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to Forth in the Java Track

Published at May 18 2019 · 0 comments
Instructions
Test suite
Solution

Implement an evaluator for a very simple subset of Forth.

Forth is a stack-based programming language. Implement a very basic evaluator for a small subset of Forth.

Your evaluator has to support the following words:

  • +, -, *, / (integer arithmetic)
  • DUP, DROP, SWAP, OVER (stack manipulation)

Your evaluator also has to support defining new words using the customary syntax: : word-name definition ;.

To keep things simple the only data type you need to support is signed integers of at least 16 bits size.

You should use the following rules for the syntax: a number is a sequence of one or more (ASCII) digits, a word is a sequence of one or more letters, digits, symbols or punctuation that is not a number. (Forth probably uses slightly different rules, but this is close enough.)

Words are case-insensitive.

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.

Submitting Incomplete Solutions

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

ForthEvaluatorTest.java

import org.junit.Before;
import org.junit.Ignore;
import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.ExpectedException;

import java.util.Arrays;
import java.util.Collections;

import static org.junit.Assert.assertEquals;

public class ForthEvaluatorTest {

    @Rule
    public ExpectedException expectedException = ExpectedException.none();

    private ForthEvaluator forthEvaluator;

    @Before
    public void setUp() {
        forthEvaluator = new ForthEvaluator();
    }

    @Test
    public void testNumbersAreJustPushedOntoTheStack() {
        assertEquals(
                Arrays.asList(1, 2, 3, 4, 5),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 2 3 4 5")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testTwoNumbersCanBeAdded() {
        assertEquals(
                Collections.singletonList(3),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 2 +")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfAdditionAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Addition requires that the stack contain at least 2 values");

        forthEvaluator.evaluateProgram(Collections.singletonList("+"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfAdditionAttemptedWithOneNumberOnTheStack() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Addition requires that the stack contain at least 2 values");

        forthEvaluator.evaluateProgram(Collections.singletonList("1 +"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testTwoNumbersCanBeSubtracted() {
        assertEquals(
                Collections.singletonList(-1),
                forthEvaluator.evaluateProgram(Collections.singletonList("3 4 -")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfSubtractionAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Subtraction requires that the stack contain at least 2 values");

        forthEvaluator.evaluateProgram(Collections.singletonList("-"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfSubtractionAttemptedWithOneNumberOnTheStack() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Subtraction requires that the stack contain at least 2 values");

        forthEvaluator.evaluateProgram(Collections.singletonList("1 -"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testTwoNumbersCanBeMultiplied() {
        assertEquals(
                Collections.singletonList(8),
                forthEvaluator.evaluateProgram(Collections.singletonList("2 4 *")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfMultiplicationAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Multiplication requires that the stack contain at least 2 values");

        forthEvaluator.evaluateProgram(Collections.singletonList("*"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfMultiplicationAttemptedWithOneNumberOnTheStack() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Multiplication requires that the stack contain at least 2 values");

        forthEvaluator.evaluateProgram(Collections.singletonList("1 *"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testTwoNumbersCanBeDivided() {
        assertEquals(
                Collections.singletonList(4),
                forthEvaluator.evaluateProgram(Collections.singletonList("12 3 /")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testThatIntegerDivisionIsUsed() {
        assertEquals(
                Collections.singletonList(2),
                forthEvaluator.evaluateProgram(Collections.singletonList("8 3 /")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfDividingByZero() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Division by 0 is not allowed");

        forthEvaluator.evaluateProgram(Collections.singletonList("4 0 /"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfDivisionAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Division requires that the stack contain at least 2 values");

        forthEvaluator.evaluateProgram(Collections.singletonList("/"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfDivisionAttemptedWithOneNumberOnTheStack() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Division requires that the stack contain at least 2 values");

        forthEvaluator.evaluateProgram(Collections.singletonList("1 /"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testCombinedAdditionAndSubtraction() {
        assertEquals(
                Collections.singletonList(-1),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 2 + 4 -")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testCombinedMultiplicationAndDivision() {
        assertEquals(
                Collections.singletonList(2),
                forthEvaluator.evaluateProgram(Collections.singletonList("2 4 * 3 /")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testDupCopiesAValueOnTheStack() {
        assertEquals(
                Arrays.asList(1, 1),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 dup")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testDupCopiesTopValueOnTheStack() {
        assertEquals(
                Arrays.asList(1, 2, 2),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 2 dup")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfDuplicatingAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Duplicating requires that the stack contain at least 1 value");

        forthEvaluator.evaluateProgram(Collections.singletonList("dup"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testDropRemovesTheTopValueOnTheStackIfItIsTheOnlyOne() {
        assertEquals(
                Collections.emptyList(),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 drop")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testDropRemovesTheTopValueOnTheStackIfItIsNotTheOnlyOne() {
        assertEquals(
                Collections.singletonList(1),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 2 drop")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfDroppingAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Dropping requires that the stack contain at least 1 value");

        forthEvaluator.evaluateProgram(Collections.singletonList("drop"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testSwapSwapsTheTopTwosValueOnTheStackIfTheyAreTheOnlyOnes() {
        assertEquals(
                Arrays.asList(2, 1),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 2 swap")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testSwapSwapsTheTopTwosValueOnTheStackIfTheyAreNotTheOnlyOnes() {
        assertEquals(
                Arrays.asList(1, 3, 2),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 2 3 swap")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfSwappingAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Swapping requires that the stack contain at least 2 values");

        forthEvaluator.evaluateProgram(Collections.singletonList("swap"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfSwappingAttemptedWithOneNumberOnTheStack() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Swapping requires that the stack contain at least 2 values");

        forthEvaluator.evaluateProgram(Collections.singletonList("1 swap"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testOverCopiesTheSecondElementIfThereAreOnlyTwo() {
        assertEquals(
                Arrays.asList(1, 2, 1),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 2 over")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testOverCopiesTheSecondElementIfThereAreMoreThanTwo() {
        assertEquals(
                Arrays.asList(1, 2, 3, 2),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 2 3 over")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfOveringAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Overing requires that the stack contain at least 2 values");

        forthEvaluator.evaluateProgram(Collections.singletonList("over"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfOveringAttemptedWithOneNumberOnTheStack() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Overing requires that the stack contain at least 2 values");

        forthEvaluator.evaluateProgram(Collections.singletonList("1 over"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testUserDefinedOperatorsCanConsistOfBuiltInOperators() {
        assertEquals(
                Arrays.asList(1, 1, 1),
                forthEvaluator.evaluateProgram(Arrays.asList(": dup-twice dup dup ;", "1 dup-twice")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testUserDefinedOperatorsAreEvaluatedInTheCorrectOrder() {
        assertEquals(
                Arrays.asList(1, 2, 3),
                forthEvaluator.evaluateProgram(Arrays.asList(": countup 1 2 3 ;", "countup")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testCanRedefineAUserDefinedOperator() {
        assertEquals(
                Arrays.asList(1, 1, 1),
                forthEvaluator.evaluateProgram(Arrays.asList(": foo dup ;", ": foo dup dup ;", "1 foo")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testCanOverrideBuiltInWordOperators() {
        assertEquals(
                Arrays.asList(1, 1),
                forthEvaluator.evaluateProgram(Arrays.asList(": swap dup ;", "1 swap")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testCanOverrideBuiltInArithmeticOperators() {
        assertEquals(
                Collections.singletonList(12),
                forthEvaluator.evaluateProgram(Arrays.asList(": + * ;", "3 4 +")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testCanUseDifferentWordsWithTheSameName() {
        assertEquals(
                Arrays.asList(5, 6),
                forthEvaluator.evaluateProgram(Arrays.asList(": foo 5 ;", ": bar foo ;", ": foo 6 ;", "bar foo")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testCanDefineWordThatUsesWordWithTheSameName() {
        assertEquals(
                Collections.singletonList(11),
                forthEvaluator.evaluateProgram(Arrays.asList(": foo 10 ;", ": foo foo 1 + ;", "foo")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testCannotRedefineNumbers() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Cannot redefine numbers");

        forthEvaluator.evaluateProgram(Collections.singletonList(": 1 2 ;"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testErrorIfEvaluatingAnUndefinedOperator() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("No definition available for operator \"foo\"");

        forthEvaluator.evaluateProgram(Collections.singletonList("foo"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testDupIsCaseInsensitive() {
        assertEquals(
                Arrays.asList(1, 1, 1, 1),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 DUP Dup dup")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testDropIsCaseInsensitive() {
        assertEquals(
                Arrays.asList(1),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 2 3 4 DROP Drop drop")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testSwapIsCaseInsensitive() {
        assertEquals(
                Arrays.asList(2, 3, 4, 1),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 2 SWAP 3 Swap 4 swap")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testOverIsCaseInsensitive() {
        assertEquals(
                Arrays.asList(1, 2, 1, 2, 1),
                forthEvaluator.evaluateProgram(Collections.singletonList("1 2 OVER Over over")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testUserDefinedWordsAreCaseInsensitive() {
        assertEquals(
                Arrays.asList(1, 1, 1, 1),
                forthEvaluator.evaluateProgram(Arrays.asList(": foo dup ;", "1 FOO Foo foo")));
    }

    @Ignore("Remove to run test")
    @Test
    public void testDefinitionsAreCaseInsensitive() {
        assertEquals(
                Arrays.asList(1, 1, 1, 1),
                forthEvaluator.evaluateProgram(Arrays.asList(": SWAP DUP Dup dup ;", "1 swap")));
    }

}

src/main/java/BuiltinFunction.java

import java.util.Deque;

abstract class BuiltinFunction {
    private final int arity;
    private final String name;

    BuiltinFunction(int arity, String name) {
        this.arity = arity;
        this.name = name;
    }

    void evaluate(Deque<Integer> stack) {
        if (stack.size() < this.arity) {
            throw new IllegalArgumentException(String.format("%s requires that the stack contain at least %d value%s",
                    this.name, this.arity, this.arity > 1 ? "s" : ""));
        }
    }
}

final class Addition extends BuiltinFunction {
    Addition() {
        super(2, "Addition");
    }

    @Override
    void evaluate(Deque<Integer> stack) {
        super.evaluate(stack);
        stack.addLast(stack.pollLast() + stack.pollLast());
    }
}

final class Subtraction extends BuiltinFunction {
    Subtraction() {
        super(2, "Subtraction");
    }

    @Override
    void evaluate(Deque<Integer> stack) {
        super.evaluate(stack);

        final var b = stack.pollLast();
        final var a = stack.pollLast();

        stack.addLast(a - b);
    }
}

final class Multiplication extends BuiltinFunction {
    Multiplication() {
        super(2, "Multiplication");
    }

    @Override
    void evaluate(Deque<Integer> stack) {
        super.evaluate(stack);
        stack.addLast(stack.pollLast() * stack.pollLast());
    }
}

final class Division extends BuiltinFunction {
    Division() {
        super(2, "Division");
    }

    @Override
    void evaluate(Deque<Integer> stack) {
        super.evaluate(stack);

        final var b = stack.pollLast();
        final var a = stack.pollLast();

        if (b == 0)
            throw new IllegalArgumentException("Division by 0 is not allowed");

        stack.addLast(a / b);
    }
}

final class Duplicating extends BuiltinFunction {
    Duplicating() {
        super(1, "Duplicating");
    }

    @Override
    void evaluate(Deque<Integer> stack) {
        super.evaluate(stack);
        stack.addLast(stack.peekLast());
    }
}

final class Dropping extends BuiltinFunction {
    Dropping() {
        super(1, "Dropping");
    }

    @Override
    void evaluate(Deque<Integer> stack) {
        super.evaluate(stack);
        stack.removeLast();
    }
}

final class Swapping extends BuiltinFunction {
    Swapping() {
        super(2, "Swapping");
    }

    @Override
    void evaluate(Deque<Integer> stack) {
        super.evaluate(stack);

        final var b = stack.pollLast();
        final var a = stack.pollLast();

        stack.addLast(b);
        stack.addLast(a);
    }
}

final class Overing extends BuiltinFunction {
    Overing() {
        super(2, "Overing");
    }

    @Override
    void evaluate(Deque<Integer> stack) {
        super.evaluate(stack);

        final var b = stack.pollLast();
        final var a = stack.pollLast();

        stack.addLast(a);
        stack.addLast(b);
        stack.addLast(a);
    }
}

src/main/java/ForthEvaluator.java

import java.util.*;
import java.util.regex.Pattern;

final class ForthEvaluator {
    private static final Pattern DECLARATION = Pattern.compile(": ([^:; ]+) ([^:;]+) ;");
    private static final Pattern NUMBER = Pattern.compile("-?\\d+");
    private static final Map<String, BuiltinFunction> FUNCTIONS = Map.ofEntries(
            Map.entry("+", new Addition()),
            Map.entry("-", new Subtraction()),
            Map.entry("*", new Multiplication()),
            Map.entry("/", new Division()),
            Map.entry("dup", new Duplicating()),
            Map.entry("drop", new Dropping()),
            Map.entry("swap", new Swapping()),
            Map.entry("over", new Overing())
    );
    private final Deque<Integer> stack = new LinkedList<>();
    private final Map<String, String> userDefined = new HashMap<>();

    List<Integer> evaluateProgram(Iterable<String> lines) {
        for (var line : lines)
            this.evaluateLine(line.toLowerCase());

        var result = new ArrayList<>(this.stack);
        this.resetState();

        return result;
    }

    private void evaluateLine(String line) {
        final var declaration = DECLARATION.matcher(line);

        if (declaration.find()) {
            this.addDeclaration(declaration.group(1).toLowerCase(), declaration.group(2));
            return;
        }

        for (var token : applySymbolReplacements(line).split(" ")) {
            if (NUMBER.matcher(token).matches()) {
                this.stack.addLast(Integer.parseInt(token));
                continue;
            }

            final var function = FUNCTIONS.get(token);

            if (function == null)
                throw new IllegalArgumentException(String.format("No definition available for operator \"%s\"", token));

            function.evaluate(this.stack);
        }
    }

    private String applySymbolReplacements(String line) {
        for (var entry : this.userDefined.entrySet())
            line = line.replace(entry.getKey(), entry.getValue());

        return line;
    }

    private void addDeclaration(String symbol, String body) {
        if (NUMBER.matcher(symbol).matches())
            throw new IllegalArgumentException("Cannot redefine numbers");

        this.userDefined.put(symbol, applySymbolReplacements(body));
    }

    private void resetState() {
        this.stack.clear();
        this.userDefined.clear();
    }
}

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