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/* reduce argument to range +/- PI */
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static const char SCCSID[]="@(#)adjlon.c 4.3 93/06/12 GIE REL";
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#define SPI 3.14159265359
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#define TWOPI 6.2831853071795864769
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#define ONEPI 3.14159265358979323846
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double adjlon (double lon) {
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if (fabs(lon) <= SPI) return( lon );
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lon += ONEPI; /* adjust to 0..2pi rad */
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lon -= TWOPI * floor(lon / TWOPI); /* remove integral # of 'revolutions'*/
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lon -= ONEPI; /* adjust back to -pi..pi rad */