From e37001aad7f6e4bbad250addba033f1eaf97d566 Mon Sep 17 00:00:00 2001 From: Jan Arve Saether Date: Wed, 31 Jul 2013 15:43:09 +0200 Subject: Say hello to qFloatDistance() This function is useful if a floating point comparison requires a certain precision. The return value can be considered as the precision. For instance, if you want to compare two 32-bit floating point values and all you need is a 24-bit precision, you can use this function like this: if (qFloatDistance(a,b) < (1 << 7)) { // The last 7 bits are not // significant // precise enough } Task-number: QTBUG-32632 Change-Id: I020a58d2f9f9452ac3c510b4bb560dc806f0d93c Reviewed-by: Shawn Rutledge Reviewed-by: Thiago Macieira --- src/corelib/global/qnumeric.cpp | 136 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 136 insertions(+) (limited to 'src/corelib/global/qnumeric.cpp') diff --git a/src/corelib/global/qnumeric.cpp b/src/corelib/global/qnumeric.cpp index 5e71753c8a..83ccb7075d 100644 --- a/src/corelib/global/qnumeric.cpp +++ b/src/corelib/global/qnumeric.cpp @@ -41,6 +41,7 @@ #include "qnumeric.h" #include "qnumeric_p.h" +#include QT_BEGIN_NAMESPACE @@ -99,4 +100,139 @@ Q_CORE_EXPORT double qQNaN() { return qt_qnan(); } Q_CORE_EXPORT double qInf() { return qt_inf(); } + +/*! + \internal + */ +static inline quint32 f2i(float f) +{ + quint32 i; + memcpy(&i, &f, sizeof(f)); + return i; +} + +/*! + Returns the number of representable floating-point numbers between \a a and \a b. + + This function provides an alternative way of doing approximated comparisons of floating-point + numbers similar to qFuzzyCompare(). However, it returns the distance between two numbers, which + gives the caller a possibility to choose the accepted error. Errors are relative, so for + instance the distance between 1.0E-5 and 1.00001E-5 will give 110, while the distance between + 1.0E36 and 1.00001E36 will give 127. + + This function is useful if a floating point comparison requires a certain precision. + Therefore, if \a a and \a b are equal it will return 0. The maximum value it will return for 32-bit + floating point numbers is 4,278,190,078. This is the distance between \c{-FLT_MAX} and + \c{+FLT_MAX}. + + The function does not give meaningful results if any of the arguments are \c Infinite or \c NaN. + You can check for this by calling qIsFinite(). + + The return value can be considered as the "error", so if you for instance want to compare + two 32-bit floating point numbers and all you need is an approximated 24-bit precision, you can + use this function like this: + + \code + if (qFloatDistance(a, b) < (1 << 7)) { // The last 7 bits are not + // significant + // precise enough + } + \endcode + + \sa qFuzzyCompare() + \relates +*/ +Q_CORE_EXPORT quint32 qFloatDistance(float a, float b) +{ + static const quint32 smallestPositiveFloatAsBits = 0x00000001; // denormalized, (SMALLEST), (1.4E-45) + /* Assumes: + * IEE754 format. + * Integers and floats have the same endian + */ + Q_STATIC_ASSERT(sizeof(quint32) == sizeof(float)); + Q_ASSERT(qIsFinite(a) && qIsFinite(b)); + if (a == b) + return 0; + if ((a < 0) != (b < 0)) { + // if they have different signs + if (a < 0) + a = -a; + else /*if (b < 0)*/ + b = -b; + return qFloatDistance(0.0F, a) + qFloatDistance(0.0F, b); + } + if (a < 0) { + a = -a; + b = -b; + } + // at this point a and b should not be negative + + // 0 is special + if (!a) + return f2i(b) - smallestPositiveFloatAsBits + 1; + if (!b) + return f2i(a) - smallestPositiveFloatAsBits + 1; + + // finally do the common integer subtraction + return a > b ? f2i(a) - f2i(b) : f2i(b) - f2i(a); +} + + +/*! + \internal + */ +static inline quint64 d2i(double d) +{ + quint64 i; + memcpy(&i, &d, sizeof(d)); + return i; +} + +/*! + Returns the number of representable floating-point numbers between \a a and \a b. + + This function serves the same purpose as \c{qFloatDistance(float, float)}, but + returns the distance between two \c double numbers. Since the range is larger + than for two \c float numbers (\c{[-DBL_MAX,DBL_MAX]}), the return type is quint64. + + + \sa qFuzzyCompare() + \relates +*/ +Q_CORE_EXPORT quint64 qFloatDistance(double a, double b) +{ + static const quint64 smallestPositiveFloatAsBits = 0x1; // denormalized, (SMALLEST) + /* Assumes: + * IEE754 format double precision + * Integers and floats have the same endian + */ + Q_STATIC_ASSERT(sizeof(quint64) == sizeof(double)); + Q_ASSERT(qIsFinite(a) && qIsFinite(b)); + if (a == b) + return 0; + if ((a < 0) != (b < 0)) { + // if they have different signs + if (a < 0) + a = -a; + else /*if (b < 0)*/ + b = -b; + return qFloatDistance(0.0, a) + qFloatDistance(0.0, b); + } + if (a < 0) { + a = -a; + b = -b; + } + // at this point a and b should not be negative + + // 0 is special + if (!a) + return d2i(b) - smallestPositiveFloatAsBits + 1; + if (!b) + return d2i(a) - smallestPositiveFloatAsBits + 1; + + // finally do the common integer subtraction + return a > b ? d2i(a) - d2i(b) : d2i(b) - d2i(a); +} + + QT_END_NAMESPACE -- cgit v1.2.3