On 8 September 2010 13:18, David McClain dbm@refined-audiometrics.com wrote:
I have a 64-bit integer obtained by the FLI from external storage. It was obtained by reading two 32-bit unsigned values and depositing them into a zero valued integer. Now I want to sign-extend the result so that if it has its MSB set, the value will be a twos-complement negative value. (defun read-int64 (ptr) (let ((v (read-uint64 ptr))) (if (logbitp 63 v) (- v #.(ash 1 64)) v))) But this looks inelegant to me, requiring the storage of a constant #.(ash 1 64) used in a subtraction operation. Been racking my brain on the BOOLE operations and the LOGNOT et al, looking for a more elegant solution to this. Not a high-value item, just curious.
(if (logbitp 63 v) (dpb v (byte 64 0) -1) v)
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