! Copyright (c) 2012 Anonymous
! See http://factorcode.org/license.txt for BSD license.
USING: accessors io kernel math math.functions math.parser
sequences ;
IN: rosetta-code.standard-deviation

! http://rosettacode.org/wiki/Standard_deviation

! Write a stateful function, class, generator or coroutine that
! takes a series of floating point numbers, one at a time, and
! returns the running standard deviation of the series. The task
! implementation should use the most natural programming style of
! those listed for the function in the implementation language;
! the task must state which is being used. Do not apply Bessel's
! correction; the returned standard deviation should always be
! computed as if the sample seen so far is the entire population.

! Use this to compute the standard deviation of this
! demonstration set, {2,4,4,4,5,5,7,9}, which is 2.

TUPLE: standard-deviator sum sum^2 n ;

: <standard-deviator> ( -- standard-deviator )
    0.0 0.0 0 standard-deviator boa ;

: current-std ( standard-deviator -- std )
    [ [ sum^2>> ] [ n>> ] bi / ]
    [ [ sum>> ] [ n>> ] bi / sq ] bi - sqrt ;

: add-value ( value standard-deviator -- )
    [ nip [ 1 + ] change-n drop ]
    [ [ + ] change-sum drop ]
    [ [ [ sq ] dip + ] change-sum^2 drop ] 2tri ;

: std-main ( -- )
    { 2 4 4 4 5 5 7 9 }
    <standard-deviator> [ [ add-value ] curry each ] keep
    current-std number>string print ;