32(a); break;
  case LBM_TYPE_U32:
    r = (uint64_t)lbm_dec_u32(a); break;
  case LBM_TYPE_FLOAT:
    r = (uint64_t)lbm_dec_float(a); break;
  case LBM_TYPE_I64:
    r = (uint64_t)lbm_dec_i64(a); break;
  case LBM_TYPE_U64:
    r = (uint64_t)lbm_dec_u64(a); break;
  case LBM_TYPE_DOUBLE:
    r = (uint64_t)lbm_dec_double(a); break;
  }
  return r;
}

lbm_uint lbm_dec_as_uint(lbm_value a) {
  lbm_uint r = 0;
  switch (lbm_type_of_functional(a)) {
  case LBM_TYPE_CHAR:
    r = (lbm_uint)lbm_dec_char(a); break;
  case LBM_TYPE_I:
    r = (lbm_uint)lbm_dec_i(a); break;
  case LBM_TYPE_U:
    r = (lbm_uint)lbm_dec_u(a); break;
  case LBM_TYPE_I32:
    r = (lbm_uint)lbm_dec_i32(a); break;
  case LBM_TYPE_U32:
    r = (lbm_uint)lbm_dec_u32(a); break;
  case LBM_TYPE_FLOAT:
    r = (lbm_uint)lbm_dec_float(a); break;
  case LBM_TYPE_I64:
    r = (lbm_uint)lbm_dec_i64(a); break;
  case LBM_TYPE_U64:
    r = (lbm_uint) lbm_dec_u64(a); break;
  case LBM_TYPE_DOUBLE:
    r = (lbm_uint)lbm_dec_double(a); break;
  }
  return r;
}

lbm_int lbm_dec_as_int(lbm_value a) {
  lbm_int r = 0;
  switch (lbm_type_of_functional(a)) {
  case LBM_TYPE_CHAR:
    r = (lbm_int)lbm_dec_char(a); break;
  case LBM_TYPE_I:
    r = (lbm_int)lbm_dec_i(a); break;
  case LBM_TYPE_U:
    r = (lbm_int)lbm_dec_u(a); break;
  case LBM_TYPE_I32:
    r = (lbm_int)lbm_dec_i32(a); break;
  case LBM_TYPE_U32:
    r = (lbm_int)lbm_dec_u32(a); break;
  case LBM_TYPE_FLOAT:
    r = (lbm_int)lbm_dec_float(a); break;
  case LBM_TYPE_I64:
    r = (lbm_int)lbm_dec_i64(a); break;
  case LBM_TYPE_U64:
    r = (lbm_int)lbm_dec_u64(a); break;
  case LBM_TYPE_DOUBLE:
    r = (lbm_int)lbm_dec_double(a); break;
  }
  return r;
}

float lbm_dec_as_float(lbm_value a) {
  float r = 0;
  switch (lbm_type_of_functional(a)) {
  case LBM_TYPE_CHAR:
    r = (float)lbm_dec_char(a); break;
  case LBM_TYPE_I:
    r = (float)lbm_dec_i(a); break;
  case LBM_TYPE_U:
    r = (float)lbm_dec_u(a); break;
  case LBM_TYPE_I32:
    r = (float)lbm_dec_i32(a); break;
  case LBM_TYPE_U32:
    r = (float)lbm_dec_u32(a); break;
  case LBM_TYPE_FLOAT:
    r = (float)lbm_dec_float(a); break;
  case LBM_TYPE_I64:
    r = (float)lbm_dec_i64(a); break;
  case LBM_TYPE_U64:
    r = (float)lbm_dec_u64(a); break;
  case LBM_TYPE_DOUBLE:
    r = (float)lbm_dec_double(a); break;
  }
  return r;
}

double lbm_dec_as_double(lbm_value a) {
  double r = 0;
  switch (lbm_type_of_functional(a)) {
  case LBM_TYPE_CHAR:
    r = (double)lbm_dec_char(a); break;
  case LBM_TYPE_I:
    r = (double)lbm_dec_i(a); break;
  case LBM_TYPE_U:
    r = (double)lbm_dec_u(a); break;
  case LBM_TYPE_I32:
    r = (double)lbm_dec_i32(a); break;
  case LBM_TYPE_U32:
    r = (double)lbm_dec_u32(a); break;
  case LBM_TYPE_FLOAT:
    r = (double)lbm_dec_float(a); break;
  case LBM_TYPE_I64:
    r = (double)lbm_dec_i64(a); break;
  case LBM_TYPE_U64:
    r = (double)lbm_dec_u64(a); break;
  case LBM_TYPE_DOUBLE:
    r = (double)lbm_dec_double(a); break;
  }
  return r;
}

/****************************************************/
/* HEAP MANAGEMENT                                  */

static int generate_freelist(size_t num_cells) {
  size_t i = 0;

  if (!lbm_heap_state.heap) return 0;

  lbm_heap_state.freelist = lbm_enc_cons_ptr(0);

  lbm_cons_t *t;

  // Add all cells to free list
  for (i = 1; i < num_cells; i ++) {
    t = lbm_ref_cell(lbm_enc_cons_ptr(i-1));
    t->car = ENC_SYM_RECOVERED;    // all cars in free list are "RECOVERED"
    t->cdr = lbm_enc_cons_ptr(i);
  }

  // Replace the incorrect pointer at the last cell.
  t = lbm_ref_cell(lbm_enc_cons_ptr(num_cells-1));
  t->cdr = ENC_SYM_NIL;

  return 1;
}

void lbm_nil_freelist(void) {
  lbm_heap_state.freelist = ENC_SYM_NIL;
  lbm_heap_state.num_alloc = lbm_heap_state.heap_size;
}

static void heap_init_state(lbm_cons_t *addr, lbm_uint num_cells,
                            lbm_uint* gc_stack_storage, lbm_uint gc_stack_size) {
  lbm_heap_state.heap         = addr;
  lbm_heap_state.heap_bytes   = (unsigned int)(num_cells * sizeof(lbm_cons_t));
  lbm_heap_state.heap_size    = num_cells;

  lbm_stack_create(&lbm_heap_state.gc_stack, gc_stack_storage, gc_stack_size);

  lbm_heap_state.num_alloc           = 0;
  lbm_heap_state.num_alloc_arrays    = 0;
  lbm_heap_state.gc_num              = 0;
  lbm_heap_state.gc_marked           = 0;
  lbm_heap_state.gc_recovered        = 0;
  lbm_heap_state.gc_recovered_arrays = 0;
  lbm_heap_state.gc_least_free       = num_cells;
  lbm_heap_state.gc_last_free        = num_cells;
}

void lbm_heap_new_freelist_length(void) {
  lbm_uint l = lbm_heap_state.heap_size - lbm_heap_state.num_alloc;
  lbm_heap_state.gc_last_free = l;
  if (l < lbm_heap_state.gc_least_free)
    lbm_heap_state.gc_least_free = l;
}

int lbm_heap_init(lbm_cons_t *addr, lbm_uint num_cells,
                  lbm_uint gc_stack_size) {

  if (((uintptr_t)addr % 8) != 0) return 0;

  memset(addr,0, sizeof(lbm_cons_t) * num_cells);

  lbm_uint *gc_stack_storage = (lbm_uint*)lbm_malloc(gc_stack_size * sizeof(lbm_uint));
  if (gc_stack_storage == NULL) return 0;

  heap_init_state(addr, num_cells,
                  gc_stack_storage, gc_stack_size);

  lbm_heaps[0] = addr;

  return generate_freelist(num_cells);
}


lbm_value lbm_heap_allocate_cell(lbm_type ptr_type, lbm_value car, lbm_value cdr) {
  lbm_value r;
  lbm_value cell = lbm_heap_state.freelist;
  if (cell) {
    lbm_uint heap_ix = lbm_dec_ptr(cell);
    lbm_heap_state.freelist = lbm_heap_state.heap[heap_ix].cdr;
    lbm_heap_state.num_alloc++;
    lbm_heap_state.heap[heap_ix].car = car;
    lbm_heap_state.heap[heap_ix].cdr = cdr;
    r = lbm_set_ptr_type(cell, ptr_type);
  } else {
    r = ENC_SYM_MERROR;
  }
  return r;
}

lbm_value lbm_heap_allocate_list(lbm_uint n) {
  if (n == 0) return ENC_SYM_NIL;
  if (lbm_heap_num_free() < n) return ENC_SYM_MERROR;

  lbm_value curr = lbm_heap_state.freelist;
  lbm_value res  = curr;
  if (lbm_type_of(curr) == LBM_TYPE_CONS) {

    lbm_cons_t *c_cell = NULL;
    lbm_uint count = 0;
    do {
      c_cell = lbm_ref_cell(curr);
      c_cell->car = ENC_SYM_NIL;
      curr = c_cell->cdr;
      count ++;
    } while (count < n);
    lbm_heap_state.freelist = curr;
    c_cell->cdr = ENC_SYM_NIL;
    lbm_heap_state.num_alloc+=count;
    return res;
  }
  return ENC_SYM_FATAL_ERROR;
}

lbm_value lbm_heap_allocate_list_init_va(unsigned int n, va_list valist) {
  if (n == 0) return ENC_SYM_NIL;
  if (lbm_heap_num_free() < n) return ENC_SYM_MERROR;

  lbm_value curr = lbm_heap_state.freelist;
  lbm_value res  = curr;
  if (lbm_type_of(curr) == LBM_TYPE_CONS) {

    lbm_cons_t *c_cell = NULL;
    unsigned int count = 0;
    do {
      c_cell = lbm_ref_cell(curr);
      c_cell->car = va_arg(valist, lbm_value);
      curr = c_cell->cdr;
      count ++;
    } while (count < n);
    lbm_heap_state.freelist = curr;
    c_cell->cdr = ENC_SYM_NIL;
    lbm_heap_state.num_alloc+=count;
    return res;
  }
  return ENC_SYM_FATAL_ERROR;
}

lbm_value lbm_heap_allocate_list_init(unsigned int n, ...) {
    va_list valist;
    va_start(valist, n);
    lbm_value r = lbm_heap_allocate_list_init_va(n, valist);
    va_end(valist);
    return r;
}

lbm_uint lbm_heap_num_allocated(void) {
  return lbm_heap_state.num_alloc;
}
lbm_uint lbm_heap_size(void) {
  return lbm_heap_state.heap_size;
}

lbm_uint lbm_heap_size_bytes(void) {
  return lbm_heap_state.heap_bytes;
}

void lbm_get_heap_state(lbm_heap_state_t *res) {
  *res = lbm_heap_state;
}

lbm_uint lbm_get_gc_stack_max(void) {
  return lbm_get_max_stack(&lbm_heap_state.gc_stack);
}

lbm_uint lbm_get_gc_stack_size(void) {
  return lbm_heap_state.gc_stack.size;
}

#ifdef USE_GC_PTR_REV
/* ************************************************************
   Deutch-Schorr-Waite (DSW) pointer reversal GC for 2-ptr cells
   with a hack-solution for the lisp-array case (n-ptr cells).

   DSW visits each branch node 3 times compared to 2 times for
   the stack based recursive mark.
   Where the stack based recursive mark performs a stack push/pop,
   DSW rearranges the, current, prev, next and a ptr field on
   the heap.

   DSW changes the structure of the heap and it introduces an
   invalid pointer (LBM_PTR_NULL) temporarily during marking.
   Since the heap will be "messed up" while marking, a mutex
   is introuded to keep other processes out of the heap while
   marking.

   TODO: See if the extra index field in arrays can be used
   to mark arrays without resorting to recursive mark calls.
*/

static inline void value_assign(lbm_value *a, lbm_value b) {
  lbm_value a_old = *a & LBM_GC_MASK;
  *a = a_old | (b & ~LBM_GC_MASK);
}

void lbm_gc_mark_phase_nm(lbm_value root) {
  bool work_to_do = true;
  if (!lbm_is_ptr(root)) return;

  lbm_value curr = root;
  lbm_value prev = lbm_enc_cons_ptr(LBM_PTR_NULL);

  while (work_to_do) {
    // follow leftwards pointers
    while (lbm_is_ptr(curr) &&
           (lbm_dec_ptr(curr) != LBM_PTR_NULL) &&
           ((curr & LBM_PTR_TO_CONSTANT_BIT) == 0) &&
           !lbm_get_gc_mark(lbm_cdr(curr))) {
      // Mark the cell if not a constant cell
      lbm_cons_t *cell = lbm_ref_cell(curr);
      cell->cdr = lbm_set_gc_mark(cell->cdr);
      if (lbm_is_cons_rw(curr)) {
        lbm_value next = 0;
        value_assign(&next, cell->car);
        value_assign(&cell->car, prev);
        value_assign(&prev,curr);
        value_assign(&curr, next);
      } else if (lbm_type_of(curr) == LBM_TYPE_LISPARRAY) {
        lbm_array_header_extended_t *arr = (lbm_array_header_extended_t*)cell->car;
        lbm_value *arr_data = (lbm_value *)arr->data;
        size_t  arr_size = (size_t)arr->size / sizeof(lbm_value);
        // C stack recursion as deep as there are nested arrays.
        // TODO: Try to do this without recursion on the C side.
        for (size_t i = 0; i < arr_size; i ++) {
          lbm_gc_mark_phase_nm(arr_data[i]);
        }
      }
      // Will jump out next iteration as gc mark is set in curr.
    }
    while (lbm_is_ptr(prev) &&
           (lbm_dec_ptr(prev) != LBM_PTR_NULL) &&
           lbm_get_gc_flag(lbm_car(prev)) ) {
      // clear the flag
      lbm_cons_t *cell = lbm_ref_cell(prev);
      cell->car = lbm_clr_gc_flag(cell->car);
      lbm_value next = 0;
      value_assign(&next, cell->cdr);
      value_assign(&cell->cdr, curr);
      value_assign(&curr, prev);
      value_assign(&prev, next);
    }
    if (lbm_is_ptr(prev) &&
        lbm_dec_ptr(prev) == LBM_PTR_NULL) {
      work_to_do = false;
    } else if (lbm_is_ptr(prev)) {
      // set the flag
      lbm_cons_t *cell = lbm_ref_cell(prev);
      cell->car = lbm_set_gc_flag(cell->car);
      lbm_value next = 0;
      value_assign(&next, cell->car);
      value_assign(&cell->car, curr);
      value_assign(&curr, cell->cdr);
      value_assign(&cell->cdr, next);
    }
  }
}

void lbm_gc_mark_phase(lbm_value root) {
    mutex_lock(&lbm_const_heap_mutex);
    lbm_gc_mark_phase_nm(root);
    mutex_unlock(&lbm_const_heap_mutex);
}

#else
/* ************************************************************
   Explicit stack "recursive" mark phase

   Trees are marked in a left subtree before rigth subtree, car first then cdr,
   way to favor lisp lists. This means that stack will grow slowly when
   marking right-leaning (cdr-recursive) data-structures while left-leaning
   (car-recursive) structures uses a lot of stack.

   Lisp arrays contain an extra book-keeping field to keep track
   of how far into the array the marking process has gone.

   TODO: DSW should be used as a last-resort if the GC stack is exhausted.
         If we use DSW as last-resort can we get away with a way smaller
         GC stack and unchanged performance (on sensible programs)?
*/

extern eval_context_t *ctx_running;
void lbm_gc_mark_phase(lbm_value root) {
  lbm_value t_ptr;
  lbm_stack_t *s = &lbm_heap_state.gc_stack;
  s->data[s->sp++] = root;

  while (!lbm_stack_is_empty(s)) {
    lbm_value curr;
    lbm_pop(s, &curr);

  mark_shortcut:

    if (!lbm_is_ptr(curr) ||
        (curr & LBM_PTR_TO_CONSTANT_BIT)) {
      continue;
    }

    lbm_cons_t *cell = &lbm_heap_state.heap[lbm_dec_ptr(curr)];

    if (lbm_get_gc_mark(cell->cdr)) {
      continue;
    }

    t_ptr = lbm_type_of(curr);

    // An array is marked in O(N) time using an additional 32bit
    // value per array that keeps track of how far into the array GC
    // has progressed.
    if (t_ptr == LBM_TYPE_LISPARRAY) {
      lbm_push(s, curr); // put array back as bookkeeping.
      lbm_array_header_extended_t *arr = (lbm_array_header_extended_t*)cell->car;
      lbm_value *arrdata = (lbm_value *)arr->data;
      uint32_t index = arr->index;

      // Potential optimization.
      // 1. CONS pointers are set to curr and recurse.
      // 2. Any other ptr is marked immediately and index is increased.
      if (lbm_is_ptr(arrdata[index]) && ((arrdata[index] & LBM_PTR_TO_CONSTANT_BIT) == 0) &&
          !((arrdata[index] & LBM_CONTINUATION_INTERNAL) == LBM_CONTINUATION_INTERNAL)) {
        lbm_cons_t *elt = &lbm_heap_state.heap[lbm_dec_ptr(arrdata[index])];
        if (!lbm_get_gc_mark(elt->cdr)) {
          curr = arrdata[index];
          goto mark_shortcut;
        }
      }
      if (index < ((arr->size/(sizeof(lbm_value))) - 1)) {
        arr->index++;
        continue;
      }

      arr->index = 0;
      cell->cdr = lbm_set_gc_mark(cell->cdr);
      lbm_heap_state.gc_marked ++;
      lbm_pop(s, &curr); // Remove array from GC stack as we are done marking it.
      continue;
    } else if (t_ptr == LBM_TYPE_CHANNEL) {
      cell->cdr = lbm_set_gc_mark(cell->cdr);
      lbm_heap_state.gc_marked ++;
      // TODO: Can channels be explicitly freed ?
      if (cell->car != ENC_SYM_NIL) {
        lbm_char_channel_t *chan = (lbm_char_channel_t *)cell->car;
        curr = chan->dependency;
        goto mark_shortcut;
      }
      continue;
    }

    cell->cdr = lbm_set_gc_mark(cell->cdr);
    lbm_heap_state.gc_marked ++;

    if (t_ptr == LBM_TYPE_CONS) {
      if (lbm_is_ptr(cell->cdr)) {
        if (!lbm_push(s, cell->cdr)) {
          lbm_critical_error();
          break;
        }
      }
      curr = cell->car;
      goto mark_shortcut; // Skip a push/pop
    }
  }
}
#endif

//Environments are proper lists with a 2 element list stored in each car.
void lbm_gc_mark_env(lbm_value env) {
  lbm_value curr = env;
  lbm_cons_t *c;

  while (lbm_is_ptr(curr)) {
    c = lbm_ref_cell(curr);
    c->cdr = lbm_set_gc_mark(c->cdr); // mark the environent list structure.
    lbm_cons_t *b = lbm_ref_cell(c->car);
    b->cdr = lbm_set_gc_mark(b->cdr); // mark the binding list head cell.
    lbm_gc_mark_phase(b->cdr);        // mark the bound object.
    lbm_heap_state.gc_marked +=2;
    curr = c->cdr;
  }
}


void lbm_gc_mark_aux(lbm_uint *aux_data, lbm_uint aux_size) {
  for (lbm_uint i = 0; i < aux_size; i ++) {
    if (lbm_is_ptr(aux_data[i])) {
      lbm_type pt_t = lbm_type_of(aux_data[i]);
      lbm_uint pt_v = lbm_dec_ptr(aux_data[i]);
      if( pt_t >= LBM_POINTER_TYPE_FIRST &&
          pt_t <= LBM_POINTER_TYPE_LAST &&
          pt_v < lbm_heap_state.heap_size) {
        lbm_gc_mark_phase(aux_data[i]);
      }
    }
  }
}

void lbm_gc_mark_roots(lbm_uint *roots, lbm_uint num_roots) {
  for (lbm_uint i = 0; i < num_roots; i ++) {
    lbm_gc_mark_phase(roots[i]);
  }
}

// Sweep moves non-marked heap objects to the free list.
int lbm_gc_sweep_phase(void) {
  unsigned int i = 0;
  lbm_cons_t *heap = (lbm_cons_t *)lbm_heap_state.heap;

  for (i = 0; i < lbm_heap_state.heap_size; i ++) {
    if ( lbm_get_gc_mark(heap[i].cdr)) {
      heap[i].cdr = lbm_clr_gc_mark(heap[i].cdr);
    } else {
      // Check if this cell is a pointer to an array
      // and free it.
      if (lbm_type_of(heap[i].cdr) == LBM_TYPE_SYMBOL) {
        switch(heap[i].cdr) {

        case ENC_SYM_IND_I_TYPE: /* fall through */
        case ENC_SYM_IND_U_TYPE:
        case ENC_SYM_IND_F_TYPE:
          lbm_memory_free((lbm_uint*)heap[i].car);
          break;
        case ENC_SYM_DEFRAG_ARRAY_TYPE:
          lbm_defrag_mem_free((lbm_uint*)heap[i].car);
          break;
        case ENC_SYM_LISPARRAY_TYPE: /* fall through */
        case ENC_SYM_ARRAY_TYPE:{
          lbm_array_header_t *arr = (lbm_array_header_t*)heap[i].car;
          lbm_memory_free((lbm_uint *)arr->data);
          lbm_heap_state.gc_recovered_arrays++;
          lbm_memory_free((lbm_uint *)arr);
        } break;
        case ENC_SYM_CHANNEL_TYPE:{
          lbm_char_channel_t *chan = (lbm_char_channel_t*)heap[i].car;
          lbm_memory_free((lbm_uint*)chan->state);
          lbm_memory_free((lbm_uint*)chan);
        } break;
        case ENC_SYM_CUSTOM_TYPE: {
          lbm_uint *t = (lbm_uint*)heap[i].car;
          lbm_custom_type_destroy(t);
          lbm_memory_free(t);
          } break;
        case ENC_SYM_DEFRAG_MEM_TYPE: {
          lbm_uint *ptr = (lbm_uint *)heap[i].car;
          lbm_defrag_mem_destroy(ptr);
          } break;
        default:
          break;
        }
      }
      // create pointer to use as new freelist
      lbm_uint addr = lbm_enc_cons_ptr(i);

      // Clear the "freed" cell.
      heap[i].car = ENC_SYM_RECOVERED;
      heap[i].cdr = lbm_heap_state.freelist;
      lbm_heap_state.freelist = addr;
      lbm_heap_state.num_alloc --;
      lbm_heap_state.gc_recovered ++;
    }
  }
  return 1;
}

void lbm_gc_state_inc(void) {
  lbm_heap_state.gc_num ++;
  lbm_heap_state.gc_recovered = 0;
  lbm_heap_state.gc_marked = 0;
}

// construct, alter and break apart
lbm_value lbm_cons(lbm_value car, lbm_value cdr) {
  return lbm_heap_allocate_cell(LBM_TYPE_CONS, car, cdr);
}

lbm_value lbm_car(lbm_value c){

  if (lbm_is_ptr(c) ){
    lbm_cons_t *cell = lbm_ref_cell(c);
    return cell->car;
  }

  if (lbm_is_symbol_nil(c)) {
    return c; // if nil, return nil.
  }

  return ENC_SYM_TERROR;
}

// TODO: Many comparisons "is this the nil symbol" can be
// streamlined a bit. NIL is 0 and cannot be confused with any other
// lbm_value.

lbm_value lbm_caar(lbm_value c) {
  if (lbm_is_ptr(c)) {
    lbm_value tmp = lbm_ref_cell(c)->car;

    if (lbm_is_ptr(tmp)) {
      return lbm_ref_cell(tmp)->car;
    } else if (lbm_is_symbol_nil(tmp)) {
      return tmp;
    }
  } else if (lbm_is_symbol_nil(c)){
    return c;
  }
  return ENC_SYM_TERROR;
}


lbm_value lbm_cadr(lbm_value c) {
  if (lbm_is_ptr(c)) {
    lbm_value tmp = lbm_ref_cell(c)->cdr;

    if (lbm_is_ptr(tmp)) {
      return lbm_ref_cell(tmp)->car;
    } else if (lbm_is_symbol_nil(tmp)) {
      return tmp;
    }
  } else if (lbm_is_symbol_nil(c)) {
    return c;
  }
  return ENC_SYM_TERROR;
}

lbm_value lbm_cdr(lbm_value c){
  if (lbm_is_ptr(c)) {
    lbm_cons_t *cell = lbm_ref_cell(c);
    return cell->cdr;
  }
  if (lbm_is_symbol_nil(c)) {
    return ENC_SYM_NIL; // if nil, return nil.
  }
  return ENC_SYM_TERROR;
}

lbm_value lbm_cddr(lbm_value c) {
  if (lbm_is_ptr(c)) {
    lbm_value tmp = lbm_ref_cell(c)->cdr;
    if (lbm_is_ptr(tmp)) {
      return lbm_ref_cell(tmp)->cdr;
    }
  }
  if (lbm_is_symbol_nil(c)) {
    return ENC_SYM_NIL;
  }
  return ENC_SYM_TERROR;
}

int lbm_set_car(lbm_value c, lbm_value v) {
  int r = 0;

  if (lbm_type_of(c) == LBM_TYPE_CONS) {
    lbm_cons_t *cell = lbm_ref_cell(c);
    cell->car = v;
    r = 1;
  }
  return r;
}

int lbm_set_cdr(lbm_value c, lbm_value v) {
  int r = 0;
  if (lbm_is_cons_rw(c)){
    lbm_cons_t *cell = lbm_ref_cell(c);
    cell->cdr = v;
    r = 1;
  }
  return r;
}

int lbm_set_car_and_cdr(lbm_value c, lbm_value car_val, lbm_value cdr_val) {
  int r = 0;
  if (lbm_is_cons_rw(c)) {
    lbm_cons_t *cell = lbm_ref_cell(c);
    cell->car = car_val;
    cell->cdr = cdr_val;
    r = 1;
  }
  return r;
}

/* calculate length of a proper list */
lbm_uint lbm_list_length(lbm_value c) {
  lbm_uint len = 0;

  while (lbm_is_cons(c)){
    len ++;
    c = lbm_cdr(c);
  }
  return len;
}

/* calculate the length of a list and check that each element
   fullfills the predicate pred */
unsigned int lbm_list_length_pred(lbm_value c, bool *pres, bool (*pred)(lbm_value)) {
  bool res = true;
  unsigned int len = 0;

  while (lbm_is_cons(c)){
    len ++;
    res = res && pred(lbm_car(c));
    c = lbm_cdr(c);
  }
  *pres = res;
  return len;
}

/* reverse a proper list */
lbm_value lbm_list_reverse(lbm_value list) {
  if (lbm_type_of(list) == LBM_TYPE_SYMBOL) {
    return list;
  }

  lbm_value curr = list;

  lbm_value new_list = ENC_SYM_NIL;
  while (lbm_is_cons(curr)) {

    new_list = lbm_cons(lbm_car(curr), new_list);
    if (lbm_type_of(new_list) == LBM_TYPE_SYMBOL) {
      return ENC_SYM_MERROR;
    }
    curr = lbm_cdr(curr);
  }
  return new_list;
}

lbm_value lbm_list_destructive_reverse(lbm_value list) {
  if (lbm_type_of(list) == LBM_TYPE_SYMBOL) {
    return list;
  }
  lbm_value curr = list;
  lbm_value last_cell = ENC_SYM_NIL;

  while (lbm_is_cons_rw(curr)) {
    lbm_value next = lbm_cdr(curr);
    lbm_set_cdr(curr, last_cell);
    last_cell = curr;
    curr = next;
  }
  return last_cell;
}


lbm_value lbm_list_copy(int *m, lbm_value list) {
  lbm_value curr = list;
  lbm_uint n = lbm_list_length(list);
  lbm_uint copy_n = n;
  if (*m >= 0 && (lbm_uint)*m < n) {
    copy_n = (lbm_uint)*m;
  } else if (*m == -1) {
    *m = (int)n; // TODO: smaller range in target variable.
  }
  if (copy_n == 0) return ENC_SYM_NIL;
  lbm_uint new_list = lbm_heap_allocate_list(copy_n);
  if (lbm_is_symbol(new_list)) return new_list;
  lbm_value curr_targ = new_list;

  while (lbm_is_cons(curr) && copy_n > 0) {
    lbm_value v = lbm_car(curr);
    lbm_set_car(curr_targ, v);
    curr_targ = lbm_cdr(curr_targ);
    curr = lbm_cdr(curr);
    copy_n --;
  }

  return new_list;
}

// Append for proper lists only
// Destructive update of list1.
lbm_value lbm_list_append(lbm_value list1, lbm_value list2) {

  if(lbm_is_list_rw(list1) &&
     lbm_is_list(list2)) {

    lbm_value curr = list1;
    while(lbm_type_of(lbm_cdr(curr)) == LBM_TYPE_CONS) {
      curr = lbm_cdr(curr);
    }
    if (lbm_is_symbol_nil(curr)) return list2;
    lbm_set_cdr(curr, list2);
    return list1;
  }
  return ENC_SYM_EERROR;
}

lbm_value lbm_list_drop(unsigned int n, lbm_value ls) {
  lbm_value curr = ls;
  while (lbm_type_of_functional(curr) == LBM_TYPE_CONS &&
         n > 0) {
    curr = lbm_cdr(curr);
    n --;
  }
  return curr;
}

lbm_value lbm_index_list(lbm_value l, int32_t n) {
  lbm_value curr = l;

  if (n < 0) {
    int32_t len = (int32_t)lbm_list_length(l);
    n = len + n;
    if (n < 0) return ENC_SYM_NIL;
  }

  while (lbm_is_cons(curr) &&
          n > 0) {
    curr = lbm_cdr(curr);
    n --;
  }
  if (lbm_is_cons(curr)) {
    return lbm_car(curr);
  } else {
    return ENC_SYM_NIL;
  }
}

// High-level arrays are just bytearrays but with a different tag and pointer type.
// These arrays will be inspected by GC and the elements of the array will be marked.

// Arrays are part of the heap module because their lifespan is managed
// by the garbage collector. The data in the array is not stored
// in the "heap of cons cells".
int lbm_heap_allocate_array_base(lbm_value *res, bool byte_array, lbm_uint size){

  lbm_uint tag = ENC_SYM_ARRAY_TYPE;
  lbm_uint type = LBM_TYPE_ARRAY;
  if (!byte_array) {
      tag = ENC_SYM_LISPARRAY_TYPE;
      type = LBM_TYPE_LISPARRAY;
      size = sizeof(lbm_value) * size;
  }
  lbm_array_header_t *array = NULL;
  if (byte_array) {
    array = (lbm_array_header_t*)lbm_malloc(sizeof(lbm_array_header_t));
  } else {
    array = (lbm_array_header_t*)lbm_malloc(sizeof(lbm_array_header_extended_t));
  }

  if (array == NULL) {
    *res = ENC_SYM_MERROR;
    return 0;
  }
  array->data = NULL;
  if ( size > 0) {
    if (!byte_array) {
      lbm_array_header_extended_t *ext_array = (lbm_array_header_extended_t*)array;
      ext_array->index = 0;
    }

    array->data = (lbm_uint*)lbm_malloc(size);

    if (array->data == NULL) {
      lbm_memory_free((lbm_uint*)array);
      *res = ENC_SYM_MERROR;
      return 0;
    }
    // It is more important to zero out high-level arrays.
    // 0 is symbol NIL which is perfectly safe for the GC to inspect.
    memset(array->data, 0, size);
  }
  array->size = size;

  // allocating a cell for array's heap-presence
  lbm_value cell = lbm_heap_allocate_cell(type, (lbm_uint) array, tag);
  if (cell == ENC_SYM_MERROR) {
    lbm_memory_free((lbm_uint*)array->data);
    lbm_memory_free((lbm_uint*)array);
    *res = ENC_SYM_MERROR;
    return 0;
  }
  *res = cell;

  lbm_heap_state.num_alloc_arrays ++;

  return 1;
}

int lbm_heap_allocate_array(lbm_value *res, lbm_uint size){
  return lbm_heap_allocate_array_base(res, true, size);
}

int lbm_heap_allocate_lisp_array(lbm_value *res, lbm_uint size) {
  return lbm_heap_allocate_array_base(res, false, size);
}

// Convert a C array into an lbm_array.
// if the array is in LBM_MEMORY, the lifetime will be managed by the GC after lifting.
int lbm_lift_array(lbm_value *value, char *data, lbm_uint num_elt) {

  lbm_array_header_t *array = NULL;
  lbm_value cell = lbm_heap_allocate_cell(LBM_TYPE_CONS, ENC_SYM_NIL, ENC_SYM_ARRAY_TYPE);

  if (cell == ENC_SYM_MERROR) {
    *value = cell;
    return 0;
  }

  array = (lbm_array_header_t*)lbm_malloc(sizeof(lbm_array_header_t));

  if (array == NULL) {
    lbm_set_car_and_cdr(cell, ENC_SYM_NIL, ENC_SYM_NIL);
    *value = ENC_SYM_MERROR;
    return 0;
  }

  array->data = (lbm_uint*)data;
  array->size = num_elt;

  lbm_set_car(cell, (lbm_uint)array);

  cell = lbm_set_ptr_type(cell, LBM_TYPE_ARRAY);
  *value = cell;
  return 1;
}

lbm_int lbm_heap_array_get_size(lbm_value arr) {

  lbm_int r = -1;
  if (lbm_is_array_r(arr)) {
    lbm_array_header_t *header = (lbm_array_header_t*)lbm_car(arr);
    if (header == NULL) {
      return r;
    }
    r = (lbm_int)header->size;
  }
  return r;
}

const uint8_t *lbm_heap_array_get_data_ro(lbm_value arr) {
  uint8_t *r = NULL;
  if (lbm_is_array_r(arr)) {
    lbm_array_header_t *header = (lbm_array_header_t*)lbm_car(arr);
    r = (uint8_t*)header->data;
  }
  return r;
}

uint8_t *lbm_heap_array_get_data_rw(lbm_value arr) {
  uint8_t *r = NULL;
  if (lbm_is_array_rw(arr)) {
    lbm_array_header_t *header = (lbm_array_header_t*)lbm_car(arr);
    r = (uint8_t*)header->data;
  }
  return r;
}


/* Explicitly freeing an array.

   This is a highly unsafe operation and can only be safely
   used if the heap cell that points to the array has not been made
   accessible to the program.

   So This function can be used to free an array in case an array
   is being constructed and some error case appears while doing so
   If the array still have not become available it can safely be
   "explicitly" freed.

   The problem is that if the "array" heap-cell is made available to
   the program, this cell can easily be duplicated and we would have
   to search the entire heap to find all cells pointing to the array
   memory in question and "null"-them out before freeing the memory
*/

int lbm_heap_explicit_free_array(lbm_value arr) {

  int r = 0;
  if (lbm_is_array_rw(arr) && lbm_cdr(arr) == ENC_SYM_ARRAY_TYPE) {
    lbm_array_header_t *header = (lbm_array_header_t*)lbm_car(arr);
    if (header == NULL) {
      return 0;
    }
    lbm_memory_free((lbm_uint*)header->data);
    lbm_memory_free((lbm_uint*)header);

    arr = lbm_set_ptr_type(arr, LBM_TYPE_CONS);
    lbm_set_car(arr, ENC_SYM_NIL);
    lbm_set_cdr(arr, ENC_SYM_NIL);
    r = 1;
  }

  return r;
}

lbm_uint lbm_size_of(lbm_type t) {
  lbm_uint s = 0;
  switch(t) {
  case LBM_TYPE_BYTE:
    s = 1;
    break;
  case LBM_TYPE_I: /* fall through */
  case LBM_TYPE_U:
  case LBM_TYPE_SYMBOL:
    s = sizeof(lbm_uint);
    break;
  case LBM_TYPE_I32: /* fall through */
  case LBM_TYPE_U32:
  case LBM_TYPE_FLOAT:
    s = 4;
    break;
  case LBM_TYPE_I64: /* fall through */
  case LBM_TYPE_U64:
  case LBM_TYPE_DOUBLE:
    s = 8;
    break;
  }
  return s;
}

static bool dummy_flash_write(lbm_uint ix, lbm_uint val) {
  (void)ix;
  (void)val;
  return false;
}

static const_heap_write_fun const_heap_write = dummy_flash_write;

int lbm_const_heap_init(const_heap_write_fun w_fun,
                        lbm_const_heap_t *heap,
                        lbm_uint *addr,
                        lbm_uint  num_words) {
  if (((uintptr_t)addr % 4) != 0) return 0;
  if ((num_words % 2) != 0) return 0;

  if (!lbm_const_heap_mutex_initialized) {
    mutex_init(&lbm_const_heap_mutex);
    lbm_const_heap_mutex_initialized = true;
  }

  if (!lbm_mark_mutex_initialized) {
    mutex_init(&lbm_mark_mutex);
    lbm_mark_mutex_initialized = true;
  }

  const_heap_write = w_fun;

  heap->heap = addr;
  heap->size = num_words;
  heap->next = 0;

  lbm_const_heap_state = heap;
  // ref_cell views the lbm_uint array as an lbm_cons_t array
  lbm_heaps[1] = (lbm_cons_t*)addr;
  return 1;
}

lbm_flash_status lbm_allocate_const_cell(lbm_value *res) {
  lbm_flash_status r = LBM_FLASH_FULL;

  mutex_lock(&lbm_const_heap_mutex);
  // waste a cell if we have ended up unaligned after writing an array to flash.
  if (lbm_const_heap_state->next % 2 == 1) {
    lbm_const_heap_state->next++;
  }

  if (lbm_const_heap_state &&
      (lbm_const_heap_state->next+1) < lbm_const_heap_state->size) {
    // A cons cell uses two words.
    lbm_value cell = lbm_const_heap_state->next;
    lbm_const_heap_state->next += 2;
    *res = (cell << LBM_ADDRESS_SHIFT) | LBM_PTR_BIT | LBM_TYPE_CONS | LBM_PTR_TO_CONSTANT_BIT;
    r = LBM_FLASH_WRITE_OK;
  }
  mutex_unlock(&lbm_const_heap_mutex);
  return r;
}

lbm_flash_status lbm_allocate_const_raw(lbm_uint nwords, lbm_uint *res) {
  lbm_flash_status r = LBM_FLASH_FULL;

  if (lbm_const_heap_state &&
      (lbm_const_heap_state->next + nwords) < lbm_const_heap_state->size) {
    lbm_uint ix = lbm_const_heap_state->next;
    *res = (lbm_uint)&lbm_const_heap_state->heap[ix];
    lbm_const_heap_state->next += nwords;
    r = LBM_FLASH_WRITE_OK;
  }
  return r;
}

lbm_flash_status lbm_write_const_raw(lbm_uint *data, lbm_uint n, lbm_uint *res) {

  lbm_flash_status r = LBM_FLASH_FULL;

  if (lbm_const_heap_state &&
      (lbm_const_heap_state->next + n) < lbm_const_heap_state->size) {
    lbm_uint ix = lbm_const_heap_state->next;

    for (unsigned int i = 0; i < n; i ++) {
      if (!const_heap_write(ix + i, ((lbm_uint*)data)[i]))
        return LBM_FLASH_WRITE_ERROR;
    }
    lbm_const_heap_state->next += n;
    *res = (lbm_uint)&lbm_const_heap_state->heap[ix];
    r = LBM_FLASH_WRITE_OK;
  }
  return r;
}

lbm_flash_status lbm_const_write(lbm_uint *tgt, lbm_uint val) {

  if (lbm_const_heap_state) {
    lbm_uint flash = (lbm_uint)lbm_const_heap_state->heap;
    lbm_uint ix = (((lbm_uint)tgt - flash) / sizeof(lbm_uint)); // byte address to ix
    if (const_heap_write(ix, val)) {
      return LBM_FLASH_WRITE_OK;
    }
    return LBM_FLASH_WRITE_ERROR;
  }
  return LBM_FLASH_FULL;
}

lbm_flash_status write_const_cdr(lbm_value cell, lbm_value val) {
  lbm_uint addr = lbm_dec_ptr(cell);
  if (const_heap_write(addr+1, val))
    return LBM_FLASH_WRITE_OK;
  return LBM_FLASH_WRITE_ERROR;
}

lbm_flash_status write_const_car(lbm_value cell, lbm_value val) {
  lbm_uint addr = lbm_dec_ptr(cell);
  if (const_heap_write(addr, val))
    return LBM_FLASH_WRITE_OK;
  return LBM_FLASH_WRITE_ERROR;
}

lbm_uint lbm_flash_memory_usage(void) {
  return lbm_const_heap_state->next;
}

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