engine: don't construct diff string if it isn't requested (#287)

* engine: don't calculate diff if it isn't requested

The original code would calculate the diff on every file regardless of
whether it was asked for because of the way the debug logging code was
structured. This PR adds a way to manually check for active debug code,
and uses that to avoid calculating the diff unnecessarily.

* clarify comment wording

Author: braydonk

engine/consecutive_engine.go

@@ -39,11 +39,16 @@ func (e *ConsecutiveEngine) FormatContent(content []byte) ([]byte, error) {
 func (e *ConsecutiveEngine) Format(paths []string) (fmt.Stringer, error) {
 	formatDiffs, formatErrs := e.formatAll(paths)
 
-	// Debug format diff output.
-	logger.Debug(
-		logger.DebugCodeDiffs,
-		fmt.Sprintf("The following files were modified:\n%s", formatDiffs.StrOutput()),
-	)
+	// Debug format diff output. Manually check for the debug code
+	// to be active since the diff string construction can be
+	// performance intensive, thus don't want to calculate it if
+	// it's not needed.
+	if logger.DebugCodeIsActive(logger.DebugCodeDiffs) {
+		logger.Debug(
+			logger.DebugCodeDiffs,
+			fmt.Sprintf("The following files were modified:\n%s", formatDiffs.StrOutput()),
+		)
+	}
 
 	if len(formatErrs) > 0 {
 		if e.ContinueOnError {

internal/logger/debug.go

@@ -45,6 +45,10 @@ func ActivateDebugCode(code string) {
 	}
 }
 
+func DebugCodeIsActive(code DebugCode) bool {
+	return activeDebugCodes.Contains(code)
+}
+
 // Debug prints a message if the given debug code is active.
 func Debug(code DebugCode, msg string, args ...any) {
 	if activeDebugCodes.Contains(code) {