sequences.extras: adding even and odd indices virtual sequences.

extra/sequences/extras/extras-tests.factor

@@ -110,3 +110,11 @@ IN: sequences.extras.tests
 { f } [ { } [ even? ] one? ] unit-test
 
 { { { 5 8 0 } { 6 9 1 } { 7 10 2 } } } [ { 5 6 7 } { 8 9 10 } [ 3array ] 2map-index ] unit-test
+
+{ { } } [ { } <evens> >array ] unit-test
+{ { 0 2 } } [ 4 iota <evens> >array ] unit-test
+{ { 0 2 4 } } [ 5 iota <evens> >array ] unit-test
+
+{ { } } [ { } <odds> >array ] unit-test
+{ { 1 3 } } [ 5 iota <odds> >array ] unit-test
+{ { 1 3 5 } } [ 6 iota <odds> >array ] unit-test

extra/sequences/extras/extras.factor

@@ -1,6 +1,6 @@
-USING: arrays grouping kernel locals math math.order math.ranges
+USING: accessors arrays grouping kernel locals math math.order
-sequences sequences.private splitting ;
+math.ranges sequences sequences.private splitting ;
-
+FROM: sequences => change-nth ;
 IN: sequences.extras
 
 : reduce1 ( seq quot -- result ) [ unclip ] dip reduce ; inline
@@ -236,3 +236,25 @@ PRIVATE>
 
 : 2map-index ( ... seq1 seq2 quot: ( ... elt1 elt2 index -- ... newelt ) -- ... newseq )
     pick [ (2each-index) ] dip map-integers ; inline
+
+TUPLE: evens seq length ;
+
+: <evens> ( seq -- evens )
+    dup length 1 + 2/ evens boa ; inline
+
+M: evens length length>> ; inline
+
+M: evens nth-unsafe [ 2 * ] [ seq>> nth-unsafe ] bi* ; inline
+
+INSTANCE: evens immutable-sequence
+
+TUPLE: odds seq length ;
+
+: <odds> ( seq -- odds )
+    dup length 2/ odds boa ; inline
+
+M: odds length length>> ; inline
+
+M: odds nth-unsafe [ 2 * 1 + ] [ seq>> nth-unsafe ] bi* ; inline
+
+INSTANCE: odds immutable-sequence