/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright 2020 Justine Alexandra Roberts Tunney                              │
│                                                                              │
│ 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; version 2 of the License.                      │
│                                                                              │
│ 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, write to the Free Software                  │
│ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA                │
│ 02110-1301 USA                                                               │
╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/assert.h"
#include "libc/bits/bits.h"
#include "libc/bits/popcnt.h"
#include "libc/bits/safemacros.h"
#include "libc/calls/calls.h"
#include "libc/runtime/buffer.h"
#include "libc/runtime/mappings.h"
#include "libc/runtime/runtime.h"
#include "libc/str/str.h"
#include "libc/sysv/consts/prot.h"

#define kGuard PAGESIZE
#define kGrain FRAMESIZE

/**
 * Allocates page-guarded buffer.
 *
 *     ┌─────────────────────────────────────┬s─i─g─s─e─g─v┐
 *     │   𝑣₀..𝑣ₙ₋₁                          │𝑣ₙ..𝑣ₙ₊₄₀₉₆₋₁│
 *     └─────────────────────────────────────┴s─i─g─s─e─g─v┘
 *
 * @param b is metadata object owned by caller, initialized to zero for
 *     first call; subsequent calls will resize
 * @param a is alignment requirement in bytes, e.g. 1,2,4,8,16,...
 * @param n is buffer size in bytes
 * @return b->p
 * @see ralloc()
 */
void *balloc(struct GuardedBuffer *b, unsigned a, size_t n) {
  int rc;
  char *p;
  size_t mi, need;
  struct AddrSize az;
  struct MemoryCoord mc;

  assert(a >= 1);
  assert(a <= kGuard);
  assert(kGuard < kGrain);
  assert(popcnt(a) == 1);
  assert(popcnt(kGuard) == 1);
  assert(popcnt(kGrain) == 1);
  assert(n < 0x800000000000ul - kGrain - kGuard);

  if (n) {
    az.addr = 0;
    az.size = 0;
    n = roundup(n, a);
    need = roundup(n + kGuard, kGrain);
    if (b->p) {
      mc = ADDRSIZE_TO_COORD(b->p, 1);
      mi = findmapping(mc.y);
      if (mi && ISOVERLAPPING(mc, _mm.p[mi - 1])) {
        az = COORD_TO_ADDRSIZE(_mm.p[mi - 1]);
        if (az.size < need) {
          rc = munmap(az.addr, az.size);
          assert(rc != -1);
          az.addr = NULL;
          az.size = 0;
        }
      }
    }
    if (!az.size) {
      if ((p = mapanon(need)) == MAP_FAILED) abort();
      if (mprotect(p + need - kGuard, kGuard, PROT_NONE) == -1) abort();
    } else {
      p = az.addr;
    }
    b->p = p + need - kGuard - n;
  }

  return b->p;
}