diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..b4fa67a
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,42 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - "**"
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        go: ["1.24", "1.25"]
+    name: Test on Go ${{ matrix.go }}
+    steps:
+      # https://github.com/actions/checkout
+      - uses: actions/checkout@v4
+        with:
+          submodules: recursive
+
+      # https://github.com/actions/setup-go
+      - uses: actions/setup-go@v5
+        with:
+          go-version: ${{ matrix.go }}
+
+      - name: Check golang version/env
+        run: |
+          set -x
+          go version
+          go env
+
+      - run: make build
+      - run: make test
+      - run: make lint
+
+      - run: make test-with-coverage
+      - run: make test-coverage-profile
+      - name: Upload coverage reports to Codecov
+        uses: codecov/codecov-action@v5
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}
+          slug: tecowl/calendatext
diff --git a/.gitignore b/.gitignore
index 4170155..b7757c8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1 @@
-/go.sum
+coverages/
diff --git a/.golangci.bck.yml b/.golangci.bck.yml
new file mode 100644
index 0000000..4920df0
--- /dev/null
+++ b/.golangci.bck.yml
@@ -0,0 +1,28 @@
+linters:
+  enable-all: true
+  disable:
+    - gosmopolitan
+    - nlreturn
+    - nolintlint
+    - tenv # The linter 'tenv' is deprecated (since v1.64.0) due to: Duplicate feature in another linter. Replaced by usetesting.
+    - testpackage
+    - varnamelen
+    - wrapcheck
+    - wsl
+
+issues:
+#   exclude-files:
+#     - example_test.go
+  exclude-rules:
+    - path: _test\.go
+      linters:
+        - depguard
+        - dupword
+        - errcheck
+        - forcetypeassert
+        - funlen
+linters-settings:
+#   cyclop:
+#     skip-tests: true
+  # lll:
+  #   line-length: 250
diff --git a/.golangci.yml b/.golangci.yml
new file mode 100644
index 0000000..a07c99b
--- /dev/null
+++ b/.golangci.yml
@@ -0,0 +1,44 @@
+version: "2"
+linters:
+  default: all
+  disable:
+    - gosmopolitan
+    - noinlineerr
+    - nlreturn
+    - nolintlint
+    - testpackage
+    - varnamelen
+    - wrapcheck
+    - wsl
+    - wsl_v5
+  exclusions:
+    generated: lax
+    presets:
+      - comments
+      - common-false-positives
+      - legacy
+      - std-error-handling
+    rules:
+      - linters:
+          - depguard
+          - dupword
+          - errcheck
+          - forcetypeassert
+          - funlen
+        path: _test\.go
+    paths:
+      - third_party$
+      - builtin$
+      - examples$
+formatters:
+  enable:
+    - gci
+    - gofmt
+    - gofumpt
+    - goimports
+  exclusions:
+    generated: lax
+    paths:
+      - third_party$
+      - builtin$
+      - examples$
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..d2f842e
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,21 @@
+.PHONY: default
+default: build lint test
+
+GOBIN=$(shell go env GOBIN)
+
+include ./Makefiles/build.mk
+include ./Makefiles/lint.mk
+include ./Makefiles/godoc.mk
+include ./Makefiles/test.mk
+include ./Makefiles/cov-unit.mk
+include ./Makefiles/cov-integration.mk
+include ./Makefiles/metadata.mk
+
+.PHONY: test
+test: test-unit
+
+# COVERAGE_GO_PACKAGES_CSV is used in Makefiles/cov-unit.mk
+COVERAGE_GO_PACKAGES_CSV=$(shell find . -type d | grep -v '.git' grep -v Makefiles | grep -v coverages | tr '\n' ',' | sed 's/,$$//')
+.PHONY: coverage-go-packages
+coverage-go-packages:
+	@echo $(COVERAGE_GO_PACKAGES_CSV)
diff --git a/Makefiles/build.mk b/Makefiles/build.mk
new file mode 100644
index 0000000..4b8e163
--- /dev/null
+++ b/Makefiles/build.mk
@@ -0,0 +1,3 @@
+.PHONY: build
+build:
+	go build ./...
diff --git a/Makefiles/cov-integration.mk b/Makefiles/cov-integration.mk
new file mode 100644
index 0000000..1a473a1
--- /dev/null
+++ b/Makefiles/cov-integration.mk
@@ -0,0 +1,28 @@
+COVERAGE_INTEGRATED_DIR=$(COVERAGES_DIR)/integrated
+$(COVERAGE_INTEGRATED_DIR):
+	mkdir -p $(COVERAGE_INTEGRATED_DIR)
+
+.PHONY: test-with-coverage
+test-with-coverage: test-unit-with-coverage
+
+COVERAGE_DIRS_CSV=$(UNIT_COVERAGE_DIR)
+
+COVERAGE_PROFILE?=$(COVERAGES_DIR)/coverage.txt
+$(COVERAGE_PROFILE): $(COVERAGE_INTEGRATED_DIR)
+	$(MAKE) test-coverage-profile
+
+.PHONY: test-coverage-profile
+test-coverage-profile: $(COVERAGE_INTEGRATED_DIR)
+	go tool covdata merge \
+		-i $(COVERAGE_DIRS_CSV) \
+		-o $(COVERAGE_INTEGRATED_DIR)
+	go tool covdata percent -i=$(COVERAGE_INTEGRATED_DIR) -o $(COVERAGE_PROFILE)
+
+COVERAGE_HTML?=$(COVERAGES_DIR)/coverage.html
+$(COVERAGE_HTML): $(COVERAGE_PROFILE)
+	go tool covdata html -i=$(COVERAGE_PROFILE) -o $(COVERAGE_HTML)
+
+.PHONY: test-coverage
+test-coverage: test-coverage-profile
+	go tool cover -html=$(COVERAGE_PROFILE) -o $(COVERAGE_HTML)
+	@command -v open && open $(COVERAGE_HTML) || echo "open $(COVERAGE_HTML)"
diff --git a/Makefiles/cov-unit.mk b/Makefiles/cov-unit.mk
new file mode 100644
index 0000000..9822446
--- /dev/null
+++ b/Makefiles/cov-unit.mk
@@ -0,0 +1,19 @@
+# Directory path from COVERAGES_DIR is used in test in sub directory. So COVERAGE_DIR must be an absolute path.
+COVERAGES_DIR=$(CURDIR)/coverages
+$(COVERAGES_DIR):
+	mkdir -p $(COVERAGES_DIR)
+
+UNIT_COVERAGE_DIR=$(COVERAGES_DIR)/unit
+$(UNIT_COVERAGE_DIR):
+	mkdir -p $(UNIT_COVERAGE_DIR)
+
+.PHONY: clean-unit-coverage
+clean-unit-coverage:
+	rm -rf $(UNIT_COVERAGE_DIR)
+
+# COVERAGE_GO_PACKAGES_CSV is defined in Makefile
+
+# See https://app.codecov.io/github/akm/go-requestid/new
+.PHONY: test-unit-with-coverage
+test-unit-with-coverage: clean-unit-coverage $(UNIT_COVERAGE_DIR)
+	go test -cover -coverpkg=$(COVERAGE_GO_PACKAGES_CSV) ./... -args -test.gocoverdir="$(UNIT_COVERAGE_DIR)"
diff --git a/Makefiles/godoc.mk b/Makefiles/godoc.mk
new file mode 100644
index 0000000..2fe104a
--- /dev/null
+++ b/Makefiles/godoc.mk
@@ -0,0 +1,12 @@
+GODOC_CLI_VERSION=latest
+GODOC_CLI_MODULE=golang.org/x/tools/cmd/godoc
+GODOC_CLI=$(GOBIN)/godoc
+$(GODOC_CLI):
+	$(MAKE) godoc-cli-install
+godoc-cli-install:
+	go install $(GODOC_CLI_MODULE)@$(GODOC_CLI_VERSION)
+
+.PHONY: godoc
+godoc: $(GODOC_CLI)
+	@echo "Open http://localhost:6060/pkg/github.com/tecowl/querybm"
+	godoc -http=:6060
diff --git a/Makefiles/lint.mk b/Makefiles/lint.mk
new file mode 100644
index 0000000..5a73331
--- /dev/null
+++ b/Makefiles/lint.mk
@@ -0,0 +1,15 @@
+GOLANGCI_LINT_CLI_VERSION?=latest
+GOLANGCI_LINT_CLI_MODULE=github.com/golangci/golangci-lint/v2/cmd/golangci-lint
+GOLANGCI_LINT_CLI=$(GOBIN)/golangci-lint
+$(GOLANGCI_LINT_CLI):
+	$(MAKE) golangci-lint-cli-install
+golangci-lint-cli-install:
+	go install $(GOLANGCI_LINT_CLI_MODULE)@$(GOLANGCI_LINT_CLI_VERSION)
+
+.PHONY: lint
+lint: $(GOLANGCI_LINT_CLI)
+	golangci-lint run
+
+.PHONY: linters-enabled
+linters-enabled: $(GOLANGCI_LINT_CLI)
+	@golangci-lint linters | awk '/^Enabled by your configuration linters:$$/{flag=1;next}/^Disabled by your configuration linters:$$/{flag=0}flag{print}'
diff --git a/Makefiles/metadata.mk b/Makefiles/metadata.mk
new file mode 100644
index 0000000..a78b11a
--- /dev/null
+++ b/Makefiles/metadata.mk
@@ -0,0 +1,7 @@
+METADATA_YAML=.project.yaml
+$(METADATA_YAML): metadata-gen
+
+METADATA_LINTERS=$(strip $(shell $(MAKE) linters-enabled --no-print-directory 2>/dev/null | grep . | wc -l))
+.PHONY: metadata-gen
+metadata-gen: 
+	@echo "linters: $(METADATA_LINTERS)" > $(METADATA_YAML)
diff --git a/Makefiles/test.mk b/Makefiles/test.mk
new file mode 100644
index 0000000..f1d531e
--- /dev/null
+++ b/Makefiles/test.mk
@@ -0,0 +1,5 @@
+GO_TEST_OPTIONS?=
+
+.PHONY: test-unit
+test-unit:
+	go test $(GO_TEST_OPTIONS) ./...
diff --git a/calendar.go b/calendar.go
index 89beab8..b7655d2 100644
--- a/calendar.go
+++ b/calendar.go
@@ -7,7 +7,7 @@ type Calendar struct {
 }
 
 func NewCalendar(start, end Date, baseEnabled bool) *Calendar {
-	return &Calendar{Period: NewPeriod(start, end), BaseEnabled: baseEnabled}
+	return &Calendar{Period: NewPeriod(start, end), BaseEnabled: baseEnabled, Patterns: Patterns{}}
 }
 
 func (c *Calendar) Dates() Dates {
diff --git a/calendar_test.go b/calendar_test.go
index ff36df4..0960b0d 100644
--- a/calendar_test.go
+++ b/calendar_test.go
@@ -4,9 +4,11 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestCalendarDays(t *testing.T) {
+	t.Parallel()
 
 	// ----- 2020-08 ------
 	// S  M  T  W  T  F  S
@@ -18,6 +20,7 @@ func TestCalendarDays(t *testing.T) {
 	// 30 31
 
 	t.Run("single date", func(t *testing.T) {
+		t.Parallel()
 		c := &Calendar{
 			Period:      NewPeriod(*NewDate(2020, 8, 1), *NewDate(2020, 8, 31)),
 			BaseEnabled: false,
@@ -55,6 +58,8 @@ func TestCalendarDays(t *testing.T) {
 }
 
 func TestCalendarParse(t *testing.T) {
+	t.Parallel()
+
 	texts := map[string]string{
 		"full-date": `
 + 平日 : 通常営業日
@@ -84,15 +89,12 @@ func TestCalendarParse(t *testing.T) {
 
 	for name, text := range texts {
 		t.Run(name, func(t *testing.T) {
+			t.Parallel()
 			c := NewCalendar(*NewDate(2020, 8, 1), *NewDate(2020, 8, 31), false)
 			err := c.ParseText(text)
-			if !assert.NoError(t, err) {
-				return
-			}
+			require.NoError(t, err)
+			require.Len(t, c.Patterns, 4)
 
-			if !assert.Equal(t, 4, len(c.Patterns)) {
-				return
-			}
 			if assert.IsType(t, (*Date)(nil), c.Patterns[0].DateMatcher) {
 				i := c.Patterns[0]
 				m := i.DateMatcher.(*Date)
diff --git a/contextual_date_parser.go b/contextual_date_parser.go
index 5b5b08a..dd4005f 100644
--- a/contextual_date_parser.go
+++ b/contextual_date_parser.go
@@ -1,12 +1,12 @@
 package calendatext
 
 import (
+	"errors"
+	"fmt"
 	"regexp"
 	"strconv"
 	"strings"
 	"time"
-
-	"github.com/pkg/errors"
 )
 
 type ContextualDateParser struct {
@@ -18,23 +18,24 @@ func NewContextualDateParser(delimiterPattern string, d *Date) *ContextualDatePa
 	if d == nil {
 		d = Today()
 	}
+
 	return &ContextualDateParser{
 		delimiter: regexp.MustCompile("[" + delimiterPattern + "]"),
 		current:   d,
 	}
 }
 
-func (cp *ContextualDateParser) Parse(s string) (*Date, error) {
-	parts := cp.delimiter.Split(strings.TrimSpace(s), 3)
+func (cp *ContextualDateParser) Parse(s string) (*Date, error) { // nolint:cyclop,funlen
+	parts := cp.delimiter.Split(strings.TrimSpace(s), 3) // nolint:mnd
 	var y, d int
 	var m time.Month
 	var err error
 	switch len(parts) {
-	case 1:
+	case 1: // nolint:mnd
 		curr := cp.current
 		d, err = strconv.Atoi(parts[0])
 		if err != nil {
-			return nil, err
+			return nil, err // nolint:wrapcheck
 		}
 		if d < curr.Day() {
 			m = curr.Month() + 1
@@ -42,16 +43,16 @@ func (cp *ContextualDateParser) Parse(s string) (*Date, error) {
 			m = curr.Month()
 		}
 		y = curr.Year()
-	case 2:
+	case 2: // nolint:mnd
 		curr := cp.current
 		v, err := strconv.Atoi(parts[0])
 		if err != nil {
-			return nil, err
+			return nil, err // nolint:wrapcheck
 		}
 		m = time.Month(v)
 		d, err = strconv.Atoi(parts[1])
 		if err != nil {
-			return nil, err
+			return nil, err // nolint:wrapcheck
 		}
 		tmpD := NewDate(curr.Year(), m, d)
 		if curr.After(tmpD) {
@@ -59,7 +60,7 @@ func (cp *ContextualDateParser) Parse(s string) (*Date, error) {
 		} else {
 			y = curr.Year()
 		}
-	case 3:
+	case 3: // nolint:mnd
 		y, err = strconv.Atoi(parts[0])
 		if err != nil {
 			return nil, err
@@ -74,10 +75,13 @@ func (cp *ContextualDateParser) Parse(s string) (*Date, error) {
 			return nil, err
 		}
 	default:
-		return nil, errors.Errorf("Something wrong to parse %v (len: %d) as Date", parts, len(parts))
+		return nil, fmt.Errorf("%w: %v (len: %d)", ErrDateParse, parts, len(parts))
 	}
 
 	r := NewDate(y, m, d)
 	cp.current = r
+
 	return r, nil
 }
+
+var ErrDateParse = errors.New("something wrong to parse date")
diff --git a/contextual_date_parser_test.go b/contextual_date_parser_test.go
index 22249d2..3cc90dd 100644
--- a/contextual_date_parser_test.go
+++ b/contextual_date_parser_test.go
@@ -5,14 +5,17 @@ import (
 	"time"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestContextualDateParser(t *testing.T) {
+	t.Parallel()
+
 	cp := NewContextualDateParser("/-", NewDate(2020, 8, 14))
 
 	{
 		d, err := cp.Parse("2020/08/15")
-		assert.NoError(t, err)
+		require.NoError(t, err)
 		assert.Equal(t, 2020, d.Year())
 		assert.Equal(t, time.Month(8), d.Month())
 		assert.Equal(t, 15, d.Day())
@@ -20,7 +23,7 @@ func TestContextualDateParser(t *testing.T) {
 
 	{
 		d, err := cp.Parse("08/16")
-		assert.NoError(t, err)
+		require.NoError(t, err)
 		assert.Equal(t, 2020, d.Year()) // 2020 が補完される
 		assert.Equal(t, time.Month(8), d.Month())
 		assert.Equal(t, 16, d.Day())
@@ -28,7 +31,7 @@ func TestContextualDateParser(t *testing.T) {
 
 	{
 		d, err := cp.Parse("2020/07/16")
-		assert.NoError(t, err)
+		require.NoError(t, err)
 		assert.Equal(t, 2020, d.Year()) // 2020 が補完される
 		assert.Equal(t, time.Month(7), d.Month())
 		assert.Equal(t, 16, d.Day())
@@ -36,7 +39,7 @@ func TestContextualDateParser(t *testing.T) {
 
 	{
 		d, err := cp.Parse("17")
-		assert.NoError(t, err)
+		require.NoError(t, err)
 		assert.Equal(t, 2020, d.Year())           // 2020 が補完される
 		assert.Equal(t, time.Month(7), d.Month()) // 7 が補完される
 		assert.Equal(t, 17, d.Day())
@@ -45,7 +48,7 @@ func TestContextualDateParser(t *testing.T) {
 	// 同じ日ならば月が進んだりしない
 	{
 		d, err := cp.Parse("17")
-		assert.NoError(t, err)
+		require.NoError(t, err)
 		assert.Equal(t, 2020, d.Year())           // 2020 が補完される
 		assert.Equal(t, time.Month(7), d.Month()) // 7 が補完される
 		assert.Equal(t, 17, d.Day())
@@ -53,7 +56,7 @@ func TestContextualDateParser(t *testing.T) {
 
 	{
 		d, err := cp.Parse("16") // 17よりも前の日なので翌月と判断される
-		assert.NoError(t, err)
+		require.NoError(t, err)
 		assert.Equal(t, 2020, d.Year())           // 2020 が補完される
 		assert.Equal(t, time.Month(8), d.Month()) // 8 が補完される
 		assert.Equal(t, 16, d.Day())
@@ -61,7 +64,7 @@ func TestContextualDateParser(t *testing.T) {
 
 	{
 		d, err := cp.Parse("08/15") // 2020/08/15 ではなく 2021/08/15 と解釈される
-		assert.NoError(t, err)
+		require.NoError(t, err)
 		assert.Equal(t, 2021, d.Year()) // 2021 が補完される
 		assert.Equal(t, time.Month(8), d.Month())
 		assert.Equal(t, 15, d.Day())
@@ -69,7 +72,7 @@ func TestContextualDateParser(t *testing.T) {
 
 	{
 		d, err := cp.Parse("31")
-		assert.NoError(t, err)
+		require.NoError(t, err)
 		assert.Equal(t, 2021, d.Year())           // 2021 が補完される
 		assert.Equal(t, time.Month(8), d.Month()) // 8 が補完される
 		assert.Equal(t, 31, d.Day())
@@ -77,7 +80,7 @@ func TestContextualDateParser(t *testing.T) {
 
 	{
 		d, err := cp.Parse("1")
-		assert.NoError(t, err)
+		require.NoError(t, err)
 		assert.Equal(t, 2021, d.Year())           // 2021 が補完される
 		assert.Equal(t, time.Month(9), d.Month()) // 9 が補完される
 		assert.Equal(t, 1, d.Day())
@@ -85,7 +88,7 @@ func TestContextualDateParser(t *testing.T) {
 
 	{
 		d, err := cp.Parse("12/24")
-		assert.NoError(t, err)
+		require.NoError(t, err)
 		assert.Equal(t, 2021, d.Year()) // 2021 が補完される
 		assert.Equal(t, time.Month(12), d.Month())
 		assert.Equal(t, 24, d.Day())
@@ -93,7 +96,7 @@ func TestContextualDateParser(t *testing.T) {
 
 	{
 		d, err := cp.Parse("3")
-		assert.NoError(t, err)
+		require.NoError(t, err)
 		assert.Equal(t, 2022, d.Year())           // 2022 が補完される
 		assert.Equal(t, time.Month(1), d.Month()) // 1 が補完される
 		assert.Equal(t, 3, d.Day())
diff --git a/date.go b/date.go
index 4cdbca0..c7153e6 100644
--- a/date.go
+++ b/date.go
@@ -1,18 +1,19 @@
 package calendatext
 
 import (
+	"errors"
+	"fmt"
 	"strconv"
 	"strings"
 	"time"
-
-	"github.com/pkg/errors"
 )
 
 // RFC3339 full-date
 // See documents about RFC3339
-//    https://www.ietf.org/rfc/rfc3339.txt
-//    https://medium.com/easyread/understanding-about-rfc-3339-for-datetime-formatting-in-software-engineering-940aa5d5f68a
-//    https://wiki.suikawiki.org/n/RFC%203339の日付形式
+//
+//	https://www.ietf.org/rfc/rfc3339.txt
+//	https://medium.com/easyread/understanding-about-rfc-3339-for-datetime-formatting-in-software-engineering-940aa5d5f68a
+//	https://wiki.suikawiki.org/n/RFC%203339の日付形式
 const DateFormat = "2006-01-02"
 
 type Date struct {
@@ -26,16 +27,21 @@ func Today() *Date {
 	return NewDate(t.Year(), t.Month(), t.Day())
 }
 
+var (
+	ErrInvalidDateFormat    = errors.New("invalid date format")
+	ErrInvalidNumberForDate = errors.New("invalid number for date")
+)
+
 func ParseDateWith(str string, delimiter string) (*Date, error) {
-	parts := strings.SplitN(str, delimiter, 3)
-	if len(parts) < 3 {
-		return nil, errors.Errorf("Invalid Date format: %q", str)
+	parts := strings.SplitN(str, delimiter, 3) // nolint:mnd
+	if len(parts) < 3 {                        // nolint:mnd
+		return nil, fmt.Errorf("%w: %q", ErrInvalidDateFormat, str)
 	}
-	nums := make([]int, 3)
+	nums := make([]int, 3) // nolint:mnd
 	for idx, s := range parts {
 		v, err := strconv.Atoi(s)
 		if err != nil {
-			return nil, errors.Errorf("Invalid number for date: %q", str)
+			return nil, fmt.Errorf("%w: %q", ErrInvalidNumberForDate, str)
 		}
 		nums[idx] = v
 	}
@@ -83,40 +89,20 @@ func (d Date) Weekday() Weekday {
 }
 
 func (d Date) MonthlyWeekNum() int {
-	r := d.Day() / 7
-	q := d.Day() % 7
+	r := d.Day() / weekdays
+	q := d.Day() % weekdays
 	if q == 0 {
 		return r
-	} else {
-		return r + 1
 	}
+	return r + 1
 }
 
-// Implement DateMatcher interface
+var _ DateMatcher = (*Date)(nil) // assert Date implements DateMatcher
+
 func (d Date) Match(other *Date) bool {
 	return d.Equal(other)
 }
 
-func (d Date) beforeAfter(other *Date, compare func(a, b int) bool, resultForSame bool) bool {
-	if other == nil {
-		return false
-	}
-	if compare(d.y, other.y) {
-		return true
-	} else if d.y == other.y {
-		if compare(int(d.m), int(other.m)) {
-			return true
-		} else if d.m == other.m {
-			if compare(d.d, other.d) {
-				return true
-			} else if d.d == other.d {
-				return resultForSame
-			}
-		}
-	}
-	return false
-}
-
 func (d Date) After(other *Date) bool {
 	return d.beforeAfter(other, func(a, b int) bool {
 		return a > b
@@ -162,7 +148,7 @@ func (d Date) NextDayOf(v int) *Date {
 }
 
 func (d Date) NextWeekOf(v int) *Date {
-	return d.NextDayOf(v * 7)
+	return d.NextDayOf(v * weekdays)
 }
 
 func (d Date) PrevDayOf(v int) *Date {
@@ -204,3 +190,26 @@ func (d Date) NextYearOf(v int) *Date {
 func (d Date) PrevYearOf(v int) *Date {
 	return d.NextYearOf(v * -1)
 }
+
+func (d Date) beforeAfter(other *Date, compare func(a, b int) bool, resultForSame bool) bool {
+	if other == nil {
+		return false
+	}
+	if compare(d.y, other.y) {
+		return true
+	}
+	if d.y != other.y {
+		return false
+	}
+	if compare(int(d.m), int(other.m)) {
+		return true
+	}
+	if d.m == other.m {
+		if compare(d.d, other.d) {
+			return true
+		} else if d.d == other.d {
+			return resultForSame
+		}
+	}
+	return false
+}
diff --git a/date_matcher.go b/date_matcher.go
index 5b41be5..ebbaf12 100644
--- a/date_matcher.go
+++ b/date_matcher.go
@@ -1,5 +1,5 @@
 package calendatext
 
 type DateMatcher interface {
-	Match(*Date) bool
+	Match(d *Date) bool
 }
diff --git a/date_test.go b/date_test.go
index fc187c9..72fdd93 100644
--- a/date_test.go
+++ b/date_test.go
@@ -7,6 +7,7 @@ import (
 )
 
 func TestNewDate(t *testing.T) {
+	t.Parallel()
 	assert.Equal(t, "2019-11-30", NewDate(2020, 0, 0).String())
 	assert.Equal(t, "2020-08-01", NewDate(2020, 8, 1).String())
 	assert.Equal(t, "2019-03-01", NewDate(2019, 2, 29).String())
@@ -15,6 +16,7 @@ func TestNewDate(t *testing.T) {
 }
 
 func TestNextDay(t *testing.T) {
+	t.Parallel()
 	d := NewDate(2020, 8, 12)
 	assert.Equal(t, "2020-07-29", d.PrevWeekOf(2).String())
 	assert.Equal(t, "2020-08-05", d.PrevWeek().String())
@@ -28,6 +30,7 @@ func TestNextDay(t *testing.T) {
 }
 
 func TestNextMonth(t *testing.T) {
+	t.Parallel()
 	d := NewDate(2020, 8, 12)
 	assert.Equal(t, "2019-11-12", d.PrevMonthOf(9).String())
 	assert.Equal(t, "2019-12-12", d.PrevMonthOf(8).String())
@@ -39,6 +42,7 @@ func TestNextMonth(t *testing.T) {
 }
 
 func TestNextYear(t *testing.T) {
+	t.Parallel()
 	d := NewDate(2020, 8, 12)
 	assert.Equal(t, "2018-08-12", d.PrevYearOf(2).String())
 	assert.Equal(t, "2019-08-12", d.PrevYear().String())
@@ -47,9 +51,11 @@ func TestNextYear(t *testing.T) {
 }
 
 func TestCompare(t *testing.T) {
+	t.Parallel()
 	t.Run("compare with myself", func(t *testing.T) {
+		t.Parallel()
 		d := NewDate(2020, 8, 12)
-		assert.True(t, d.Equal(d))
+		assert.True(t, d.Equal(d.Clone()))
 		assert.True(t, d.BeforeEqual(d))
 		assert.True(t, d.AfterEqual(d))
 		assert.False(t, d.Before(d))
@@ -58,6 +64,7 @@ func TestCompare(t *testing.T) {
 
 	subTestForNext := func(d0, d1 *Date) func(t *testing.T) {
 		return func(t *testing.T) {
+			t.Helper()
 			assert.False(t, d0.Equal(d1))
 			assert.True(t, d0.BeforeEqual(d1))
 			assert.True(t, d0.Before(d1))
@@ -68,6 +75,7 @@ func TestCompare(t *testing.T) {
 
 	subTestForPrev := func(d0, d1 *Date) func(t *testing.T) {
 		return func(t *testing.T) {
+			t.Helper()
 			assert.False(t, d0.Equal(d1))
 			assert.False(t, d0.BeforeEqual(d1))
 			assert.False(t, d0.Before(d1))
@@ -77,15 +85,16 @@ func TestCompare(t *testing.T) {
 	}
 
 	d := NewDate(2020, 8, 12)
-	t.Run("compare with next day", subTestForNext(d, d.NextDay()))
-	t.Run("compare with next month", subTestForNext(d, d.NextMonth()))
-	t.Run("compare with next year", subTestForNext(d, d.NextYear()))
-	t.Run("compare with prev day", subTestForPrev(d, d.PrevDay()))
-	t.Run("compare with prev month", subTestForPrev(d, d.PrevMonth()))
-	t.Run("compare with prev year", subTestForPrev(d, d.PrevYear()))
+	t.Run("compare with next day", subTestForNext(d, d.NextDay()))     // nolint:paralleltest
+	t.Run("compare with next month", subTestForNext(d, d.NextMonth())) // nolint:paralleltest
+	t.Run("compare with next year", subTestForNext(d, d.NextYear()))   // nolint:paralleltest
+	t.Run("compare with prev day", subTestForPrev(d, d.PrevDay()))     // nolint:paralleltest
+	t.Run("compare with prev month", subTestForPrev(d, d.PrevMonth())) // nolint:paralleltest
+	t.Run("compare with prev year", subTestForPrev(d, d.PrevYear()))   // nolint:paralleltest
 }
 
 func TestDateMonthlyWeekNum(t *testing.T) {
+	t.Parallel()
 	// ----- 2020-08 ------
 	// S  M  T  W  T  F  S
 	//                    1
diff --git a/go.mod b/go.mod
index 7661464..2f67d82 100644
--- a/go.mod
+++ b/go.mod
@@ -1,8 +1,11 @@
 module github.com/tecowl/calendatext
 
-go 1.13
+go 1.24.5
+
+require github.com/stretchr/testify v1.6.1
 
 require (
-	github.com/pkg/errors v0.9.1
-	github.com/stretchr/testify v1.6.1
+	github.com/davecgh/go-spew v1.1.0 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c // indirect
 )
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..afe7890
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,11 @@
+github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
+github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/testify v1.6.1 h1:hDPOHmpOpP40lSULcqw7IrRb/u7w6RpDC9399XyoNd0=
+github.com/stretchr/testify v1.6.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c h1:dUUwHk2QECo/6vqA44rthZ8ie2QXMNeKRTHCNY2nXvo=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/main_test.go b/main_test.go
index eefd238..206cd63 100644
--- a/main_test.go
+++ b/main_test.go
@@ -1,12 +1,14 @@
 package calendatext_test
 
 import (
+	"testing"
+
 	"github.com/stretchr/testify/assert"
 	"github.com/tecowl/calendatext"
-	"testing"
 )
 
 func TestCalendar(t *testing.T) {
+	t.Parallel()
 
 	// ----- 2020-12 ------  ----- 2021-01 ------
 	//  S  M  T  W  T  F  S    S  M  T  W  T  F  S
@@ -76,5 +78,4 @@ func TestCalendar(t *testing.T) {
 		},
 		cal.Dates().Strings(),
 	)
-
 }
diff --git a/monthly_day_test.go b/monthly_day_test.go
index 49d71a6..ed66f7f 100644
--- a/monthly_day_test.go
+++ b/monthly_day_test.go
@@ -8,6 +8,7 @@ import (
 )
 
 func TestCalendarWithMonthlyDay(t *testing.T) {
+	t.Parallel()
 
 	// ----- 2020-12 ------  ----- 2021-01 ------
 	//  S  M  T  W  T  F  S    S  M  T  W  T  F  S
@@ -45,5 +46,4 @@ func TestCalendarWithMonthlyDay(t *testing.T) {
 		},
 		cal.Dates().Strings(),
 	)
-
 }
diff --git a/monthly_weekday_test.go b/monthly_weekday_test.go
index cd34fa1..2b1b162 100644
--- a/monthly_weekday_test.go
+++ b/monthly_weekday_test.go
@@ -8,6 +8,7 @@ import (
 )
 
 func TestCalendarWithMonthlyWeekday(t *testing.T) {
+	t.Parallel()
 
 	// ----- 2020-12 ------  ----- 2021-01 ------
 	//  S  M  T  W  T  F  S    S  M  T  W  T  F  S
@@ -43,5 +44,4 @@ func TestCalendarWithMonthlyWeekday(t *testing.T) {
 		},
 		cal.Dates().Strings(),
 	)
-
 }
diff --git a/pattern.go b/pattern.go
index 77028a0..05541b7 100644
--- a/pattern.go
+++ b/pattern.go
@@ -1,9 +1,10 @@
 package calendatext
 
 type Pattern struct {
+	DateMatcher
+
 	Enabled     bool
 	Description string
-	DateMatcher
 }
 
 func NewPattern(enabled bool, description string, matcher DateMatcher) *Pattern {
diff --git a/period.go b/period.go
index d3e5898..f19c1f1 100644
--- a/period.go
+++ b/period.go
@@ -16,8 +16,9 @@ func (pd *Period) Include(d *Date) bool {
 	return pd.Start.BeforeEqual(d) && pd.End.AfterEqual(d)
 }
 
-// Implement DateMatcher interface
-func (pd Period) Match(other *Date) bool {
+var _ DateMatcher = (*Period)(nil) // assert Period implements DateMatcher
+
+func (pd *Period) Match(other *Date) bool {
 	return pd.Include(other)
 }
 
diff --git a/period_test.go b/period_test.go
index 9c55968..2b3363f 100644
--- a/period_test.go
+++ b/period_test.go
@@ -7,7 +7,9 @@ import (
 )
 
 func TestPeriodInclude(t *testing.T) {
+	t.Parallel()
 	t.Run("single date", func(t *testing.T) {
+		t.Parallel()
 		d := NewDate(2020, 8, 6)
 		pd := NewPeriod(*d, *d)
 		assert.False(t, pd.Include(d.PrevYear()))
@@ -22,6 +24,7 @@ func TestPeriodInclude(t *testing.T) {
 	})
 
 	t.Run("normal", func(t *testing.T) {
+		t.Parallel()
 		d1 := NewDate(2020, 8, 6)
 		d2 := NewDate(2020, 8, 20)
 		pd := NewPeriod(*d1, *d2)
diff --git a/text_parser.go b/text_parser.go
index f8b5913..1ce949d 100644
--- a/text_parser.go
+++ b/text_parser.go
@@ -1,11 +1,11 @@
 package calendatext
 
 import (
+	"errors"
+	"fmt"
 	"regexp"
 	"strconv"
 	"strings"
-
-	"github.com/pkg/errors"
 )
 
 type BuildMatcher func(s string) (DateMatcher, error)
@@ -44,24 +44,25 @@ func (tp *textParser) Run(s string) error {
 
 func (tp *textParser) parseLine(line string) (*Pattern, error) {
 	var enabled bool
-	if strings.HasPrefix(line, "+") {
+	switch {
+	case strings.HasPrefix(line, "+"):
 		enabled = true
-	} else if strings.HasPrefix(line, "-") {
+	case strings.HasPrefix(line, "-"):
 		enabled = false
-	} else {
-		return nil, errors.Errorf("Invalid first charactor. It must be '+' or '-': %q\n", line)
+	default:
+		return nil, fmt.Errorf("%w. It must be '+' or '-': %q", ErrInvalidFirstCharacter, line)
 	}
 
 	line = line[1:]
-	bodies := strings.SplitN(line, ":", 2)
+	bodies := strings.SplitN(line, ":", 2) // nolint:mnd
 	description := ""
-	if len(bodies) == 2 {
+	if len(bodies) == 2 { // nolint:mnd
 		description = strings.TrimSpace(bodies[1])
 	}
 
 	matcher, err := tp.parseMatcher(strings.TrimSpace(bodies[0]))
 	if err != nil {
-		return nil, errors.WithMessagef(err, "Failed to build matcher for %s", description)
+		return nil, fmt.Errorf("%w for %s: %w", ErrMatcherBuild, description, err)
 	}
 	return &Pattern{
 		Enabled:     enabled,
@@ -70,7 +71,7 @@ func (tp *textParser) parseLine(line string) (*Pattern, error) {
 	}, nil
 }
 
-func (tp *textParser) parseMatcher(body string) (DateMatcher, error) {
+func (tp *textParser) parseMatcher(body string) (DateMatcher, error) { // nolint:ireturn
 	for _, build := range tp.matcherBuilders {
 		m, err := build(body)
 		if err != nil {
@@ -80,7 +81,7 @@ func (tp *textParser) parseMatcher(body string) (DateMatcher, error) {
 			return m, nil
 		}
 	}
-	return nil, errors.Errorf("No build function found for %q", body)
+	return nil, fmt.Errorf("%w for %q", ErrBuildFunctionNotFound, body)
 }
 
 var (
@@ -89,9 +90,15 @@ var (
 	weeklyRE         = regexp.MustCompile(`\A毎週`)
 	monthlyDayRE     = regexp.MustCompile(`\A毎月[^\d]*(\d+)日`)
 	monthlyWeekdayRE = regexp.MustCompile(`\A毎月.*第(\d)(.+)`)
+
+	ErrInvalidFirstCharacter = errors.New("invalid first character")
+	ErrSomethingWrongToParse = errors.New("something wrong to parse")
+	ErrPeriodSplit           = errors.New("failed to split string as Period")
+	ErrMatcherBuild          = errors.New("failed to build matcher")
+	ErrBuildFunctionNotFound = errors.New("no build function found")
 )
 
-func newMatcherBuilders(date *Date) []BuildMatcher {
+func newMatcherBuilders(date *Date) []BuildMatcher { // nolint:gocognit,cyclop,funlen
 	delimiter := "/"
 	contextualParser := NewContextualDateParser(delimiter, date)
 
@@ -126,12 +133,12 @@ func newMatcherBuilders(date *Date) []BuildMatcher {
 			if len(m) < 1 {
 				return nil, nil
 			}
-			if len(m[0]) < 2 {
-				return nil, errors.Errorf("something wrong to parse %q", s)
+			if len(m[0]) < 2 { // nolint:mnd
+				return nil, fmt.Errorf("%w %q", ErrSomethingWrongToParse, s)
 			}
 			d, err := strconv.ParseInt(m[0][1], 10, 10)
 			if err != nil {
-				return nil, err
+				return nil, err // nolint:wrapcheck
 			}
 			return MonthlyDay(d), nil
 		},
@@ -142,8 +149,8 @@ func newMatcherBuilders(date *Date) []BuildMatcher {
 			if len(m) < 1 {
 				return nil, nil
 			}
-			if len(m[0]) < 3 {
-				return nil, errors.Errorf("something wrong to parse %q", s)
+			if len(m[0]) < 3 { // nolint:mnd
+				return nil, fmt.Errorf("%w %q", ErrSomethingWrongToParse, s)
 			}
 			n, err := strconv.Atoi(m[0][1])
 			if err != nil {
@@ -167,9 +174,9 @@ func newMatcherBuilders(date *Date) []BuildMatcher {
 			if !slashPeriodRE.MatchString(s) {
 				return nil, nil
 			}
-			parts := strings.SplitN(s, "-", 2)
-			if len(parts) < 2 {
-				return nil, errors.Errorf("Failed to split string as Period: %q", s)
+			parts := strings.SplitN(s, "-", 2) // nolint:mnd
+			if len(parts) < 2 {                // nolint:mnd
+				return nil, fmt.Errorf("%w %q", ErrPeriodSplit, s)
 			}
 
 			st, err := contextualParser.Parse(strings.TrimSpace(parts[0]))
diff --git a/weekday.go b/weekday.go
index f8409cc..cb831b6 100644
--- a/weekday.go
+++ b/weekday.go
@@ -1,9 +1,9 @@
 package calendatext
 
 import (
+	"errors"
+	"fmt"
 	"time"
-
-	"github.com/pkg/errors"
 )
 
 type Weekday time.Weekday
@@ -18,6 +18,8 @@ const (
 	Saturday  = Weekday(time.Saturday)
 )
 
+const weekdays = 7
+
 func (wd Weekday) Match(d *Date) bool {
 	if d == nil {
 		return false
@@ -25,7 +27,8 @@ func (wd Weekday) Match(d *Date) bool {
 	return time.Weekday(wd) == d.Time().Weekday()
 }
 
-var WeekdayNameMap = map[Weekday]rune{
+// See https://github.com/tecowl/calendatext/issues/5
+var WeekdayNameMap = map[Weekday]rune{ // nolint:gochecknoglobals
 	Sunday:    '日',
 	Monday:    '月',
 	Tuesday:   '火',
@@ -35,6 +38,8 @@ var WeekdayNameMap = map[Weekday]rune{
 	Saturday:  '土',
 }
 
+var ErrUnknownWeekdayName = errors.New("unknown weekday name")
+
 func ParseWeekdayName(s string) (*Weekday, error) {
 	c := ([]rune(s))[0]
 	for d, name := range WeekdayNameMap {
@@ -42,5 +47,5 @@ func ParseWeekdayName(s string) (*Weekday, error) {
 			return &d, nil
 		}
 	}
-	return nil, errors.Errorf("Unknown Weekday name %q", s)
+	return nil, fmt.Errorf("%w %q", ErrUnknownWeekdayName, s)
 }
diff --git a/weekday_test.go b/weekday_test.go
index a4bf836..c860064 100644
--- a/weekday_test.go
+++ b/weekday_test.go
@@ -8,6 +8,7 @@ import (
 )
 
 func TestCalendarWithWeekday(t *testing.T) {
+	t.Parallel()
 
 	// ----- 2020-12 ------  ----- 2021-01 ------
 	//  S  M  T  W  T  F  S    S  M  T  W  T  F  S
@@ -63,5 +64,4 @@ func TestCalendarWithWeekday(t *testing.T) {
 		},
 		cal.Dates().Strings(),
 	)
-
 }