59 lines
1.7 KiB
C++
59 lines
1.7 KiB
C++
/*
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* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/.
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*
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* Copyright 2016 Danny Robson <danny@nerdcruft.net>
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*/
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#pragma once
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#include "../maths.hpp"
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#include <cmath>
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namespace cruft::roots {
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/// find a root of a function using the bisection method
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///
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/// the user is responsible for ensuring there is in fact a root. there
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/// is no iteration limit currently, so poorly considered arguments could
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/// result in an infinite loop.
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///
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/// a: lower bound
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/// b: upper bound
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/// f: real function
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/// tolerance: minimum range termination condition
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template <typename T>
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T
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bisection (T a, T b, T (*f)(T), T tolerance)
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{
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// to simplify the exit conditions we assume that a..b is an
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// increasing range.
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if (a > b)
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std::swap (a, b);
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while (b - a > tolerance) {
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// calculate the midpoint
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auto c = (a + b) / T{2};
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auto f_c = f (c);
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// return early if we happened across the root
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if (exactly_zero (f_c))
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return c;
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// check which interval we fell into, and divide for the next
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// iteration
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auto f_a = f (a);
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if (samesign (f_a, f_c))
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a = c;
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else
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b = c;
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}
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// we didn't find it exactly, but it's somewhere in this range so take
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// a punt and return the middle of the range. not sure if this is
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// valid mathematically.
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return (a + b) / T{2};
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}
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}
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