class Expression {
  private static readonly INVALID_REQUEST: string = "400: Bad request";

  value: string;

  l: Expression | null = null;
  r: Expression | null = null;

  private splitBy(operation: string): boolean {
    let splitOperands = this.value.split(operation);
    if (splitOperands.length < 2) {
      return false;
    }

    this.value = operation;
    this.r = new Expression(splitOperands.pop());
    this.l = new Expression(splitOperands.join(operation));
    return true;
  }

  constructor(expr: string) {
    this.value = expr;

    for (let operation of "+-*$") {
      if (this.splitBy(operation)) {
        return;
      }
    }
  }

  public evaluate(): (number | string) {
    if (this.l === null && this.r === null) {
      // process constants
      if (![...this.value].every(c => "0123456789.".includes(c))) {
        return Expression.INVALID_REQUEST;
      }

      const val = Number.parseFloat(this.value);
      return val;
    }

    if (this.l === null || this.r === null || !("+-$*".includes(this.value))) {
      return Expression.INVALID_REQUEST;
    }

    const left = this.l.evaluate();
    const right = this.r.evaluate();

    if (typeof left !== "number" || typeof right !== "number") {
      return Expression.INVALID_REQUEST;
    }

    let operation = null;
    switch (this.value) {
      case "+":
        return left + right;
      case "-":
        return left - right;
      case "$":
        return left / right;
      case "*":
        return left * right;
    }
  }
}

export const calculate = (sum: string): (number | string) => {
  return new Expression(sum).evaluate();
}