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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package ring implements operations on circular lists.
package ring
// A Ring is an element of a circular list, or ring.
// Rings do not have a beginning or end; a pointer to any ring element
// serves as reference to the entire ring. Empty rings are represented
// as nil Ring pointers. The zero value for a Ring is a one-element
// ring with a nil Value.
//
type Ring struct {
next, prev *Ring
Value interface{} // for use by client; untouched by this library
}
func (r *Ring) init() *Ring {
r.next = r
r.prev = r
return r
}
// Next returns the next ring element. r must not be empty.
func (r *Ring) Next() *Ring {
if r.next == nil {
return r.init()
}
return r.next
}
// Prev returns the previous ring element. r must not be empty.
func (r *Ring) Prev() *Ring {
if r.next == nil {
return r.init()
}
return r.prev
}
// Move moves n % r.Len() elements backward (n < 0) or forward (n >= 0)
// in the ring and returns that ring element. r must not be empty.
//
func (r *Ring) Move(n int) *Ring {
if r.next == nil {
return r.init()
}
switch {
case n < 0:
for ; n < 0; n++ {
r = r.prev
}
case n > 0:
for ; n > 0; n-- {
r = r.next
}
}
return r
}
// New creates a ring of n elements.
func New(n int) *Ring {
if n <= 0 {
return nil
}
r := new(Ring)
p := r
for i := 1; i < n; i++ {
p.next = &Ring{prev: p}
p = p.next
}
p.next = r
r.prev = p
return r
}
// Link connects ring r with ring s such that r.Next()
// becomes s and returns the original value for r.Next().
// r must not be empty.
//
// If r and s point to the same ring, linking
// them removes the elements between r and s from the ring.
// The removed elements form a subring and the result is a
// reference to that subring (if no elements were removed,
// the result is still the original value for r.Next(),
// and not nil).
//
// If r and s point to different rings, linking
// them creates a single ring with the elements of s inserted
// after r. The result points to the element following the
// last element of s after insertion.
//
func (r *Ring) Link(s *Ring) *Ring {
n := r.Next()
if s != nil {
p := s.Prev()
// Note: Cannot use multiple assignment because
// evaluation order of LHS is not specified.
r.next = s
s.prev = r
n.prev = p
p.next = n
}
return n
}
// Unlink removes n % r.Len() elements from the ring r, starting
// at r.Next(). If n % r.Len() == 0, r remains unchanged.
// The result is the removed subring. r must not be empty.
//
func (r *Ring) Unlink(n int) *Ring {
if n <= 0 {
return nil
}
return r.Link(r.Move(n + 1))
}
// Len computes the number of elements in ring r.
// It executes in time proportional to the number of elements.
//
func (r *Ring) Len() int {
n := 0
if r != nil {
n = 1
for p := r.Next(); p != r; p = p.next {
n++
}
}
return n
}
// Do calls function f on each element of the ring, in forward order.
// The behavior of Do is undefined if f changes *r.
func (r *Ring) Do(f func(interface{})) {
if r != nil {
f(r.Value)
for p := r.Next(); p != r; p = p.next {
f(p.Value)
}
}
}