// Physical memory allocator, for user processes,
// kernel stacks, page-table pages,
// and pipe buffers. Allocates whole 4096-byte pages.

#include "types.h"
#include "param.h"
#include "memlayout.h"
#include "spinlock.h"
#include "riscv.h"
#include "defs.h"

void freerange(void *pa_start, void *pa_end);

extern char end[]; // first address after kernel.
                   // defined by kernel.ld.
int refcount[(PHYSTOP - KERNBASE) >> PGSHIFT];

struct run {
  struct run *next;
};

struct {
  struct spinlock lock;
  struct run *freelist;
} kmem;

void
kinit()
{
  initlock(&kmem.lock, "kmem");
  memset(refcount, 0, sizeof(refcount));
  freerange(end, (void*)PHYSTOP);
}
int total_free;
int alloc_count;
int free_count;
int count_ref() {
  int count = 0;
  for (int i = 0; i < NELEM(refcount); ++ i) {
    count += (refcount[i] > 0);
  }
  return count;
}

void show_page_count()
{
  int ref_count = count_ref();
  printf("count ref: %d / %d; alloc=%d, free=%d\n", ref_count, total_free, alloc_count, free_count);
}

void
freerange(void *pa_start, void *pa_end)
{
  char *p;
  total_free = alloc_count = free_count = 0;
  p = (char*)PGROUNDUP((uint64)pa_start);
  for(; p + PGSIZE <= (char*)pa_end; p += PGSIZE, total_free ++){
    refcount_add((uint64)p, 1);
    kfree(p);
  }
  show_page_count();
}

// Free the page of physical memory pointed at by pa,
// which normally should have been returned by a
// call to kalloc().  (The exception is when
// initializing the allocator; see kinit above.)
void
kfree(void *pa)
{
  struct run *r;

  if(((uint64)pa % PGSIZE) != 0 || (char*)pa < end || (uint64)pa >= PHYSTOP)
    panic("kfree");

//* [COW] if still has refcount after dec, keep it
  refcount_add((uint64)pa, -1);
  free_count++;
  acquire(&kmem.lock);
  if (refcount_get((uint64)pa) > 0) {
    release(&kmem.lock);
    return;
  } else if (refcount_get((uint64)pa) < 0) {
    panic("kfree: invalid refcount");
  }
  refcount[((uint64)pa - KERNBASE) >> PGSHIFT] = 0;
  release(&kmem.lock);

  // Fill with junk to catch dangling refs.
  memset(pa, 1, PGSIZE);

  r = (struct run*)pa;

  acquire(&kmem.lock);
  if (alloc_count) {
    if (r == 0) panic("kfree r");
    if (kmem.freelist == 0) panic("kfree");
  }
  r->next = kmem.freelist;
  kmem.freelist = r;
  release(&kmem.lock);
}

// Allocate one 4096-byte page of physical memory.
// Returns a pointer that the kernel can use.
// Returns 0 if the memory cannot be allocated.
void *
kalloc(void)
{
  struct run *r;

  acquire(&kmem.lock);
  r = kmem.freelist;
  if(r) {
    kmem.freelist = r->next;
    int idx = ((uint64)r - KERNBASE) >> PGSHIFT;
    if (idx < 0 || idx >= NELEM(refcount)) panic("!!!");
    refcount[idx] = 1;
    alloc_count ++;
  } else {
    show_page_count();
  }
  release(&kmem.lock);

  if(r)
    memset((char*)r, 5, PGSIZE); // fill with junk
  return (void*)r;
}

void
refcount_add(uint64 pa, int val)
{
  acquire(&kmem.lock);
  int idx = (pa - KERNBASE) >> PGSHIFT;
  if (idx < 0 || idx >= NELEM(refcount)) panic("!!!");
  refcount[idx] += val;
  release(&kmem.lock);
}

int
refcount_get(uint64 pa)
{
  int idx = (pa - KERNBASE) >> PGSHIFT;
  if (idx < 0 || idx >= NELEM(refcount)) panic("!!!");
  return refcount[idx];
}