Expose Matcher.Prefix()

Sometimes label matchers know that they match values with a specific
prefix. This information can be valuable in some downstream storage
implementations.

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

model/labels/matcher.go

@@ -128,3 +128,12 @@ func (m *Matcher) SetMatches() []string {
 	}
 	return m.re.setMatches
 }
+
+// Prefix returns the required prefix of the value to match, if possible.
+// It will be empty if it's an equality matcher or if the prefix can't be determined.
+func (m *Matcher) Prefix() string {
+	if m.re == nil {
+		return ""
+	}
+	return m.re.prefix
+}

model/labels/matcher_test.go

@@ -14,13 +14,14 @@
 package labels
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/stretchr/testify/require"
 )
 
 func mustNewMatcher(t *testing.T, mType MatchType, value string) *Matcher {
-	m, err := NewMatcher(mType, "", value)
+	m, err := NewMatcher(mType, "test_label_name", value)
 	require.NoError(t, err)
 	return m
 }
@@ -133,6 +134,58 @@ func TestInverse(t *testing.T) {
 	}
 }
 
+func TestPrefix(t *testing.T) {
+	for i, tc := range []struct {
+		matcher *Matcher
+		prefix  string
+	}{
+		{
+			matcher: mustNewMatcher(t, MatchEqual, "abc"),
+			prefix:  "",
+		},
+		{
+			matcher: mustNewMatcher(t, MatchNotEqual, "abc"),
+			prefix:  "",
+		},
+		{
+			matcher: mustNewMatcher(t, MatchRegexp, "abc.+"),
+			prefix:  "abc",
+		},
+		{
+			matcher: mustNewMatcher(t, MatchRegexp, "abcd|abc.+"),
+			prefix:  "abc",
+		},
+		{
+			matcher: mustNewMatcher(t, MatchNotRegexp, "abcd|abc.+"),
+			prefix:  "abc",
+		},
+		{
+			matcher: mustNewMatcher(t, MatchRegexp, "abc(def|ghj)|ab|a."),
+			prefix:  "a",
+		},
+		{
+			matcher: mustNewMatcher(t, MatchRegexp, "foo.+bar|foo.*baz"),
+			prefix:  "foo",
+		},
+		{
+			matcher: mustNewMatcher(t, MatchRegexp, "abc|.*"),
+			prefix:  "",
+		},
+		{
+			matcher: mustNewMatcher(t, MatchRegexp, "abc|def"),
+			prefix:  "",
+		},
+		{
+			matcher: mustNewMatcher(t, MatchRegexp, ".+def"),
+			prefix:  "",
+		},
+	} {
+		t.Run(fmt.Sprintf("%d: %s", i, tc.matcher), func(t *testing.T) {
+			require.Equal(t, tc.prefix, tc.matcher.Prefix())
+		})
+	}
+}
+
 func BenchmarkMatchType_String(b *testing.B) {
 	for i := 0; i <= b.N; i++ {
 		_ = MatchType(i % int(MatchNotRegexp+1)).String()