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path/filepath: optimize isReservedName

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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 <dneil@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@google.com>
This commit is contained in:
Joe Tsai 2022-09-22 16:54:19 -07:00 committed by Gopher Robot
parent c2ede92a0d
commit 2551324cd0
1 changed files with 14 additions and 14 deletions
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28
src/path/filepath/path_windows.go
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@ -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