How to get the whole number part of either a `Double` or `Decimal`?

import Foundation

extension Decimal {
    var wholePart: Self {
        self    // ??? what to do here?
    }
}

extension Double {
    // https://stackoverflow.com/questions/44909637/how-to-get-the-integer-part-and-fractional-part-of-a-number-in-swift
    var wholePart: Self {
          // all of these seem to work
//        modf(self).0
//        floor(self)
        self.rounded(.towardZero)
    }
}

typealias DataType = Double    // can be changed to either Decimal or Double
let someValue: DataType = 123.456
// how to get its whole part (123)?
print(someValue.wholePart)

You're looking for NSDecimalRound; unfortunately, I don't believe that it supports rounding toward zero, so you have to fake that:

extension Decimal {
  var wholePart: Self {
    var result = Decimal()
    var mutableSelf = self
    NSDecimalRound(&result, &mutableSelf, 0, self >= 0 ? .down : .up)
    return result
  }
}
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