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to Forth in the Kotlin Track

Published at Jan 01 2019 · 0 comments
Instructions
Test suite
Solution

Note:

This exercise has changed since this solution was written.

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.

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ForthEvaluatorTest.kt

import org.junit.Before
import org.junit.Ignore
import org.junit.Rule
import org.junit.Test
import org.junit.rules.ExpectedException
import kotlin.test.assertEquals

class ForthEvaluatorTest {

    @Rule
    @JvmField
    var expectedException: ExpectedException = ExpectedException.none()

    private lateinit var forthEvaluator: ForthEvaluator

    @Before
    fun setUp() {
        forthEvaluator = ForthEvaluator()
    }

    @Test
    fun testNumbersAreJustPushedOntoTheStack() {
        assertEquals(
                listOf(1, 2, 3, 4, 5),
                forthEvaluator.evaluateProgram(listOf("1 2 3 4 5")))
    }

    @Ignore
    @Test
    fun testTwoNumbersCanBeAdded() {
        assertEquals(
                listOf(3),
                forthEvaluator.evaluateProgram(listOf("1 2 +")))
    }

    @Ignore
    @Test
    fun testErrorIfAdditionAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Addition requires that the stack contain at least 2 values")

        forthEvaluator.evaluateProgram(listOf("+"))
    }

    @Ignore
    @Test
    fun testErrorIfAdditionAttemptedWithOneNumberOnTheStack() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Addition requires that the stack contain at least 2 values")

        forthEvaluator.evaluateProgram(listOf("1 +"))
    }

    @Ignore
    @Test
    fun testTwoNumbersCanBeSubtracted() {
        assertEquals(
                listOf(-1),
                forthEvaluator.evaluateProgram(listOf("3 4 -")))
    }

    @Ignore
    @Test
    fun testErrorIfSubtractionAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Subtraction requires that the stack contain at least 2 values")

        forthEvaluator.evaluateProgram(listOf("-"))
    }

    @Ignore
    @Test
    fun testErrorIfSubtractionAttemptedWithOneNumberOnTheStack() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Subtraction requires that the stack contain at least 2 values")

        forthEvaluator.evaluateProgram(listOf("1 -"))
    }

    @Ignore
    @Test
    fun testTwoNumbersCanBeMultiplied() {
        assertEquals(
                listOf(8),
                forthEvaluator.evaluateProgram(listOf("2 4 *")))
    }

    @Ignore
    @Test
    fun testErrorIfMultiplicationAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Multiplication requires that the stack contain at least 2 values")

        forthEvaluator.evaluateProgram(listOf("*"))
    }

    @Ignore
    @Test
    fun testErrorIfMultiplicationAttemptedWithOneNumberOnTheStack() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Multiplication requires that the stack contain at least 2 values")

        forthEvaluator.evaluateProgram(listOf("1 *"))
    }

    @Ignore
    @Test
    fun testTwoNumbersCanBeDivided() {
        assertEquals(
                listOf(4),
                forthEvaluator.evaluateProgram(listOf("12 3 /")))
    }

    @Ignore
    @Test
    fun testThatIntegerDivisionIsUsed() {
        assertEquals(
                listOf(2),
                forthEvaluator.evaluateProgram(listOf("8 3 /")))
    }

    @Ignore
    @Test
    fun testErrorIfDividingByZero() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Division by 0 is not allowed")

        forthEvaluator.evaluateProgram(listOf("4 0 /"))
    }

    @Ignore
    @Test
    fun testErrorIfDivisionAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Division requires that the stack contain at least 2 values")

        forthEvaluator.evaluateProgram(listOf("/"))
    }

    @Ignore
    @Test
    fun testErrorIfDivisionAttemptedWithOneNumberOnTheStack() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Division requires that the stack contain at least 2 values")

        forthEvaluator.evaluateProgram(listOf("1 /"))
    }

    @Ignore
    @Test
    fun testCombinedAdditionAndSubtraction() {
        assertEquals(
                listOf(-1),
                forthEvaluator.evaluateProgram(listOf("1 2 + 4 -")))
    }

    @Ignore
    @Test
    fun testCombinedMultiplicationAndDivision() {
        assertEquals(
                listOf(2),
                forthEvaluator.evaluateProgram(listOf("2 4 * 3 /")))
    }

    @Ignore
    @Test
    fun testDupCopiesTheTopValueOnTheStack() {
        assertEquals(
                listOf(1, 1),
                forthEvaluator.evaluateProgram(listOf("1 dup")))
    }

    @Ignore
    @Test
    fun testDupParsingIsCaseInsensitive() {
        assertEquals(
                listOf(1, 2, 2),
                forthEvaluator.evaluateProgram(listOf("1 2 dup")))
    }

    @Ignore
    @Test
    fun testErrorIfDuplicatingAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Duplicating requires that the stack contain at least 1 value")

        forthEvaluator.evaluateProgram(listOf("dup"))
    }

    @Ignore
    @Test
    fun testDropRemovesTheTopValueOnTheStackIfItIsTheOnlyOne() {
        assertEquals(
                emptyList(),
                forthEvaluator.evaluateProgram(listOf("1 drop")))
    }

    @Ignore
    @Test
    fun testDropRemovesTheTopValueOnTheStackIfItIsNotTheOnlyOne() {
        assertEquals(
                listOf(1),
                forthEvaluator.evaluateProgram(listOf("1 2 drop")))
    }

    @Ignore
    @Test
    fun testErrorIfDroppingAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Dropping requires that the stack contain at least 1 value")

        forthEvaluator.evaluateProgram(listOf("drop"))
    }

    @Ignore
    @Test
    fun testSwapSwapsTheTopTwosValueOnTheStackIfTheyAreTheOnlyOnes() {
        assertEquals(
                listOf(2, 1),
                forthEvaluator.evaluateProgram(listOf("1 2 swap")))
    }

    @Ignore
    @Test
    fun testSwapSwapsTheTopTwosValueOnTheStackIfTheyAreNotTheOnlyOnes() {
        assertEquals(
                listOf(1, 3, 2),
                forthEvaluator.evaluateProgram(listOf("1 2 3 swap")))
    }

    @Ignore
    @Test
    fun testErrorIfSwappingAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Swapping requires that the stack contain at least 2 values")

        forthEvaluator.evaluateProgram(listOf("swap"))
    }

    @Ignore
    @Test
    fun testErrorIfSwappingAttemptedWithOneNumberOnTheStack() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Swapping requires that the stack contain at least 2 values")

        forthEvaluator.evaluateProgram(listOf("1 swap"))
    }

    @Ignore
    @Test
    fun testOverCopiesTheSecondElementIfThereAreOnlyTwo() {
        assertEquals(
                listOf(1, 2, 1),
                forthEvaluator.evaluateProgram(listOf("1 2 over")))
    }

    @Ignore
    @Test
    fun testOverCopiesTheSecondElementIfThereAreMoreThanTwo() {
        assertEquals(
                listOf(1, 2, 3, 2),
                forthEvaluator.evaluateProgram(listOf("1 2 3 over")))
    }

    @Ignore
    @Test
    fun testErrorIfOveringAttemptedWithNothingOnTheStack() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Overing requires that the stack contain at least 2 values")

        forthEvaluator.evaluateProgram(listOf("over"))
    }

    @Ignore
    @Test
    fun testErrorIfOveringAttemptedWithOneNumberOnTheStack() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Overing requires that the stack contain at least 2 values")

        forthEvaluator.evaluateProgram(listOf("1 over"))
    }

    @Ignore
    @Test
    fun testUserDefinedOperatorsCanConsistOfBuiltInOperators() {
        assertEquals(
                listOf(1, 1, 1),
                forthEvaluator.evaluateProgram(listOf(": dup-twice dup dup ;", "1 dup-twice")))
    }

    @Ignore
    @Test
    fun testUserDefinedOperatorsAreEvaluatedInTheCorrectOrder() {
        assertEquals(
                listOf(1, 2, 3),
                forthEvaluator.evaluateProgram(listOf(": countup 1 2 3 ;", "countup")))
    }

    @Ignore
    @Test
    fun testCanRedefineAUserDefinedOperator() {
        assertEquals(
                listOf(1, 1, 1),
                forthEvaluator.evaluateProgram(listOf(": foo dup ;", ": foo dup dup ;", "1 foo")))
    }

    @Ignore
    @Test
    fun testCanOverrideBuiltInWordOperators() {
        assertEquals(
                listOf(1, 1),
                forthEvaluator.evaluateProgram(listOf(": swap dup ;", "1 swap")))
    }

    @Ignore
    @Test
    fun testCanOverrideBuiltInArithmeticOperators() {
        assertEquals(
                listOf(12),
                forthEvaluator.evaluateProgram(listOf(": + * ;", "3 4 +")))

    }

    @Ignore
    @Test
    fun testCanUseDifferentWordsWithSameName(){
        assertEquals(
                listOf(5,6),
                forthEvaluator.evaluateProgram(listOf(": foo 5;", ": bar foo ;", ": foo 6 ;", "bar foo")))
    }

    @Ignore
    @Test
    fun testCanDefineWordThatUsesWordWithTheSameName(){
        assertEquals(
                listOf(11),
                forthEvaluator.evaluateProgram(listOf(": foo 10;", ": foo foo 1 + ;", "foo")))
    }

    @Ignore
    @Test
    fun testCannotRedefineNumbers() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("Cannot redefine numbers")

        forthEvaluator.evaluateProgram(listOf(": 1 2 ;"))
    }

    @Ignore
    @Test
    fun testErrorIfEvaluatingAnUndefinedOperator() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("No definition available for operator \"foo\"")

        forthEvaluator.evaluateProgram(listOf("foo"))
    }

    @Ignore
    @Test
    fun testCaseInsensitivity(){
        assertEquals(
                listOf(1,1,1,1),
                forthEvaluator.evaluateProgram(listOf("1 DUP Dup dup")))
    }

    @Ignore
    @Test
    fun testDropIsCaseInsensitive(){
        assertEquals(
                listOf(1),
                forthEvaluator.evaluateProgram(listOf("1 2 3 4 DROP Drop drop")))
    }

    @Ignore
    @Test
    fun testSwapIsCaseInsensitive(){
        assertEquals(
                listOf(2,3,4,1),
                forthEvaluator.evaluateProgram(listOf("1 2 SWAP 3 Swap 4 swap")))
    }

    @Ignore
    @Test
    fun testOverIsCaseInsensitive(){
        assertEquals(
                listOf(1,2,1,2,1),
                forthEvaluator.evaluateProgram(listOf("1 2 OVER Over over")))
    }

    @Ignore
    @Test
    fun testUserDefinedWordsAreCaseInsensitive(){
        assertEquals(
                listOf(1,1,1,1),
                forthEvaluator.evaluateProgram(listOf(": foo dup;", "1 FOO Foo foo")))
    }

    @Ignore
    @Test
    fun testDefinitionsAreCaseInsensitive(){
        assertEquals(
                listOf(1,1,1,1),
                forthEvaluator.evaluateProgram(listOf(": SWAP DUP Dup dup ;", "1 swap")))
    }
}
import java.util.Stack

class ForthEvaluator {
  private val stack = Stack<Int>()
  private val wordDefinitions = mutableMapOf<String, String>()
  private val definitionPattern = Regex(": (\\S+) ([^;]+);")

  fun evaluateProgram(statements: List<String>): List<Int> {
    for (rawStatement in statements) {
      val statement = rawStatement.toLowerCase()
      val result = definitionPattern.matchEntire(statement)

      if (result == null) {
        canonicalizeStatement(statement).split(" ").forEach(this::evaluateToken)
      } else {
        val word = result.groupValues[1]
        val definition = canonicalizeStatement(result.groupValues[2].trim())
        require(word.toIntOrNull() == null) { "Cannot redefine numbers" }

        wordDefinitions.put(word, definition)
      }
    }

    return stack.toList()
  }

  private fun canonicalizeStatement(statement: String) = wordDefinitions
      .entries
      .fold(statement) { acc, (word, definition) -> acc.replace(word, definition) }

  private fun evaluateToken(token: String) {
    when (token) {
      "+" -> processTwoArguments("Addition") { a, b -> stack.push(a + b) }
      "*" -> processTwoArguments("Multiplication") { a, b -> stack.push(a * b) }
      "-" -> processTwoArguments("Subtraction") { a, b -> stack.push(b - a) }

      "/" -> processTwoArguments("Division") { a, b ->
        require(a != 0) { "Division by 0 is not allowed" }
        stack.push(b / a)
      }

      "dup" -> processOneArgument("Duplicating") { a ->
        stack.push(a)
        stack.push(a)
      }

      "drop" -> processOneArgument("Dropping") { _ -> }

      "swap" -> processTwoArguments("Swapping") { a, b ->
        stack.push(a)
        stack.push(b)
      }

      "over" -> processTwoArguments("Overing") { a, b ->
        stack.push(b)
        stack.push(a)
        stack.push(b)
      }

      else -> {
        val number = token.toIntOrNull()
        require(number != null) { "No definition available for operator \"$token\"" }
        stack.push(number)
      }
    }
  }

  private fun processTwoArguments(name: String, operation: (Int, Int) -> Unit) {
    require(stack.size > 1) { "$name requires that the stack contain at least 2 values" }
    operation(stack.pop(), stack.pop())
  }

  private fun processOneArgument(name: String, operation: (Int) -> Unit) {
    require(!stack.empty()) { "$name requires that the stack contain at least 1 value" }
    operation(stack.pop())
  }
}

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