std/heapqueue

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

Basic usage

Example:

import std/heapqueue
var heap = [8, 2].toHeapQueue
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 objects

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

Example:

import std/heapqueue
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: A heap queue, commonly known as a priority queue. Source Edit

Procs

proc `$`[T](heap: HeapQueue[T]): string: Turns a heap into its string representation.
Example:

let heap = [1, 2].toHeapQueue assert $heap == "[1, 2]"
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proc `[]`[T](heap: HeapQueue[T]; i: Natural): lent T {.inline.}: Accesses the i-th element of heap. Source Edit
proc clear[T](heap: var HeapQueue[T]): Removes all elements from heap, making it empty.
Example:

var heap = [9, 5, 8].toHeapQueue heap.clear() assert heap.len == 0
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proc contains[T](heap: HeapQueue[T]; x: T): bool: Returns true if x is in heap or false if not found. This is a shortcut for find(heap, x) >= 0. 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 = [9, 5, 8].toHeapQueue heap.del(1) assert heap[0] == 5 assert heap[1] == 8
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proc find[T](heap: HeapQueue[T]; x: T): int: Linear scan to find the index of the item x or -1 if not found.
Example:

let heap = [9, 5, 8].toHeapQueue assert heap.find(5) == 0 assert heap.find(9) == 1 assert heap.find(777) == -1
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proc initHeapQueue[T](): HeapQueue[T]: Creates a new empty heap.

Heaps are initialized by default, so it is not necessary to call this function explicitly.

See also:

toHeapQueue proc

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proc len[T](heap: HeapQueue[T]): int {.inline.}: Returns the number of elements of heap.
Example:

let heap = [9, 5, 8].toHeapQueue assert heap.len == 3
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proc pop[T](heap: var HeapQueue[T]): T: Pops and returns the smallest item from heap, maintaining the heap invariant.
Example:

var heap = [9, 5, 8].toHeapQueue assert heap.pop() == 5
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proc push[T](heap: var HeapQueue[T]; item: sink T): Pushes item onto heap, maintaining the heap invariant. Source Edit
proc pushpop[T](heap: var HeapQueue[T]; item: sink T): T: Fast version of a push() followed by a pop().

See also:

replace proc

Example:

var heap = [5, 12].toHeapQueue assert heap.pushpop(6) == 5 assert heap.len == 2 assert heap[0] == 6 assert heap.pushpop(4) == 4
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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.

See also:

pushpop proc

Example:

var heap = [5, 12].toHeapQueue assert heap.replace(6) == 5 assert heap.len == 2 assert heap[0] == 6 assert heap.replace(4) == 6
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proc toHeapQueue[T](x: openArray[T]): HeapQueue[T]: Creates a new HeapQueue that contains the elements of x.

See also:

initHeapQueue proc

Example:

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

iterator items[T](heap: HeapQueue[T]): lent T {.inline.}: Iterates over each item of heap. Source Edit