diff --git a/cii/include/cii/ring.h b/cii/include/cii/ring.h
index 82e1b246..2a54527f 100644
--- a/cii/include/cii/ring.h
+++ b/cii/include/cii/ring.h
@@ -100,4 +100,17 @@ extern void* ring_remove_low(ring_t* ring);
 
 extern void* ring_remove_high(ring_t* ring);
 
+/**
+ * Renumbers the ring by rotating it left or right.
+ *
+ * If |n| is positive, rotate to right, clockwise.
+ * Else rotate to left, counterclockwise.
+ *
+ * It is a checked runtime error if absolute of |n| is larger than length of ring.
+ *
+ * @param ring
+ * @param n
+ */
+extern void ring_rotate(ring_t* ring, int n);
+
 #endif  // CII_RING_H_
diff --git a/cii/src/ring.c b/cii/src/ring.c
index 62a8725c..c02ac8a3 100644
--- a/cii/src/ring.c
+++ b/cii/src/ring.c
@@ -2,4 +2,218 @@
 // Use of this source is governed by GNU General Public License
 // that can be found in the LICENSE file.
 
-#include "cii/ring.h"
\ No newline at end of file
+#include "cii/ring.h"
+
+#include <stdarg.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "cii/assert.h"
+#include "cii/mem.h"
+
+/**
+ * Double linked list to hold the value of the ring.
+ */
+struct node_s {
+  struct node_s* left;
+  struct node_s* right;
+  void* value;
+};
+typedef struct node_s node_t;
+
+struct ring_s {
+  size_t length;
+  // |head| points to the zero indexed node.
+  node_t* head;
+};
+
+ring_t* ring_new(void) {
+  ring_t* ring;
+  NEW(ring);
+  ring->length = 0;
+  ring->head = NULL;
+
+  return ring;
+}
+
+ring_t* ring_ring(void* x, ...) {
+  ring_t* ring = ring_new();
+
+  va_list ap;
+  va_start(ap, x);
+  for (/* empty */ ; x != NULL; x = va_arg(ap, void*)) {
+    ring_add_high(ring, x);
+  }
+
+  va_end(ap);
+
+  return ring;
+}
+
+// First deallocates the node structures.
+// Then deallocates the head.
+void ring_free(ring_t** ring) {
+  assert(ring != NULL && *ring != NULL);
+  if ((*ring)->head != NULL) {
+    node_t* p = (*ring)->head;
+    node_t* q = NULL;
+    for (size_t i = 0; i < (*ring)->length; ++i) {
+      q = p->right;
+      FREE(p);
+      p = q;
+    }
+  }
+
+  FREE(*ring);
+}
+
+size_t ring_length(ring_t* ring) {
+  assert(ring != NULL);
+  return ring->length;
+}
+
+node_t* ring_get_node(ring_t* ring, size_t index) {
+  node_t* p = ring->head;
+  if (index <= ring->length / 2) {
+    for (size_t i = 0; i < index; ++i) {
+      p = p->right;
+    }
+  } else {
+    for (size_t i = ring->length - index; i > 0; --i) {
+      p = p->left;
+    }
+  }
+  return p;
+}
+
+void* ring_get(ring_t* ring, size_t index) {
+  assert(ring != NULL);
+  assert(index < ring->length);
+
+  node_t* p = ring_get_node(ring, index);
+  return p->value;
+}
+
+void* ring_put(ring_t* ring, size_t index, void* value) {
+  assert(ring != NULL);
+  assert(index < ring->length);
+
+  node_t* p = ring_get_node(ring, index);
+  void* prev = p->value;
+  p->value = value;
+  return prev;
+}
+
+void* ring_add_low(ring_t* ring, void* value) {
+  assert(ring != NULL);
+  ring_add_high(ring, value);
+  // Rotate the ring one value to the right.
+  ring->head = ring->head->left;
+  return value;
+}
+
+void* ring_add_high(ring_t* ring, void* value) {
+  assert(ring != NULL);
+  node_t* p;
+  NEW(p);
+  if (ring->head != NULL) {
+    node_t* q = ring->head;
+    // Insert p to the left of q.
+    p->left = q->left;
+    p->right = q;
+    q->left->right = p;
+    q->left = p;
+  } else {
+    // p is the only value in ring.
+    p->left = p;
+    p->right = p;
+    ring->head = p;
+  }
+  ring->length += 1;
+  return value;
+}
+
+void* ring_add(ring_t* ring, int pos, void* value) {
+  assert(ring != NULL);
+  assert(pos >= -ring->length && pos <= ring->length + 1);
+
+  if (pos == 1 || pos == -ring->length) {
+    return ring_add_low(ring, value);
+  } else if (pos == 0 || pos == ring->length + 1) {
+    return ring_add_high(ring, value);
+  }
+
+  size_t index = (pos < 0) ? (pos + ring->length) : (pos - 1);
+  node_t* q = ring_get_node(ring, index);
+  node_t* p;
+  NEW(p);
+  // Insert p to the left of q.
+  p->left = q->left;
+  p->right = q;
+  q->left->right = p;
+  q->left = p;
+
+  p->value = value;
+  ring->length += 1;
+  return value;
+}
+
+static void ring_remove_node(ring_t* ring, node_t* q) {
+  // Delete node q.
+  q->left->right = q->right;
+  q->right->left = q->left;
+  FREE(q);
+  ring->length -= 1;
+  if (ring->length == 0) {
+    ring->head = NULL;
+  }
+}
+
+void* ring_remove(ring_t* ring, size_t index) {
+  assert(ring != NULL);
+  assert(ring->length > 0);
+  assert(index < ring->length);
+  node_t* q = ring_get_node(ring, index);
+  if (index == 0) {
+    ring->head = ring->head->right;
+  }
+  void* value = q->value;
+  ring_remove_node(ring, q);
+
+  return value;
+}
+
+void* ring_remove_low(ring_t* ring) {
+  assert(ring != NULL);
+  assert(ring->length > 0);
+  // Rotate the ring one value right.
+  ring->head = ring->head->right;
+  return ring_remove_high(ring);
+}
+
+void* ring_remove_high(ring_t* ring) {
+  assert(ring != NULL);
+  assert(ring->length > 0);
+
+  node_t* q = ring->head->left;
+  void* value = q->value;
+  ring_remove_node(ring, q);
+
+  return value;
+}
+
+void ring_rotate(ring_t* ring, int n) {
+  assert(ring != NULL);
+  assert(ring->length > 0);
+  assert(abs(n) <= ring->length);
+  size_t index;
+  if (n >= 0) {
+    index = n % ring->length;
+  } else {
+    index = n + ring->length;
+  }
+
+  node_t* q = ring_get_node(ring, index);
+  assert(q != NULL);
+  ring->head = q;
+}
\ No newline at end of file