Avatar of jlacar

jlacar's solution

to Darts in the Java Track

Published at May 01 2019 · 3 comments
Instructions
Test suite
Solution

Note:

This exercise has changed since this solution was written.

Write a function that returns the earned points in a single toss of a Darts game.

Darts is a game where players throw darts to a target.

In our particular instance of the game, the target rewards with 4 different amounts of points, depending on where the dart lands:

  • If the dart lands outside the target, player earns no points (0 points).
  • If the dart lands in the outer circle of the target, player earns 1 point.
  • If the dart lands in the middle circle of the target, player earns 5 points.
  • If the dart lands in the inner circle of the target, player earns 10 points.

The outer circle has a radius of 10 units (This is equivalent to the total radius for the entire target), the middle circle a radius of 5 units, and the inner circle a radius of 1. Of course, they are all centered to the same point (That is, the circles are concentric) defined by the coordinates (0, 0).

Write a function that given a point in the target (defined by its real cartesian coordinates x and y), returns the correct amount earned by a dart landing in that point.

Setup

Go through the setup instructions for Java to install the necessary dependencies:

https://exercism.io/tracks/java/installation

Running the tests

You can run all the tests for an exercise by entering the following in your terminal:

$ gradle test

Use gradlew.bat if you're on Windows

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by removing the @Ignore("Remove to run test") annotation.

Source

Inspired by an exercise created by a professor Della Paolera in Argentina

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

DartsTest.java

import org.junit.Ignore;
import org.junit.Test;

import static org.junit.Assert.assertEquals;

public class DartsTest {

    @Test
    public void testDartOutsideTarget() {
        Darts darts = new Darts(-9, 9);
        assertEquals(0, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testDartLandsOnBorderOfTarget() {
        Darts darts = new Darts(0, 10);
        assertEquals(1, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testDartLandsInOuterCircle() {
        Darts darts = new Darts(4, 4);
        assertEquals(1, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testDartLandsInBorderBetweenOuterAndMiddleCircles() {
        Darts darts = new Darts(5, 0);
        assertEquals(5, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testDartLandsOnMiddleOfCircle() {
        Darts darts = new Darts(0.8, -0.8);
        assertEquals(5, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testDartLandsOnBorderBetweenMiddleAndInnerCircles() {
        Darts darts = new Darts(0, -1);
        assertEquals(10, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testDartLandsInTheInnerCircle() {
        Darts darts = new Darts(-0.1, -0.1);
        assertEquals(10, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testCoordinatesSumLargerOneWithRadiusToOriginSmallerOrEqualOneScoredInnerCircle() {
        Darts darts = new Darts(0.4, 0.8);
        assertEquals(10, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testCoordinatesSumLargerFiveWithRadiusToOriginSmallerOrEqualFiveScoredMiddleCircle() {
        Darts darts = new Darts(2, 4);
        assertEquals(5, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testCoordinatesSumLargerTenWithRadiusToOriginSmallerOrEqualTenScoredOuterCircle() {
        Darts darts = new Darts(4, 8);
        assertEquals(1, darts.score());
    }

}
import java.util.List;
import java.util.function.Function;

import static java.lang.Math.*;
import static java.util.Comparator.comparing;

/* Stream version */

class Darts {

    private final int score;

    Darts(double x, double y) {
        double hit = sqrt(x * x + y * y);
        score = List.of(ring(1.0, 10),
                        ring(5.0, 5),
                        ring(10.0, 1))
                    .stream()
                    .map(fn -> fn.apply(hit))
                    .max(comparing(Integer::valueOf))
                    .get();
    }

    int score() {
        return this.score;
    }

    /* Creates a closure for a ring with given radius and equivalent score */
    private static Function<Double, Integer> ring(double radius, int score) {
        return hit -> (hit <= radius) ? score : 0;
    }
}

Community comments

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Avatar of jlacar
Solution Author
commented 172 days ago

Note: It looks more complicated than a straightforward imperative solution, which I did for my first iteration. I wanted to see and compare a stream-based solution so this is what I came up with. I kind of like it, even though it may take a few seconds to figure out. Note also that there is no explicit if-statement although one could argue that there is a conditional branch in the max() method. <shrug></shrug>

(edited 172 days ago)
Avatar of jlacar
Solution Author
commented 172 days ago

Note: Score is calculated in the constructor itself and the x and y cartesian coordinates are not stored as fields. This is because the API suggests that Darts is immutable. If x and y never change, then the score will never change. If the score never changes, why should we calculate it every time it is queried? Better to calculate it once in the constructor. I did the same thing with my imperative solution.

Avatar of jlacar
Solution Author
commented 171 days ago

Note: Someone elsewhere pointed out that the Math.hypot() method can be used instead of sqrt() - I think that was an excellent suggestion. Thanks, Campbell R.!

(edited 171 days ago)

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