math.statistics: Fix out of bounds on quantiles that should return the first element.

basis/math/statistics/statistics-tests.factor

@@ -224,3 +224,9 @@ IN: math.statistics.tests
     H{ { 0 V{ 0 3 6 9 } } { 1 V{ 1 4 7 10 } } { 2 V{ 2 5 8 } } }
 }
 [ 600 610 [a,b] [ 3 mod ] collect-index-by ] unit-test
+
+
+{ { 1 } } [
+    { 1 2 3 4 5 10 21 12 12 12 12203 3403 030 3022 2 2 }
+    { 1/1000 } quantile5
+] unit-test

basis/math/statistics/statistics.factor

@@ -158,7 +158,7 @@ PRIVATE>
 ! could subtract 1 from a
 
 : quantile-x ( a b N q -- x )
-    [ + ] dip * + 1 - ; inline
+    [ + ] dip * + 1 - 0 max ; inline
 
 ! 2+1/4 frac is 1/4
 : frac ( x -- x' )