math.extras: Add gini and concentration coefficient.

extra/math/extras/extras-tests.factor

@@ -33,3 +33,30 @@ IN: math.extras.test
 { { 0 2/3 2/3 0 } } [ 4 bartlett ] unit-test
 { { 0 1/2 1 1/2 0 } } [ 5 bartlett ] unit-test
 { { 0 2/5 4/5 4/5 2/5 0 } } [ 6 bartlett ] unit-test
+
+{ 2819/3914 } [
+    {
+        998,000
+        20,000
+        17,500
+        70,000
+        23,500
+        45,200
+    } gini
+] unit-test
+
+{ 8457/9785 } [
+    {
+        998,000
+        20,000
+        17,500
+        70,000
+        23,500
+        45,200
+    } concentration-coefficient
+] unit-test
+
+{ 0 } [ { 1 } gini ] unit-test
+{ 0 } [ { 1 1 1 1 1 1 } gini ] unit-test
+{ 0 } [ { 10 10 10 10 } gini ] unit-test
+{ 0 } [ { } gini ] unit-test

extra/math/extras/extras.factor

@@ -4,7 +4,7 @@
 USING: combinators.short-circuit grouping kernel locals math
 math.combinatorics math.constants math.functions math.order
 math.primes math.ranges math.statistics math.vectors memoize
-sequences ;
+sequences sequences.extras sorting assocs ;
 
 IN: math.extras
 
@@ -154,3 +154,20 @@ PRIVATE>
 
 : until-zero ( n quot -- )
     [ dup zero? ] swap until drop ; inline
+
+<PRIVATE
+
+:: (gini) ( seq -- x )
+    seq natural-sort :> sorted
+    seq length :> len
+    len [1,b] sorted zip 0 [ * + ] assoc-reduce :> sum0
+    2 sum0 * sorted sum len * / :> G
+    G 1 - 1 len / - ; inline
+
+PRIVATE>
+
+: gini ( seq -- x )
+    dup length 1 <= [ drop 0 ] [ (gini) ] if ;
+
+: concentration-coefficient ( seq -- x )
+    [ gini ] [ length [ ] [ 1 - ] bi / ] bi * ;