package com.tofvesson.async;

import java.lang.reflect.Constructor;
import java.lang.reflect.Method;

@SuppressWarnings({"WeakerAccess", "unused", "unchecked"})
public class Async<T> {

    /**
     * Thread running background task.
     */
    Thread task;

    /**
     * Return value/ constructed object.
     */
    volatile T ret;

    /**
     * Status indicators.
     */
    volatile boolean complete = false, failed = false; // Used by anonymous class, therefore not private

    /**
     * Exception to throw in case something goes wrong.
     */
    volatile Throwable t;

    /**
     * Initiates an async task that invokes the defined method. If object is null, the method must be static.
     * Note that this is optimized for larger tasks id est tasks that take more than 5 milliseconds to process (preferably a minimum of 10 ms).
     * @param o Object to invoke method on.
     * @param method Method to invoke.
     * @param params Required parameters.
     */
    public Async(final Object o, final Method method, final Object... params){
        method.setAccessible(true); // Ensure that no crash occurs
        task = new Thread(()-> { // Create a new thread
            try {
                ret = (T) method.invoke(o, params); // Invoke given method
                complete = true;                    // Notify all threads who are checking
            } catch (Throwable t1) {                // Prepare for failure
                if(!failed) {                       // Checks if task was canceled
                    failed = true;                  // Notifies all threads that task failed
                    t=t1;                           // Makes error accessible to be thrown
                }
            }
        });
        task.setDaemon(true);                       // Ensure that process dies with program
        task.start();                               // Start task
    }

    /**
     * Dispatches an asynchronous call to supplied method on specified object with no parameters.
     * @param o Object to call method on.
     * @param m Method to call. Must be parameter-less!
     */
    public Async(Object o, Method m){ this(o, m, (Object[]) null); }

    /**
     * Dispatches an asynchronous call to supplied static method.
     * @param m Static method to call.
     * @param params Parameters to supply to method.
     */
    public Async(Method m, Object... params){ this(null, m, params); }

    /**
     * Dispatches an asynchronous, static, parameter-less method call.
     * @param m Method to call.
     */
    public Async(Method m){ this(null, m, (Object[]) null); }

    /**
     * Create a new async task for instantiating an object.
     * @param c Constructor to use when instantiating object.
     * @param params Parameters to use when instantiaing object.
     */
    public Async(final Constructor<T> c, final Object... params){
        c.setAccessible(true);                  // Ensure that constructor can be called
        task = new Thread(() -> {               // Creates a new thread for asynchronous execution
            try {
                ret = c.newInstance(params);    // Create a new instance: invoke "<init>" method
                complete = true;                // Notify all threads that async is finished
            } catch (Throwable t1) {            // Handle crash
                if(!failed) {                   // Ensure that crash wasn't called by cancel()
                    failed = true;              // Notify all threads that error has been encountered
                    t=t1;                       // Make throwable accessible to be thrown in caller thread
                }
            }
        });
        task.setDaemon(true);                   // Ensure that thread dies with program
        task.start();                           // Start construction
    }

    /**
     * Dispatch an asynchronous construction of the object corresponding to the passed constructor with no parameters.
     * @param c Constructor to call.
     */
    public Async(Constructor<T> c){ this(c, (Object[]) null); }

    /**
     * WARNING: Package-scoped because it should only be used when overriding standard construction. Should not bw used haphazardly!
     */
    Async() {task = null;}

    /**
     * Await completion of async task. Blocks thread if task isn't complete.
     * @return Return value from async method call. Return is null if {@link #cancel()} is called before this method and async task wan't finished.
     */
    public T await(){
        //noinspection StatementWithEmptyBody
        while(!failed && !complete); // Check using variables rather than checking if worker thread is alive since method calls are more computationally expensive
        if(ret==null && t!=null) throw new RuntimeException(t); // Detect a unique error state, get error and throw in caller thread
        //noinspection unchecked
        return ret; // Don't bother resetting values since this object is only intended to be recycled after value is gotten
    }

    /**
     * Checks if async task is still running.
     * @return True if it's still running.
     */
    public boolean isAlive(){ return task.isAlive(); } // Check if thread is still alive which directly determines if process is alive since Async is a wrapper of Thread

    /**
     * Cancels async operation if it's still alive.
     */
    public void cancel(){
        if(task.isAlive()){
                            // Set values before interrupting to prevent InterruptedException from
                            // being propagated and thrown in the main thread
            t=null;
            failed = true;  // Creates a unique and identifiable state
                            //noinspection deprecation
            task.stop();    // Force-stop thread
        }
    }
}