From 08b6556ee6270a4910e43c8922dab95264fadb56 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Fabian=20St=C3=A4ber?= <fabian@fstab.de>
Date: Fri, 12 Aug 2016 20:21:04 +0200
Subject: [PATCH] Assume counters start at zero after reset.

---
 promql/functions.go            |  2 +-
 promql/testdata/functions.test | 12 ++++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/promql/functions.go b/promql/functions.go
index 251838607..e4d896e23 100644
--- a/promql/functions.go
+++ b/promql/functions.go
@@ -68,7 +68,7 @@ func extrapolatedRate(ev *evaluator, arg Expr, isCounter bool, isRate bool) mode
 		for _, sample := range samples.Values {
 			currentValue := sample.Value
 			if isCounter && currentValue < lastValue {
-				counterCorrection += lastValue - currentValue
+				counterCorrection += lastValue
 			}
 			lastValue = currentValue
 		}
diff --git a/promql/testdata/functions.test b/promql/testdata/functions.test
index cbc789fd5..7ee67adad 100644
--- a/promql/testdata/functions.test
+++ b/promql/testdata/functions.test
@@ -69,6 +69,18 @@ eval instant at 50m increase(http_requests[100m])
 
 clear
 
+# Test for increase() with counter reset.
+# When the counter is reset, it always starts at 0.
+# So the sequence 3 2 (decreasing counter = reset) is interpreted the same as 3 0 1 2.
+# Prometheus assumes it missed the intermediate values 0 and 1.
+load 5m
+	http_requests{path="/foo"}	0 1 2 3 2 3 4
+
+eval instant at 30m increase(http_requests[30m])
+    {path="/foo"} 7
+
+clear
+
 # Tests for irate().
 load 5m
 	http_requests{path="/foo"}	0+10x10