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sources.go
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sources.go
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package iterutil
import (
"cmp"
"iter"
"maps"
"slices"
"golang.org/x/exp/constraints"
)
// Empty returns an empty iterator.
func Empty[E any]() iter.Seq[E] {
return func(_ func(E) bool) {}
}
// SeqOf returns an iterator composed of elems.
func SeqOf[E any](elems ...E) iter.Seq[E] {
return func(yield func(E) bool) {
for _, e := range elems {
if !yield(e) {
return
}
}
}
}
// Between, if step is nonzero, returns an iterator
// ranging from n (inclusive) to m (exclusive) in increments of step;
// otherwise, it panics.
func Between[I constraints.Signed](n, m, step I) iter.Seq[I] {
switch cmp.Compare(step, 0) {
default:
panic("step cannot be zero")
case 1: // ascending
return func(yield func(I) bool) {
for ; n < m && yield(n); n += step {
// deliberately empty
}
}
case -1: // descending
return func(yield func(I) bool) {
for ; n > m && yield(n); n += step {
// deliberately empty
}
}
}
}
// Repeat returns an iterator whose values are invariably e.
// The resulting iterator, if count is non-negative, is of length count;
// otherwise, it's infinite.
func Repeat[I constraints.Integer, E any](e E, count I) iter.Seq[E] {
if 0 <= count {
return func(yield func(E) bool) {
for i := I(0); i < count; i++ {
if !yield(e) {
return
}
}
}
}
return func(yield func(E) bool) {
for yield(e) {
// deliberately empty
}
}
}
// Iterate returns an infinite iterator composed of repeated applications
// of f to e.
func Iterate[E any](e E, f func(E) E) iter.Seq[E] {
return func(yield func(E) bool) {
for yield(e) {
e = f(e)
}
}
}
// Cycle returns an iterator that infinitely repeats seq.
func Cycle[E any](seq iter.Seq[E]) iter.Seq[E] {
return func(yield func(E) bool) {
for {
for e := range seq {
if !yield(e) {
return
}
}
}
}
}
// SortedFromMap returns an iterator over the key-value pairs in m
// ordered by its keys.
func SortedFromMap[M ~map[K]V, K cmp.Ordered, V any](m M) iter.Seq2[K, V] {
return func(yield func(K, V) bool) {
// One possibility would be to simply iterate over
// slices.Sorted(maps.Keys(m)),
// but doing so would incur unnecessary allocations;
// we can do better since we already know the number of keys.
// See https://github.com/golang/go/issues/61899#issuecomment-2198727055.
ks := keys(m)
slices.Sort(ks)
for _, k := range slices.Sorted(maps.Keys(m)) {
if !yield(k, m[k]) {
return
}
}
}
}
// SortedFuncFromMap returns an iterator over the key-value pairs in m
// ordered by its keys, using cmp as comparison function.
//
// Note that, for a deterministic behavior,
// cmp must define a [total order] on K;
// for more details, see the testable example labeled "incorrect".
//
// [total order]: https://en.wikipedia.org/wiki/Total_order
func SortedFuncFromMap[M ~map[K]V, K comparable, V any](m M, cmp func(K, K) int) iter.Seq2[K, V] {
return func(yield func(K, V) bool) {
// see implementation comment in SortedFuncFromMap
ks := keys(m)
slices.SortFunc(ks, cmp)
for _, k := range ks {
if !yield(k, m[k]) {
return
}
}
}
}
func keys[K comparable, V any](m map[K]V) []K {
ks := make([]K, 0, len(m))
for k := range m {
ks = append(ks, k)
}
return ks
}