Scheme Programmer's Manual
NAME
bitwiselength, fxlength  bitwise length of an integer
LIBRARY
(import (rnrs)) ;R6RS
(import (rnrs arithmetic bitwise)) ;R6RS
(import (rnrs arithmetic fixnums)) ;R6RS
SYNOPSIS
(bitwiselength ei)
(fxlength fx)
DESCRIPTION
Returns the number of bits needed to represent
ei
if it is positive, and the number of bits needed to represent
(bitwisenot ei) if it is negative.
The
fxlength
variant constrains the argument and return value to the
fixnum range.
IMPLEMENTATION NOTES
There are no noteworthy differences between implementations of
these procedures, apart from performance differences. R6RS gives
this standard definition:
(do ((result 0 (+ result 1))
(bits (if (negative? ei)
(bitwisenot ei)
ei)
(bitwisearithmeticshift bits 1)))
((zero? bits)
result))
This implementation runs in O(n) where n is the bitwise length of the
input. An implementation which is O(1) is possible if the implementer
has access to the internal structure of bignums. Furthermore, CPUs
often provide instructions that speed up the fixnum case.
RETURN VALUES
Returns a single exact integer object.
EXAMPLES
(bitwiselength #b1000)
=> 4
(fxlength #xff)
=> 8
(fxlength 1)
=> 0
(fxlength #b1000)
=> 3
(bitwiselength #x70)
=> 7
APPLICATION USAGE
This procedure is commonly used to compute the number bytes needed to
represent an integer during serialization to an external format.
RATIONALE
This procedure can be implemented more efficiently by the Scheme
runtime than by portable code, and it has some use in applications.
COMPATIBILITY
These procedures are specified by R6RS. The generic variant is
called
integerlength
in Common Lisp, SRFI 33, SRFI 60 and SRFI 151.
The usual compatibility concerns surrounding fixnums apply to the
fixnum variant.
ERRORS
This procedure can raise exceptions with the following condition types:
 &assertion (R6RS)

The wrong number of arguments was passed or an argument was outside its domain.
SEE ALSO
bitwisefirstbitset(3),
log(3)
STANDARDS
R6RS
AUTHORS
This page is part of the
schememanpages
project.
It includes materials from the RnRS documents.
More information can be found at
Index
 NAME

 LIBRARY

 SYNOPSIS

 DESCRIPTION

 IMPLEMENTATION NOTES

 RETURN VALUES

 EXAMPLES

 APPLICATION USAGE

 RATIONALE

 COMPATIBILITY

 ERRORS

 SEE ALSO

 STANDARDS

 AUTHORS

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