Reduced the calls to the complex value parsing in units.py

Now we call it just once for each component and store the result.
Then we use the stored value instead of computing it over and over.
In this way we save time and warnings about malformed values are
reported once. It reduced the number of warnings in the test case
from 39 to 3.

kiplot/bom/bom.py

@@ -35,7 +35,7 @@ DNC = {
     "no change": 1,
     "fixed": 1
 }
-# RV == Resistor Variable
+# RV == Resistor Variable or Varistor
 # RN == Resistor 'N'(Pack)
 # RT == Thermistor
 RLC_PREFIX = {'R': 1, 'L': 1, 'C': 1, 'RV': 1, 'RN': 1, 'RT': 1}
@@ -47,7 +47,7 @@ def compare_value(c1, c2, cfg):
     if c1.value.lower() == c2.value.lower():
         return True
     # Otherwise, perform a more complicated value comparison
-    if compare_values(c1.value, c2.value):
+    if compare_values(c1, c2):
         return True
     # Ignore value if both components are connectors
     # Note: Is common practice to use the "Value" field of connectors to denote its use.
@@ -313,18 +313,16 @@ def test_reg_include(cfg, c):
 
 def get_value_sort(comp):
     """ Try to better sort R, L and C components """
-    pref = comp.ref_prefix
-    if pref in RLC_PREFIX:
-        res = comp_match(comp.value)
-        if res:
-            value, mult, unit = res
-            if pref in "CL":
-                # fempto Farads
-                value = "{0:15d}".format(int(value * 1e15 * mult + 0.1))
-            else:
-                # milli Ohms
-                value = "{0:15d}".format(int(value * 1000 * mult + 0.1))
-            return value
+    res = comp.value_sort
+    if res:
+        value, mult, unit = res
+        if comp.ref_prefix in "CL":
+            # fempto Farads
+            value = "{0:15d}".format(int(value * 1e15 * mult + 0.1))
+        else:
+            # milli Ohms
+            value = "{0:15d}".format(int(value * 1000 * mult + 0.1))
+        return value
     return comp.value
 
 
@@ -338,6 +336,11 @@ def group_components(cfg, components):
                 continue
             if test_reg_exclude(cfg, c):
                 continue
+        # Cache the value used to sort
+        if c.ref_prefix in RLC_PREFIX:
+            c.value_sort = comp_match(c.value)
+        else:
+            c.value_sort = None
         # Try to add the component to an existing group
         found = False
         for g in groups:

kiplot/bom/units.py

@@ -166,8 +166,9 @@ def comp_match(component):
 def compare_values(c1, c2):
     """ Compare two values """
 
-    r1 = comp_match(c1)
-    r2 = comp_match(c2)
+    # These are the results from comp_match()
+    r1 = c1.value_sort
+    r2 = c2.value_sort
 
     if not r1 or not r2:
         return False