Require power-of-two size. Add ring_poke.

ring.c

@@ -1,7 +1,11 @@
 #include "ring.h"
+#include "insist.h"
 #include <jemalloc/jemalloc.h>
 
-struct ring *ring_new_size(size_t size) {
+struct ring *ring_new_size(uint32_t size) {
+  insist((size & (size - 1)) == 0,
+         "size must be a power of two, %d is not.", size);
+
   struct ring *r = malloc(sizeof(struct ring));
   r->writer = 0;
   r->reader = 0;
@@ -16,35 +20,45 @@ inline int ring_is_empty(struct ring *ring) {
   return ring->count == 0;
 } /* ring_is_empty */
 
-int ring_is_full(struct ring *ring) {
+inline int ring_is_full(struct ring *ring) {
   return ring->count == ring->size;
 } /* ring_is_full */
 
-int ring_pop(struct ring *ring, void **object_ret) {
-  if (ring_is_empty(ring)) {
+inline int ring_pop(struct ring *ring, void **object_ret) {
+  int rc = ring_peek(ring, 0, object_ret);
+  if (rc != RING_OK) {
     return RING_IS_EMPTY;
   }
 
-  *object_ret = ring->buffer[ring->reader];
-
-  ring->reader++;
+  /* increment reader position and wrap write if necessary */
+  ring->reader = (ring->reader + 1) & (ring->size - 1);
   ring->count--;
-  if (ring->reader == ring->size) {
-    ring->reader = 0; /* wrap around */
-  }
   return RING_OK;
 } /* ring_pop */
 
-int ring_push(struct ring *ring, void *object) {
+inline int ring_push(struct ring *ring, void *object) {
   if (ring_is_full(ring)) {
     return RING_IS_FULL;
   }
 
   ring->buffer[ring->writer] = object;
-  ring->writer++;
+  /* increment write position and wrap write if necessary */
+  ring->writer = (ring->writer + 1) & (ring->size - 1);
   ring->count++;
-  if (ring->writer == ring->size) {
-    ring->writer = 0; /* wrap around */
-  }
   return RING_OK;
 } /* ring_push */
+
+inline uint32_t ring_count(struct ring *ring) {
+  return ring->count;
+} /* ring count */
+
+inline int ring_peek(struct ring *ring, uint32_t i, void **object_ret) {
+  if (i >= ring->count) {
+    return RING_INDEX_OUT_OF_BOUNDS;
+  }
+  /* item 0 is the next one after the reader 
+   * we mask with 'size - 1' as a way of wrapping the value since we enforce
+   * power-of-two-ness */
+  *object_ret = ring->buffer[(ring->reader + i) & (ring->size - 1)];
+  return RING_OK;
+} /* ring_peek */

ring.h

@@ -1,25 +1,29 @@
 #ifndef _RING_H_
 #define _RING_H_
-#include <sys/types.h>
+#include <stdint.h>
 
 struct ring {
-  size_t writer;
-  size_t reader;
-  size_t size;
-  size_t count;
+  uint32_t writer; /* write position */
+  uint32_t reader; /* read position */
+  uint32_t size; /* maximum number of items */
+  uint32_t count; /* current count of items */
   void **buffer; /* array of pointers to whatever objects we're storing */
 };
 
 #define RING_OK 0x00
 #define RING_IS_EMPTY 0x01
 #define RING_IS_FULL 0x02
+#define RING_INDEX_OUT_OF_BOUNDS 0x03
 
-struct ring *ring_new_size(size_t count);
+struct ring *ring_new_size(uint32_t count);
 
 int ring_is_empty(struct ring *ring);
 int ring_is_full(struct ring *ring);
 
+uint32_t ring_count(struct ring *ring);
+
 int ring_pop(struct ring *ring, void **object_ret);
+int ring_peek(struct ring *ring, uint32_t index, void **object_ret);
 int ring_push(struct ring *ring, void *object);
 
 #endif /* _RING_H_ */