public Class CoroutineManager

Support the coroutine design pattern.

A coroutine set is a very simple cooperative non-preemptive multitasking model, where the switch from one task to another is performed via an explicit request. Coroutines interact according to the following rules:

• One coroutine in the set has control, which it retains until it either exits or resumes another coroutine.
• A coroutine is activated when it is resumed by some other coroutine for the first time.
• An active coroutine that gives up control by resuming another in the set retains its context -- including call stack and local variables -- so that if/when it is resumed, it will proceed from the point at which it last gave up control.

Coroutines can be thought of as falling somewhere between pipes and subroutines. Like call/return, there is an explicit flow of control from one coroutine to another. Like pipes, neither coroutine is actually "in charge", and neither must exit in order to transfer control to the other.

One classic application of coroutines is in compilers, where both the parser and the lexer are maintaining complex state information. The parser resumes the lexer to process incoming characters into lexical tokens, and the lexer resumes the parser when it has reached a point at which it has a reliably interpreted set of tokens available for semantic processing. Structuring this as call-and-return would require saving and restoring a considerable amount of state each time. Structuring it as two tasks connected by a queue may involve higher overhead (in systems which can optimize the coroutine metaphor), isn't necessarily as clear in intent, may have trouble handling cases where data flows in both directions, and may not handle some of the more complex cases where more than two coroutines are involved.

Most coroutine systems also provide a way to pass data between the source and target of a resume operation; this is sometimes referred to as "yielding" a value. Others rely on the fact that, since only one member of a coroutine set is running at a time and does not lose control until it chooses to do so, data structures may be directly shared between them with only minimal precautions.

"Note: This should not be taken to mean that producer/consumer problems should be always be done with coroutines." Queueing is often a better solution when only two threads of execution are involved and full two-way handshaking is not required. It's a bit difficult to find short pedagogical examples that require coroutines for a clear solution.

The fact that only one of a group of coroutines is running at a time, and the control transfer between them is explicit, simplifies their possible interactions, and in some implementations permits them to be implemented more efficiently than general multitasking. In some situations, coroutines can be compiled out entirely; in others, they may only require a few instructions more than a simple function call.

This version is built on top of standard Java threading, since that's all we have available right now. It's been encapsulated for code clarity and possible future optimization.

(Two possible approaches: wait-notify based and queue-based. Some folks think that a one-item queue is a cleaner solution because it's more abstract -- but since coroutine _is_ an abstraction I'm not really worried about that; folks should be able to switch this code without concern.)

%TBD% THIS SHOULD BE AN INTERFACE, to facilitate building other implementations... perhaps including a true coroutine system someday, rather than controlled threading. Arguably Coroutine itself should be an interface much like Runnable, but I think that can be built on top of this.