uhd: Harmonize fuzzy frequency comparisons

Throughout UHD, we often do floating-point comparisons for frequency
ranges that require resilience to floating point rounding errors. Most
of the time the checks look like this:

```cpp
if (fp_compare_epsilon<double>(freq) > boundary) {
    // ...
}
```

The exception is the N320 daughterboard control, which uses a custom
epsilon:
```cpp
if (fp_compare_epsilon<double>(freq,
        RHODIUM_FREQ_COMPARE_EPSILON) > boundary) {
    // ...
}
```

This was, for the most part, not by design, but because authors simply
didn't think about which epsilon value was appropriate for the frequency
comparison. This was complicated by the fact that fp_compare_epsilon
previously had some issues.

This patch introduces FREQ_COMPARE_EPSILON, which is a sensible default
value for fp_compare_epsilon when doing frequency comparisons (note that
fp_compare_delta already had such a value).
Also, it introduces freq_compare_epsilon(x), which is a shorthand for
fp_compare_epsilon<double>(x, FREQ_COMPARE_EPSILON).

We then replace all occurrences of fp_compare_epsilon<double> which are
specific to frequency checks with freq_compare_epsilon.

1 files changed, 26 insertions, 0 deletions

diff --git a/host/include/uhd/utils/math.hpp b/host/include/uhd/utils/math.hpp
index 6ee46e98a..c44aa4db9 100644
--- a/host/include/uhd/utils/math.hpp
+++ b/host/include/uhd/utils/math.hpp
@@ -51,6 +51,16 @@ static const double PI = 3.14159265358979323846;
 static const float SINGLE_PRECISION_EPSILON  = 1.19e-7f;
 static const double DOUBLE_PRECISION_EPSILON = 2.22e-16;
 
+//! Epsilon value for when using fp_compare_epsilon to compare frequencies
+//
+// Note that this is a UHD/USRP-specific constant. In UHD, frequencies are on
+// the order of 1e9 (GHz) or below. We will declare frequencies as equal if they
+// are within a millihertz.  For the purpose of
+// comparing frequencies, we "lose" 9 decimal places for the integer
+// component of the frequency, so we choose this epsilon value for floating
+// point comparison of frequencies.
+static constexpr double FREQ_COMPARE_EPSILON = 1e-12;
+
 namespace fp_compare {
 
 /*!
@@ -142,6 +152,22 @@ UHD_INLINE bool operator>(double lhs, fp_compare_epsilon<float_t> rhs);
 template <typename float_t>
 UHD_INLINE bool operator>=(double lhs, fp_compare_epsilon<float_t> rhs);
 
+//! An alias for fp_compare_epsilon, but with defaults for frequencies
+class freq_compare_epsilon : public fp_compare_epsilon<double>
+{
+public:
+    UHD_INLINE freq_compare_epsilon(double value)
+        : fp_compare_epsilon<double>(value, FREQ_COMPARE_EPSILON)
+    {
+    }
+
+    UHD_INLINE freq_compare_epsilon(const freq_compare_epsilon& copy)
+        : fp_compare_epsilon<double>(copy)
+    {
+    }
+};
+
+
 } // namespace fp_compare