[External] : Re: RFR: 8263087: Add a MethodHandle combinator that switches over a set of MethodHandles

[John Rose]

On Apr 9, 2021, at 4:00 PM, John Rose <john.r.rose at oracle.com<mailto:john.r.rose at oracle.com>> wrote:

The MH combinator for lookupswitch can use a data-driven
reverse lookup in a (frozen/stable) int[] array, using binary
search.  The bytecode emitter can render such a thing as
an internal lookupswitch, if that seems desirable.  But
the stable array with binary search scales to other types
besides int, so it’s the right primitive.

This may be confusing on a couple of points.
First, the mapping function I’m talking about is not
a MH combinator, but rather a MH factory, which takes
a non-MH argument, probably an unsorted array or List
of items of any type.  It then DTRT (internal hash map
or tree map or flat binary search or flat table lookup or
lookupswitch or any combination thereof) to get
an algorithm to classify the array or List elements
into a compact enumeration [0,N).

Second, when the input array or List is of int (or
Integer) then it *might* be a lookupswitch internally,
and I’m abusing the terminology to call this particular
case a lookupswitch.  But it’s really just a classifier
factory, whose output is a MH of type K → [0,N) for
some K.  The output might also be ToIntFunction<K>
for all I care; that can be inter-converted with a MH.