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Optimisation: Use heapify in MutableBinaryHeap #712

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Optimisation: Use heapify in MutableBinaryHeap #712

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1396ead
use heapify instead of iteratively inserting elements
rushabh-v Dec 10, 2020
5aa2e2a
use heapify! instead of heapify
rushabh-v Dec 11, 2020
c19e8c6
use separate copies of arrays for each test
rushabh-v Dec 11, 2020
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7 changes: 3 additions & 4 deletions src/heaps/mutable_binary_heap.jl
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Original file line number Diff line number Diff line change
@@ -1,5 +1,7 @@
# Binary heap

include("arrays_as_heaps.jl")

struct MutableBinaryHeapNode{T}
value::T
handle::Int
Expand Down Expand Up @@ -131,10 +133,10 @@ end

function _make_mutable_binary_heap(ord::Ordering, ty::Type{T}, values) where T
# make a static binary index tree from a list of values

n = length(values)
nodes = Vector{MutableBinaryHeapNode{T}}(undef, n)
nodemap = Vector{Int}(undef, n)
values = heapify!(values, ord)

i::Int = 0
for v in values
Expand All @@ -143,9 +145,6 @@ function _make_mutable_binary_heap(ord::Ordering, ty::Type{T}, values) where T
@inbounds nodemap[i] = i
end

for i = 1 : n
_heap_bubble_up!(ord, nodes, nodemap, i)
end
return nodes, nodemap
end

Expand Down
15 changes: 12 additions & 3 deletions test/test_mutable_binheap.jl
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Expand Up @@ -52,11 +52,12 @@ end

@testset "MutableBinheap" begin

vs = [4, 1, 3, 2, 16, 9, 10, 14, 8, 7]
vs2 = collect(enumerate(vs))
ordering = Base.Order.By(last)

@testset "construct heap" begin
vs = [4, 1, 3, 2, 16, 9, 10, 14, 8, 7]
vs2 = collect(enumerate(vs))
Comment on lines +58 to +59
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@rushabh-v rushabh-v Dec 11, 2020 •
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The new implementation was overwriting vs. So, the tests were failing because of updated vs (The value of vs was not as expected because it was updated during some other test). So, I updated it to use separate vs and vs2 for each test (the values are still the same).

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It is not OK for MutableBinaryMinHeap(vs) to mutate vs.
We should add a copy to the constructor if that is the case.

(we can consider later adding a MutableBinaryMinHeap! or something that does do so. but lets not worry about it in this PR)

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list_values related tests (i.e., this one) started failing after adding copy in the constructor. the reason for that is because nodemap is not updated as I mentioned here. Those tests were not failing before because vs was being mutated.

I am trying to think if there is an efficient way to update nodemap after the heapify! call.

It might be that not being able to update nodemap efficiently (with O(n) implementation) is the reason they implemented it in O(nlogn) in the first place.


MutableBinaryHeap{Int, Base.ForwardOrdering}()
MutableBinaryHeap{Int, Base.ForwardOrdering}(vs)

Expand Down Expand Up @@ -88,6 +89,7 @@ end
end

@testset "make mutable binary minheap" begin
vs = [4, 1, 3, 2, 16, 9, 10, 14, 8, 7]
h = MutableBinaryMinHeap(vs)

@test length(h) == 10
Expand All @@ -99,17 +101,20 @@ end
end

@testset "make mutable binary maxheap" begin
vs = [4, 1, 3, 2, 16, 9, 10, 14, 8, 7]
h = MutableBinaryMaxHeap(vs)

@test length(h) == 10
@test !isempty(h)
@test first(h) == 16
@test isequal(list_values(h), vs)
@test isequal(heap_values(h), [16, 14, 10, 8, 7, 3, 9, 1, 4, 2])
@test isequal(heap_values(h), [16, 14, 10, 8, 7, 9, 3, 2, 4, 1])
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@rushabh-v rushabh-v Dec 11, 2020 •
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I think this is just due to the difference in the implementation of heapify and the previous method _heap_bubble_up. Otherwise, there is no problem IMO, so I updated the test. (Let me know if that's not the case)

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Do these both follow the heap rule about being larger than children?
(I don't know how to read heaps when written in array form TBH)

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Yes, they both are valid heaps. I also confirmed it using this function here.

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seems fine then

@test sizehint!(h, 100) === h
end

@testset "make mutable binary custom ordering heap" begin
vs = [4, 1, 3, 2, 16, 9, 10, 14, 8, 7]
vs2 = collect(enumerate(vs))
h = MutableBinaryHeap(ordering, vs2)

@test length(h) == 10
Expand All @@ -121,6 +126,7 @@ end
end

@testset "hmin / push! / pop!" begin
vs = [4, 1, 3, 2, 16, 9, 10, 14, 8, 7]
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this is a good change anyway. Testsets hould be indipendent.
Though maybe we move this down to after ss is declared?

hmin = MutableBinaryMinHeap{Int}()
@test length(hmin) == 0
@test isempty(hmin)
Expand Down Expand Up @@ -152,6 +158,7 @@ end
end

@testset "hmax / push! / pop!" begin
vs = [4, 1, 3, 2, 16, 9, 10, 14, 8, 7]
hmax = MutableBinaryMaxHeap{Int}()
@test length(hmax) == 0
@test isempty(hmax)
Expand Down Expand Up @@ -183,6 +190,8 @@ end
end

@testset "Custom ordering push! / pop!" begin
vs = [4, 1, 3, 2, 16, 9, 10, 14, 8, 7]
vs2 = collect(enumerate(vs))
heap = MutableBinaryHeap{Tuple{Int,Int}}(ordering)
@test length(heap) == 0
@test isempty(heap)
Expand Down