3 files changed, 557 insertions, 0 deletions
diff --git a/src/container/ring/example_test.go b/src/container/ring/example_test.go
new file mode 100644
index 0000000..4b659d2
--- /dev/null
+++ b/src/container/ring/example_test.go
@@ -0,0 +1,193 @@
+// Copyright 2017 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_test
+
+import (
+	"container/ring"
+	"fmt"
+)
+
+func ExampleRing_Len() {
+	// Create a new ring of size 4
+	r := ring.New(4)
+
+	// Print out its length
+	fmt.Println(r.Len())
+
+	// Output:
+	// 4
+}
+
+func ExampleRing_Next() {
+	// Create a new ring of size 5
+	r := ring.New(5)
+
+	// Get the length of the ring
+	n := r.Len()
+
+	// Initialize the ring with some integer values
+	for i := 0; i < n; i++ {
+		r.Value = i
+		r = r.Next()
+	}
+
+	// Iterate through the ring and print its contents
+	for j := 0; j < n; j++ {
+		fmt.Println(r.Value)
+		r = r.Next()
+	}
+
+	// Output:
+	// 0
+	// 1
+	// 2
+	// 3
+	// 4
+}
+
+func ExampleRing_Prev() {
+	// Create a new ring of size 5
+	r := ring.New(5)
+
+	// Get the length of the ring
+	n := r.Len()
+
+	// Initialize the ring with some integer values
+	for i := 0; i < n; i++ {
+		r.Value = i
+		r = r.Next()
+	}
+
+	// Iterate through the ring backwards and print its contents
+	for j := 0; j < n; j++ {
+		r = r.Prev()
+		fmt.Println(r.Value)
+	}
+
+	// Output:
+	// 4
+	// 3
+	// 2
+	// 1
+	// 0
+}
+
+func ExampleRing_Do() {
+	// Create a new ring of size 5
+	r := ring.New(5)
+
+	// Get the length of the ring
+	n := r.Len()
+
+	// Initialize the ring with some integer values
+	for i := 0; i < n; i++ {
+		r.Value = i
+		r = r.Next()
+	}
+
+	// Iterate through the ring and print its contents
+	r.Do(func(p any) {
+		fmt.Println(p.(int))
+	})
+
+	// Output:
+	// 0
+	// 1
+	// 2
+	// 3
+	// 4
+}
+
+func ExampleRing_Move() {
+	// Create a new ring of size 5
+	r := ring.New(5)
+
+	// Get the length of the ring
+	n := r.Len()
+
+	// Initialize the ring with some integer values
+	for i := 0; i < n; i++ {
+		r.Value = i
+		r = r.Next()
+	}
+
+	// Move the pointer forward by three steps
+	r = r.Move(3)
+
+	// Iterate through the ring and print its contents
+	r.Do(func(p any) {
+		fmt.Println(p.(int))
+	})
+
+	// Output:
+	// 3
+	// 4
+	// 0
+	// 1
+	// 2
+}
+
+func ExampleRing_Link() {
+	// Create two rings, r and s, of size 2
+	r := ring.New(2)
+	s := ring.New(2)
+
+	// Get the length of the ring
+	lr := r.Len()
+	ls := s.Len()
+
+	// Initialize r with 0s
+	for i := 0; i < lr; i++ {
+		r.Value = 0
+		r = r.Next()
+	}
+
+	// Initialize s with 1s
+	for j := 0; j < ls; j++ {
+		s.Value = 1
+		s = s.Next()
+	}
+
+	// Link ring r and ring s
+	rs := r.Link(s)
+
+	// Iterate through the combined ring and print its contents
+	rs.Do(func(p any) {
+		fmt.Println(p.(int))
+	})
+
+	// Output:
+	// 0
+	// 0
+	// 1
+	// 1
+}
+
+func ExampleRing_Unlink() {
+	// Create a new ring of size 6
+	r := ring.New(6)
+
+	// Get the length of the ring
+	n := r.Len()
+
+	// Initialize the ring with some integer values
+	for i := 0; i < n; i++ {
+		r.Value = i
+		r = r.Next()
+	}
+
+	// Unlink three elements from r, starting from r.Next()
+	r.Unlink(3)
+
+	// Iterate through the remaining ring and print its contents
+	r.Do(func(p any) {
+		fmt.Println(p.(int))
+	})
+
+	// Output:
+	// 0
+	// 4
+	// 5
+}
diff --git a/src/container/ring/ring.go b/src/container/ring/ring.go
new file mode 100644
index 0000000..268670b
--- /dev/null
+++ b/src/container/ring/ring.go
@@ -0,0 +1,136 @@
+// 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      any // 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(any)) {
+	if r != nil {
+		f(r.Value)
+		for p := r.Next(); p != r; p = p.next {
+			f(p.Value)
+		}
+	}
+}
diff --git a/src/container/ring/ring_test.go b/src/container/ring/ring_test.go
new file mode 100644
index 0000000..28acbbc
--- /dev/null
+++ b/src/container/ring/ring_test.go
@@ -0,0 +1,228 @@
+// 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
+
+import (
+	"fmt"
+	"testing"
+)
+
+// For debugging - keep around.
+func dump(r *Ring) {
+	if r == nil {
+		fmt.Println("empty")
+		return
+	}
+	i, n := 0, r.Len()
+	for p := r; i < n; p = p.next {
+		fmt.Printf("%4d: %p = {<- %p | %p ->}\n", i, p, p.prev, p.next)
+		i++
+	}
+	fmt.Println()
+}
+
+func verify(t *testing.T, r *Ring, N int, sum int) {
+	// Len
+	n := r.Len()
+	if n != N {
+		t.Errorf("r.Len() == %d; expected %d", n, N)
+	}
+
+	// iteration
+	n = 0
+	s := 0
+	r.Do(func(p any) {
+		n++
+		if p != nil {
+			s += p.(int)
+		}
+	})
+	if n != N {
+		t.Errorf("number of forward iterations == %d; expected %d", n, N)
+	}
+	if sum >= 0 && s != sum {
+		t.Errorf("forward ring sum = %d; expected %d", s, sum)
+	}
+
+	if r == nil {
+		return
+	}
+
+	// connections
+	if r.next != nil {
+		var p *Ring // previous element
+		for q := r; p == nil || q != r; q = q.next {
+			if p != nil && p != q.prev {
+				t.Errorf("prev = %p, expected q.prev = %p\n", p, q.prev)
+			}
+			p = q
+		}
+		if p != r.prev {
+			t.Errorf("prev = %p, expected r.prev = %p\n", p, r.prev)
+		}
+	}
+
+	// Next, Prev
+	if r.Next() != r.next {
+		t.Errorf("r.Next() != r.next")
+	}
+	if r.Prev() != r.prev {
+		t.Errorf("r.Prev() != r.prev")
+	}
+
+	// Move
+	if r.Move(0) != r {
+		t.Errorf("r.Move(0) != r")
+	}
+	if r.Move(N) != r {
+		t.Errorf("r.Move(%d) != r", N)
+	}
+	if r.Move(-N) != r {
+		t.Errorf("r.Move(%d) != r", -N)
+	}
+	for i := 0; i < 10; i++ {
+		ni := N + i
+		mi := ni % N
+		if r.Move(ni) != r.Move(mi) {
+			t.Errorf("r.Move(%d) != r.Move(%d)", ni, mi)
+		}
+		if r.Move(-ni) != r.Move(-mi) {
+			t.Errorf("r.Move(%d) != r.Move(%d)", -ni, -mi)
+		}
+	}
+}
+
+func TestCornerCases(t *testing.T) {
+	var (
+		r0 *Ring
+		r1 Ring
+	)
+	// Basics
+	verify(t, r0, 0, 0)
+	verify(t, &r1, 1, 0)
+	// Insert
+	r1.Link(r0)
+	verify(t, r0, 0, 0)
+	verify(t, &r1, 1, 0)
+	// Insert
+	r1.Link(r0)
+	verify(t, r0, 0, 0)
+	verify(t, &r1, 1, 0)
+	// Unlink
+	r1.Unlink(0)
+	verify(t, &r1, 1, 0)
+}
+
+func makeN(n int) *Ring {
+	r := New(n)
+	for i := 1; i <= n; i++ {
+		r.Value = i
+		r = r.Next()
+	}
+	return r
+}
+
+func sumN(n int) int { return (n*n + n) / 2 }
+
+func TestNew(t *testing.T) {
+	for i := 0; i < 10; i++ {
+		r := New(i)
+		verify(t, r, i, -1)
+	}
+	for i := 0; i < 10; i++ {
+		r := makeN(i)
+		verify(t, r, i, sumN(i))
+	}
+}
+
+func TestLink1(t *testing.T) {
+	r1a := makeN(1)
+	var r1b Ring
+	r2a := r1a.Link(&r1b)
+	verify(t, r2a, 2, 1)
+	if r2a != r1a {
+		t.Errorf("a) 2-element link failed")
+	}
+
+	r2b := r2a.Link(r2a.Next())
+	verify(t, r2b, 2, 1)
+	if r2b != r2a.Next() {
+		t.Errorf("b) 2-element link failed")
+	}
+
+	r1c := r2b.Link(r2b)
+	verify(t, r1c, 1, 1)
+	verify(t, r2b, 1, 0)
+}
+
+func TestLink2(t *testing.T) {
+	var r0 *Ring
+	r1a := &Ring{Value: 42}
+	r1b := &Ring{Value: 77}
+	r10 := makeN(10)
+
+	r1a.Link(r0)
+	verify(t, r1a, 1, 42)
+
+	r1a.Link(r1b)
+	verify(t, r1a, 2, 42+77)
+
+	r10.Link(r0)
+	verify(t, r10, 10, sumN(10))
+
+	r10.Link(r1a)
+	verify(t, r10, 12, sumN(10)+42+77)
+}
+
+func TestLink3(t *testing.T) {
+	var r Ring
+	n := 1
+	for i := 1; i < 10; i++ {
+		n += i
+		verify(t, r.Link(New(i)), n, -1)
+	}
+}
+
+func TestUnlink(t *testing.T) {
+	r10 := makeN(10)
+	s10 := r10.Move(6)
+
+	sum10 := sumN(10)
+
+	verify(t, r10, 10, sum10)
+	verify(t, s10, 10, sum10)
+
+	r0 := r10.Unlink(0)
+	verify(t, r0, 0, 0)
+
+	r1 := r10.Unlink(1)
+	verify(t, r1, 1, 2)
+	verify(t, r10, 9, sum10-2)
+
+	r9 := r10.Unlink(9)
+	verify(t, r9, 9, sum10-2)
+	verify(t, r10, 9, sum10-2)
+}
+
+func TestLinkUnlink(t *testing.T) {
+	for i := 1; i < 4; i++ {
+		ri := New(i)
+		for j := 0; j < i; j++ {
+			rj := ri.Unlink(j)
+			verify(t, rj, j, -1)
+			verify(t, ri, i-j, -1)
+			ri.Link(rj)
+			verify(t, ri, i, -1)
+		}
+	}
+}
+
+// Test that calling Move() on an empty Ring initializes it.
+func TestMoveEmptyRing(t *testing.T) {
+	var r Ring
+
+	r.Move(1)
+	verify(t, &r, 1, 0)
+}