[vpython] Add conditional packages based on PEP425

vpython/spec/match.go

Index: vpython/spec/match.go
diff --git a/vpython/spec/match.go b/vpython/spec/match.go
new file mode 100644
index 0000000000000000000000000000000000000000..03869059b0a4973323082e55b133349494182909
--- /dev/null
+++ b/vpython/spec/match.go
@@ -0,0 +1,55 @@
+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package spec
+
+import (
+	"github.com/luci/luci-go/vpython/api/vpython"
+)
+
+// PackageMatches returns true if any of the match qualifiers in the supplied
+// package match at least one of the supplied PEP425 tags.
+//
+// As a special case, if the package doesn't specify any match tags, it will
+// always match regardless of the supplied PEP425 tags.
+//
+// See PEP425Matches for more information.
+func PackageMatches(pkg *vpython.Spec_Package, tags []*vpython.Pep425Tag) bool {
+	if len(pkg.MatchTag) == 0 {
+		return true
+	}
+
+	for _, matchTag := range pkg.MatchTag {
+		if PEP425Matches(matchTag, tags) {
+			return true
+		}
+	}
+	return false
+}
+
+// PEP425Matches returns true if match matches at least one of the tags in tags.
+//
+// A match is determined if the non-zero fields in match equal the equivalent
+// fields in a tag.
+func PEP425Matches(match *vpython.Pep425Tag, tags []*vpython.Pep425Tag) bool {
+	// Special case: empty match matches nothing.
+	if match.IsZero() {
+		return false
+	}
+
+	for _, tag := range tags {
+		if v := match.Version; v != "" && tag.Version != v {
+			continue
+		}
+		if v := match.Abi; v != "" && tag.Abi != v {
+			continue
+		}
+		if v := match.Arch; v != "" && tag.Arch != v {
+			continue
+		}
+		return true
+	}
+
+	return false
+}