problem18a.js

const fs = require('fs');

const data = fs.readFileSync('input18.txt', 'utf8');
// const data = `
// 2 * 3 + (4 * 5)
// 5 + (8 * 3 + 9 + 3 * 4 * 3)
// 5 * 9 * (7 * 3 * 3 + 9 * 3 + (8 + 6 * 4))
// ((2 + 4 * 9) * (6 + 9 * 8 + 6) + 6) + 2 + 4 * 2
// `;

const lines = data.split('\n').map(line => line.trim()).filter(line => line.length);

// problem -> expr
// expr -> term (operator term)*
// term -> \d+
// term -> ( expr )

let sum = 0;
for (const line of lines) {
  let tokens = line.split(/([\s()+\-*/])/).filter(token => token.replace(/\s+/, '').length).map(token => {
    if (token.match(/^\d+$/)) {
      return parseInt(token);
    } else {
      return token;
    }
  });
  const ast = parseExpr();
  // console.log(JSON.stringify(ast, null, '  '));
  sum += evalAst(ast);
  function parseExpr() {
    const termOps = [];
    const term = parseTerm();
    while (true) {
      const operator = accept('+', '-', '*', '/');
      if (!operator) {
        break;
      }
      const term = parseTerm();
      termOps.push({
        operator,
        term
      });
    }
    return {
      type: 'expr',
      initial: term,
      termOps
    };
  }
  function parseTerm() {
    if (accept('(')) {
      const inner = parseExpr();
      expect(')');
      return inner;
    }
    const value = expect('number');
    return {
      type: 'number',
      value: value.number
    };
  }
  function accept(...options) {
    const token = tokens[0];
    if (!token) {
      return false;
    }
    for (const option of options) {
      if (token === option) {
        consume();
        return token;
      }
      if ((option === 'number') && ((typeof token) === option)) {
        consume();
        return { number: token };
      }
    }
    return false;
  }
  function expect(option) {
    const result = accept(option);
    if (!result) {
      throw `Expected ${option}`;
    }
    return result;
  }
  function consume() {
    tokens.shift();
  }
}

console.log(sum);

function evalAst(ast) {
  if (ast.type === 'expr') {
    let value = evalAst(ast.initial);
    for (const termOp of ast.termOps) {
      if (termOp.operator === '*') {
        value *= evalAst(termOp.term);
      } else if (termOp.operator === '+') {
        value += evalAst(termOp.term);
      } else if (termOp.operator === '-') {
        value -= evalAst(termOp.term);
      } else if (termOp.operator === '/') {
        value /= evalAst(termOp.term);
      }
    }
    return value;
  } else if (ast.type === 'number') {
    return ast.value;
  }
}