Cache sorted flags

As you can't delete flags in FlagSet, this method works

Gainful performance improvement:
benchmark                      old ns/op     new ns/op     delta
BenchmarkVisitAll10Flags-4     1549          33.9          -97.81%
BenchmarkVisit10Flags-4        1704          34.2          -97.99%

benchmark                      old allocs     new allocs     delta
BenchmarkVisitAll10Flags-4     3              0              -100.00%
BenchmarkVisit10Flags-4        3              0              -100.00%

benchmark                      old bytes     new bytes     delta
BenchmarkVisitAll10Flags-4     272           0             -100.00%
BenchmarkVisit10Flags-4        272           0             -100.00%

1 files changed, 19 insertions, 2 deletions

diff --git a/flag.go b/flag.go
index 3a2e255..2bfedff 100644
--- a/flag.go
+++ b/flag.go
@@ -142,8 +142,10 @@ type FlagSet struct {
 	parsed            bool
 	actual            map[NormalizedName]*Flag
 	orderedActual     []*Flag
+	sortedActual      []*Flag
 	formal            map[NormalizedName]*Flag
 	orderedFormal     []*Flag
+	sortedFormal      []*Flag
 	shorthands        map[byte]*Flag
 	args              []string // arguments after flags
 	argsLenAtDash     int      // len(args) when a '--' was located when parsing, or -1 if no --
@@ -200,6 +202,7 @@ func sortFlags(flags map[NormalizedName]*Flag) []*Flag {
 // "--getUrl" which may also be translated to "geturl" and everything will work.
 func (f *FlagSet) SetNormalizeFunc(n func(f *FlagSet, name string) NormalizedName) {
 	f.normalizeNameFunc = n
+	f.sortedFormal = f.sortedFormal[:0]
 	for k, v := range f.orderedFormal {
 		delete(f.formal, NormalizedName(v.Name))
 		nname := f.normalizeFlagName(v.Name)
@@ -240,9 +243,16 @@ func (f *FlagSet) SetOutput(output io.Writer) {
 // in primordial order if f.SortFlags is false, calling fn for each.
 // It visits all flags, even those not set.
 func (f *FlagSet) VisitAll(fn func(*Flag)) {
+	if len(f.formal) == 0 {
+		return
+	}
+
 	var flags []*Flag
 	if f.SortFlags {
-		flags = sortFlags(f.formal)
+		if len(f.formal) != len(f.sortedFormal) {
+			f.sortedFormal = sortFlags(f.formal)
+		}
+		flags = f.sortedFormal
 	} else {
 		flags = f.orderedFormal
 	}
@@ -279,9 +289,16 @@ func VisitAll(fn func(*Flag)) {
 // in primordial order if f.SortFlags is false, calling fn for each.
 // It visits only those flags that have been set.
 func (f *FlagSet) Visit(fn func(*Flag)) {
+	if len(f.actual) == 0 {
+		return
+	}
+
 	var flags []*Flag
 	if f.SortFlags {
-		flags = sortFlags(f.actual)
+		if len(f.actual) != len(f.sortedActual) {
+			f.sortedActual = sortFlags(f.actual)
+		}
+		flags = f.sortedActual
 	} else {
 		flags = f.orderedActual
 	}