32 lines
699 B
C
32 lines
699 B
C
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#include <math.h>
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#include <errno.h>
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#include "fastmath.h"
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/* atanh (x) = 0.5 * log ((1.0 + x)/(1.0 - x)) */
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double atanh(double x)
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{
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double z;
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if isnan (x)
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return x;
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z = fabs (x);
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if (z == 1.0)
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{
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errno = ERANGE;
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return (x > 0 ? INFINITY : -INFINITY);
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}
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if (z > 1.0)
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{
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errno = EDOM;
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return nan("");
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}
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/* Rearrange formula to avoid precision loss for small x.
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atanh(x) = 0.5 * log ((1.0 + x)/(1.0 - x))
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= 0.5 * log1p ((1.0 + x)/(1.0 - x) - 1.0)
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= 0.5 * log1p ((1.0 + x - 1.0 + x) /(1.0 - x))
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= 0.5 * log1p ((2.0 * x ) / (1.0 - x)) */
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z = 0.5 * __fast_log1p ((z + z) / (1.0 - z));
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return x >= 0 ? z : -z;
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}
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