queue: add fixed size circular buffer

This commit introduces a fixed size circular buffer  which stores
elements in a fixed size underlying array, wrapping to overwrite items
when the buffer gets full.

queue/circular_buf.go

@@ -0,0 +1,116 @@
+package queue
+
+import (
+	"errors"
+)
+
+// errInvalidSize is returned when an invalid size for a buffer is provided.
+var errInvalidSize = errors.New("buffer size must be > 0")
+
+// CircularBuffer is a buffer which retains a set of values in memory, and
+// overwrites the oldest item in the buffer when a new item needs to be
+// written.
+type CircularBuffer struct {
+	// total is the total number of items that have been added to the
+	// buffer.
+	total int
+
+	// items is the set of buffered items.
+	items []interface{}
+}
+
+// NewCircularBuffer returns a new circular buffer with the size provided. It
+// will fail if a zero or negative size parameter is provided.
+func NewCircularBuffer(size int) (*CircularBuffer, error) {
+	if size <= 0 {
+		return nil, errInvalidSize
+	}
+
+	return &CircularBuffer{
+		total: 0,
+
+		// Create a slice with length and capacity equal to the size of
+		// the buffer so that we do not need to resize the underlying
+		// array when we add items.
+		items: make([]interface{}, size),
+	}, nil
+}
+
+// index returns the index that should be written to next.
+func (c *CircularBuffer) index() int {
+	return c.total % len(c.items)
+}
+
+// Add adds an item to the buffer, overwriting the oldest item if the buffer
+// is full.
+func (c *CircularBuffer) Add(item interface{}) {
+	// Set the item in the next free index in the items array.
+	c.items[c.index()] = item
+
+	// Increment the total number of items that we have stored.
+	c.total++
+}
+
+// List returns a copy of the items in the buffer ordered from the oldest to
+// newest item.
+func (c *CircularBuffer) List() []interface{} {
+	size := cap(c.items)
+	index := c.index()
+
+	switch {
+	// If no items have been stored yet, we can just return a nil list.
+	case c.total == 0:
+		return nil
+
+	// If we have added fewer items than the buffer size, we can simply
+	// return the total number of items from the beginning of the list
+	// to the index. This special case is added because the oldest item
+	// is at the beginning of the underlying array, not at the index when
+	// we have not filled the array yet.
+	case c.total < size:
+		resp := make([]interface{}, c.total)
+		copy(resp, c.items[:c.index()])
+		return resp
+	}
+
+	resp := make([]interface{}, size)
+
+	// Get the items in the underlying array from index to end, the first
+	// item in this slice will be the oldest item in the list.
+	firstHalf := c.items[index:]
+
+	// Copy the first set into our response slice from index 0, so that
+	// the response returned is from oldest to newest.
+	copy(resp, firstHalf)
+
+	// Get the items in the underlying array from beginning until the write
+	// index, the last item in this slice will be the newest item in the
+	// list.
+	secondHalf := c.items[:index]
+
+	// Copy the second set of items into the response slice offset by the
+	// length of the first set of items so that we return a response which
+	// is ordered from oldest to newest entry.
+	copy(resp[len(firstHalf):], secondHalf)
+
+	return resp
+}
+
+// Total returns the total number of items that have been added to the buffer.
+func (c *CircularBuffer) Total() int {
+	return c.total
+}
+
+// Latest returns the item that was most recently added to the buffer.
+func (c *CircularBuffer) Latest() interface{} {
+	// If no items have been added yet, return nil.
+	if c.total == 0 {
+		return nil
+	}
+
+	// The latest item is one before our total, mod by length.
+	latest := (c.total - 1) % len(c.items)
+
+	// Return the latest item added.
+	return c.items[latest]
+}