From a6614ee974aa770fc0a8e756acdb01542e5008ff Mon Sep 17 00:00:00 2001
From: John Benediktsson <mrjbq7@gmail.com>
Date: Fri, 29 Mar 2013 07:10:27 -0700
Subject: [PATCH] sequences.extras: faster even-indices and odd-indices.

---
 extra/sequences/extras/extras.factor | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/extra/sequences/extras/extras.factor b/extra/sequences/extras/extras.factor
index aa2e169f82..31a0bbeebd 100644
--- a/extra/sequences/extras/extras.factor
+++ b/extra/sequences/extras/extras.factor
@@ -90,9 +90,15 @@ IN: sequences.extras
 : filter-index ( ... seq quot: ( ... elt i -- ... ? ) -- ... seq' )
     over filter-index-as ; inline
 
-: even-indices ( seq -- seq' ) [ nip even? ] filter-index ;
+: even-indices ( seq -- seq' )
+    [ length 1 + 2/ ] keep [
+        [ [ 2 * ] dip nth-unsafe ] curry
+    ] keep map-integers ;
 
-: odd-indices ( seq -- seq' ) [ nip odd? ] filter-index ;
+: odd-indices ( seq -- seq' )
+    [ length 2/ ] keep [
+        [ [ 2 * 1 + ] dip nth-unsafe ] curry
+    ] keep map-integers ;
 
 : compact ( seq quot elt -- seq' )
     [ split-when harvest ] dip join ; inline