/* jshint esnext: true */

// fn should be a repeat-access function that returns an m.request.
export default memo( fn => {
	let outcome;
	let pending;
	let resolved = false;

	return ( ...args ) => {
		// If the pendant is executed with arguments, we call fn.
		// If it has already been invoked, we return the previous
		// outcome.
		if( !pending || args.length ){
			// Outcome is an m.prop and a thennable
			outcome = fn( ...args );
			pending = true;

			outcome.then( () => pending = false );

			if( !resolved ) outcome.then( () => resolved = true );
		}

		// Return the outcome, extended with state accessors
		return Object.assign( outcome, {
			pending  : () => pending,
			resolved : () => resolved,
			// valueOf avoids the need for `myPendant()()`,
			// which becomes `myPendant().valueOf()`.
			valueOf  : () => outcome()
		} );
	}
} )

// Returns a function that will return previously cached results
// (instead of executing again) if the input is identical to that
// of a previous execution
function memo( fn ){
	var cache = WeakMap();

	return input => cache.has( input )
		? cache.get( input )
		: cache.set( input, fn( input ) ).get( input );
}