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choose.go
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choose.go
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/*
Copyright IBM Corp. All Rights Reserved.
SPDX-License-Identifier: Apache-2.0
*/
package graph
type orderedSet struct {
elements []interface{}
}
func (s *orderedSet) add(o interface{}) {
s.elements = append(s.elements, o)
}
type indiceSet struct {
indices []int
}
type indiceSets []*indiceSet
func factorial(n int) int {
m := 1
for i := 1; i <= n; i++ {
m *= i
}
return m
}
func nChooseK(n, k int) int {
a := factorial(n)
b := factorial(n-k) * factorial(k)
return a / b
}
func chooseKoutOfN(n, k int) indiceSets {
var res indiceSets
subGroups := &orderedSet{}
choose(n, k, 0, nil, subGroups)
for _, el := range subGroups.elements {
res = append(res, el.(*indiceSet))
}
return res
}
func choose(n int, targetAmount int, i int, currentSubGroup []int, subGroups *orderedSet) {
// Check if we have enough elements in our current subgroup
if len(currentSubGroup) == targetAmount {
subGroups.add(&indiceSet{indices: currentSubGroup})
return
}
// Return early if not enough remaining candidates to pick from
itemsLeftToPick := n - i
if targetAmount-len(currentSubGroup) > itemsLeftToPick {
return
}
// We either pick the current element
choose(n, targetAmount, i+1, append(currentSubGroup, i), subGroups)
// Or don't pick it
choose(n, targetAmount, i+1, currentSubGroup, subGroups)
}