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184 lines
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>12.8. Byte Manipulation Functions</TITLE>
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<meta name="description" value=" Byte Manipulation Functions">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html3175 HREF="node133.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3173 HREF="node121.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3167 HREF="node131.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3177 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3178 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3176 HREF="node133.html"> Random Numbers</A>
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<B>Up:</B> <A NAME=tex2html3174 HREF="node121.html"> Numbers</A>
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<B> Previous:</B> <A NAME=tex2html3168 HREF="node131.html"> Logical Operations on </A>
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<HR> <P>
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<H1><A NAME=SECTION001680000000000000000>12.8. Byte Manipulation Functions</A></H1>
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<P>
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Several functions are provided for dealing with an arbitrary-width field of
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contiguous bits appearing anywhere in an integer.
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Such a contiguous set of bits is called a <i>byte</i>.
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Here the term <i>byte</i> does not imply some fixed number of bits
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(such as eight), rather a field of arbitrary and user-specifiable width.
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<P>
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The byte-manipulation functions use objects called <i>byte specifiers</i> to
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designate a specific byte position within an integer.
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The representation of a byte specifier is implementation-dependent;
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in particular, it may or may not be a number.
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It is sufficient to know that the function <tt>byte</tt> will construct one,
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and that the byte-manipulation functions will accept them.
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The function <tt>byte</tt> accepts two integers representing
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the <i>position</i> and <i>size</i> of the byte and returns
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a byte specifier.
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Such a specifier designates a byte whose width is <i>size</i>
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and whose bits have weights <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap41863.gif">
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through <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap41864.gif">.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>byte <i>size</i> <i>position</i></tt><P><tt>byte</tt> takes two integers representing the size and position
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of a byte and returns a byte specifier suitable for use
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as an argument to byte-manipulation functions.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>byte-size <i>bytespec</i> <BR></tt><tt>byte-position <i>bytespec</i></tt><P>Given a byte specifier, <tt>byte-size</tt> returns the size specified as an
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integer; <tt>byte-position</tt> similarly returns the position.
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For example:
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<P><pre>
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(byte-size (byte <i>j</i> <i>k</i>)) == <i>j</i>
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(byte-position (byte <i>j</i> <i>k</i>)) == <i>k</i>
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</pre><P>
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<P>
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<BR><b>[Function]</b><BR>
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<tt>ldb <i>bytespec</i> <i>integer</i></tt><P><i>bytespec</i> specifies a byte of <i>integer</i> to be extracted.
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The result is returned as a non-negative integer.
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For example:
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<P><pre>
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(logbitp <i>j</i> (ldb (byte <i>s</i> <i>p</i>) <i>n</i>)) == (and (< <i>j</i> <i>s</i>) (logbitp (+ <i>j</i> <i>p</i>) <i>n</i>))
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</pre><P>
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The name of the function <tt>ldb</tt> means ``load byte.''
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<P>
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<hr>
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<b>Compatibility note:</b> The MacLisp function <tt>haipart</tt> can be
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implemented in terms of <tt>ldb</tt> as follows:
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<P><pre>
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(defun haipart (integer count)
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(let ((x (abs integer)))
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(if (minusp count)
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(ldb (byte (- count) 0) x)
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(ldb (byte count (max 0 (- (integer-length x) count)))
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x))))
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</pre><P>
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<hr>
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<P>
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If the argument <i>integer</i> is specified by a form that is a <i>place</i> form
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acceptable to <tt>setf</tt>, then
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<tt>setf</tt> may be used with <tt>ldb</tt> to modify
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a byte within the integer that is stored
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in that <i>place</i>.
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The effect is to perform a <tt>dpb</tt> operation
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and then store the result back into the <i>place</i>.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>ldb-test <i>bytespec</i> <i>integer</i></tt><P><tt>ldb-test</tt> is a predicate that is true if any of
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the bits designated by the byte specifier <i>bytespec</i> are 1's in <i>integer</i>;
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that is, it is true if the designated field is non-zero.
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<P><pre>
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(ldb-test <i>bytespec</i> <i>n</i>) == (not (zerop (ldb <i>bytespec</i> <i>n</i>)))
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</pre><P>
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<P>
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<BR><b>[Function]</b><BR>
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<tt>mask-field <i>bytespec</i> <i>integer</i></tt><P>This is similar to <tt>ldb</tt>; however, the result contains
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the specified byte
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of <i>integer</i> in the position specified by <i>bytespec</i>,
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rather than in position 0 as with <tt>ldb</tt>.
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The result therefore agrees with <i>integer</i> in the byte specified
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but has zero-bits everywhere else.
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For example:
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<P><pre>
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(ldb <i>bs</i> (mask-field <i>bs</i> <i>n</i>)) == (ldb <i>bs</i> <i>n</i>)
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(logbitp <i>j</i> (mask-field (byte <i>s</i> <i>p</i>) <i>n</i>))
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== (and (>= <i>j</i> <i>p</i>) (< <i>j</i> (+ <i>p</i> <i>s</i>)) (logbitp <i>j</i> <i>n</i>))
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(mask-field <i>bs</i> <i>n</i>) == (logand <i>n</i> (dpb -1 <i>bs</i> 0))
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</pre><P>
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<P>
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If the argument <i>integer</i> is specified by a form that is a <i>place</i> form
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acceptable to <tt>setf</tt>,
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then <tt>setf</tt> may be used with <tt>mask-field</tt>
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to modify a byte within the integer that is stored
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in that <i>place</i>.
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The effect is to perform a <tt>deposit-field</tt> operation
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and then store the result back into the <i>place</i>.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>dpb <i>newbyte</i> <i>bytespec</i> <i>integer</i></tt><P>This returns a number that is the same as <i>integer</i> except in the
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bits specified by <i>bytespec</i>. Let <i>s</i> be the size specified
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by <i>bytespec</i>; then the low <i>s</i> bits of <i>newbyte</i> appear in
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the result in the byte specified by <i>bytespec</i>.
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The integer <i>newbyte</i> is therefore interpreted as
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being right-justified, as if it were the result of <tt>ldb</tt>.
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For example:
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<P><pre>
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(logbitp <i>j</i> (dpb <i>m</i> (byte <i>s</i> <i>p</i>) <i>n</i>))
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== (if (and (>= <i>j</i> <i>p</i>) (< <i>j</i> (+ <i>p</i> <i>s</i>)))
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(logbitp (- <i>j</i> <i>p</i>) <i>m</i>)
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(logbitp <i>j</i> <i>n</i>))
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</pre><P>
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The name of the function <tt>dpb</tt> means ``deposit byte.''
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<P>
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<BR><b>[Function]</b><BR>
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<tt>deposit-field <i>newbyte</i> <i>bytespec</i> <i>integer</i></tt><P>This function is to <tt>mask-field</tt> as <tt>dpb</tt> is to <tt>ldb</tt>.
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The result is an integer that contains the bits of <i>newbyte</i>
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within the byte specified by <i>bytespec</i>, and elsewhere contains the bits
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of <i>integer</i>.
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For example:
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<P><pre>
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(logbitp <i>j</i> (deposit-field <i>m</i> (byte <i>s</i> <i>p</i>) <i>n</i>))
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== (if (and (>= <i>j</i> <i>p</i>) (< <i>j</i> (+ <i>p</i> <i>s</i>)))
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(logbitp <i>j</i> <i>m</i>)
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(logbitp <i>j</i> <i>n</i>))
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</pre><P>
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<P>
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<hr>
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<b>Implementation note:</b> If the <i>bytespec</i> is a constant, one may of course
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construct, at compile time, an equivalent mask <i>m</i>, for example
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by computing <tt>(deposit-field -1 <i>bytespec</i> 0)</tt>. Given
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this mask <i>m</i>, one may then compute
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<P><pre>
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(deposit-field <i>newbyte</i> <i>bytespec</i> <i>integer</i>)
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</pre><P>
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by computing
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<P><pre>
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(logior (logand <i>newbyte</i> <i>m</i>) (logand <i>integer</i> (lognot <i>m</i>)))
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</pre><P>
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where the result of <tt>(lognot <i>m</i>)</tt> can of course also be computed
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at compile time. However, the following expression
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may also be used and may require fewer
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temporary registers in some situations:
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<P><pre>
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(logxor <i>integer</i> (logand <i>m</i> (logxor <i>integer</i> <i>newbyte</i>)))
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</pre><P>
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A related, though possibly less useful, trick is that
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<P><pre>
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(let ((z (logand (logxor x y) m)))
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(setq x (logxor z x))
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(setq y (logxor z y)))
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</pre><P>
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interchanges those bits of <tt>x</tt> and <tt>y</tt> for which the mask <tt>m</tt> is
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<tt>1</tt>, and leaves alone those bits of <tt>x</tt> and <tt>y</tt> for which <tt>m</tt> is
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<tt>0</tt>.
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<hr>
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<BR> <HR><A NAME=tex2html3175 HREF="node133.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3173 HREF="node121.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3167 HREF="node131.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3177 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3178 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3176 HREF="node133.html"> Random Numbers</A>
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<B>Up:</B> <A NAME=tex2html3174 HREF="node121.html"> Numbers</A>
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<B> Previous:</B> <A NAME=tex2html3168 HREF="node131.html"> Logical Operations on </A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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