From c9b7de3b70298367aa427e81146c9f549480e960 Mon Sep 17 00:00:00 2001
From: Ray Speth <speth@mit.edu>
Date: Sun, 18 Nov 2018 23:07:11 -0500
Subject: [PATCH] Add support for metric prefixes to class Units

---
 include/cantera/base/Units.h |  2 ++
 src/base/Units.cpp           | 35 +++++++++++++++++++++++++++++++++--
 test/general/test_units.cpp  |  8 ++++++++
 3 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/include/cantera/base/Units.h b/include/cantera/base/Units.h
index d573f83e0..b9fac71a5 100644
--- a/include/cantera/base/Units.h
+++ b/include/cantera/base/Units.h
@@ -79,6 +79,8 @@ private:
  *     m*s^-2
  *     J/kg/K
  *
+ * Metric prefixes are recognized for all units, e.g. nm, hPa, mg, EJ, mL, kcal.
+ *
  * @ingroup inputfiles
  */
 class UnitSystem
diff --git a/src/base/Units.cpp b/src/base/Units.cpp
index 010b3cea2..6d4a3e6d2 100644
--- a/src/base/Units.cpp
+++ b/src/base/Units.cpp
@@ -70,6 +70,27 @@ const std::map<std::string, Units> knownUnits{
     {"coulomb", Units(1.0, 0, 0, 1, 0, 1)}, // A*s
 };
 
+const std::map<std::string, double> prefixes{
+    {"Y", 1e24},
+    {"Z", 1e21},
+    {"E", 1e18},
+    {"P", 1e15},
+    {"T", 1e12},
+    {"G", 1e9},
+    {"M", 1e6},
+    {"k", 1e3},
+    {"h", 1e2},
+    {"d", 1e-1},
+    {"c", 1e-2},
+    {"m", 1e-3},
+    {"u", 1e-6},
+    {"n", 1e-9},
+    {"p", 1e-12},
+    {"f", 1e-15},
+    {"a", 1e-18},
+    {"z", 1e-21},
+    {"y", 1e-24}
+};
 }
 
 namespace Cantera
@@ -121,8 +142,18 @@ Units::Units(const std::string& name)
             // Incorporate the unit defined by the current group
             *this *= knownUnits.at(unit).pow(exponent);
         } else {
-            throw CanteraError("Units::Units(string)",
-                "Unknown unit '{}' in unit string '{}'", unit, name);
+            // See if the unit looks like a prefix + base unit
+            std::string prefix = unit.substr(0, 1);
+            std::string suffix = unit.substr(1);
+            if (prefixes.find(prefix) != prefixes.end() &&
+                knownUnits.find(suffix) != knownUnits.end()) {
+                Units u = knownUnits.at(suffix);
+                u.scale(prefixes.at(prefix));
+                *this *= u.pow(exponent);
+            } else {
+                throw CanteraError("Units::Units(string)",
+                    "Unknown unit '{}' in unit string '{}'", unit, name);
+            }
         }
 
         start = stop+1;
diff --git a/test/general/test_units.cpp b/test/general/test_units.cpp
index 1853a0b0f..133c575c0 100644
--- a/test/general/test_units.cpp
+++ b/test/general/test_units.cpp
@@ -30,3 +30,11 @@ TEST(Units, basic_conversions) {
     EXPECT_DOUBLE_EQ(U.convert(2, "m^3", "l"), 2000);
     EXPECT_DOUBLE_EQ(U.convert(1, "atm", "Pa"), 101325);
 }
+
+TEST(Units, prefixes) {
+    UnitSystem U;
+    EXPECT_DOUBLE_EQ(U.convert(1.0, "MJ", "J"), 1e6);
+    EXPECT_DOUBLE_EQ(U.convert(1.0, "nm", "cm"), 1e-7);
+    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);
+}