heapqueue

The heapqueue module implements a heap data structure that can be used as a priority queue. Heaps are arrays for which a[k] <= a[2*k+1] and a[k] <= a[2*k+2] for all k, counting elements from 0. The interesting property of a heap is that a[0] is always its smallest element.

Basic usage

Example:

var heap = initHeapQueue[int]()
heap.push(8)
heap.push(2)
heap.push(5)
# The first element is the lowest element
assert heap[0] == 2
# Remove and return the lowest element
assert heap.pop() == 2
# The lowest element remaining is 5
assert heap[0] == 5

Usage with custom object

To use a HeapQueue with a custom object, the < operator must be implemented.

Example:

type Job = object
  priority: int

proc `<`(a, b: Job): bool = a.priority < b.priority

var jobs = initHeapQueue[Job]()
jobs.push(Job(priority: 1))
jobs.push(Job(priority: 2))

assert jobs[0].priority == 1

Imports

since

Types

HeapQueue[T] = object data: seq[T]: A heap queue, commonly known as a priority queue. Source Edit

Procs

proc initHeapQueue[T](): HeapQueue[T]

Creates a new empty heap.

See also:

initHeapQueue proc

Example:

var heap = toHeapQueue([9, 5, 8])
assert heap.pop() == 5
assert heap[0] == 8

Source Edit

proc pop[T](heap: var HeapQueue[T]): T

Pops and returns the smallest item from heap, maintaining the heap invariant.

Example:

var heap = toHeapQueue([9, 5, 8])
assert heap.pop() == 5

Source Edit

proc find[T](heap: HeapQueue[T]; x: T): int

Linear scan to find index of item x or -1 if not found.

Example:

var heap = toHeapQueue([9, 5, 8])
assert heap.find(5) == 0
assert heap.find(9) == 1
assert heap.find(777) == -1

Source Edit

proc del[T](heap: var HeapQueue[T]; index: Natural)

Removes the element at index from heap, maintaining the heap invariant.

Example:

var heap = toHeapQueue([9, 5, 8])
heap.del(1)
assert heap[0] == 5
assert heap[1] == 8

Source Edit

proc replace[T](heap: var HeapQueue[T]; item: sink T): T

Pops and returns the current smallest value, and add the new item. This is more efficient than pop() followed by push(), and can be more appropriate when using a fixed-size heap. Note that the value returned may be larger than item! That constrains reasonable uses of this routine unless written as part of a conditional replacement:

Example:

var heap = initHeapQueue[int]()
heap.push(5)
heap.push(12)
assert heap.replace(6) == 5
assert heap.len == 2
assert heap[0] == 6
assert heap.replace(4) == 6

Source Edit

proc pushpop[T](heap: var HeapQueue[T]; item: sink T): T

Fast version of a push followed by a pop.

Example:

var heap = initHeapQueue[int]()
heap.push(5)
heap.push(12)
assert heap.pushpop(6) == 5
assert heap.len == 2
assert heap[0] == 6
assert heap.pushpop(4) == 4

Source Edit

proc clear[T](heap: var HeapQueue[T])

Removes all elements from heap, making it empty.

Example:

var heap = initHeapQueue[int]()
heap.push(1)
heap.clear()
assert heap.len == 0

Source Edit

proc `$`[T](heap: HeapQueue[T]): string

Turns a heap into its string representation.

Example:

var heap = initHeapQueue[int]()
heap.push(1)
heap.push(2)
assert $heap == "[1, 2]"

Source Edit