/* Step 6: does OCaml's collector touch memory it does not own?
 *
 * Flan's arenas, Vecs and Maps are plain malloc'd memory. The claim is that
 * OCaml never sees them, so a compaction cannot move or scribble on them. The
 * probe: fill an arena with a checkable pattern, hold raw interior pointers
 * into it across a full major collection AND a compaction, then verify every
 * byte and every pointer.
 *
 * What this proves is narrow and worth stating narrowly: OCaml traces its own
 * roots only. It does NOT license storing an OCaml `value` in this arena --
 * that would need caml_register_global_root, and is the way the assumption
 * actually breaks.
 */
#include <caml/callback.h>
#include <caml/mlvalues.h>
#include <caml/memory.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

#define ARENA (8u << 20) /* 8 MiB, the shape of a Flan arena */

static uint8_t *arena;
static uint64_t *interior[64];

CAMLprim value spike_note_arena(value p) { (void)p; return Val_unit; }

static uint8_t pattern(size_t i) { return (uint8_t)(i * 31u + 7u); }

int main(int argc, char **argv) {
  const value *f;
  size_t i, bad = 0;
  uint8_t *before;
  (void)argc;

  arena = malloc(ARENA);
  if (!arena) return 1;
  for (i = 0; i < ARENA; i++) arena[i] = pattern(i);
  for (i = 0; i < 64; i++) interior[i] = (uint64_t *)(arena + i * 4096);
  before = arena;

  caml_startup(argv);

  f = caml_named_value("spike_churn");
  if (!f) { fprintf(stderr, "spike: churn not registered\n"); return 1; }
  printf("%s\n", String_val(caml_callback(*f, Val_int(200))));

  for (i = 0; i < ARENA; i++) if (arena[i] != pattern(i)) bad++;
  printf("arena base %s (%p -> %p)\n", before == arena ? "unmoved" : "MOVED",
         (void *)before, (void *)arena);
  printf("arena bytes altered by the GC: %zu of %u\n", bad, ARENA);

  bad = 0;
  for (i = 0; i < 64; i++)
    if (interior[i] != (uint64_t *)(arena + i * 4096)) bad++;
  printf("raw interior pointers invalidated: %zu of 64\n", bad);
  printf("spike: %s\n", bad == 0 ? "foreign memory is invisible to the collector"
                                 : "FOREIGN MEMORY WAS DISTURBED");
  free(arena);
  return 0;
}