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Diffstat (limited to 'pl/math/tools/log10.sollya')
-rw-r--r-- | pl/math/tools/log10.sollya | 44 |
1 files changed, 44 insertions, 0 deletions
diff --git a/pl/math/tools/log10.sollya b/pl/math/tools/log10.sollya new file mode 100644 index 0000000..85d1d15 --- /dev/null +++ b/pl/math/tools/log10.sollya @@ -0,0 +1,44 @@ +// polynomial for approximating log10(1+x) +// +// Copyright (c) 2019-2023, Arm Limited. +// SPDX-License-Identifier: MIT OR Apache-2.0 WITH LLVM-exception + +deg = 6; // poly degree +// |log10(1+x)| > 0x1p-5 outside the interval +a = -0x1.p-5; +b = 0x1.p-5; + +ln10 = evaluate(log(10),0); +invln10hi = double(1/ln10 + 0x1p21) - 0x1p21; // round away last 21 bits +invln10lo = double(1/ln10 - invln10hi); + +// find log10(1+x)/x polynomial with minimal relative error +// (minimal relative error polynomial for log10(1+x) is the same * x) +deg = deg-1; // because of /x + +// f = log(1+x)/x; using taylor series +f = 0; +for i from 0 to 60 do { f = f + (-x)^i/(i+1); }; +f = f/ln10; + +// return p that minimizes |f(x) - poly(x) - x^d*p(x)|/|f(x)| +approx = proc(poly,d) { + return remez(1 - poly(x)/f(x), deg-d, [a;b], x^d/f(x), 1e-10); +}; + +// first coeff is fixed, iteratively find optimal double prec coeffs +poly = invln10hi + invln10lo; +for i from 1 to deg do { + p = roundcoefficients(approx(poly,i), [|D ...|]); + poly = poly + x^i*coeff(p,0); +}; +display = hexadecimal; +print("invln10hi:", invln10hi); +print("invln10lo:", invln10lo); +print("rel error:", accurateinfnorm(1-poly(x)/f(x), [a;b], 30)); +print("in [",a,b,"]"); +print("coeffs:"); +for i from 0 to deg do coeff(poly,i); + +display = decimal; +print("in [",a,b,"]"); |