BroadcastTime string to rate quarters error handling (#716)

Returns zero from string_to_rate_quarters when there is a parse error on the input string.
Parse error is defined as not being able to take any prefix of the input string as a valid number.
Thus an empty string or "Freddy" turns into zero, but "123asdasd" turns into 123. 

===

* Move from stl stof() to libc strtof().

* Add string_to_rate_quarters unit test.

src/openlcb/BroadcastTimeDefs.cxx

@@ -139,7 +139,15 @@ bool BroadcastTimeDefs::string_to_time(
 //
 int16_t BroadcastTimeDefs::string_to_rate_quarters(const std::string &srate)
 {
-    float rate = std::stof(srate);
+    const char *rate_c_str = srate.c_str();
+    char *end;
+    float rate = strtof(rate_c_str, &end);
+
+    if (end == rate_c_str)
+    {
+        // None of the string processed.
+        return 0;
+    }
     if (rate < -512)
     {
         // set to minimum valid rate

src/openlcb/BroadcastTimeDefs.cxxtest

@@ -0,0 +1,46 @@
+#include "utils/async_if_test_helper.hxx"
+
+#include "openlcb/BroadcastTimeDefs.hxx"
+
+#if 0
+#define PRINT_ALL_PACKETS() print_all_packets()
+#else
+#define PRINT_ALL_PACKETS()
+#endif
+
+namespace openlcb
+{
+
+TEST(BroadcastTimeDefs, string_to_rate_quarters)
+{
+    int16_t rate;
+
+    rate = BroadcastTimeDefs::string_to_rate_quarters("Freddy");
+    EXPECT_EQ(0, rate);
+
+    rate = BroadcastTimeDefs::string_to_rate_quarters("2000");
+    EXPECT_EQ(2047, rate);
+
+    rate = BroadcastTimeDefs::string_to_rate_quarters("-2000");
+    EXPECT_EQ(-2048, rate);
+
+    rate = BroadcastTimeDefs::string_to_rate_quarters("");
+    EXPECT_EQ(0, rate);
+
+    rate = BroadcastTimeDefs::string_to_rate_quarters("-");
+    EXPECT_EQ(0, rate);
+
+    rate = BroadcastTimeDefs::string_to_rate_quarters("    ");
+    EXPECT_EQ(0, rate);
+
+    rate = BroadcastTimeDefs::string_to_rate_quarters("511,75");
+    EXPECT_EQ(2044, rate);
+
+    rate = BroadcastTimeDefs::string_to_rate_quarters("1.75");
+    EXPECT_EQ(7, rate);
+
+    rate = BroadcastTimeDefs::string_to_rate_quarters("-1.75");
+    EXPECT_EQ(-7, rate);
+}
+
+} // namespace openlcb

src/openlcb/BroadcastTimeDefs.hxx

@@ -362,7 +362,9 @@ struct BroadcastTimeDefs
 
     /// Convert a string (float) to rate quarters.
     /// @param srate rate in the form of a string float
-    /// @return rate in the form of an int16_t in rate quarters
+    /// @return Rate in the form of an int16_t in rate quarters. If a
+    ///         conversion error occurs, then the returned value will be 0,
+    ///         which is at least a valid rate.
     static int16_t string_to_rate_quarters(const std::string &srate);
 };