predicate-batch-iterator.raku

use v6.*;

class PredicateBatchIterator does Iterator {
    class UnPrimed {}

    has Iterator $.input is required;
    has &.predicate is required;
    has $.stashed is rw = UnPrimed;

    submethod TWEAK { }
    method pull-one {
        my @a;
        my $val;
        my $prevval = UnPrimed;
            
        return IterationEnd if $.stashed.WHICH === IterationEnd.WHICH;

        if $.stashed !=== UnPrimed { 
            @a.push($.stashed);
            $.stashed = UnPrimed;
        }

        loop  {
            $val := $.input.pull-one;

            if $val =:= IterationEnd {
                $.stashed = IterationEnd;
                last;
            }

            if $prevval === UnPrimed {
                @a.push: $val;
                $prevval = $val;
                next;
            }

            if &.predicate.($val) eqv &.predicate.($prevval) {
                @a.push: $val;
                $prevval = $val;
            } else {
                $.stashed = $val;
                last;
            }
        }

        @a
    }
}

multi sub batch($_, &predicate) {
    Seq.new: PredicateBatchIterator.new: input => .iterator, predicate => &predicate;
}
.say for [0..1000].&batch(*.chars);