[Proposal] Add methods ShiftLeft(positions), ShiftRight(positions) to System.Range struct

[ichensky]

In algorithms using a sliding window, it's very useful to have methods that shift the window. The proposal is to add two methods to the System.Range struct that will return a new, shifted Range:

struct Range
{ 
   ...

    public static Range ShiftRight(int positions){
        return (this.Start.Value + positions)..(this.End.Value + positions);
    }

    public static Range ShiftLeft(int positions){
        return (this.Start.Value - positions)..(this.End.Value - positions);
    }
}

[huoyaoyuan]

Range and Index can represent index either from start or end. Typically in sliding window algorithms, only index from start will be used. Range is not the correct type to use here.

Moreover, Range serves more as a temporary query parameter, not a persisted local variable. Using two separated integers or a tuple (int start, int end) is usually better.

---

The internal representation of Range and Index can't represent out-of-bound position, means such shifting method must always perform a overflow checking, instead of delaying the check when accessing array or collection.

[tannergooding]

The internal representation of Range and Index can't represent out-of-bound position, means such shifting method must always perform a overflow checking, instead of delaying the check when accessing array or collection.

This depends on what you mean by out of bounds.

Index can represent any location [0, int.MaxValue] from the start or end, it isn't tied to a specific collection. Range likewise just uses two Index representing "start" and "end" positions. We don't really do anything outside very basic "validation" until the index/range is "used" with a collection type. It's the Index.GetOffset and Range.GetOffsetAndLength that do that since the "validity" is dependent on the collection.Length that its being used with.

it's very useful to have methods that shift the window

Shift is likely the wrong term and is liable to cause confusion. Users may expect this does (x << y) instead

What you're asking for is essentially a simpler way to do Index +/- Index and Range +/- Index. The current approach requires:

return new Index(previousIndex.Value + change, previousIndex.IsFromEnd);

Now whether or not this is worthwhile to add to the core libraries is a different question and likely warrants more weigh-in. However, if we were to add it I'd think we'd just want to use operator + and operator -.

[ichensky]

Shift is likely the wrong term and is liable to cause confusion. Users may expect this does (x << y) instead

Agree, better names should be used for the methods or overload + - operators.

use operator + and operator -

With +- operators, the syntax for the Range will look like:

var input = 0..4;
var target = input + 1; // 1..5
target = target - 1 // 0..4 

target += 1; // 1..5
target -= 1; // 0..4

input++; // 1..5
input--; // 0..4
++input; // 1..5
--input; // 0..4

The syntax is easy to understand and remember.