DynamicArray.subArray creates array with length bound+1

[DarinM223]

Description

Running this in the REPL:

val arr = DynamicArray.array (10, 0)
val () = DynamicArray.update (arr, 0, 1)
val () = DynamicArray.update (arr, 1, 1)
val () = DynamicArray.update (arr, 2, 1)
val subarr = DynamicArray.toList (DynamicArray.subArray (arr, 0, 2))

raises a Subscript exception, even though the values in the range [0, 2] are set.
Similarly, running this in the REPL:

val arr = DynamicArray.array (10, 0)
val () = DynamicArray.update (arr, 0, 1)
val () = DynamicArray.update (arr, 1, 1)
val () = DynamicArray.update (arr, 2, 1)
val () = DynamicArray.truncate (arr, 2)

will cause arr to be in an invalid state, so running DynamicArray.toList arr or DynamicArray.toVector arr will raise a Subscript exception.

This is because subArray creates an array with the length of the new array bound, even though the comment in the file says that the array should have at least bound+1 elements.

I also modified truncate to set the bound reference even when the array size <= 3 * sz. With this change, truncate can be used to consistently lower the bound of the dynamic array.

Related Issue

Motivation and Context

I want to use DynamicArray.truncate to lower the bound, and I want to use DynamicArray.bound + 1 as the length of all the set elements in the array. Right now truncate often results in an invalid state, and it only lowers the bound in certain cases, which means that the elements from [0, bound] may include zeroed out values.

How Has This Been Tested?

I loaded the updated smlnj-lib.cm file into the REPL and ran this:

> val arr = DynamicArray.array (10, 0);
val arr = - : int DynamicArray.array
> DynamicArray.update (arr, 0, 1);
val it = (): unit
> DynamicArray.update (arr, 1, 1);
val it = (): unit
> DynamicArray.update (arr, 2, 1);
val it = (): unit
> DynamicArray.toList (DynamicArray.subArray (arr, 0, 2));
val it = [1, 1, 1]: int list

> val arr = DynamicArray.array (10, 0);
val arr = - : int DynamicArray.array
> DynamicArray.update (arr, 0, 1);
val it = (): unit
> DynamicArray.update (arr, 1, 1);
val it = (): unit
> DynamicArray.update (arr, 2, 1);
val it = (): unit
> DynamicArray.truncate (arr, 2);
val it = (): unit
> DynamicArray.truncate (arr, 1);
val it = (): unit
> DynamicArray.truncate (arr, 0);
val it = (): unit

- val arr = DynamicArray.array (3, 0);
val arr = - : int DynamicArray.array
- DynamicArray.update (arr, 0, 1);
val it = () : unit
- DynamicArray.update (arr, 1, 1);
val it = () : unit
- DynamicArray.update (arr, 2, 1);
val it = () : unit
- DynamicArray.bound arr;
val it = 2 : int
- DynamicArray.truncate (arr, 2);
val it = () : unit
- DynamicArray.bound arr;
val it = 1 : int

[JohnReppy]

These changes look good to me.