const Decimal = require('decimal.js');

const
  WS = Symbol('ws'),
  NUMBER = Symbol('num'),
  PLUS = Symbol('plus'),
  MINUS = Symbol('minus'),
  MULTIPLY = Symbol('mul'),
  DIVIDE = Symbol('div'),
  MOD = Symbol('mod'),
  LEFT_PAR = Symbol('lpar'),
  RIGHT_PAR = Symbol('rpar'),
  IDENTIFIER = Symbol('id'),
  EQUAL = Symbol('eq'),
  SEMICOLON = Symbol('semicolon'),
  UNKNOWN = Symbol('unknown'),
  END_OF_FILE = Symbol('eof');

function* tokenize(input, sym = {}) {
  const matchers = [
    { type: NUMBER, re: /^[0-9\.]+/ },
    { type: PLUS, re: /^\+/ },
    { type: MINUS, re: /^[-−]/ },
    { type: MULTIPLY, re: /^[*×⋅·]/ },
    { type: DIVIDE, re: /^\// },
    { type: MOD, re: /^%/ },
    { type: WS, re: /^[\s]+/ },
    { type: LEFT_PAR, re: /^\(/ },
    { type: RIGHT_PAR, re: /^\)/ },
    { type: IDENTIFIER, re: /^[_a-zA-Z][_a-zA-Z0-9]*/ },
    { type: EQUAL, re: /^=/ },
    { type: SEMICOLON, re: /^;/ },
    { type: END_OF_FILE, re: /^$/ }
  ];
  let pos = 0;
  let unknownFrom = -1;
  while (input) {
    const posBefore = pos;
    for (const { type, re } of matchers) {
      let match = re.exec(input.slice(pos))?.[0];
      if (typeof(match) === 'string') {
        if (unknownFrom >= 0) {
          // flush the unknown input as one token
          yield {
            type: UNKNOWN,
            pos: unknownFrom,
            match: input.slice(unknownFrom, pos),
          };
          unknownFrom = -1;
        }
        if (type === IDENTIFIER) {
          for (const name in sym) {
            if (match.startsWith(name)) match = name;
          }
        }
        yield { type, pos, match };
        if (type === END_OF_FILE) input = null; // breaks outer loop
        pos += match.length;
        break;
      }
    }
    if (input && posBefore === pos) {
      // nothing matched, track unknown input
      if (unknownFrom < 0) unknownFrom = pos;
      pos++;
    }
  }
}

class ExpressionCalc {
  #symbols = {
    pi: new Decimal(Math.PI),
    e: new Decimal(Math.E),
    sin: d => d.sin(),
    cos: d => d.cos(),
  };
  #stream;
  #token;

  constructor(input) {
    this.#stream = tokenize(input, this.#symbols);
    this.#next();
  }

  calc() {
    let result;
    do {
      result = this.#expression();
    } while (this.#accept(SEMICOLON));
    this.#expect(END_OF_FILE);
    return result;
  }

  #error(err) {
    throw new Error(`${err}, pos=${this.#token?.pos}, token=${this.#token.match}, type=${this.#token.type.description}`);
  }

  #next() {
    do {
      this.#token = this.#stream.next()?.value;
    } while (this.#token?.type === WS);
  }

  #check(...types) {
    return types.includes(this.#token?.type);
  }

  #accept(type) {
    let match;
    if (this.#check(type)) {
      match = this.#token.match;
      this.#next();
      return match || true;
    }
    return false;
  }

  #expect(type) {
    return this.#accept(type) || this.#error(`expected ${type.description}`);
  }

  #factor() {
    let result;

    let text;
    if (this.#accept(LEFT_PAR)) { // parenthesis (but not function calls)
      result = this.#expression();
      this.#expect(RIGHT_PAR);
    } else if (text = this.#accept(NUMBER)) { // numeric literals
      result = new Decimal(text);
    } else if (this.#accept(PLUS)) { // unary plus
      result = this.#factor();
    } else if (this.#accept(MINUS)) { // unary minus
      result = this.#factor().mul(new Decimal(-1));
    } else if (text = this.#accept(IDENTIFIER)) { // identifiers (named constants and function calls)
      text = text.toLowerCase();
      if (this.#accept(EQUAL)) {
        this.#symbols[text] = this.#expression();
      }
      const sym = this.#symbols[text];
      if (sym instanceof Decimal) result = sym;
      else if (typeof sym === 'function') {
        this.#expect(LEFT_PAR);
        result = sym(this.#expression());
        this.#expect(RIGHT_PAR);
      }
      else this.#error(`unknown id ${text}`);
    } else {
      this.#error('unexpected input');
    }

    return result
  }

  #term() {
    let result = this.#factor();

    while (true) {
      if (this.#accept(MULTIPLY) || this.#check(NUMBER, IDENTIFIER, LEFT_PAR)) result = result.mul(this.#factor());
      else if (this.#accept(DIVIDE)) result = result.div(this.#factor());
      else if (this.#accept(MOD)) result = result.mod(this.#factor());
      else break;
    }

    return result;
  }

  #expression() {
    let result = this.#term();

    while (true) {
      if (this.#accept(PLUS)) result = result.add(this.#term());
      else if (this.#accept(MINUS)) result = result.sub(this.#term());
      else break;
    }

    return result;
  }
}

console.log(new ExpressionCalc('2+2').calc().eq(new Decimal('4')));
console.log(new ExpressionCalc('2+3-4').calc().eq(new Decimal('1')));
console.log(new ExpressionCalc('2+2*2').calc().eq(new Decimal('6')));
console.log(new ExpressionCalc('3+4*5').calc().eq(new Decimal('23')));
console.log(new ExpressionCalc('3/2+4*5').calc().eq(new Decimal('21.5')));
console.log(new ExpressionCalc('(2+2)*2').calc().eq(new Decimal('8')));
console.log(new ExpressionCalc('(02. + 0002.) × 002.000').calc().eq(new Decimal('8')));
console.log(new ExpressionCalc('3%2').calc().eq(new Decimal('1')));
console.log(new ExpressionCalc('+1').calc().eq(new Decimal('1')));
console.log(new ExpressionCalc('-(2+3)').calc().eq(new Decimal('-5')));
console.log(new ExpressionCalc('cos(2*pi)').calc().eq(new Decimal('1')));
console.log(new ExpressionCalc('-2.1+ .355 / (cos(pi % 3) + sin(0.311))').calc().eq(new Decimal('-1.8260809473359577169')));
console.log(new ExpressionCalc('2pi').calc().eq(new Decimal(Math.PI * 2)));
console.log(new ExpressionCalc('3(5cos(1)2)').calc().eq(new Decimal('16.209069176044191522')));
console.log(new ExpressionCalc('2 2').calc().eq(new Decimal('4')));
console.log(new ExpressionCalc('(2 + 3) 2').calc().eq(new Decimal('10')));
console.log(new ExpressionCalc('(2)(2)3(4)\t3').calc().eq(new Decimal('144')));
console.log(new ExpressionCalc('a=7').calc().eq(new Decimal('7')))
console.log(new ExpressionCalc('a=b=c=7').calc().eq(new Decimal('7')))
console.log(new ExpressionCalc('2*((a=1)+(b=1))').calc().eq(new Decimal('4')));
console.log(new ExpressionCalc('a=1;b=2').calc().eq(new Decimal('2')));
console.log(new ExpressionCalc('a=1; b=2; x=pi/3; a*b*cos(x)').calc().eq(new Decimal('1.0000000000000001376')));
console.log(new ExpressionCalc('a=1; b=2; x=pi/3; abcos(x)').calc().eq(new Decimal('1.0000000000000001376')));