path/filepath: optimize isReservedName

A linear search through a list of 22 strings takes ~80ns.
A quick check for 3-4 byte strings reduces this check to 2ns
for a vast majority of inputs.
In the event of a name match, the new logic is either just
as fast (for "CON") or 10x faster (for "LPT9").

Change-Id: I412fa73beebd7c81dc95f9ed12c35ca1d5d6baf0
Reviewed-on: https://go-review.googlesource.com/c/go/+/433175
Reviewed-by: Damien Neil <[email protected]>
Run-TryBot: Joseph Tsai <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Joseph Tsai <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

src/path/filepath/path_windows.go

@@ -13,24 +13,24 @@ func isSlash(c uint8) bool {
  return c == '\\' || c == '/'
 }
 
-// reservedNames lists reserved Windows names. Search for PRN in
-// https://docs.microsoft.com/en-us/windows/desktop/fileio/naming-a-file
-// for details.
-var reservedNames = []string{
- "CON", "PRN", "AUX", "NUL",
- "COM1", "COM2", "COM3", "COM4", "COM5", "COM6", "COM7", "COM8", "COM9",
- "LPT1", "LPT2", "LPT3", "LPT4", "LPT5", "LPT6", "LPT7", "LPT8", "LPT9",
-}
-
 // isReservedName returns true, if path is Windows reserved name.
 // See reservedNames for the full list.
 func isReservedName(path string) bool {
- if len(path) == 0 {
- return false
+ toUpper := func(c byte) byte {
+ if 'a' <= c && c <= 'z' {
+ return c - ('a' - 'A')
+ }
+ return c
  }
- for _, reserved := range reservedNames {
- if strings.EqualFold(path, reserved) {
- return true
+
+ // For details, search for PRN in
+ // https://docs.microsoft.com/en-us/windows/desktop/fileio/naming-a-file.
+ if 3 <= len(path) && len(path) <= 4 {
+ switch string([]byte{toUpper(path[0]), toUpper(path[1]), toUpper(path[2])}) {
+ case "CON", "PRN", "AUX", "NUL":
+ return len(path) == 3
+ case "COM", "LPT":
+ return len(path) == 4 && '1' <= path[3] && path[3] <= '9'
  }
  }
  return false