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authorKevin Ryde <user42@zip.com.au>2003-05-10 01:11:58 +0000
committerKevin Ryde <user42@zip.com.au>2003-05-10 01:11:58 +0000
commit7f5efb18b8216cf97f774635957bb5f8d14e07bf (patch)
tree32b8191bf1cc01d98efface25b3eaeb662c24061 /doc/ref/scheme-data.texi
parent6cced6fea84c93c0f395003fee67086dc2fd46c5 (diff)
downloadguile-7f5efb18b8216cf97f774635957bb5f8d14e07bf.tar.gz
(Bitwise Operations): Note negatives are treated as
infinite precision twos complement. Revise `ash' to emphasise this for right shifts of negatives. Describe integer-length behaviour on negatives. Add `...' to logand, logior, logxor since they take multiple parameters.
Diffstat (limited to 'doc/ref/scheme-data.texi')
-rwxr-xr-xdoc/ref/scheme-data.texi51
1 files changed, 33 insertions, 18 deletions
diff --git a/doc/ref/scheme-data.texi b/doc/ref/scheme-data.texi
index 0542493c8..1873b8275 100755
--- a/doc/ref/scheme-data.texi
+++ b/doc/ref/scheme-data.texi
@@ -1055,7 +1055,13 @@ Naturally, these C functions expect and return @code{double} arguments.
@node Bitwise Operations
@subsection Bitwise Operations
-@deffn {Scheme Procedure} logand n1 n2
+For the following bitwise functions, negative numbers are treated as
+infinite precision twos-complements. For instance @math{-6} is bits
+@math{@dots{}111010}, with infinitely many ones on the left. It can
+be seen that adding 6 (binary 110) to such a bit pattern gives all
+zeros.
+
+@deffn {Scheme Procedure} logand n1 n2 @dots{}
Return the bitwise @sc{and} of the integer arguments.
@lisp
@@ -1065,7 +1071,7 @@ Return the bitwise @sc{and} of the integer arguments.
@end lisp
@end deffn
-@deffn {Scheme Procedure} logior n1 n2
+@deffn {Scheme Procedure} logior n1 n2 @dots{}
Return the bitwise @sc{or} of the integer arguments.
@lisp
@@ -1075,9 +1081,10 @@ Return the bitwise @sc{or} of the integer arguments.
@end lisp
@end deffn
-@deffn {Scheme Procedure} logxor n1 n2
+@deffn {Scheme Procedure} logxor n1 n2 @dots{}
Return the bitwise @sc{xor} of the integer arguments. A bit is
set in the result if it is set in an odd number of arguments.
+
@lisp
(logxor) @result{} 0
(logxor 7) @result{} 7
@@ -1124,20 +1131,24 @@ argument, ie.@: each 0 bit is changed to 1 and each 1 bit to 0.
@deffn {Scheme Procedure} ash n cnt
@deffnx {C Function} scm_ash (n, cnt)
-The function @code{ash} performs an arithmetic shift left by @var{cnt}
-bits (or shift right, if @var{cnt} is negative). `Arithmetic'
-means that the function does not guarantee to keep the bit
-structure of @var{n}, but rather guarantees that the result
-will always be rounded towards minus infinity. Therefore, the
-results of @code{ash} and a corresponding bitwise shift will differ if
-@var{n} is negative.
+Return @var{n} shifted left by @var{cnt} bits, or shifted right if
+@var{cnt} is negative. This is an ``arithmetic'' shift.
-Formally, the function returns an integer equivalent to
-@code{(inexact->exact (floor (* @var{n} (expt 2 @var{cnt}))))}.
+This is effectively a multiplication by @m{2^{cnt}, 2^@var{cnt}}, and
+when @var{cnt} is negative it's a division, rounded towards negative
+infinity. (Note that this is not the same rounding as @code{quotient}
+does.)
+
+With @var{n} viewed as an infinite precision twos complement,
+@code{ash} means a left shift introducing zero bits, or a right shift
+dropping bits.
@lisp
(number->string (ash #b1 3) 2) @result{} "1000"
(number->string (ash #b1010 -1) 2) @result{} "101"
+
+;; -23 is bits ...11101001, -6 is bits ...111010
+(ash -23 -2) @result{} -6
@end lisp
@end deffn
@@ -1162,13 +1173,17 @@ representation are counted. If 0, 0 is returned.
@deffnx {C Function} scm_integer_length (n)
Return the number of bits necessary to represent @var{n}.
+For positive @var{n} this is how many bits to the most significant one
+bit. For negative @var{n} it's how many bits to the most significant
+zero bit in twos complement form.
+
@lisp
-(integer-length #b10101010)
- @result{} 8
-(integer-length 0)
- @result{} 0
-(integer-length #b1111)
- @result{} 4
+(integer-length #b10101010) @result{} 8
+(integer-length #b1111) @result{} 4
+(integer-length 0) @result{} 0
+(integer-length -1) @result{} 0
+(integer-length -256) @result{} 8
+(integer-length -257) @result{} 9
@end lisp
@end deffn