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Diffstat (limited to 'src/math/log10l.c')
| -rw-r--r-- | src/math/log10l.c | 186 |
1 files changed, 186 insertions, 0 deletions
diff --git a/src/math/log10l.c b/src/math/log10l.c new file mode 100644 index 00000000..3a858833 --- /dev/null +++ b/src/math/log10l.c @@ -0,0 +1,186 @@ +/* origin: OpenBSD /usr/src/lib/libm/src/ld80/e_log10l.c */ +/* + * Copyright (c) 2008 Stephen L. Moshier <steve@moshier.net> + * + * Permission to use, copy, modify, and distribute this software for any + * purpose with or without fee is hereby granted, provided that the above + * copyright notice and this permission notice appear in all copies. + * + * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES + * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF + * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR + * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES + * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN + * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF + * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. + */ +/* + * Common logarithm, long double precision + * + * + * SYNOPSIS: + * + * long double x, y, log10l(); + * + * y = log10l( x ); + * + * + * DESCRIPTION: + * + * Returns the base 10 logarithm of x. + * + * The argument is separated into its exponent and fractional + * parts. If the exponent is between -1 and +1, the logarithm + * of the fraction is approximated by + * + * log(1+x) = x - 0.5 x**2 + x**3 P(x)/Q(x). + * + * Otherwise, setting z = 2(x-1)/x+1), + * + * log(x) = z + z**3 P(z)/Q(z). + * + * + * ACCURACY: + * + * Relative error: + * arithmetic domain # trials peak rms + * IEEE 0.5, 2.0 30000 9.0e-20 2.6e-20 + * IEEE exp(+-10000) 30000 6.0e-20 2.3e-20 + * + * In the tests over the interval exp(+-10000), the logarithms + * of the random arguments were uniformly distributed over + * [-10000, +10000]. + * + * ERROR MESSAGES: + * + * log singularity: x = 0; returns MINLOG + * log domain: x < 0; returns MINLOG + */ + +#include "libm.h" + +#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024 +long double log10l(long double x) +{ + return log10(x); +} +#elif LDBL_MANT_DIG == 64 && LDBL_MAX_EXP == 16384 +/* Coefficients for log(1+x) = x - x**2/2 + x**3 P(x)/Q(x) + * 1/sqrt(2) <= x < sqrt(2) + * Theoretical peak relative error = 6.2e-22 + */ +static long double P[] = { + 4.9962495940332550844739E-1L, + 1.0767376367209449010438E1L, + 7.7671073698359539859595E1L, + 2.5620629828144409632571E2L, + 4.2401812743503691187826E2L, + 3.4258224542413922935104E2L, + 1.0747524399916215149070E2L, +}; +static long double Q[] = { +/* 1.0000000000000000000000E0,*/ + 2.3479774160285863271658E1L, + 1.9444210022760132894510E2L, + 7.7952888181207260646090E2L, + 1.6911722418503949084863E3L, + 2.0307734695595183428202E3L, + 1.2695660352705325274404E3L, + 3.2242573199748645407652E2L, +}; + +/* Coefficients for log(x) = z + z^3 P(z^2)/Q(z^2), + * where z = 2(x-1)/(x+1) + * 1/sqrt(2) <= x < sqrt(2) + * Theoretical peak relative error = 6.16e-22 + */ +static long double R[4] = { + 1.9757429581415468984296E-3L, +-7.1990767473014147232598E-1L, + 1.0777257190312272158094E1L, +-3.5717684488096787370998E1L, +}; +static long double S[4] = { +/* 1.00000000000000000000E0L,*/ +-2.6201045551331104417768E1L, + 1.9361891836232102174846E2L, +-4.2861221385716144629696E2L, +}; +/* log10(2) */ +#define L102A 0.3125L +#define L102B -1.1470004336018804786261e-2L +/* log10(e) */ +#define L10EA 0.5L +#define L10EB -6.5705518096748172348871e-2L + +#define SQRTH 0.70710678118654752440L + +long double log10l(long double x) +{ + long double y; + volatile long double z; + int e; + + if (isnan(x)) + return x; + if(x <= 0.0L) { + if(x == 0.0L) + return -1.0L / (x - x); + return (x - x) / (x - x); + } + if (x == INFINITY) + return INFINITY; + /* separate mantissa from exponent */ + /* Note, frexp is used so that denormal numbers + * will be handled properly. + */ + x = frexpl(x, &e); + + /* logarithm using log(x) = z + z**3 P(z)/Q(z), + * where z = 2(x-1)/x+1) + */ + if (e > 2 || e < -2) { + if (x < SQRTH) { /* 2(2x-1)/(2x+1) */ + e -= 1; + z = x - 0.5L; + y = 0.5L * z + 0.5L; + } else { /* 2 (x-1)/(x+1) */ + z = x - 0.5L; + z -= 0.5L; + y = 0.5L * x + 0.5L; + } + x = z / y; + z = x*x; + y = x * (z * __polevll(z, R, 3) / __p1evll(z, S, 3)); + goto done; + } + + /* logarithm using log(1+x) = x - .5x**2 + x**3 P(x)/Q(x) */ + if (x < SQRTH) { + e -= 1; + x = ldexpl(x, 1) - 1.0L; /* 2x - 1 */ + } else { + x = x - 1.0L; + } + z = x*x; + y = x * (z * __polevll(x, P, 6) / __p1evll(x, Q, 7)); + y = y - ldexpl(z, -1); /* -0.5x^2 + ... */ + +done: + /* Multiply log of fraction by log10(e) + * and base 2 exponent by log10(2). + * + * ***CAUTION*** + * + * This sequence of operations is critical and it may + * be horribly defeated by some compiler optimizers. + */ + z = y * (L10EB); + z += x * (L10EB); + z += e * (L102B); + z += y * (L10EA); + z += x * (L10EA); + z += e * (L102A); + return z; +} +#endif |