1 files changed, 95 insertions, 0 deletions
diff --git a/testing/microprom/handler_test.go b/testing/microprom/handler_test.go
new file mode 100644
index 0000000..93027e3
--- /dev/null
+++ b/testing/microprom/handler_test.go
@@ -0,0 +1,95 @@
+// SPDX-FileCopyrightText: 2026 Stefan Majewsky <majewsky@gmx.net>
+// SPDX-License-Identifier: Apache-2.0
+
+package microprom_test
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"maps"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+	"go.xyrillian.de/gg/assert"
+	"go.xyrillian.de/gg/microprom"
+)
+
+func TestHandlerFunctionallyIdenticalToPromhttp(t *testing.T) {
+	// build a microprom.Handler rendering two metric families
+	h1 := microprom.Handler{
+		Families: map[microprom.MetricFamilyName]microprom.MetricFamilyInfo{
+			"events": {
+				Type: microprom.MetricTypeCounter,
+				Help: "Counts events that happened.",
+			},
+			"memory_usage_bytes": {
+				Type: microprom.MetricTypeGauge,
+				Help: "How much memory is currently used.",
+			},
+		},
+		SortOutput: true,
+		Collect: func(ctx context.Context, ms *microprom.MetricSet) error {
+			labelNames := microprom.NewLabelNames("shard", "type")
+			for idx := range 5 {
+				labels := ms.FormatLabels(labelNames, fmt.Sprintf("node%d", idx), "update")
+				ms.Add("events", labels, float64(10*idx))
+			}
+			ms.Add("memory_usage_bytes", "", 42<<20)
+			return nil
+		},
+	}
+
+	// build a promhttp.Handler rendering the same metric families
+	eventsCounter := prometheus.NewCounterVec(prometheus.CounterOpts{
+		Name: "events_total",
+		Help: "Counts events that happened.",
+	}, []string{"type", "shard"})
+	for idx := range 5 {
+		eventsCounter.With(prometheus.Labels{
+			"type":  "update",
+			"shard": fmt.Sprintf("node%d", idx),
+		}).Add(float64(10 * idx))
+	}
+	memoryUsageBytesGauge := prometheus.NewGauge(prometheus.GaugeOpts{
+		Name: "memory_usage_bytes",
+		Help: "How much memory is currently used.",
+	})
+	memoryUsageBytesGauge.Set(42 << 20)
+	r := prometheus.NewRegistry()
+	r.MustRegister(eventsCounter)
+	r.MustRegister(memoryUsageBytesGauge)
+	h2 := promhttp.HandlerFor(r, promhttp.HandlerOpts{EnableOpenMetrics: true})
+
+	// test identical behavior for Prometheus Text Format
+	body1, headers1 := getMetrics(t, h1, nil)
+	body2, headers2 := getMetrics(t, h2, nil)
+	assert.Equal(t, strings.Split(body1, "\n"), strings.Split(body2, "\n"))
+	assert.Equal(t, headers1, headers2)
+
+	// test identical behavior for OpenMetrics 1.0 text format
+	body1, headers1 = getMetrics(t, h1, http.Header{"Accept": {"application/openmetrics-text; version=1.0.0"}})
+	body2, headers2 = getMetrics(t, h2, http.Header{"Accept": {"application/openmetrics-text; version=1.0.0"}})
+	assert.Equal(t, strings.Split(body1, "\n"), strings.Split(body2, "\n"))
+	assert.Equal(t, headers1, headers2)
+}
+
+func getMetrics(t *testing.T, h http.Handler, requestHeaders http.Header) (responseBody string, responseHeaders http.Header) {
+	t.Helper()
+	r := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/metrics", nil)
+	maps.Copy(r.Header, requestHeaders)
+
+	w := httptest.NewRecorder()
+	h.ServeHTTP(w, r)
+	resp := w.Result()
+
+	buf, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatal(err.Error())
+	}
+	return string(buf), resp.Header
+}