From a99d7b7e2c57380fbcf85c571194df26d86f9d76 Mon Sep 17 00:00:00 2001
From: Grégoire Duchêne <gduchene@awhk.org>
Date: Sun, 26 Jun 2022 00:31:10 +0100
Subject: Add a few flag helper functions

---
 flag.go      | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 flag_test.go | 50 ++++++++++++++++++++++++++++++++++++
 2 files changed, 134 insertions(+)
 create mode 100644 flag.go
 create mode 100644 flag_test.go

diff --git a/flag.go b/flag.go
new file mode 100644
index 0000000..bed32eb
--- /dev/null
+++ b/flag.go
@@ -0,0 +1,84 @@
+// SPDX-FileCopyrightText: © 2022 Grégoire Duchêne <gduchene@awhk.org>
+// SPDX-License-Identifier: ISC
+
+package core
+
+import (
+	"flag"
+	"fmt"
+	"time"
+)
+
+func FlagVar[T any](fs *flag.FlagSet, name, usage string, parse ParseFunc[T]) *T {
+	v := &flagValue[T]{Parse: parse, Value: new(T)}
+	fs.Var(v, name, usage)
+	return v.Value
+}
+
+func FlagVarPtr[T any](fs *flag.FlagSet, name, usage string, parse ParseFunc[T], val *T) {
+	fs.Var(&flagValue[T]{Parse: parse, Value: val}, name, usage)
+}
+
+func FlagVarSlice[T any](fs *flag.FlagSet, name, usage string, parse ParseFunc[T]) *[]T {
+	v := &flagValueSlice[T]{Parse: parse, Values: new([]T)}
+	fs.Var(v, name, usage)
+	return v.Values
+}
+
+func FlagVarSlicePtr[T any](fs *flag.FlagSet, name, usage string, parse ParseFunc[T], vals *[]T) {
+	fs.Var(&flagValueSlice[T]{Parse: parse, Values: vals}, name, usage)
+}
+
+// ParseString returns the string passed with no error set.
+func ParseString(s string) (string, error) {
+	return s, nil
+}
+
+// ParseTime parses a string according to the time.RFC3339 format.
+func ParseTime(s string) (time.Time, error) {
+	return time.Parse(time.RFC3339, s)
+}
+
+// ParseFunc describes functions that will parse a string and return a
+// value or an error.
+type ParseFunc[T any] func(string) (T, error)
+
+type flagValue[T any] struct {
+	Parse ParseFunc[T]
+	Value *T
+}
+
+var _ flag.Value = &flagValue[any]{}
+
+func (f *flagValue[T]) Set(s string) error {
+	val, err := f.Parse(s)
+	if err != nil {
+		return err
+	}
+	*f.Value = val
+	return nil
+}
+
+func (f *flagValue[T]) String() string {
+	return fmt.Sprintf("%v", f.Value)
+}
+
+type flagValueSlice[T any] struct {
+	Parse  ParseFunc[T]
+	Values *[]T
+}
+
+var _ flag.Value = &flagValueSlice[any]{}
+
+func (f *flagValueSlice[T]) Set(s string) error {
+	val, err := f.Parse(s)
+	if err != nil {
+		return err
+	}
+	*f.Values = append(*f.Values, val)
+	return nil
+}
+
+func (f *flagValueSlice[T]) String() string {
+	return fmt.Sprintf("%v", f.Values)
+}
diff --git a/flag_test.go b/flag_test.go
new file mode 100644
index 0000000..56711d5
--- /dev/null
+++ b/flag_test.go
@@ -0,0 +1,50 @@
+// SPDX-FileCopyrightText: © 2022 Grégoire Duchêne <gduchene@awhk.org>
+// SPDX-License-Identifier: ISC
+
+package core_test
+
+import (
+	"flag"
+	"strconv"
+	"testing"
+
+	"go.awhk.org/core"
+)
+
+func TestFlagVar(s *testing.T) {
+	t := core.T{T: s}
+
+	fs := flag.NewFlagSet("", flag.ContinueOnError)
+	fl := core.FlagVar(fs, "test", "", strconv.ParseBool)
+	t.AssertErrorIs(nil, fs.Parse([]string{"-test=true"}))
+	t.AssertEqual(true, *fl)
+}
+
+func TestFlagVarPtr(s *testing.T) {
+	t := core.T{T: s}
+
+	fs := flag.NewFlagSet("", flag.ContinueOnError)
+	fl := false
+	core.FlagVarPtr(fs, "test", "", strconv.ParseBool, &fl)
+	t.AssertErrorIs(nil, fs.Parse([]string{"-test=true"}))
+	t.AssertEqual(true, fl)
+}
+
+func TestFlagVarSlice(s *testing.T) {
+	t := core.T{T: s}
+
+	fs := flag.NewFlagSet("", flag.ContinueOnError)
+	fl := core.FlagVarSlice(fs, "test", "", strconv.Atoi)
+	t.AssertErrorIs(nil, fs.Parse([]string{"-test=1", "-test=2", "-test=42"}))
+	t.AssertEqual([]int{1, 2, 42}, *fl)
+}
+
+func TestFlagVarSlicePtr(s *testing.T) {
+	t := core.T{T: s}
+
+	fs := flag.NewFlagSet("", flag.ContinueOnError)
+	fl := []int{}
+	core.FlagVarSlicePtr(fs, "test", "", strconv.Atoi, &fl)
+	t.AssertErrorIs(nil, fs.Parse([]string{"-test=1", "-test=2", "-test=42"}))
+	t.AssertEqual([]int{1, 2, 42}, fl)
+}
-- 
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