From efb0b2fb4d5d3e8feff70e10f273dbdd175ef504 Mon Sep 17 00:00:00 2001
From: David Kerkeslager <kerkeslager@gmail.com>
Date: Sat, 30 Jul 2016 12:58:05 -0400
Subject: [PATCH] Added a multi producer single consumer queue

---
 vm/vm.c | 123 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 123 insertions(+)
 create mode 100644 vm/vm.c

diff --git a/vm/vm.c b/vm/vm.c
new file mode 100644
index 0000000..bc6588c
--- /dev/null
+++ b/vm/vm.c
@@ -0,0 +1,123 @@
+#include <stdbool.h>
+#include <stdlib.h>
+
+struct Queue;
+typedef struct Queue Queue;
+struct QueueNode;
+typedef struct QueueNode QueueNode;
+
+void initializeQueue(Queue* queue);
+void enqueue(Queue* queue, void* item);
+void* dequeue(Queue* queue);
+void freeQueue(Queue* queue, void (*itemFreer)(void*));
+
+struct Queue
+{
+  // This implements a multiple-producer, single-consumer lockless concurrent
+  // queue.
+  //
+  // After initialization, `head` and `tail` always points to a valid memory
+  // location--they are never `NULL`. To achieve this we initialize `head` and
+  // `tail` with a dummy node. This has two negative implications:
+  //
+  // 1. Unintuitively, `head->item` actually represents the item most recently
+  //    dequeued. In fact, `head->item` may have been freed after it was
+  //    dequeued, so we should never dereference it.
+  // 2. At initialization, no item has been dequeued yet, so we must allocate
+  //    an extra `QueueNode` at initialization which will never be used to
+  //    store an item.
+  //
+  // However, this extra memory allocation at initialization means that we
+  // never have to check whether `head` or `tail` are `NULL`, or initialize
+  // these variables concurrently, which are expensive operations.
+  QueueNode* head;
+  QueueNode* tail;
+};
+
+struct QueueNode
+{
+  // This implements a singly-linked list.
+  void* item;
+  QueueNode* next;
+};
+
+void initializeQueue(Queue* queue)
+{
+  // Initialize the queue with a node that fakes the previously dequeued item.
+  // We don't have to initialize `dummyNode->item` because it will never be
+  // used.
+  QueueNode* dummyNode = malloc(sizeof(QueueNode));
+  dummyNode->next = NULL;
+  queue->head = dummyNode;
+  queue->tail = dummyNode;
+}
+
+bool isEmpty(Queue* queue)
+{
+  // In a normal singly-linked list, the list is empty when the `head` is
+  // `NULL`, but here the queue is empty when the `head` is the same as the
+  // `tail`.
+  return queue->head == queue->tail;
+}
+
+void enqueue(Queue* queue, void* item)
+{
+  // Create the node that will be the new tail.
+  QueueNode* node = malloc(sizeof(QueueNode));
+  node->item = item;
+  node->next = NULL;
+
+  // Append the node to the list. Note that this is blocked as long as
+  // `queue->tail` is not actually pointing to the last node in the list
+  // because then `queue->tail->next` will not be `NULL`. This situation
+  // occurs when another thread has called `enqueue` and appended a node to
+  // the list, but hasn't yet updated `queue->tail`.
+  while(!__sync_bool_compare_and_swap(&(queue->tail->next), NULL, node));
+
+  // Move `queue->tail` forward until it points to the end of the list.
+  // Other threads attempting to eqneue will be unable to do so until this
+  // is complete. Additionally, other threads attempting to dequeue will be
+  // blocked until `queue->tail` has advanced by at least one node.
+  //
+  // We don't have to worry that another thread might have appended more
+  // nodes; advancing `queue->tail` to the end of the linked list is always
+  // the correct thing to do.
+  QueueNode* tail;
+  while((tail = queue->tail)->next != NULL)
+  {
+    __sync_val_compare_and_swap(&(queue->tail), tail, tail->next);
+  }
+}
+
+void* dequeue(Queue* queue)
+{
+  if(isEmpty(queue))
+  {
+    return NULL; // TODO Provide ability to return a defined default.
+  }
+
+  QueueNode* oldHead = queue->head;
+  queue->head = oldHead->next;
+  free(oldHead);
+
+  return queue->head->item;
+}
+
+void freeQueue(Queue* queue, void (*itemFreer)(void*))
+{
+  QueueNode* current = queue->head;
+  free(queue);
+
+  while(current != NULL)
+  {
+    itemFreer(current->item);
+    QueueNode* temp = current->next;
+    free(current);
+    current = temp;
+  }
+}
+
+int main(int argc, char** argv)
+{
+  return EXIT_SUCCESS;
+}
-- 
2.20.1