gForth examples improved

gForth examples improved

\ http://www.complang.tuwien.ac.at/forth/gforth/Docs-html/General-Loops-Tutorial.html

: halve-n ( n -- n/2 ) 2/ ;

: init-counter-for-number-of-halves-needed ( -- 0 ) 0 ;

: insure-n-is-greater-than-or-equal-to-0 ( n counter -- n counter flag ) 
  over 0> ;

: halve-n-and-increment-counter { n counter -- n/2 counter+1 }
  n 2/ counter 1+ ;

: log2 ( +n1 -- n2 ) 
  assert( dup 0> )	
  halve-n 
  init-counter-for-number-of-halves-needed 
  BEGIN
    insure-n-is-greater-than-or-equal-to-0
    WHILE \ n >= 0
      halve-n-and-increment-counter 
  REPEAT 
  NIP \ n, leaving the counter
  ;

\ http://www.complang.tuwien.ac.at/forth/gforth/Docs-html/Counted-loops-Tutorial.html

: ^ { n power }
  n 1 power 0 
  U+DO
    OVER *
  LOOP
  NIP ;

\ http://www.complang.tuwien.ac.at/forth/gforth/Docs-html/Counted-loops-Tutorial.html

: fac { u -- u! }
    1 \ seed the factorial multiplication
    u 1+ 1 U+DO \ loop over range [1,u+1) 
      i *
    LOOP ;
 
\ recursion

: fib { n -- fibn }
  assert( n 0>= ) 
  n CASE
    0 OF 0 ENDOF
    1 OF 1 ENDOF
    2 OF 1 ENDOF	
    ( otherwise ) n 1 - recurse n 2 - recurse + 
  SWAP ENDCASE ;