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// -*- Mode: Go; indent-tabs-mode: t -*-
/*
* Copyright (C) 2021 Canonical Ltd
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 3 as
* published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
*/
package strutil
// Intersection computes the intersection of a set of slices, treating each
// slice as a set. It does not mutate any of the input slices and returns a new
// slice. It is recursive.
func Intersection(slices ...[]string) []string {
// handle trivial cases
switch len(slices) {
case 0:
return nil
case 1:
return slices[0]
case 2:
// actually perform the intersection
l1 := slices[0]
l2 := slices[1]
guessLen := len(l1)
if len(l1) > len(l2) {
guessLen = len(l2)
}
alreadyAdded := map[string]bool{}
result := make([]string, 0, guessLen)
for _, item := range l1 {
if !alreadyAdded[item] && ListContains(l2, item) {
result = append(result, item)
alreadyAdded[item] = true
}
}
return result
}
// all other cases require some recursion operating on smaller chunks
// we take advantage of the fact that intersection is commutative and
// iteratively perform an intersection between a running intersection of
// all previous lists and the next list in the total set of slices
// TODO: this could be sped up with maps or any number of things, but
// hopefully this is only ever used on a few lists that are small in size
// so we can get away with this inefficient implementation
result := slices[0]
for _, s := range slices[1:] {
result = Intersection(result, s)
}
return result
}
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