buildtags: add Evaluate method

Updates #3

buildtags/buildtags.go

@@ -34,6 +34,7 @@ import (
 type Interface interface {
 	ConstraintsConvertable
 	fmt.GoStringer
+	Evaluate(v map[string]bool) bool
 	Validate() error
 }
 
@@ -66,6 +67,15 @@ func (cs Constraints) Validate() error {
 	}
 	return nil
 }
+
+func (cs Constraints) Evaluate(v map[string]bool) bool {
+	r := true
+	for _, c := range cs {
+		r = r && c.Evaluate(v)
+	}
+	return r
+}
+
 func (cs Constraints) GoString() string {
 	s := ""
 	for _, c := range cs {
@@ -86,6 +96,14 @@ func (c Constraint) Validate() error {
 	return nil
 }
 
+func (c Constraint) Evaluate(v map[string]bool) bool {
+	r := false
+	for _, o := range c {
+		r = r || o.Evaluate(v)
+	}
+	return r
+}
+
 func (c Constraint) GoString() string {
 	s := "// +build"
 	for _, o := range c {
@@ -107,6 +125,14 @@ func (o Option) Validate() error {
 	return nil
 }
 
+func (o Option) Evaluate(v map[string]bool) bool {
+	r := true
+	for _, t := range o {
+		r = r && t.Evaluate(v)
+	}
+	return r
+}
+
 func (o Option) GoString() string {
 	var ts []string
 	for _, t := range o {
@@ -119,22 +145,24 @@ func (t Term) ToConstraints() Constraints { return t.ToOption().ToConstraints()
 func (t Term) ToConstraint() Constraint   { return t.ToOption().ToConstraint() }
 func (t Term) ToOption() Option           { return Option{t} }
 
-func (t Term) Validate() error {
-	name := string(t)
+func (t Term) IsNegated() bool { return strings.HasPrefix(string(t), "!") }
 
+func (t Term) Name() string {
+	return strings.TrimPrefix(string(t), "!")
+}
+
+func (t Term) Validate() error {
 	// Reference: https://github.com/golang/go/blob/204a8f55dc2e0ac8d27a781dab0da609b98560da/src/cmd/go/internal/imports/build.go#L110-L112
 	//
 	//		if strings.HasPrefix(name, "!!") { // bad syntax, reject always
 	//			return false
 	//		}
 	//
-	if strings.HasPrefix(name, "!!") {
+	if strings.HasPrefix(string(t), "!!") {
 		return errors.New("at most one '!' allowed")
 	}
 
-	name = strings.TrimPrefix(name, "!")
-
-	if len(name) == 0 {
+	if len(t.Name()) == 0 {
 		return errors.New("empty tag name")
 	}
 
@@ -148,7 +176,7 @@ func (t Term) Validate() error {
 	//			}
 	//		}
 	//
-	for _, c := range name {
+	for _, c := range t.Name() {
 		if !unicode.IsLetter(c) && !unicode.IsDigit(c) && c != '_' && c != '.' {
 			return fmt.Errorf("character '%c' disallowed in tags", c)
 		}
@@ -157,6 +185,10 @@ func (t Term) Validate() error {
 	return nil
 }
 
+func (t Term) Evaluate(v map[string]bool) bool {
+	return (t.Validate() == nil) && (v[t.Name()] == !t.IsNegated())
+}
+
 func (t Term) GoString() string { return string(t) }
 
 func Not(ident string) Term {
@@ -202,3 +234,11 @@ func ParseConstraint(expr string) (Constraint, error) {
 	}
 	return c, nil
 }
+
+func SetTags(names ...string) map[string]bool {
+	v := map[string]bool{}
+	for _, n := range names {
+		v[n] = true
+	}
+	return v
+}

buildtags/buildtags_test.go

@@ -65,10 +65,7 @@ func TestParseConstraintRoundTrip(t *testing.T) {
 		"linux,386 darwin,!cgo",
 	}
 	for _, expr := range exprs {
-		c, err := ParseConstraint(expr)
-		if err != nil {
-			t.Fatalf("error parsing expression %q: %q", expr, err)
-		}
+		c := AssertParseConstraint(t, expr)
 		got := c.GoString()
 		expect := "// +build " + expr + "\n"
 		if got != expect {
@@ -84,3 +81,62 @@ func TestParseConstraintError(t *testing.T) {
 		t.Fatalf("expected error parsing %q", expr)
 	}
 }
+
+func TestEvaluate(t *testing.T) {
+	cases := []struct {
+		Constraint Interface
+		Values     map[string]bool
+		Expect     bool
+	}{
+		{Term("a"), SetTags("a"), true},
+		{Term("!a"), SetTags("a"), false},
+		{Term("!a"), SetTags(), true},
+		{Term("inval-id"), SetTags("inval-id"), false},
+
+		{Opt(Term("a"), Term("b")), SetTags(), false},
+		{Opt(Term("a"), Term("b")), SetTags("a"), false},
+		{Opt(Term("a"), Term("b")), SetTags("b"), false},
+		{Opt(Term("a"), Term("b")), SetTags("a", "b"), true},
+		{Opt(Term("a"), Term("b-a-d")), SetTags("a", "b-a-d"), false},
+
+		{
+			Any(Opt(Term("linux"), Term("386")), Opt("darwin", Not("cgo"))),
+			SetTags("linux", "386"),
+			true,
+		},
+		{
+			Any(Opt(Term("linux"), Term("386")), Opt("darwin", Not("cgo"))),
+			SetTags("darwin"),
+			true,
+		},
+		{
+			Any(Opt(Term("linux"), Term("386")), Opt("darwin", Not("cgo"))),
+			SetTags("linux", "darwin", "cgo"),
+			false,
+		},
+
+		{
+			And(Any(Term("linux"), Term("darwin")), Term("386")),
+			SetTags("darwin", "386"),
+			true,
+		},
+	}
+	for _, c := range cases {
+		got := c.Constraint.Evaluate(c.Values)
+		if c.Constraint.Validate() != nil && got {
+			t.Fatal("invalid expressions must evaluate false")
+		}
+		if got != c.Expect {
+			t.Errorf("%#v evaluated with %#v got %v expect %v", c.Constraint, c.Values, got, c.Expect)
+		}
+	}
+}
+
+func AssertParseConstraint(t *testing.T, expr string) Constraint {
+	t.Helper()
+	c, err := ParseConstraint(expr)
+	if err != nil {
+		t.Fatalf("error parsing expression %q: %q", expr, err)
+	}
+	return c
+}