SimpleLisp.java

package com.simplelisp;


import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

public class SimpleLisp {

  //(+ 3 (+ 3 2) (+ 4 5))
  // (+
  private static HashMap<String, Integer> assignmentMap = new HashMap<>();

  public static void main(String[] args) {
    String inputA = "(+ 3 (* 2 3) (+ 2 4))"; //(function arg1 arg2); this is one expression
    System.out.println(inputA + " -> " + eval(inputA));

    String inputB = "(let x 5)";
    System.out.println(inputB + " -> " + eval(inputB));

    String inputC = "(+ x 5)";
    System.out.println(inputC + " -> "+eval(inputC));
  }

  private static String eval(String expression) {
    /** we are going to parse the function type out */
    StringBuilder sb = new StringBuilder("");
    int pointer = 0;
    while (expression.charAt(pointer) != ' ') {
      if (expression.charAt(pointer) != '(') {
        sb.append(expression.charAt(pointer));
      }
      pointer++;
    }
    String function = sb.toString();
    sb.delete(0, sb.length());
    pointer++; //we are now past the space and on the first expression/value

    /** we are going to parse our list of expressions */
    List<String> arguments = new ArrayList<>();
    while(pointer <= (expression.length() - 1)) {

      if(expression.charAt(pointer) != '(') {
        while(pointer <= (expression.length() - 1) && expression.charAt(pointer) != ' '
                && expression.charAt(pointer) != ')') {
          sb.append(expression.charAt(pointer));
          pointer++;
        }
        String argument = sb.toString();
        if(assignmentMap.containsKey(argument)) {
          arguments.add(assignmentMap.get(argument).toString());
        } else {
          arguments.add(argument);
        }
        sb.delete(0, sb.length());
      } else {
        int parens = 1;
        sb.append('(');
        pointer++;
        while(parens != 0) {
          if(expression.charAt(pointer) == '(') parens++;
          if(expression.charAt(pointer) == ')') parens--;
          sb.append(expression.charAt(pointer));
          pointer++;
        }
        arguments.add(eval(sb.toString()));
        sb.delete(0, sb.length());
      }

      pointer++;
    }

    /** operate the function over the list of expressions */
    switch(function) {
      case "+": {
        Integer returnVal = 0;
        for(String arg : arguments) {
          returnVal += Integer.parseInt(arg);
        }
        return returnVal.toString();
      }
      case "-": {
        Integer returnVal = 0;
        for(String arg : arguments) {
          returnVal -= Integer.parseInt(arg);
        }
        return returnVal.toString();
      }
      case "*": {
        Integer returnVal = 1;
        for(String arg : arguments) {
          returnVal *= Integer.parseInt(arg);
        }
        return returnVal.toString();
      }
      case "let": {
        Integer val = Integer.parseInt(arguments.get(1));
        assignmentMap.put(arguments.get(0), val);
        return arguments.get(0)+"="+val;
      }
    }
    return "";
  }

}