Making struct bitfield readers fast
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! 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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@ -6,7 +6,8 @@ combinators.smart cpu.architecture definitions functors.backend
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fry generalizations generic.parser kernel kernel.private lexer
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libc locals macros make math math.order parser quotations
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sequences slots slots.private specialized-arrays vectors words
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summary namespaces assocs vocabs.parser math.functions bit-arrays ;
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summary namespaces assocs vocabs.parser math.functions
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classes.struct.bit-accessors bit-arrays ;
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QUALIFIED: math
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IN: classes.struct
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@ -89,23 +90,14 @@ MACRO: <struct-boa> ( class -- quot: ( ... -- struct ) )
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: pad-struct-slots ( values class -- values' class )
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[ struct-slots [ initial>> ] map over length tail append ] keep ;
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: bits@ ( slot -- beginning end )
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[ offset>> ] [ bits>> ] bi dupd + ;
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QUALIFIED: math.bits
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: bytes>bits ( byte-array -- bit-array )
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[ 8 math.bits:<bits> ] { } map-as ?{ } join ;
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: read-bits ( beginning end byte-array -- n )
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! This is absurdly inefficient
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bytes>bits subseq bit-array>integer ;
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: sign-extend ( n bits -- n' )
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! formula from:
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! http://guru.multimedia.cx/fast-sign-extension/
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1 - -1 swap shift [ + ] keep bitxor ; inline
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: sign-extender ( signed? bits -- quot )
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'[ _ [ _ sign-extend ] when ] ;
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GENERIC: (reader-quot) ( slot -- quot )
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M: struct-slot-spec (reader-quot)
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@ -113,10 +105,10 @@ M: struct-slot-spec (reader-quot)
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[ offset>> [ >c-ptr ] swap suffix ] bi prepend ;
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M: struct-bit-slot-spec (reader-quot)
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[ bits@ ] [ signed?>> ] [ bits>> ] tri '[
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[ _ _ ] dip (underlying)>> read-bits
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_ [ _ sign-extend ] when
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] ;
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[ [ offset>> ] [ bits>> ] bi bit-reader ]
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[ [ signed?>> ] [ bits>> ] bi sign-extender ]
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bi compose
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[ >c-ptr ] prepose ;
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GENERIC: (writer-quot) ( slot -- quot )
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@ -124,6 +116,11 @@ M: struct-slot-spec (writer-quot)
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[ type>> c-setter ]
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[ offset>> [ >c-ptr ] swap suffix ] bi prepend ;
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QUALIFIED: math.bits
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: bytes>bits ( byte-array -- bit-array )
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[ 8 math.bits:<bits> ] { } map-as ?{ } join ;
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: (write-bits) ( value offset end byte-array -- )
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! This is absurdly inefficient
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[
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@ -131,6 +128,9 @@ M: struct-slot-spec (writer-quot)
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replace-slice ?{ } like underlying>>
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] keep 0 swap copy ;
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: bits@ ( slot -- beginning end )
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[ offset>> ] [ bits>> ] bi dupd + ;
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M: struct-bit-slot-spec (writer-quot) ( slot -- quot )
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bits@ '[ [ _ _ ] dip (underlying)>> (write-bits) ] ;
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