[swift-evolution] protocol-oriented integers (take 2)

[Brent Royal-Gordon]

> On Jan 13, 2017, at 12:47 PM, Max Moiseev via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Protocol-oriented integers (take 2)
> 
> 	• Proposal: SE-NNNN
> 	• Authors: Dave Abrahams, Maxim Moiseev
> 	• Review Manager: TBD
> 	• Status: Awaiting review
> 	• Bug: SR-3196
> 	• Previous Proposal: SE-0104

Definitely liking what I'm seeing here.

> public protocol Arithmetic : Equatable, ExpressibleByIntegerLiteral 
> {

Is there a reason `Arithmetic` is not `Hashable`? (I think `Comparable` isn't here because complex numbers can't be compared, but correct me if I'm wrong about that.)

> /// A type that can represent the absolute value of any possible value of the
>   /// conforming type.
>   associatedtype Magnitude : Equatable, ExpressibleByIntegerLiteral

Is there a reason not to have this be `Arithmetic`? Maybe just the circularity problem?

> /// Returns the n-th word, counting from the least significant to most
>   /// significant, of this value's binary representation.
>   ///
>   /// The `word(at:)` method returns negative values in two's complement
>   /// representation, regardless of a type's underlying implementation. If `n`
>   /// is greater than the number of words in this value's current
>   /// representation, the result is `0` for positive numbers and `~0` for
>   /// negative numbers.
>   ///
>   /// - Parameter n: The word to return, counting from the least significant to
>   ///   most significant. `n` must be greater than or equal to zero.
>   /// - Returns: An word-sized, unsigned integer with the bit pattern of the
>   ///   n-th word of this value.
>   func word(at n: Int) -> UInt

How does the client know how many words there are? Are they supposed to calculate that from `bitWidth`?

Oh, I see:

> good catch; countRepresentedWords is in the prototype
> (https://github.com/apple/swift/blob/new-integer-protocols/stdlib/public/core/Integers.swift.gyb#L1521),
> and it should be in the proposal.

That looks fine to me.

> /// The number of bits in the current binary representation of this value.
>   ///
>   /// This property is a constant for instances of fixed-width integer
>   /// types.
>   var bitWidth : Int { get }

So, um, I'm a little confused about this one. Is this the physical number of bits in the value, or is it the number of bits you need to get from `word(at:)` in order to get all bits representing the value?

For instance, when you call `UInt32.bitWidth`, does it return `32`, the physical number of bits in the value, or `33`, the number of bits including the (always zero) sign bit?

>   static func doubleWidthMultiply(_ lhs: Self, _ rhs: Self) -> (high: Self, low: Magnitude)
>   static func doubleWidthDivide(_ lhs: (high: Self, low: Magnitude), _ rhs: Self) -> (quotient: Self, remainder: Self)

Could these take/return a single `DoubleWidth<Self>` value instead of two separate `Self` and `Magnitude` values? Or would that present circularity problems?

> /// The number of bits equal to 1 in this value's binary representation.
>   ///
>   /// For example, in a fixed-width integer type with a `bitWidth` value of 8,
>   /// the number 31 has five bits equal to 1.
>   ///
>   ///     let x: Int8 = 0b0001_1111
>   ///     // x == 31
>   ///     // x.popcount == 5
>   var popcount: Int { get }

I'm not super-fond of this name; I assume it's a term of art, but it's a pretty obscure one. Maybe `numberOfOnes`? `onesWithin`?

> DoubleWidth
> 
> The DoubleWidth<T> type allows to create wider fixed-width integer types from the ones available in the standard library.

I'm glad you're planning to include `DoubleWidth` this time.

> 	• Deprecation of the BitwiseOperations protocol. We find it hard to imagine a type that conforms to this protocol, but is not a binary integer type.

While I'm sure any such type *could* be a binary integer type, I'm not convinced they necessary *should* be. For instance, suppose I have a "bit vector"; I know I never want to perform arithmetic on it, but I *do* want to manipulate bits separately, so I make it look like a `RandomAccessCollection` of `Bool`s. It might make a great deal of sense to support bitwise operations on this type, even though I don't want to clutter it up with arithmetic.

-- 
Brent Royal-Gordon
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