Avoid two unnecessary string.Substring calls in PunycodeDecode (#18546)

stephentoub · web-flow · commit 90850134cb27 · 2018-06-19T17:28:32.000-04:00
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/IdnMapping.cs b/src/System.Private.CoreLib/shared/System/Globalization/IdnMapping.cs
@@ -638,10 +638,10 @@ private static string PunycodeDecode(string ascii)
 
                 // See if this section's ASCII or ACE
                 if (ascii.Length < c_strAcePrefix.Length + iAfterLastDot || 
-                    !ascii.Substring(iAfterLastDot,c_strAcePrefix.Length).Equals(c_strAcePrefix, StringComparison.OrdinalIgnoreCase))
+                    string.Compare(ascii, iAfterLastDot, c_strAcePrefix, 0, c_strAcePrefix.Length, StringComparison.OrdinalIgnoreCase) != 0)
                 {
                     // Its ASCII, copy it
-                    output.Append(ascii.Substring(iAfterLastDot, iNextDot - iAfterLastDot));
+                    output.Append(ascii, iAfterLastDot, iNextDot - iAfterLastDot);
                 }
                 else
                 {

Original file line number	Diff line number	Diff line change
`@@ -638,10 +638,10 @@ private static string PunycodeDecode(string ascii)`
`638`	`638`
`639`	`639`	`// See if this section's ASCII or ACE`
`640`	`640`	`if (ascii.Length < c_strAcePrefix.Length + iAfterLastDot \|\|`
`641`		`- !ascii.Substring(iAfterLastDot,c_strAcePrefix.Length).Equals(c_strAcePrefix, StringComparison.OrdinalIgnoreCase))`
	`641`	`+ string.Compare(ascii, iAfterLastDot, c_strAcePrefix, 0, c_strAcePrefix.Length, StringComparison.OrdinalIgnoreCase) != 0)`
`642`	`642`	`{`
`643`	`643`	`// Its ASCII, copy it`
`644`		`- output.Append(ascii.Substring(iAfterLastDot, iNextDot - iAfterLastDot));`
	`644`	`+ output.Append(ascii, iAfterLastDot, iNextDot - iAfterLastDot);`
`645`	`645`	`}`
`646`	`646`	`else`
`647`	`647`	`{`