container/heap: use sort.Interface where possible

[EdSchouten]

Functions heap.Pop(), heap.Push() and heap.Remove() use type
heap.Interface. This type is equal to sort.Interface, with Push() and
Pop() methods added. This is necessary, because these functions affect
the size of the resulting heap.

Functions heap.Fix() and heap.Init() are different. They may compare and
swap elements in the underlying container, but never alter its size. It
is therefore possible to just use sort.Interface. This would make these
functions a lot more pleasant to use in situations where you know the
number of elements in the heap is fixed.

Consider the case where you use these functions to merge the results of
n infinitely long monotonically increasing sequences. For example, logs
received from multiple sources that need to be turned into a single
stream of events sorted by time. This can now be implemented as follows:

type Value ...
type Sequence <-chan Value

type PeekedSequence struct {
        First     Value
        Remainder Sequence
}

type SequenceHeap []PeekedSequence

func (h SequenceHeap) Len() int           { return len(h) }
func (h SequenceHeap) Less(i, j int) bool { return h[i].First < h[j].First }
func (h SequenceHeap) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }

func MergeSequences(sequences []Sequence, out chan<- Value) {
        h := make(SequenceHeap, 0, len(sequences))
        for _, s := range sequences {
                h = append(h, PeekedSequence{
                        First:     <-s,
                        Remainder: s,
                })
        }
        heap.Init(h)
        for {
                out <- h[0].First
                h[0].First = <-h[0].Remainder
                heap.Fix(h, 0)
        }
}

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Robert Griesemer:

Patch Set 2:

(1 comment)

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