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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 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)
                }
        }
}