LeggedRaces

Motivation

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.

You will complete four methods that cover two different types of races. The first three versions are an "n+1 legged race". This is where all participants are a part of the same group. Each person's legs are tied to their neighbors, and thus it is impossible for one person to take a second step before everyone else has taken their first step. The final method is a "3 legged race". Participants have a partner who they are tied to and can continue to take steps as long as their partner is keeping up with them.

Code To Use

Phaser

class Phaser (Guide to the Java Phaser)

register

bulkRegister

arriveAndAwaitAdvance

arrive

awaitAdvance use via PhaserUtils.awaitAdvanceForPhase(phaser, phase)

PhaserUtils

PhaserUtils

awaitAdvanceForPhase

If you are on or past the phase you want to await, then calling phaser.awaitAdvance(phase) directly is fine. If you might be on a prior phase, then invoke PhaserUtils.awaitAdvanceForPhase(phaser, phase). If in doubt, invoke PhaserUtils.awaitAdvanceForPhase(phaser, phase) as it will await until the phase is arrived upon no matter what.

PhaserUtils

public static int awaitAdvanceForPhase(Phaser phaser, int phase) { return awaitAdvanceForPhase(phaser, phase, () -> Thread.yield()); } public static int awaitAdvanceForPhase(Phaser phaser, int phase, Runnable runnable) { while (true) { int currentPhase = phaser.awaitAdvance(phase); if (currentPhase < 0 || currentPhase > phase) { return currentPhase; } else { if (runnable != null) { runnable.run(); } } } }

Legged Races

interface Participant

takeStep

Code To Implement

Single Everyone Bound Together Races

AllTogetherLeggedRace

class:	AllTogetherLeggedRace.java
methods:	takeSteps
package:	leggedrace.exercise
source folder:	student/src/main/java

method: public void takeSteps(Participant[] participants, int stepCount) (parallel implementation required)

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 fork loop so that the program only progresses to the next stepIndex after every participant has performed a takeStep with the current index value.

X10PhasedAllTogetherLeggedRace

class:	X10PhasedAllTogetherLeggedRace.java
methods:	takeSteps
package:	leggedrace.exercise
source folder:	student/src/main/java

PhasedAllTogetherLeggedRace

class:	PhasedAllTogetherLeggedRace.java
methods:	takeSteps
package:	leggedrace.exercise
source folder:	student/src/main/java

method: public void takeSteps(Participant[] participants, int stepCount) (parallel implementation required)

In this solution, you will flip your sequentialLoop/parallelLoop pattern into a parallelLoop/sequentialLoop pattern with a phaser. For this, there should only be one Phaser, which is bulk registered such that each participant can wait on the same Phaser. In order to simplify the code, look into arriveAndAwaitAdvance().

Teams Of 2 Races

As Daniel Tiger correctly observed, it is very, very, very hard to wait. It is also wasteful. Here we will use multiple Phasers to simulate a 3-legged race with multiple teams participating.

PhaserPerTeamLeggedRace

class:	PhaserPerTeamLeggedRace.java
methods:	constructor phaserCreator takeSteps
package:	leggedrace.exercise
source folder:	student/src/main/java

method: public void takeSteps(Participant[] participants, int stepCount) (parallel implementation required)

Warning:When you create an array, the contents will initially be null.

Testing Your Solution

Visualization

class:	LeggedRaceViz.java	VIZ
package:	leggedrace.viz
source folder:	student/src/main/java

Click on the buttons on the right to visualize your solutions when you have implemented them. Here are examples of what the correct visualization should look like for each method.

Correctness

class:	__LeggedRaceTestSuite.java
package:	leggedrace.exercise
source folder:	testing/src/test/java

Pledge, Acknowledgments, Citations

file:

exercise-legged-races-pledge-acknowledgments-citations.txt

More info about the Honor Pledge