A Zipper in Go using "container/list" | https://en.wikipedia.org/wiki/Zipper_(data_structure)

zipper.go

package zipper

import l "container/list"

type Zipper struct {
    nzip   int
    list   *l.List
    back   *l.Element
    sel    *l.Element
    front  *l.Element
}

func New(list *l.List) *Zipper {
    z := new(Zipper)
    z.nzip = 0
    z.list = list
    z.sel = list.Front()
    z.back = z.sel.Prev()
    z.front = z.sel.Next()
    return z
}

func (z *Zipper) Zip() *Zipper {
    if z.front != nil {
        z.sel = z.sel.Next()
        z.back = z.sel.Prev()
        z.front = z.sel.Next()
        z.nzip++
        return z
    }
    return nil
}

func (z *Zipper) Unzip() *Zipper {
    if z.back != nil {
        z.sel = z.sel.Prev()
        z.back = z.sel.Prev()
        z.front = z.sel.Next()
        z.nzip--
        return z
    }
    return nil
}

func (z *Zipper) Len() int {
    return z.list.Len()
}

func (z *Zipper) LenZipped() int {
    return z.nzip
}

func (z *Zipper) LenUnzipped() int {
    return z.Len() - z.LenZipped()
}

func (z *Zipper) Top() *l.Element {
    return z.sel
}