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Discussion on: Daily Challenge #10 - Calculator

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Kevin Sullivan
interface Number {
  type: "NUMBER" | string;
  value: number;
}

interface Operation {
  type: "OPERATION";
  value: "*" | "/" | "+" | "-";
}

const OPERATIONS = {
  "*": multiply,
  "/": divide,
  "+": add,
  "-": subtract
} as const;

const parse = (input: string): (Operation | Number)[] =>
  input
    .split(" ")
    .map(
      (input: string): Number | Operation =>
        input === "*"
          ? { type: "OPERATION", value: "*" }
          : input === "/"
          ? { type: "OPERATION", value: "/" }
          : input === "+"
          ? { type: "OPERATION", value: "+" }
          : input === "-"
          ? { type: "OPERATION", value: "-" }
          : { type: "NUMBER", value: Number(input) }
    );

export const combine = ({ value }: Operation, a: Number, b: Number) => ({
  type: "NUMBER",
  value: OPERATIONS[value](a.value, b.value)
});

export const doOperations = (operations: ("*" | "/" | "+" | "-")[]) => (
  input: (Operation | Number)[]
) => {
  let result = input;

  const firstOperationIndex = (list: (Operation | Number)[]) =>
    list.findIndex(
      ({ type, value }) =>
        type === "OPERATION" && operations.includes(value as any)
    );

  const operationsRemaining = (result: (Operation | Number)[]) =>
    ~firstOperationIndex(result);

  while (operationsRemaining(result)) {
    const operationIndex = firstOperationIndex(result);
    const operation = result[operationIndex] as Operation;

    const firstNumberIndex = operationIndex - 1;
    const firstNumber = result[firstNumberIndex] as Number;

    const secondNumberIndex = operationIndex + 1;
    const secondNumber = result[secondNumberIndex] as Number;

    result = result.reduce(
      (result: (Operation | Number)[], current: Operation | Number, index) => {
        return index === operationIndex
          ? [...result, combine(operation, firstNumber, secondNumber)]
          : [firstNumberIndex, secondNumberIndex].includes(index)
          ? result
          : [...result, current];
      },
      []
    );
  }

  return result;
};

export const calculate = pipe(
  parse,
  doOperations(["*", "/"]),
  doOperations(["+", "-"]),
  map(prop("value")),
  head
);