58 lines
1.3 KiB
Forth
58 lines
1.3 KiB
Forth
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\ compute the nth fibonacci number, starting at 0
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: fib ( n -- n2 )
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dup ( n n )
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2 < if
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drop 1 ( 1 )
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else
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dup 2 - recurse ( n fib[n-2] )
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swap ( fib[n-2] n )
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1 - recurse ( fib[n-2] fib[n-1] )
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+ ( n2 )
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then ;
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create memo 100 cells allot
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\ initialize it
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: init ( n -- )
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dup ( n n )
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if ( n )
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dup 0 swap ( n 0 n )
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1 - ( n 0 n-1 )
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cells memo + ( n n-1+memo )
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1 - ( n 0 n-1+memo)
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! ( n )
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1 - ( n-1 )
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recurse
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else
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drop
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then ;
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: fib2x ( n -- n2 )
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dup ( n n )
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2 < if ( n )
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drop 1 ( 1 )
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else
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dup cells memo + ( n n+memo )
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@ ( n ${n+memo} )
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dup 0 > if ( n ${n+memo} )
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swap drop ( ${n+memo} )
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else
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drop ( n )
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dup dup ( n n n )
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2 - recurse ( n n fib2[n-2] )
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swap ( n fib2[n-2] n )
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1 - recurse ( n fib2[n-2] fib2[n-1] )
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+ ( n fib2[n] )
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dup rot ( fib2[n] fib2[n] n )
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cells memo + ! ( fib2[n] )
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then
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then ;
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: fib2
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\ allocate a lookup table and run fib2x
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100 init fib2x ;
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45 fib2
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.s cr
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bye
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