Introduce Iterator.skip-one

A method that can be overriden if you're writing an iterator that can
be significantly cheaper when skipping a generated value.  Use case,
the :10nth parameter with Str.match (aka, give me only the 10th match).
By default, it is just a slightly more expensive .pull-one.

Also use let Iterator.skip-at-least-pull-one use skip-one, as well
as in Rakudo::Internals.SeqSkipNFromIterator .

src/core/Iterator.pm

@@ -8,6 +8,14 @@ my role Iterator {
     # methods in this role, they'll all end up falling back to using this.
     method pull-one() { ... }
 
+    # Skip one value from the iterator.  Should return a true-like value to
+    # indicate the skip was successful.  Override this method if you can
+    # make an iterator that has significantly less to do when skipping a
+    # generated value.
+    method skip-one() {
+        nqp::not_i(nqp::eqaddr(self.pull-one,IterationEnd))
+    }
+
     # Has the iterator produce a certain number of values and push them into
     # the target. The only time the iterator may push less values than asked
     # for is when it reaches the end of the iteration. It may never push more
@@ -68,17 +76,18 @@ my role Iterator {
 
     # Skip the given number of values produced before returning the next
     # pulled value.  Given 0 it is an expensive way to do .pull-one
-    method skip-at-least-pull-one(\todrop) is raw {
+    method skip-at-least-pull-one(\toskip) is raw {
         nqp::stmts(
-          (my int $topull = todrop + 1),
-          nqp::while(
-            nqp::isge_i(($topull = nqp::sub_i($topull,1)),0),
-            nqp::if(
-              nqp::eqaddr((my $pulled := self.pull-one),IterationEnd),
-              ($topull = 0),
-            )
+          (my int $left = toskip),
+          nqp::while(   # skipping
+            nqp::isge_i(($left = nqp::sub_i($left,1)),0) && self.skip-one,
+            Nil
           ),
-          $pulled
+          nqp::if(
+            nqp::islt_i($left,0),   # could skip all?
+            self.pull-one,
+            IterationEnd
+          )
         )
     }
 

src/core/Rakudo/Internals.pm

@@ -308,34 +308,29 @@ my class Rakudo::Internals {
             method new(\i,\s) { nqp::create(self)!SET-SELF(i,s) }
             method pull-one() is raw {
                 nqp::if(
-                  nqp::islt_i($!skipping,-1),
-                  IterationEnd,                  # exhausted before
-                  nqp::stmts(                    # not exhausted yet
-                    nqp::if(
+                  $!iterator,
+                  nqp::stmts(
+                    nqp::while(
                       nqp::isgt_i($!skipping,0),
-                      nqp::until(                # still skipping
-                        nqp::iseq_i($!skipping,0),
-                        nqp::if(
-                          nqp::eqaddr($!iterator.pull-one,IterationEnd),
-                          nqp::stmts(            # exhausted now
-                            ($!skipping = -1),
-                            (return IterationEnd)
-                          ),
-                          ($!skipping = nqp::sub_i($!skipping,1)),
+                      nqp::if(
+                        $!iterator.skip-one,   
+                        ($!skipping = nqp::sub_i($!skipping,1)), # skipped ok
+                        nqp::stmts(                              # exhausted
+                          ($!iterator := Mu),
+                          (return IterationEnd)
                         )
                       )
                     ),
-                    nqp::stmts(                  # no longer skipping
-                      nqp::if(
-                        nqp::eqaddr(
-                          (my $pulled := $!iterator.pull-one),
-                          IterationEnd
-                        ),
-                        ($!skipping = -1),
+                    nqp::if(                                     # done skipping
+                      nqp::eqaddr(
+                        (my $pulled := $!iterator.pull-one),
+                        IterationEnd
                       ),
-                      $pulled
-                    )
-                  )
+                      ($!iterator := Mu)                         # exhausted
+                    ),
+                    $pulled
+                  ),
+                  IterationEnd                   # exhausted before
                 )
             }
         }.new(iterator,skipping))