median used the wrong algorithm. now it runs in O(n) time. add kth-smallest word, used to implement median

basis/math/statistics/statistics-tests.factor

@@ -13,6 +13,24 @@ IN: math.statistics.tests
 [ 2 ] [ { 1 2 3 } median ] unit-test
 [ 5/2 ] [ { 1 2 3 4 } median ] unit-test
 
+[ { } median ] must-fail
+[ { } upper-median ] must-fail
+[ { } lower-median ] must-fail
+
+[ 2 ] [ { 1 2 3 4 } lower-median ] unit-test
+[ 3 ] [ { 1 2 3 4 } upper-median ] unit-test
+[ 3 ] [ { 1 2 3 4 5 } lower-median ] unit-test
+[ 3 ] [ { 1 2 3 4 5 } upper-median ] unit-test
+
+
+[ 1 ] [ { 1 } lower-median ] unit-test
+[ 1 ] [ { 1 } upper-median ] unit-test
+[ 1 ] [ { 1 } median ] unit-test
+
+[ 1 ] [ { 1 2 } lower-median ] unit-test
+[ 2 ] [ { 1 2 } upper-median ] unit-test
+[ 3/2 ] [ { 1 2 } median ] unit-test
+
 [ 1 ] [ { 1 2 3 } var ] unit-test
 [ 1.0 ] [ { 1 2 3 } std ] unit-test
 [ t ] [ { 1 2 3 4 } ste 0.6454972243679028 - .0001 < ] unit-test

basis/math/statistics/statistics.factor

@@ -1,7 +1,8 @@
 ! Copyright (C) 2008 Doug Coleman, Michael Judge.
 ! See http://factorcode.org/license.txt for BSD license.
 USING: arrays combinators kernel math math.analysis
-math.functions math.order sequences sorting ;
+math.functions math.order sequences sorting locals
+sequences.private ;
 IN: math.statistics
 
 : mean ( seq -- n )
@@ -13,13 +14,55 @@ IN: math.statistics
 : harmonic-mean ( seq -- n )
     [ recip ] sigma recip ;
 
-: median ( seq -- n )
+: slow-median ( seq -- n )
     natural-sort dup length even? [
         [ midpoint@ dup 1 - 2array ] keep nths mean
     ] [
         [ midpoint@ ] keep nth
     ] if ;
 
+:: kth-smallest ( seq k -- elt )
+    #! Wirth's method, Algorithm's + Data structues = Programs p. 84
+    #! The algorithm modifiers seq, so we clone it
+    seq clone :> seq
+    0 :> i!
+    0 :> j!
+    0 :> l!
+    0 :> x!
+    seq length 1 - :> m!
+    [ l m < ]
+    [
+        k seq nth x!
+        l i!
+        m j!
+        [ i j <= ]
+        [
+            [ i seq nth-unsafe x < ] [ i 1 + i! ] while
+            [ x j seq nth-unsafe < ] [ j 1 - j! ] while
+            i j <= [
+                i j seq exchange
+                i 1 + i!
+                j 1 - j!
+            ] when
+        ] do while
+
+        j k < [ i l! ] when
+        k i < [ j m! ] when
+    ] while
+    k seq nth ; inline
+
+: lower-median ( seq -- elt )
+    dup dup length odd? [ midpoint@ ] [ midpoint@ 1 - ] if kth-smallest ;
+
+: upper-median ( seq -- elt )
+    dup midpoint@ kth-smallest ;
+
+: medians ( seq -- lower upper )
+    [ lower-median ] [ upper-median ] bi ;
+
+: median ( seq -- x )
+    dup length odd? [ lower-median ] [ medians + 2 / ] if ;
+
 : minmax ( seq -- min max )
     #! find the min and max of a seq in one pass
     [ 1/0. -1/0. ] dip [ [ min ] [ max ] bi-curry bi* ] each ;
@@ -32,15 +75,13 @@ IN: math.statistics
     dup length 1 <= [
         drop 0
     ] [
-        [ [ mean ] keep [ - sq ] with sigma ] keep
-        length 1 - /
+        [ [ mean ] keep [ - sq ] with sigma ]
+        [ length 1 - ] bi /
     ] if ;
 
-: std ( seq -- x )
-    var sqrt ;
+: std ( seq -- x ) var sqrt ;
 
-: ste ( seq -- x )
-    [ std ] [ length ] bi sqrt / ;
+: ste ( seq -- x ) [ std ] [ length ] bi sqrt / ;
 
 : ((r)) ( mean(x) mean(y) {x} {y} -- (r) )
     ! finds sigma((xi-mean(x))(yi-mean(y))
@@ -64,4 +105,3 @@ IN: math.statistics
     [ (r) ] 2keep ! stack is mean(x) mean(y) r sx sy
     swap / * ! stack is mean(x) mean(y) beta
     [ swapd * - ] keep ;
-