Document log10 usage in TraceDiagnosticRepresentation.java

Signed-off-by: ascopes <73482956+ascopes@users.noreply.github.com>

Author: ascopes

java-compiler-testing/src/main/java/io/github/ascopes/jct/repr/TraceDiagnosticRepresentation.java

@@ -208,6 +208,12 @@ private Snippet(String text, long startLine, long startOffset, long endOffset) {
       this.endOffset = endOffset;
 
       // Width of the line number part of the output.
+      // Use log10 to determine the number of decimal digits
+      // to use. This allows us to produce a consistent width
+      // margin that will fit every line number. We only show a
+      // few lines in the snippet usually, so adding 1 more
+      // digit to this provides us safety if the snippet were
+      // to start on line 99 and move into line 100.
       lineNumberWidth = (int) Math.ceil(Math.log10(startLine)) + 1;
     }