Source file src/pkg/container/ring/ring.go

// 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.

// The ring package 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
}


func (r *Ring) Iter() <-chan interface{} {
    c := make(chan interface{})
    go func() {
        if r != nil {
            c <- r.Value
            for p := r.Next(); p != r; p = p.next {
                c <- p.Value
            }
        }
        close(c)
    }()
    return c
}