Scheme Programmer's Manual

NAME

bitwise-bit-count - population count

LIBRARY

(import (rnrs))                     ;R6RS
(import (rnrs arithmetic bitwise))  ;R6RS
(import (rnrs arithmetic fixnums))  ;R6RS

SYNOPSIS

(bitwise-bit-count ei)
(fxbit-count fx)

DESCRIPTION

If the argument is non-negative, this procedure returns the number of 1 bits in its two's complement representation. Otherwise it returns the result of the following computation: (bitwise-not (bitwise-bit-count (bitwise-not ei)))

RETURN VALUES

Returns a single exact integer object.

EXAMPLES

(bitwise-bit-count #b1)
=> 1

(bitwise-bit-count #b1000)
=> 1

(bitwise-bit-count #b1001)
=> 2

(bitwise-bit-count #b-1001)
=> -2

APPLICATION USAGE

This procedure is used to compute the Hamming distance. It's also used in Hash Array Mapped Tries and other applications.

COMPATIBILITY

This procedure is new in R6RS. SRFI-151 has the similar procedure bit-count.

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. For fxbit-count the argument must be a fixnum.

STANDARDS

R6RS

HISTORY

This procedure can be implemented with the popcount procedure in x86-64 processors. This instruction appeared even in some early computers like the IBM Stretch (1961).

AUTHORS

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