filter: Reduce redundant path and pattern splitting

A single call to filter.List will split the path only once and also
split each search pattern only once and use it for both match and
childMatch.

name                          old time/op    new time/op    delta
FilterPatterns/Relative-4       62.1ms ±15%    30.3ms ±10%  -51.22%  (p=0.000 n=9+10)
FilterPatterns/Absolute-4        111ms ±10%      49ms ± 3%  -56.08%  (p=0.000 n=10+8)
FilterPatterns/Wildcard-4        393ms ±15%     345ms ± 9%  -12.30%  (p=0.000 n=10+10)
FilterPatterns/ManyNoMatch-4     10.0s ± 3%      3.9s ± 2%  -60.53%  (p=0.000 n=10+9)

name                          old alloc/op   new alloc/op   delta
FilterPatterns/Relative-4       16.4MB ± 0%     4.6MB ± 0%  -71.76%  (p=0.000 n=10+9)
FilterPatterns/Absolute-4       31.4MB ± 0%     8.5MB ± 0%  -72.77%  (p=0.000 n=9+10)
FilterPatterns/Wildcard-4        168MB ± 0%     146MB ± 0%  -13.19%  (p=0.000 n=10+9)
FilterPatterns/ManyNoMatch-4    3.23GB ± 0%    0.91GB ± 0%  -71.96%  (p=0.000 n=10+9)

name                          old allocs/op  new allocs/op  delta
FilterPatterns/Relative-4         178k ± 0%       67k ± 0%  -62.50%  (p=0.000 n=10+10)
FilterPatterns/Absolute-4         266k ± 0%       89k ± 0%  -66.67%  (p=0.000 n=10+10)
FilterPatterns/Wildcard-4        1.87M ± 0%     1.70M ± 0%   -9.47%  (p=0.000 n=10+10)
FilterPatterns/ManyNoMatch-4     17.7M ± 0%      4.5M ± 0%  -74.87%  (p=0.000 n=9+10)

internal/filter/filter.go

@@ -11,6 +11,32 @@ import (
 // second argument.
 var ErrBadString = errors.New("filter.Match: string is empty")
 
+type filterPattern []string
+
+func prepareStr(str string) ([]string, error) {
+	if str == "" {
+		return nil, ErrBadString
+	}
+
+	// convert file path separator to '/'
+	if filepath.Separator != '/' {
+		str = strings.Replace(str, string(filepath.Separator), "/", -1)
+	}
+
+	return strings.Split(str, "/"), nil
+}
+
+func preparePattern(pattern string) filterPattern {
+	pattern = filepath.Clean(pattern)
+
+	// convert file path separator to '/'
+	if filepath.Separator != '/' {
+		pattern = strings.Replace(pattern, string(filepath.Separator), "/", -1)
+	}
+
+	return strings.Split(pattern, "/")
+}
+
 // Match returns true if str matches the pattern. When the pattern is
 // malformed, filepath.ErrBadPattern is returned. The empty pattern matches
 // everything, when str is the empty string ErrBadString is returned.
@@ -26,21 +52,13 @@ func Match(pattern, str string) (matched bool, err error) {
 		return true, nil
 	}
 
-	pattern = filepath.Clean(pattern)
+	patterns := preparePattern(pattern)
+	strs, err := prepareStr(str)
 
-	if str == "" {
+	if err != nil {
-		return false, ErrBadString
+		return false, err
 	}
 
-	// convert file path separator to '/'
-	if filepath.Separator != '/' {
-		pattern = strings.Replace(pattern, string(filepath.Separator), "/", -1)
-		str = strings.Replace(str, string(filepath.Separator), "/", -1)
-	}
-
-	patterns := strings.Split(pattern, "/")
-	strs := strings.Split(str, "/")
-
 	return match(patterns, strs)
 }
 
@@ -59,21 +77,13 @@ func ChildMatch(pattern, str string) (matched bool, err error) {
 		return true, nil
 	}
 
-	pattern = filepath.Clean(pattern)
+	patterns := preparePattern(pattern)
+	strs, err := prepareStr(str)
 
-	if str == "" {
+	if err != nil {
-		return false, ErrBadString
+		return false, err
 	}
 
-	// convert file path separator to '/'
-	if filepath.Separator != '/' {
-		pattern = strings.Replace(pattern, string(filepath.Separator), "/", -1)
-		str = strings.Replace(str, string(filepath.Separator), "/", -1)
-	}
-
-	patterns := strings.Split(pattern, "/")
-	strs := strings.Split(str, "/")
-
 	return childMatch(patterns, strs)
 }
 
@@ -162,17 +172,26 @@ func match(patterns, strs []string) (matched bool, err error) {
 // List returns true if str matches one of the patterns. Empty patterns are
 // ignored.
 func List(patterns []string, str string) (matched bool, childMayMatch bool, err error) {
+	if len(patterns) == 0 {
+		return false, false, nil
+	}
+
+	strs, err := prepareStr(str)
+	if err != nil {
+		return false, false, err
+	}
 	for _, pat := range patterns {
 		if pat == "" {
 			continue
 		}
 
-		m, err := Match(pat, str)
+		pats := preparePattern(pat)
+		m, err := match(pats, strs)
 		if err != nil {
 			return false, false, err
 		}
 
-		c, err := ChildMatch(pat, str)
+		c, err := childMatch(pats, strs)
 		if err != nil {
 			return false, false, err
 		}