Implement changes() function.

changes() takes a range vector and returns the number of times a value
has changed in the given time window for each time series as an instant
vector.

promql/functions.go

@@ -532,6 +532,33 @@ func funcResets(ev *evaluator, args Expressions) Value {
 	return out
 }
 
+// === changes(matrix ExprMatrix) Vector ===
+func funcChanges(ev *evaluator, args Expressions) Value {
+	in := ev.evalMatrix(args[0])
+	out := make(Vector, 0, len(in))
+
+	for _, samples := range in {
+		changes := 0
+		prev := clientmodel.SampleValue(samples.Values[0].Value)
+		for _, sample := range samples.Values[1:] {
+			current := sample.Value
+			if current != prev {
+				changes++
+			}
+			prev = current
+		}
+
+		rs := &Sample{
+			Metric:    samples.Metric,
+			Value:     clientmodel.SampleValue(changes),
+			Timestamp: ev.Timestamp,
+		}
+		rs.Metric.Delete(clientmodel.MetricNameLabel)
+		out = append(out, rs)
+	}
+	return out
+}
+
 var functions = map[string]*Function{
 	"abs": {
 		Name:       "abs",
@@ -563,6 +590,12 @@ var functions = map[string]*Function{
 		ReturnType: ExprVector,
 		Call:       funcCeil,
 	},
+	"changes": {
+		Name:       "changes",
+		ArgTypes:   []ExprType{ExprMatrix},
+		ReturnType: ExprVector,
+		Call:       funcChanges,
+	},
 	"count_over_time": {
 		Name:       "count_over_time",
 		ArgTypes:   []ExprType{ExprMatrix},

promql/testdata/functions.test

@@ -1,21 +1,51 @@
+# Testdata for resets() and changes().
 load 5m
 	http_requests{path="/foo"}	1 2 3 0 1 0 0 1 2 0
 	http_requests{path="/bar"}	1 2 3 4 5 1 2 3 4 5
+	http_requests{path="/biz"}	0 0 0 0 0 1 1 1 1 1
 
+# Tests for resets().
 eval instant at 50m resets(http_requests[5m])
 	{path="/foo"} 0
 	{path="/bar"} 0
+	{path="/biz"} 0
 
 eval instant at 50m resets(http_requests[20m])
 	{path="/foo"} 1
 	{path="/bar"} 0
+	{path="/biz"} 0
 
 eval instant at 50m resets(http_requests[30m])
 	{path="/foo"} 2
 	{path="/bar"} 1
+	{path="/biz"} 0
 
 eval instant at 50m resets(http_requests[50m])
 	{path="/foo"} 3
 	{path="/bar"} 1
+	{path="/biz"} 0
 
 eval instant at 50m resets(nonexistent_metric[50m])
+
+# Tests for changes().
+eval instant at 50m changes(http_requests[5m])
+	{path="/foo"} 0
+	{path="/bar"} 0
+	{path="/biz"} 0
+
+eval instant at 50m changes(http_requests[20m])
+	{path="/foo"} 3
+	{path="/bar"} 3
+	{path="/biz"} 0
+
+eval instant at 50m changes(http_requests[30m])
+	{path="/foo"} 4
+	{path="/bar"} 5
+	{path="/biz"} 1
+
+eval instant at 50m changes(http_requests[50m])
+	{path="/foo"} 8
+	{path="/bar"} 9
+	{path="/biz"} 1
+
+eval instant at 50m changes(nonexistent_metric[50m])