package org.simantics.graphviz.continuation;

import java.util.ArrayList;
import java.util.concurrent.CancellationException;
import java.util.concurrent.CountDownLatch;

public abstract class Computation<T> {
    T result;
    Exception exception;
    ArrayList<Continuation<T>> continuations = new ArrayList<Continuation<T>>(); 
    CountDownLatch doneGate = new CountDownLatch(1); 

    public void addContinuation(Continuation<T> continuation) {
        synchronized(doneGate) {
            if(doneGate.getCount() == 0) {
                if(exception == null)
                    continuation.succeeded(result);
                else
                    continuation.failed(exception);
            }
            else
                continuations.add(continuation);
        }
    }

    public boolean isDone() {
        return doneGate.getCount() == 0;
    }
    
    protected void failWith(Exception exception) {
        this.exception = exception;
        ArrayList<Continuation<T>> cs;
        synchronized (doneGate) {
            cs = this.continuations;
            this.continuations = null;
            doneGate.countDown();
        }
        if(cs != null)
            for(Continuation<T> continuation : cs)
                continuation.failed(exception);
    }
    
    protected void doneWith(T result) {
        this.result = result;
        ArrayList<Continuation<T>> cs;
        synchronized (doneGate) {
            cs = this.continuations;
            this.continuations = null;
            doneGate.countDown();
        }
        if(cs != null)
            for(Continuation<T> continuation : cs)
                continuation.succeeded(result);
    }
    
    public void cancel() {
        failWith(new CancellationException());
    }
    
    public T get() throws Exception {
        doneGate.await();
        if(exception != null)
            throw exception;
        return result;
    }
}