59 lines
1.6 KiB
C
59 lines
1.6 KiB
C
#ifndef lint
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static char sccsid[] = "@(#)ldexp.c 1.1 92/07/30 SMI"; /* from S5R2 2.7 */
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#endif
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/*LINTLIBRARY*/
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/*
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* double ldexp (value, exp)
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* double value;
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* int exp;
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*
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* Ldexp returns value * 2**exp, if that result is in range.
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* If underflow occurs, it returns zero. If overflow occurs,
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* it returns a value of appropriate sign and largest single-
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* precision magnitude. In case of underflow or overflow,
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* the external int "errno" is set to ERANGE. Note that errno is
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* not modified if no error occurs, so if you intend to test it
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* after you use ldexp, you had better set it to something
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* other than ERANGE first (zero is a reasonable value to use).
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*/
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#include <values.h>
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#include <errno.h>
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/* Largest signed long int power of 2 */
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#define MAXSHIFT (BITSPERBYTE * sizeof(long) - 2)
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extern double frexp();
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double
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ldexp(value, exp)
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register double value;
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register int exp;
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{
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int old_exp;
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if (exp == 0 || value == 0.0) /* nothing to do for zero */
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return (value);
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#if !(pdp11 || u3b5) /* pdp11 "cc" can't handle cast of
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double to void on pdp11 or 3b5 */
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(void)
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#endif
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frexp(value, &old_exp);
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if (exp > 0) {
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if (exp + old_exp > MAXBEXP) { /* overflow */
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errno = ERANGE;
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return (value < 0 ? -1.0e999 : 1.0e999);
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}
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for ( ; exp > MAXSHIFT; exp -= MAXSHIFT)
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value *= (1L << MAXSHIFT);
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return (value * (1L << exp));
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}
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if (exp + old_exp < MINBEXP) { /* underflow */
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errno = ERANGE;
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return (0.0);
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}
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for ( ; exp < -MAXSHIFT; exp += MAXSHIFT)
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value *= 1.0/(1L << MAXSHIFT); /* mult faster than div */
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return (value / (1L << -exp));
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}
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