filter: ability to use negative patterns

This is quite similar to gitignore. If a pattern is suffixed by an
exclamation mark and match a file that was previously matched by a
regular pattern, the match is cancelled. Notably, this can be used
with `--exclude-file` to cancel the exclusion of some files.

Like for gitignore, once a directory is excluded, it is not possible
to include files inside the directory. For example, a user wanting to
only keep `*.c` in some directory should not use:

    ~/work
    !~/work/*.c

But:

    ~/work/*
    !~/work/*.c

I didn't write documentation or changelog entry. I would like to get
feedback if this is the right approach for excluding/including files
at will for backups. I use something like this as an exclude file to
backup my home:

    $HOME/**/*
    !$HOME/Documents
    !$HOME/code
    !$HOME/.emacs.d
    !$HOME/games
    # [...]
    node_modules
    *~
    *.o
    *.lo
    *.pyc
    # [...]
    $HOME/code/linux/*
    !$HOME/code/linux/.git
    # [...]

There are some limitations for this change:

 - Patterns are not mixed accross methods: patterns from file are
   handled first and if a file is excluded with this method, it's not
   possible to reinclude it with `--exclude !something`.

 - Patterns starting with `!` are now interpreted as a negative
   pattern. I don't think anyone was relying on that.

 - The whole list of patterns is walked for each match. We may
   optimize later by exiting early if we know no pattern is starting
   with `!`.

Fix #233

internal/filter/filter.go

@@ -18,7 +18,8 @@ type patternPart struct {
 
 // Pattern represents a preparsed filter pattern
 type Pattern struct {
-	parts []patternPart
+	parts     []patternPart
+	isNegated bool
 }
 
 func prepareStr(str string) ([]string, error) {
@@ -29,6 +30,12 @@ func prepareStr(str string) ([]string, error) {
 }
 
 func preparePattern(patternStr string) Pattern {
+	var negate bool
+	if patternStr[0] == '!' {
+		negate = true
+		patternStr = patternStr[1:]
+	}
+
 	pathParts := splitPath(filepath.Clean(patternStr))
 	parts := make([]patternPart, len(pathParts))
 	for i, part := range pathParts {
@@ -41,7 +48,7 @@ func preparePattern(patternStr string) Pattern {
 		parts[i] = patternPart{part, isSimple}
 	}
 
-	return Pattern{parts}
+	return Pattern{parts, negate}
 }
 
 // Split p into path components. Assuming p has been Cleaned, no component
@@ -123,7 +130,7 @@ func childMatch(pattern Pattern, strs []string) (matched bool, err error) {
 	} else {
 		l = len(strs)
 	}
-	return match(Pattern{pattern.parts[0:l]}, strs)
+	return match(Pattern{pattern.parts[0:l], pattern.isNegated}, strs)
 }
 
 func hasDoubleWildcard(list Pattern) (ok bool, pos int) {
@@ -151,7 +158,7 @@ func match(pattern Pattern, strs []string) (matched bool, err error) {
 			}
 			newPat = append(newPat, pattern.parts[pos+1:]...)
 
-			matched, err := match(Pattern{newPat}, strs)
+			matched, err := match(Pattern{newPat, pattern.isNegated}, strs)
 			if err != nil {
 				return false, err
 			}
@@ -234,7 +241,9 @@ func ListWithChild(patterns []Pattern, str string) (matched bool, childMayMatch
 	return list(patterns, true, str)
 }
 
-// List returns true if str matches one of the patterns. Empty patterns are ignored.
+// list returns true if str matches one of the patterns. Empty patterns are ignored.
+// Patterns prefixed by "!" are negated: any matching file excluded by a previous pattern
+// will become included again.
 func list(patterns []Pattern, checkChildMatches bool, str string) (matched bool, childMayMatch bool, err error) {
 	if len(patterns) == 0 {
 		return false, false, nil
@@ -260,11 +269,12 @@ func list(patterns []Pattern, checkChildMatches bool, str string) (matched bool,
 			c = true
 		}
 
-		matched = matched || m
-		childMayMatch = childMayMatch || c
-
-		if matched && childMayMatch {
-			return true, true, nil
+		if pat.isNegated {
+			matched = matched && !m
+			childMayMatch = childMayMatch && !m
+		} else {
+			matched = matched || m
+			childMayMatch = childMayMatch || c
 		}
 	}
 

internal/filter/filter_test.go

@@ -259,7 +259,20 @@ var filterListTests = []struct {
 	{[]string{"/*/*/bar/test.*"}, "/foo/bar/bar", false, true},
 	{[]string{"/*/*/bar/test.*", "*.go"}, "/foo/bar/test.go", true, true},
 	{[]string{"", "*.c"}, "/foo/bar/test.go", false, true},
+	{[]string{"!**", "*.go"}, "/foo/bar/test.go", true, true},
+	{[]string{"!**", "*.c"}, "/foo/bar/test.go", false, true},
+	{[]string{"/foo/*/test.*", "!*.c"}, "/foo/bar/test.c", false, false},
+	{[]string{"/foo/*/test.*", "!*.c"}, "/foo/bar/test.go", true, true},
+	{[]string{"/foo/*/*", "!test.*", "*.c"}, "/foo/bar/test.go", false, true},
+	{[]string{"/foo/*/*", "!test.*", "*.c"}, "/foo/bar/test.c", true, true},
+	{[]string{"/foo/*/*", "!test.*", "*.c"}, "/foo/bar/file.go", true, true},
+	{[]string{"/**/*", "!/foo", "/foo/*", "!/foo/bar"}, "/foo/other/test.go", true, true},
+	{[]string{"/**/*", "!/foo", "/foo/*", "!/foo/bar"}, "/foo/bar", false, false},
+	{[]string{"/**/*", "!/foo", "/foo/*", "!/foo/bar"}, "/foo/bar/test.go", false, false},
+	{[]string{"/**/*", "!/foo", "/foo/*", "!/foo/bar"}, "/foo/bar/test.go/child", false, false},
+	{[]string{"/**/*", "!/foo", "/foo/*", "!/foo/bar", "/foo/bar/test*"}, "/foo/bar/test.go/child", true, true},
 	{[]string{"/foo/bar/*"}, "/foo", false, true},
+	{[]string{"/foo/bar/*", "!/foo/bar/[a-m]*"}, "/foo", false, true},
 	{[]string{"/foo/**/test.c"}, "/foo/bar/foo/bar/test.c", true, true},
 	{[]string{"/foo/*/test.c"}, "/foo/bar/foo/bar/test.c", false, false},
 }