LeggedRaces

Background

In a legged race, a group of people must try to move forward as quickly as possible with their legs tied to each other. In other words, the group can only be as quick as its slowest link and they must try to move in unison. In many ways, this concept mirrors parallel phasers. In this studio, you will use phasers and forall loops in order to simulate a legged race.

Where to Start

Test Suite

In the src/test/java folder, leggedrace.studio package, run LeggedRaceTestSuite.java.

Visualization

In the src/visualization/java folder, leggedrace.viz package, run LeggedRaceVisualizationApp.java.

Click on the buttons on the right to visualize your solutions when you have implemented them.

Legged Race Implementations

Navigate your way to the src/main/java folder and then to the leggedrace.studio package. You will have to implement four separate approaches to this problem: sequential, forall, forall_phased (which just uses next() barriers), and forall_phased_point_to_point (which uses an array of phasers to signal and wait). For each of these implementations, you must just complete the takeSteps method.

SequentialLeggedRace

In this implementation, try to solve the problem sequentially. Your method should go through the array of participants and make each one takeStep until you do this for every stepIndex up until the stepCount for every participant.

Hint: Basically, you will need two for loops. One that iterates up until the stepCount and another which goes through the array of participants.

Java For Loop

Java For Each Loop

ForallLeggedRace

In this implementation, the participants should all take each individual step in parallel. However, the steps should be taken each in turn sequentially. Think about where to place the forall loop so that the program only progresses to the next stepIndex after every participant has performed a takeStep with the current index value.

Java For Loop

Java For Each Loop

forall

ForallPhasedLeggedRace

In this solution, you will flip your sequentialLoop/parallelLoop pattern into a parallelPhasedLoop/sequentialLoop pattern with phaserNext().

Java For Loop

Java For Each Loop

forall phased()

phased()

phaserNext()

ForallPhasedPointToPointLeggedRace

In this implementation, you will build on what you did with the forall implementation to incorporate an array of phasers. Start this off by instantiating an array of phasers at DEFAULT_MODE so that the phasers can both signal and wait. Afterwards, takeStep as you did before with each participant, but remember that the partner phaser should wait until the participant phaser signals something. Remember that you must still ensure that every participant must take a step at the current index value before moving on to the next index.

For this implementation there is no incantation of the parallel forall loop we can use. Rather, we will use a normal Java for loop to spawn async(phased(...),()->{ ... })s wrapped in a finish().

Use provided int getPartnerIndex(int) to find the partner for a participant.

	private static int getPartnerIndex(int index) {
		boolean isOddIndex = (index & 0x1) == 0x1;
		if (isOddIndex) {
			return index - 1;
		} else {
			return index + 1;
		}
	}