30 lines
1021 B
Factor
30 lines
1021 B
Factor
! Copyright (C) 2009 Daniel Ehrenberg
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! See http://factorcode.org/license.txt for BSD license.
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USING: kernel sequences math fry locals math.order alien.accessors ;
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IN: classes.struct.bit-accessors
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! Bitfield accessors are little-endian on all platforms
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! Why not? It's platform-dependent in C
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: ones-between ( start end -- n )
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[ 2^ 1 - ] bi@ swap bitnot bitand ;
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:: read-bits ( offset bits -- quot: ( byte-array -- n ) shift-amount offset' bits' )
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offset 8 /mod :> start-bit :> i
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start-bit bits + 8 min :> end-bit
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start-bit end-bit ones-between :> mask
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end-bit start-bit - :> used-bits
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! The code generated for this isn't optimal
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! To improve the code, algebraic simplifications should
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! have interval information available
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[ i alien-unsigned-1 mask bitand start-bit neg shift ]
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used-bits
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i 1 + 8 *
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bits used-bits - ;
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: bit-reader ( offset bits -- quot: ( alien -- n ) )
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read-bits dup zero? [ 3drop ] [
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bit-reader swap '[ _ _ bi _ shift bitor ]
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] if ;
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