Include the non-prefixed BaseUnits in the BaseUnitPrefixes (#1492)

Include the non-prefixed `BaseUnits` in the `BaseUnitPrefixes`:
- the `BaseUnitPrefixes` now also handles the case where prefixing an
already prefixed base unit results in a non-prefixed base unit
- regenerating the quantities resulted in a number of new `BaseUnits` to
become available (as well as some which got their scales reduced)

CodeGen/Helpers/PrefixBuilder/BaseUnitPrefix.cs

@@ -6,12 +6,13 @@
 namespace CodeGen.Helpers.PrefixBuilder;
 
 /// <summary>
-///     Represents a unique key that combines a base unit and a prefix.
+///     Represents a unique combination of a base unit and an optional prefix, used to identify a prefixed unit.
 /// </summary>
 /// <param name="BaseUnit">
-///     The base unit associated with the prefix. For example, "Gram".
+///     The base unit associated with the prefix, such as "Gram".
 /// </param>
 /// <param name="Prefix">
-///     The prefix applied to the base unit. For example, <see cref="JsonTypes.Prefix.Kilo" />.
+///     The prefix applied to the base unit, such as <see cref="JsonTypes.Prefix.Kilo" />.
+///     <para>If the prefix exponent value is 0, this parameter will be <c>null</c>.</para>
 /// </param>
-internal readonly record struct BaseUnitPrefix(string BaseUnit, Prefix Prefix);
+internal readonly record struct BaseUnitPrefix(string BaseUnit, Prefix? Prefix);

CodeGen/Helpers/PrefixBuilder/BaseUnitPrefixes.cs

@@ -77,6 +77,7 @@ public static BaseUnitPrefixes FromBaseUnits(IEnumerable<Unit> baseUnits)
         {
             var unitName = baseUnit.SingularName;
             prefixedStringFactors[unitName] = new PrefixScaleFactor(unitName, 0);
+            baseUnitPrefixConversions[new BaseUnitPrefix(unitName, null)] = unitName;
             foreach (Prefix prefix in baseUnit.Prefixes)
             {
                 var prefixedUnitName = prefix + unitName.ToCamelCase();
@@ -123,10 +124,20 @@ internal bool TryGetMatchingPrefix(string unitName, int exponent, Prefix prefix,
         {
             var (quotient, remainder) = int.DivRem(prefixFactor, exponent);
             // Ensure the prefix factor is divisible by the exponent without a remainder and that there is a valid prefix matching the target scale
-            if (remainder == 0 && TryGetPrefixWithScale(targetPrefixFactor.ScaleFactor + quotient, out Prefix calculatedPrefix))
+            if (remainder == 0)
             {
-                matchingPrefix = new BaseUnitPrefix(targetPrefixFactor.BaseUnit, calculatedPrefix);
-                return true;
+                if (targetPrefixFactor.ScaleFactor + quotient == 0)
+                {
+                    // when the resulting exponent is 0: return the non-prefixed BaseUnit
+                    matchingPrefix = new BaseUnitPrefix(targetPrefixFactor.BaseUnit, null);
+                    return true;
+                }
+
+                if (TryGetPrefixWithScale(targetPrefixFactor.ScaleFactor + quotient, out Prefix calculatedPrefix))
+                {
+                    matchingPrefix = new BaseUnitPrefix(targetPrefixFactor.BaseUnit, calculatedPrefix);
+                    return true;
+                }
             }
         }