UnitSystem class supports default source units for conversions

include/cantera/base/Units.h

@@ -63,6 +63,7 @@ private:
     double m_temperature_dim;
     double m_current_dim;
     double m_quantity_dim;
+    double m_pressure_dim; //!< pseudo-dimension to track explicit pressure units
 
     friend class UnitSystem;
 };
@@ -70,6 +71,11 @@ private:
 
 //! Unit conversion utility
 /*!
+ * Provides functions for converting dimensional values from a given unit system.
+ * The main use is for converting values specified in input files to Cantera's
+ * native unit system, which is SI units except for the use of kmol as the base
+ * unit of quantity, i.e. kilogram, meter, second, kelvin, ampere, and kmol.
+ *
  * String representations of units can be written using multiplication,
  * division, and exponentiation. Spaces are ignored. Positive, negative, and
  * decimal exponents are permitted. Examples:
@@ -86,10 +92,45 @@ private:
 class UnitSystem
 {
 public:
+    //! Create a unit system with the specified default units
+    UnitSystem(std::initializer_list<std::string> units={});
+
+    //! Set the default units to convert from when explicit units are not
+    //! provided. Defaults can be set for mass, length, time, quantity, and
+    //! pressure. Conversion using the pressure unit is done only when the
+    //! target units explicitly contain pressure units.
+    //!
+    //! * To use SI+kmol: `setDefaults({"kg", "m", "s", "kmol"});`
+    //! * To use CGS+mol: `setDefaults({"cm", "g", "mol"});`
+    void setDefaults(std::initializer_list<std::string> units);
+
     //! Convert `value` from the units of `src` to the units of `dest`.
     double convert(double value, const std::string& src,
                    const std::string& dest) const;
     double convert(double value, const Units& src, const Units& dest) const;
+    //! Convert `value` from this unit system (defined by `setDefaults`) to the
+    //! specified units.
+    double convert(double value, const std::string& dest) const;
+    double convert(double value, const Units& dest) const;
+
+private:
+    //! Factor to convert mass from this unit system to kg
+    double m_mass_factor;
+
+    //! Factor to convert length from this unit system to meters
+    double m_length_factor;
+
+    //! Factor to convert time from this unit system to seconds
+    double m_time_factor;
+
+    //! Factor to convert energy from this unit system to Joules
+    double m_energy_factor;
+
+    //! Factor to convert pressure from this unit system to Pa
+    double m_pressure_factor;
+
+    //! Factor to convert quantity from this unit system to kmol
+    double m_quantity_factor;
 };
 
 }

src/base/Units.cpp

@@ -105,7 +105,13 @@ Units::Units(double factor, double mass, double length, double time,
     , m_temperature_dim(temperature)
     , m_current_dim(current)
     , m_quantity_dim(quantity)
+    , m_pressure_dim(0)
 {
+    if (mass != 0 && length == -mass && time == -2 * mass
+        && temperature == 0 && current == 0 && quantity == 0) {
+        // Dimension looks like Pa^n
+        m_pressure_dim = mass;
+    }
 }
 
 Units::Units(const std::string& name)
@@ -116,6 +122,7 @@ Units::Units(const std::string& name)
     , m_temperature_dim(0)
     , m_current_dim(0)
     , m_quantity_dim(0)
+    , m_pressure_dim(0)
 {
     size_t start = 0;
     while (true) {
@@ -182,6 +189,7 @@ Units& Units::operator*=(const Units& other)
     m_temperature_dim += other.m_temperature_dim;
     m_current_dim += other.m_current_dim;
     m_quantity_dim += other.m_quantity_dim;
+    m_pressure_dim += other.m_pressure_dim;
     return *this;
 }
 
@@ -202,6 +210,41 @@ std::string Units::str() const {
 }
 
 
+UnitSystem::UnitSystem(std::initializer_list<std::string> units)
+    : m_mass_factor(1.0)
+    , m_length_factor(1.0)
+    , m_time_factor(1.0)
+    , m_energy_factor(1.0)
+    , m_pressure_factor(1.0)
+    , m_quantity_factor(1.0)
+{
+    setDefaults(units);
+}
+
+void UnitSystem::setDefaults(std::initializer_list<std::string> units)
+{
+    for (const auto& name : units) {
+        auto unit = Units(name);
+        if (unit.convertible(knownUnits.at("kg"))) {
+            m_mass_factor = unit.factor();
+        } else if (unit.convertible(knownUnits.at("m"))) {
+            m_length_factor = unit.factor();
+        } else if (unit.convertible(knownUnits.at("s"))) {
+            m_time_factor = unit.factor();
+        } else if (unit.convertible(knownUnits.at("kmol"))) {
+            m_quantity_factor = unit.factor();
+        } else if (unit.convertible(knownUnits.at("Pa"))) {
+            m_pressure_factor = unit.factor();
+        } else if (unit.convertible(knownUnits.at("K"))
+                   || unit.convertible(knownUnits.at("A"))) {
+            // Do nothing -- no other scales are supported for temperature and current
+        } else {
+            throw CanteraError("UnitSystem::setDefaults",
+                "Unable to match unit '{}' to a basic dimension", name);
+        }
+    }
+}
+
 double UnitSystem::convert(double value, const std::string& src,
                            const std::string& dest) const
 {
@@ -218,4 +261,19 @@ double UnitSystem::convert(double value, const Units& src,
     return value * src.factor() / dest.factor();
 }
 
+double UnitSystem::convert(double value, const std::string& dest) const
+{
+    return convert(value, Units(dest));
+}
+
+double UnitSystem::convert(double value, const Units& dest) const
+{
+    return value / dest.factor()
+        * pow(m_mass_factor, dest.m_mass_dim - dest.m_pressure_dim)
+        * pow(m_length_factor, dest.m_length_dim + dest.m_pressure_dim)
+        * pow(m_time_factor, dest.m_time_dim + 2*dest.m_pressure_dim)
+        * pow(m_quantity_factor, dest.m_quantity_dim)
+        * pow(m_pressure_factor, dest.m_pressure_dim);
+}
+
 }

test/general/test_units.cpp

@@ -38,3 +38,14 @@ TEST(Units, prefixes) {
     EXPECT_DOUBLE_EQ(U.convert(1.0, "m^2", "cm^2"), 1e4);
     EXPECT_DOUBLE_EQ(U.convert(1.0, "m/s", "km/hr"), 3.6);
 }
+
+TEST(Units, with_defaults) {
+    UnitSystem U({"cm", "g", "mol", "atm"});
+    EXPECT_DOUBLE_EQ(U.convert(1.0, "m"), 0.01);
+    EXPECT_DOUBLE_EQ(U.convert(1.0, "kmol/m^3"), 1000);
+    EXPECT_DOUBLE_EQ(U.convert(1.0, "kg/kmol"), 1.0);
+    EXPECT_DOUBLE_EQ(U.convert(1.0, "cm^2"), 1.0);
+    EXPECT_DOUBLE_EQ(U.convert(1.0, "Pa"), 101325);
+    EXPECT_DOUBLE_EQ(U.convert(1.0, "hPa"), 1013.25);
+    EXPECT_DOUBLE_EQ(U.convert(1.0, "Pa*m^6/kmol"), 101325*1e-12*1000);
+}