diff --git a/.circleci/config.yml b/.circleci/config.yml
index 68881e3..bb1ad6a 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -1,15 +1,39 @@
-version: 2
+version: 2.1
+
 jobs:
     build:
         docker:
-            - image: circleci/golang:1.9
-        working_directory: /go/src/github.com/powerman/check
+            - image: circleci/golang:1
+        environment:
+            GOLANGCI_LINT_VER: 1.12.2
         steps:
             - checkout
-            - run: go get -v -t -d ./...
-            - run: go test -v -race ./...
             - run:
-                name: Report coverage
+                name: Initialize
                 command: |
+                    rmdir /go/*/ # fix owner/permission
+                    env | grep _VER | sort > /tmp/tools.ver
+            - restore_cache:
+                keys:
+                    - v1-{{ checksum "/tmp/tools.ver" }}-{{ checksum "go.mod" }}-{{ .Branch }}
+                    - v1-{{ checksum "/tmp/tools.ver" }}-{{ checksum "go.mod" }}-
+                    - v1-{{ checksum "/tmp/tools.ver" }}-
+                    - v1-
+            - run:
+                name: Install tools
+                command: |
+                    cd /
+                    golangci-lint --version | tee /dev/stderr | grep -wq $GOLANGCI_LINT_VER ||
+                        curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b /go/bin v$GOLANGCI_LINT_VER
                     go get -v github.com/mattn/goveralls
-                    goveralls -service=circle-ci
+            - run: go test -mod=readonly -v -race ./...
+            - run: golangci-lint run
+            - run: goveralls -service=circle-ci
+            - save_cache:
+                when: always
+                key: v1-{{ checksum "/tmp/tools.ver" }}-{{ checksum "go.mod" }}-{{ .Branch }}
+                paths:
+                    - /go/bin/
+                    - /go/pkg/
+                    - /go/src/
+                    - ~/.cache/go-build/
diff --git a/.golangci.yml b/.golangci.yml
new file mode 100644
index 0000000..3ccc639
--- /dev/null
+++ b/.golangci.yml
@@ -0,0 +1,67 @@
+linters-settings:
+    errcheck:
+        check-type-assertions: false
+        check-blank: false
+    govet:
+        check-shadowing: true
+    golint:
+        min-confidence: 0 # 0.8
+    gocyclo:
+        min-complexity: 12 # 30
+    maligned:
+        suggest-new: false
+    dupl:
+        threshold: 150
+    goconst:
+        min-len: 3
+        min-occurrences: 3
+    unused:
+        check-exported: false
+    unparam:
+        algo: cha
+        check-exported: false
+    nakedret:
+        max-func-lines: 30
+    prealloc:
+        simple: true
+        range-loops: true
+        for-loops: false
+    gocritic:
+        enabled-checks:
+            - appendAssign
+            - appendCombine
+            - assignOp
+            - boolExprSimplify
+            - caseOrder
+            - defaultCaseOrder
+            - deprecatedComment
+            - dupArg
+            - dupBranchBody
+            - dupCase
+            - flagDeref
+            - ifElseChain
+            - indexAlloc
+            - methodExprCall
+            - rangeExprCopy
+            # - rangeValCopy
+            - regexpMust
+            - singleCaseSwitch
+            - sloppyLen
+            # - switchTrue
+            - typeSwitchVar
+            - underef
+            # - unlabelStmt
+            - unlambda
+            - unslice
+
+linters:
+    enable-all: true
+    disable:
+        - gofmt
+        - goimports
+        - depguard
+        - dupl
+        - gochecknoglobals
+        - gochecknoinits
+        - lll
+        - scopelint
diff --git a/check.go b/check.go
index f6e942b..fc0ffc7 100644
--- a/check.go
+++ b/check.go
@@ -220,11 +220,12 @@ func (t *C) report2(actual, expected interface{}, msg []interface{}, ok bool) bo
 func (t *C) report3(actual, expected1, expected2 interface{}, msg []interface{}, ok bool) bool {
 	t.Helper()
 	checker, arg2Name, arg3Name := callerFuncName(1), "arg1", "arg2"
-	if strings.Contains(checker, "Between") {
+	switch {
+	case strings.Contains(checker, "Between"):
 		arg2Name, arg3Name = "Min", "Max"
-	} else if strings.Contains(checker, "Delta") {
+	case strings.Contains(checker, "Delta"):
 		arg2Name, arg3Name = nameExpected, "Delta"
-	} else if strings.Contains(checker, "SMAPE") {
+	case strings.Contains(checker, "SMAPE"):
 		arg2Name, arg3Name = nameExpected, "SMAPE"
 	}
 	return t.report(ok, msg,
@@ -398,8 +399,9 @@ func isEqual(actual, expected interface{}) bool {
 	switch actual := actual.(type) {
 	case time.Time:
 		return actual.Equal(expected.(time.Time))
+	default:
+		return actual == expected
 	}
-	return actual == expected
 }
 
 // EQ is a synonym for Equal.
diff --git a/check_test.go b/check_test.go
index 947911f..3a7548a 100644
--- a/check_test.go
+++ b/check_test.go
@@ -1249,8 +1249,7 @@ func TestCheckerApprox(t *testing.T) {
 }
 
 func half(v interface{}) interface{} {
-	switch v := v.(type) {
-	case time.Duration:
+	if v, ok := v.(time.Duration); ok {
 		return v / 2
 	}
 	switch val := reflect.ValueOf(v); val.Kind() {
@@ -1336,7 +1335,7 @@ func TestCheckerSubstring(t *testing.T) {
 		{[]byte("Sunday"), []byte("Monday")},
 		{[]rune("Sunday"), []rune("Monday")},
 		{time.Sunday, time.Monday},
-		{errors.New("Sunday"), errors.New("Monday")},
+		{errors.New("Sunday"), errors.New("Monday")}, // nolint:golint
 	}
 
 	t.Run("HasPrefix", func(tt *testing.T) {
diff --git a/dump.go b/dump.go
index cc81854..3d2b116 100644
--- a/dump.go
+++ b/dump.go
@@ -66,7 +66,7 @@ func (v dump) diff(expected dump) string {
 // - []byte: same as string instead of hexdump for valid utf8
 // - []rune: use quoted char instead of number for valid runes in list
 // - json.RawMessage: indent, then same as string
-func newDump(i interface{}) (d dump) {
+func newDump(i interface{}) (d dump) { // nolint:gocyclo
 	d.dump = spewCfg.Sdump(i)
 
 	if i == nil {