factor/contrib/lazy-lists/lists.factor

! Copyright (C) 2004 Chris Double.
! See http://factorcode.org/license.txt for BSD license.
!
! Updated by Matthew Willis, July 2006
! Updated by Chris Double, September 2006
!
! TODO: 
!   lazy-map and lazy-subset don't memoize the computed car.
!   demonstrated by the following fib definition:
!   : fib ( n -- ) dup 2 < [ drop 1 ] [ dup 1 - "fibs" get nth swap 2 - "fibs" get nth + ] if ;
!   naturals [ fib ] lmap "fibs" set
!   25 fib . 25 fib .
!   The second call should be faster than the first.
!    
USING: kernel sequences math vectors arrays namespaces generic ;
IN: lazy-lists

TUPLE: promise quot forced? value ;

C: promise ( quot -- promise ) [ set-promise-quot ] keep ;

: force ( promise -- value )
    #! Force the given promise leaving the value of calling the
    #! promises quotation on the stack. Re-forcing the promise
    #! will return the same value and not recall the quotation.
    dup promise-forced? [
        dup promise-quot call over set-promise-value
        t over set-promise-forced?
    ] unless
    promise-value ;

TUPLE: cons car cdr ;
GENERIC: car  ( cons -- car )
GENERIC: cdr  ( cons -- cdr )
GENERIC: nil? ( cons -- bool )

C: cons ( car cdr -- list ) 
    [ set-cons-cdr ] keep 
    [ set-cons-car ] keep ;

M: cons car ( cons -- car )
    cons-car ;    

M: cons cdr ( cons -- cdr )
    cons-cdr ;    

: nil ( -- cons )
  T{ cons f f f } ;

M: cons nil? ( cons -- bool )
    nil eq? ;

: cons ( car cdr -- list )
    <cons> ;

: 1list ( obj -- cons )
    nil <cons> ;

: 2list ( a b -- cons )
    nil <cons> <cons> ;

! Both 'car' and 'cdr' are promises  
: lazy-cons ( car cdr -- promise ) 
    >r <promise> r> <promise> <cons> 
    T{ promise f f t f } clone [ set-promise-value ] keep ;

M: promise car ( promise -- car )
  force car force ;

M: promise cdr ( promise -- cdr )
  force cdr force ;

M: promise nil? ( cons -- bool )
  force nil? ;

DEFER: lunit 
DEFER: lnth 
TUPLE: list ;

: 1lazy-list ( a -- lazy-cons )
  [ nil ] lazy-cons ;

: 2lazy-list ( a b -- lazy-cons )
  1lazy-list unit lazy-cons ;

: 3lazy-list ( a b c -- lazy-cons )
  2lazy-list unit lazy-cons ;

: lnth ( n list -- elt ) 
  swap [ cdr ] times car ;

: uncons ( cons -- car cdr )
    #! Return the car and cdr of the lazy list
    dup car swap cdr ;

: leach ( list quot -- )
  swap dup nil? [ 
    2drop 
  ] [
    uncons swap pick call swap leach
  ] if ;

: 2curry ( a b quot -- quot )
  curry curry ;

TUPLE: lazy-map cons quot ;

: lmap ( list quot -- result )
    over nil? [ 2drop nil ] [ <lazy-map> ] if ;

M: lazy-map car ( lazy-map -- car )
  [ lazy-map-cons car ] keep
  lazy-map-quot call ;

M: lazy-map cdr ( lazy-map -- cdr )
  [ lazy-map-cons cdr ] keep
  lazy-map-quot lmap ;

M: lazy-map nil? ( lazy-map -- bool )
  lazy-map-cons nil? ;

TUPLE: lazy-take n cons ;

: ltake ( n list -- result )
    over zero? [ 2drop nil ] [ <lazy-take> ] if ;
     
M: lazy-take car ( lazy-take -- car )
  lazy-take-cons car ;

M: lazy-take cdr ( lazy-take -- cdr )
  [ lazy-take-n 1- ] keep
  lazy-take-cons cdr ltake ;

M: lazy-take nil? ( lazy-take -- bool )
  lazy-take-n zero? ;

TUPLE: lazy-subset cons quot ;

: lsubset ( list quot -- list )
    over nil? [ 2drop nil ] [ <lazy-subset> ] if ;

: car-subset?  ( lazy-subset -- )
  [ lazy-subset-cons car ] keep
  lazy-subset-quot call ;

: skip ( lazy-subset -- )
  [ lazy-subset-cons cdr ] keep
  set-lazy-subset-cons ;

M: lazy-subset car ( lazy-subset -- car )
  dup car-subset? [
    lazy-subset-cons car
  ] [
    dup skip car
  ] if ;

M: lazy-subset cdr ( lazy-subset -- cdr )
  dup car-subset? [
    [ lazy-subset-cons cdr ] keep
    lazy-subset-quot lsubset
  ] [
    dup skip cdr
  ] if ;

M: lazy-subset nil? ( lazy-subset -- bool )
  dup lazy-subset-cons nil? [
    drop t
  ] [
    dup car-subset? [
      drop f
    ] [
      dup skip nil?
    ] if 
  ] if ;

: list>vector ( list -- vector )
  [ [ , ] leach ] V{ } make ;

: list>array ( list -- array )
  [ [ , ] leach ] { } make ;

TUPLE: lazy-append list1 list2 ;

: lappend ( list1 list2 -- result )
  {
    { [ over nil? over nil? and ] [ 2drop nil ] }
    { [ over nil? ] [ nip ] }
    { [ dup nil? ] [ drop ] }
    { [ t ] [ <lazy-append> ] }
  } cond ;

M: lazy-append car ( lazy-append -- car )
  lazy-append-list1 car ;

M: lazy-append cdr ( lazy-append -- cdr )
  [ lazy-append-list1 cdr  ] keep
  lazy-append-list2 lappend ;

M: lazy-append nil? ( lazy-append -- bool )
  dup lazy-append-list1 nil? [
    drop t 
  ] [
    lazy-append-list2 nil? 
  ] if ;

TUPLE: lazy-from-by n quot ;

: lfrom-by ( n quot -- list )
  <lazy-from-by> ;
    
: lfrom ( n -- list )
  [ 1 + ] lfrom-by ;

M: lazy-from-by car ( lazy-from-by -- car )
  lazy-from-by-n ;

M: lazy-from-by cdr ( lazy-from-by -- cdr )
  [ lazy-from-by-n ] keep
  lazy-from-by-quot dup >r call r> lfrom-by ;

M: lazy-from-by nil? ( lazy-from-by -- bool )
  drop f ;