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42 lines
1.4 KiB
C++
42 lines
1.4 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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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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/* Implementations of FloatingPoint functions */
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#include "mozilla/FloatingPoint.h"
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#include <cfloat> // for FLT_MAX
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namespace mozilla {
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bool IsFloat32Representable(double aValue) {
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// NaNs and infinities are representable.
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if (!std::isfinite(aValue)) {
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return true;
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}
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// If it exceeds finite |float| range, casting to |double| is always undefined
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// behavior per C++11 [conv.double]p1 last sentence.
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if (Abs(aValue) > FLT_MAX) {
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return false;
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}
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// But if it's within finite range, then either it's 1) an exact value and so
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// representable, or 2) it's "between two adjacent destination values" and
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// safe to cast to "an implementation-defined choice of either of those
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// values".
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auto valueAsFloat = static_cast<float>(aValue);
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// Per [conv.fpprom] this never changes value.
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auto valueAsFloatAsDouble = static_cast<double>(valueAsFloat);
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// Finally, in 1) exact representable value equals exact representable value,
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// or 2) *changed* value does not equal original value, ergo unrepresentable.
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return valueAsFloatAsDouble == aValue;
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}
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} /* namespace mozilla */
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