// Variables:
// chunk = getBuffer()
// localNextByteOffset == getPosition()
// chunkSize = getChunkSize()
//
// Case 1: localNextByteOffset == 0 && remoteNextByteOffset == 0:
// we are retrying from first chunk start from 0 offset.
//
// Case 2: localNextByteOffset == remoteNextByteOffset:
// Special case of Case 1 when a chunk is retried.
//
// Case 3: localNextByteOffset < remoteNextByteOffset
//             && driftOffset < chunkSize:
// Upload progressed and localNextByteOffset is not in-sync with
// remoteNextByteOffset and driftOffset is less than chunkSize.
// driftOffset must be less than chunkSize for it to retry using
// chunk maintained in memory.
// Find the driftOffset by subtracting localNextByteOffset from
// remoteNextByteOffset.
// Use driftOffset to determine where to restart from using the chunk in
// memory.
//
// Case 4: localNextByteOffset < remoteNextByteOffset
//            && driftOffset == chunkSize:
// Special case of Case 3.
// If chunkSize is equal to driftOffset then remoteNextByteOffset has moved on
// to the next chunk.
//
// Case 5: localNextByteOffset < remoteNextByteOffset
//            && driftOffset > chunkSize:
// Throw exception as remoteNextByteOffset has drifted beyond the retriable
// chunk maintained in memory. This is not possible unless there's multiple
// clients uploading to the same resumable upload session.
//
// Case 6: localNextByteOffset > remoteNextByteOffset:
// For completeness, this case is not possible because it would require retrying
// a 400 status code which is not allowed.