internal/sdkio: add RingBuffer data structure to the sdk (#417)

Adds a RingBuffer data structure. RingBuffer acts as a revolving buffer of a predefined length. It implements io.ReadWriter interface.

Author: skotambkar

CHANGELOG_PENDING.md

@@ -17,6 +17,9 @@ SDK Features
 
 SDK Enhancements
 ---
+* `internal/sdkio`: Adds RingBuffer data structure to the sdk [#417](https://github.com/aws/aws-sdk-go-v2/pull/417)
+  * Adds an implementation of RingBuffer data structure which acts as a revolving buffer of a predefined length. The RingBuffer implements io.ReadWriter interface.
+  * Adds unit tests to test the behavior of the ring buffer. 
 * `aws/ec2metadata`: Adds support for EC2Metadata client to use secure tokens provided by the IMDS ([#453](https://github.com/aws/aws-sdk-go-v2/pull/453)) 
   * Modifies and adds tests to verify the behavior of the EC2Metadata client.
 * `service/dynamodb/dynamodbattribute`: Adds clarifying docs on dynamodbattribute.UnixTime ([#464](https://github.com/aws/aws-sdk-go-v2/pull/464))

internal/sdkio/ringbuffer.go

@@ -0,0 +1,72 @@
+package sdkio
+
+import "io"
+
+// RingBuffer struct satisfies io.ReadWrite interface.
+//
+// ReadBuffer is a revolving buffer data structure, which can be used to store snapshots of data in a
+// revolving window.
+type RingBuffer struct {
+	slice []byte
+	start int
+	end   int
+	size  int
+}
+
+// NewRingBuffer method takes in a byte slice as an input and returns a RingBuffer.
+func NewRingBuffer(slice []byte) *RingBuffer {
+	ringBuf := RingBuffer{
+		slice: slice,
+	}
+	return &ringBuf
+}
+
+// Write method inserts the elements in a byte slice, and returns the number of bytes written along with an error.
+func (r *RingBuffer) Write(p []byte) (int, error) {
+	for _, b := range p {
+		// check if end points to invalid index, we need to circle back
+		if r.end == len(r.slice) {
+			r.end = 0
+		}
+		// check if start points to invalid index, we need to circle back
+		if r.start == len(r.slice) {
+			r.start = 0
+		}
+		// if ring buffer is filled, increment the start index
+		if r.size == len(r.slice) {
+			r.size--
+			r.start++
+		}
+
+		r.slice[r.end] = b
+		r.end++
+		r.size++
+	}
+	return r.size, nil
+}
+
+// Read copies the data on the ring buffer into the byte slice provided to the method.
+// Returns the read count along with Error encountered while reading
+func (r *RingBuffer) Read(p []byte) (int, error) {
+	// readCount keeps track of the number of bytes read
+	var readCount int
+	for j := 0; j < len(p); j++ {
+		// if ring buffer is empty or completely read
+		// return EOF error.
+		if r.size == 0 {
+			return readCount, io.EOF
+		}
+
+		p[j] = r.slice[r.start]
+		readCount++
+		// increment the start pointer for ring buffer
+		r.start++
+		// decrement the size of ring buffer
+		r.size--
+
+		if r.start == len(r.slice) {
+			r.start = 0
+		}
+	}
+	return readCount, nil
+}