function tokenizer(input) {
  var current = 0;
  var tokens = [];

  while (current < input.length) {
    var char = input[current];

    if (char === '(') {
      tokens.push({
        type: 'paren',
        value: '('
      });
      current++;
      continue;
    }

    if (char === ')') {
      tokens.push({
        type: 'paren',
        value: ')'
      });
      current++;
      continue;
    }

    var WHITESPACE = /\s/;
    if (WHITESPACE.test(char)) {
      current++;
      continue;
    }

    var NUMBERS = /[0-9]/;
    if (NUMBERS.test(char)) {
      var value = '';

      while (NUMBERS.test(char)) {
        value += char;
        char = input[++current];
      }

      tokens.push({
        type: 'number',
        value: value
      });

      continue;
    }

    var LETTERS = /[a-z]i/;
    if (LETTERS.test(char)) {
      var value = '';

      while (LETTERS.test(char)) {
        value += char;
        char = input[++current];
      }

      tokens.push({
        type: 'name',
        value: value
      });

      continue;
    }

    throw new TypeError('I dont know what this character is: ' + char);
  }

  return tokens;
}

function parser(tokens) {
  var current = 0;

  function walk() {
    var token = tokens[current];

    if (token.type === 'number') {
      current++;

      return {
        type: 'NumberLiteral',
        value: token.value
      };
    }

    if (
      token.type === 'paren' &&
      token.value === '('
    ) {
      token = tokens[++current];

      var node = {
        type: 'CallExpression',
        name: token.value,
        params: []
      };

      token = tokens[++current];

      while (
        (token.type !== 'paren') ||
        (token.type === 'paren' && token.value !== ')')
      ) {
        node.params.push(walk());
        token = tokens[current];
      }

      current++;

      return node;
    }

    throw new TypeError(token.type);
  }

  var ast = {
    type: 'Program',
    body: []
  };

  while (current < tokens.length) {
    ast.body.push(walk());
  }

  return ast;
}

function traverser(ast, visitor) {
  function traverseArray(array, parent) {
    array.forEach(function(child) {
      traverseNode(child, parent);
    });
  }

  function traverseNode(node, parent) {
    var method = visitor[node.type];

    if (method) {
      method(node, parent);
    }

    switch (node.type) {
      case 'Program':
        traverseArray(node.body, node);
        break;

      case 'CallExpression':
        traverseArray(node.params, node);
        break;

      case 'NumberLiteral':
        break;

      default:
        throw new TypeError(node.type);
    }
  }

  traverseNode(ast, null);
}

function transformer(ast) {
  var newAst = {
    type: 'Program',
    body: []
  };

  ast._context = newAst.body;

  traverser(ast, {
    NumberLiteral: function(node, parent) {
      parent._context.push({
        type: 'NumberLiteral',
        value: node.value
      });
    },

    CallExpression: function(node, parent) {
      var expression = {
        type: 'CallExpression',
        callee: {
          type: 'Identifier',
          name: node.name
        },
        arguments: []
      };

      node._context = expression.arguments;

      if (parent.type !== 'CallExpression') {
        expression = {
          type: 'ExpressionStatement',
          expression: expression
        };
      }

      parent._context.push(expression);
    }
  });

  return newAst;
}

function codeGenerator(node) {
  switch (node.type) {
    case 'Program':
      return node.body.map(codeGenerator)
        .join('\n');

    case 'ExpressionStatement':
      return (
        codeGenerator(node.expression) +
        ';'
      );

    case 'CallExpression':
      return (
        codeGenerator(node.callee) +
        '(' +
        node.arguments.map(codeGenerator)
          .join(', ') +
        ')'
      );

    case 'Identifier':
      return node.name;

    case 'NumberLiteral':
      return node.value;

    default:
      throw new TypeError(node.type);
  }
}

function compiler(input) {
  var tokens = tokenizer(input);
  var ast    = parser(tokens);
  var newAst = transformer(ast);
  var output = codeGenerator(newAst);

  return output;
}

module.exports = {
  tokenizer: tokenizer,
  parser: parser,
  transformer: transformer,
  codeGenerator: codeGenerator,
  compiler: compiler
};