Review feedback from @gregtatcam: STAmount

- Fix a couple of typos.
- Make fewer unneeded copies in roundToScale.

include/xrpl/protocol/STAmount.h

@@ -701,7 +701,7 @@ getRate(STAmount const& offerOut, STAmount const& offerIn);
  */
 STAmount
 roundToScale(
-    STAmount value,
+    STAmount const& value,
     std::int32_t scale,
     Number::rounding_mode rounding = Number::getround());
 
@@ -729,7 +729,7 @@ roundToAsset(
     STAmount const ret{asset, value};
     if (ret.integral())
         return ret;
-    // Not that the ctor will round integral types (XRP, MPT) via canonicalize,
+    // Note that the ctor will round integral types (XRP, MPT) via canonicalize,
     // so no extra work is needed for those.
     return roundToScale(ret, scale);
 }

src/libxrpl/protocol/STAmount.cpp

@@ -1462,9 +1462,12 @@ canonicalizeRoundStrict(
 }
 
 STAmount
-roundToScale(STAmount value, std::int32_t scale, Number::rounding_mode rounding)
+roundToScale(
+    STAmount const& value,
+    std::int32_t scale,
+    Number::rounding_mode rounding)
 {
-    // Nothing to do for intgral types.
+    // Nothing to do for integral types.
     if (value.integral())
         return value;
 
@@ -1474,16 +1477,15 @@ roundToScale(STAmount value, std::int32_t scale, Number::rounding_mode rounding)
     if (value.exponent() >= scale)
         return value;
 
-    STAmount referenceValue{
+    STAmount const referenceValue{
         value.asset(), STAmount::cMinValue, scale, value.negative()};
 
     NumberRoundModeGuard mg(rounding);
-    // With an IOU, the total will be truncated to the precision of the
-    // larger value: referenceValue
-    value += referenceValue;
-    // Remove the reference value, and we're left with the rounded value.
-    value -= referenceValue;
-    return value;
+    // With an IOU, the the result of addition will be truncated to the
+    // precision of the larger value, which in this case is referenceValue. Then
+    // remove the reference value via subtraction, and we're left with the
+    // rounded value.
+    return (value + referenceValue) - referenceValue;
 }
 
 namespace {