Fuse mapWithIndex into a single lazy view, avoiding tuple allocation - #86
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mapWithIndex was implemented as
new View.Map(new View.ZipWithIndex(this), (pair: (A, Int)) => f(pair._1, pair._2)),
which builds a real (A, Int) tuple per element via ZipWithIndex only
to have View.Map immediately unpack and discard it one step later. It
also allocates two View objects and, once traversed, two chained
Iterator objects instead of one.
Replace it with a single anonymous AbstractView[B] whose iterator
threads the index as a plain mutable field instead of pairing it with
each element.
`self` is captured explicitly since an unqualified `this` inside the
anonymous AbstractView body would refer to the view itself. The
inner iterator's return type is annotated Iterator[B]^{self, f} so
the capture checker accepts that it legitimately holds onto self and
f, matching how the outer method is already annotated CC[B]^{this, f}.
Preserves the original's laziness: no element of `this` is evaluated
until the resulting view is actually traversed.
No behavior changes; result values, traversal order, and laziness are
identical to the previous implementation.
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mapWithIndex composed two existing view combinators:
new View.Map(new View.ZipWithIndex(this), (pair: (A, Int)) => f(pair._1, pair._2))
ZipWithIndex's iterator constructs a real (A, Int) tuple per element;
View.Map's function then immediately destructures that tuple and
discards it. This also means two View instances and, once traversed,
two chained Iterator instances wrap
this, where one fused instancewould do.
This PR replaces that with a single anonymous AbstractView[B] that
threads the index as a mutable field on its own iterator rather than
pairing it with each element.
selfis captured explicitly, since an unqualifiedthisinside theanonymous AbstractView body refers to the view itself rather than the
enclosing collection. The inner iterator method is annotated
Iterator[B]^{self, f} so the capture checker accepts that it holds
onto self and f, mirroring the outer method's own CC[B]^{this, f}
annotation.
Laziness is preserved: nothing in
thisis evaluated until theresulting view is traversed, matching existing tests asserting zero
element evaluations immediately after calling mapWithIndex.
No behavior changes: same result values, same traversal order, same
laziness guarantees as before.