/* Calcite, src/kernel/heap.c
 * Copyright 2025 Benji Dial
 *
 * 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 <https://www.gnu.org/licenses/>.
 */

#include "paging.h"
#include "heap.h"

struct dealloc_record {
  //0 for unused
  uint64_t length;
  void *start;
};

struct dealloc_record_page {
  struct dealloc_record records[255];
  struct dealloc_record_page *prev_page;
};

static struct dealloc_record_page *last_page;

static void *map_pages(uint64_t page_count) {

  void *vma = find_free_kernel_region(page_count * 4096);

  for (uint64_t i = 0; i < page_count; ++i) {
    uint64_t pma = take_free_physical_page();
    map_in_kernel_page_table(pma, vma + i * 4096, 1, 0);
  }

  return vma;

}

static struct dealloc_record *get_unused_record() {

  for (struct dealloc_record_page *page = last_page;
       page != 0; page = page->prev_page)
    for (int i = 0; i < 255; ++i)
      if (page->records[i].length == 0)
        return &page->records[i];

  struct dealloc_record_page *page = map_pages(1);
  page->prev_page = last_page;
  last_page = page;

  page->records[0].start = (void *)page + 4096 - 8;
  page->records[0].length = 8;

  for (int i = 2; i < 255; ++i)
    page->records[i].length = 0;

  return &page->records[1];

}

void *heap_alloc(uint64_t length) {

  for (struct dealloc_record_page *page = last_page;
       page != 0; page = page->prev_page)
    for (int i = 0; i < 255; ++i)
      if (page->records[i].length == length) {
        page->records[i].length = 0;
        return page->records[i].start;
      }

  for (struct dealloc_record_page *page = last_page;
       page != 0; page = page->prev_page)
    for (int i = 0; i < 255; ++i)
      if (page->records[i].length > length) {
        page->records[i].length -= length;
        return page->records[i].start + page->records[i].length;
      }

  uint64_t pages = (length - 1) / 4096 + 1;
  void *vma = map_pages(pages);

  if (pages * 4096 != length) {
    struct dealloc_record *record = get_unused_record();
    record->start = vma + length;
    record->length = pages * 4096 - length;
  }

  return vma;

}

void heap_dealloc(void *start, uint64_t length) {
  struct dealloc_record *record = get_unused_record();
  record->start = start;
  record->length = length;
}