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clarified bitwise-bit-field and bitwise-bit-set?

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svn: r18225
This commit is contained in:
Robby Findler 2010-02-20 15:20:27 +00:00
parent eb46f95246
commit ac2c537b8f
1 changed files with 20 additions and 6 deletions
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26
collects/scribblings/reference/numbers.scrbl
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@ -586,8 +586,12 @@ produces @scheme[+nan.0] in the case that neither @scheme[y] nor
@defproc[(bitwise-bit-set? [n exact-integer?] [m exact-nonnegative-integer?])
boolean?]{
Returns @scheme[(not (zero? (bitwise-and n (arithmetic-shift 1 m))))],
but faster and in constant time when @scheme[n] is positive.
Returns @scheme[#t] when the @scheme[m]th bit of @scheme[n] is set in @scheme[n]'s
(semi-infinite) two's complement representation.
This is equivalent to
@scheme[(not (zero? (bitwise-and n (arithmetic-shift 1 m))))],
but is faster and runs in constant time when @scheme[n] is positive.
@mz-examples[(bitwise-bit-set? 5 0) (bitwise-bit-set? 5 2) (bitwise-bit-set? -5 (expt 2 700))]}
@ -598,20 +602,30 @@ but faster and in constant time when @scheme[n] is positive.
(start . <= . end))])
exact-integer?]{
Returns
Extracts the bits between position @scheme[start] and @scheme[(- end 1)] (inclusive)
from @scheme[n] and shifts them down to the least significant portion of the number.
This is equivalent to this computation,
@schemeblock[
(bitwise-and (sub1 (arithmetic-shift 1 (- end start)))
(arithmetic-shift n (- start)))
]
but in constant time when @scheme[n] is positive, @scheme[start] and
but it runs in constant time when @scheme[n] is positive, @scheme[start] and
@scheme[end] are fixnums, and @scheme[(- end start)] is no more than
the maximum width of a fixnum.
@mz-examples[(bitwise-bit-field 13 1 1)
Each pair of examples below uses the same numbers, but shows the result in
both binary and as integers.
@mz-examples[(format "~b" (bitwise-bit-field (string->number "1101" 2) 1 1))
(bitwise-bit-field 13 1 1)
(format "~b" (bitwise-bit-field (string->number "1101" 2) 1 3))
(bitwise-bit-field 13 1 3)
(bitwise-bit-field 13 1 4)]}
(format "~b" (bitwise-bit-field (string->number "1101" 2) 1 4))
(bitwise-bit-field 13 1 4)]
}
@defproc[(arithmetic-shift [n exact-integer?] [m exact-integer?])