lazy iterables in go

filters.go

package main

import (
	"fmt"
	"math/rand"
)

type DataSource interface {
	Next() Element
}

type Element interface {
	DataSource
	Val() uint32
}

type LazyList struct {
	ds   DataSource
	val  uint32
	next Element
}

func (l *LazyList) Val() uint32 {
	return l.val
}

func (l *LazyList) Next() Element {
	if l.next == nil {
		l.next = l.ds.Next()
	}
	return l.next
}

func NewLazyList(ds DataSource) DataSource {
	return &LazyList{ds, 0, nil}
}

type Predicate func(Element) bool
type LazyFilter struct {
	ds   DataSource
	val  uint32
	next Element
	pred Predicate
}

func (l *LazyFilter) Val() uint32 {
	return l.val
}

func (l *LazyFilter) Next() Element {
	if l.next == nil {
		r := l.ds.Next()
		for r != nil {
			if l.pred(r) {
				l.next = &LazyFilter{r, r.Val(), nil, l.pred}
				break
			}
			r = r.Next()
		}
	}
	return l.next
}

func NewLazyFilter(ds DataSource, pred Predicate) Element {
	return &LazyFilter{ds, 0, nil, pred}
}

func main() {
	incr := Incrementor(0)
	even := NewLazyFilter(incr, func(e Element) bool {
		return (e.Val() % 3) == 0
	})
	deca := NewLazyFilter(even, func(e Element) bool {
		return (e.Val() % 10) == 0
	})
	rang := NewLazyFilter(deca, func(e Element) bool {
		return e.Val() < 200000000 && e.Val() > 2200000
	})
	feed := NewLazyList(rang)
	cnt := 0
	for elem := feed.Next(); cnt <= 40; elem = elem.Next() {
		fmt.Println(elem.Val())
		cnt++
	}
}

type Increment uint32
func (i Increment) Val() uint32         { return uint32(i) }
func (i Increment) Next() Element       { return Increment(rand.Uint32()) }
func Incrementor(val uint32) DataSource { return Increment(val) }