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A heap is a set of slots. Contiguous sets of slots with the same status (free
or allocated) are grouped into blocks.
Example Heap Activity
=====================
|
|
malloc
___
| |
|___|
malloc
_______
| . |
|_______|
malloc
___________
| . . |
|___________|
free 1
___ ___ ___
| | F | |
|___|___|___|
malloc
___________
| . . |
|___________|
malloc
_______________
| . . . |
|_______________|
malloc
___________________
| . . . . |
|___________________|
malloc
_______________________
| . . . . . |
|_______________________|
malloc
___________________________
| . . . . . . |
|___________________________|
malloc
_______________________________
| . . . . . . . |
|_______________________________|
free 1
___ ___ _______________________
| | F | . . . . . |
|___|___|_______________________|
free 2
___ _______ ___________________
| | F . | . . . . |
|___|_______|___________________|
free 6
___ _______ ___________ ___ ___
| | F . | . . | F | |
|___|_______|___________|___|___|
free 4
___ _______ ___ ___ ___ ___ ___
| | F . | | F | | F | |
|___|_______|___|___|___|___|___|
free 5
___ _______ ___ ___________ ___
| | F . | | F . . | |
|___|_______|___|___________|___|
malloc
_______ ___ ___ ___________ ___
| . | F | | F . . | |
|_______|___|___|___________|___|
malloc
_______________ ___________ ___
| . . . | F . . | |
|_______________|___________|___|
malloc
___________________ _______ ___
| . . . . | F . | |
|___________________|_______|___|
Superpseudocode
===============
malloc
if no top
set heap->top
set heap[heap->top] size=1 prev=null, next=null
return top
malloc
if no top
set and return top
else if no free head
grow and return top
else (free blocks exist)
if free head size == 1
move free head to next block
return old block
else
move free block start to head+1, --size
return old block
free
if ptr+1 free
if heap[ptr+1]->prev + size = ptr
merge prev with ptr+1
else
merge ptr with ptr+1
elif ptr-1 free
merge ptr-1 with ptr (just ++size)
else
mark ptr free
Pseudocode
==========
`__malloc(heap)`:
if heap->top = null
# Empty heap, grow
heap->top = 0
return 0
elif heap->free_head = null
# No free blocks, grow heap
return ++heap->top
else
# Free blocks exist, reallocate
if heap[heap->free_head]->size == 1
# Take whole block, point to next block
new = heap->free_head
heap->free_head = heap[heap->free_head]->next
heap[heap->free_head]->prev = null
return new
else
# Shrink block
new = heap->free_head
++heap->free_head
heap[heap->free_head]->size = heap[new]->size - 1
heap[heap->free_head]->prev = null
heap[heap->free_head]->next = heap[new]->next
unset heap[new]->size
unset heap[new]->prev
unset heap[new]->next
return new
`__free(heap, ptr)`:
if ptr < heap->top and heap[ptr + 1]->size != null
elif ptr > 0 and heap[ptr - 1]->size != null
++heap[ptr - 1]->size
else
heap->ptr
if ptr > 0 and heap[ptr - 1]->size != null
++heap[ptr - 1]->size
elif ptr < heap->top and heap[ptr + 1]->size != null
heap[ptr]->size = heap[ptr + 1]->size + 1
heap[ptr]->prev = heap[ptr + 1]->prev
heap[ptr]->next = heap[ptr + 1]->next
|
