From 2da917e49e599c2a14e2ad953a247783403251cf Mon Sep 17 00:00:00 2001
From: Arno Geurts <arno.geurts@sqills.com>
Date: Mon, 7 Dec 2020 15:44:42 +0100
Subject: [PATCH] Ability to use encoding.TextUnmarshaler

---
 decode_hooks.go      | 24 ++++++++++++++++++++++++
 decode_hooks_test.go | 26 ++++++++++++++++++++++++++
 2 files changed, 50 insertions(+)

diff --git a/decode_hooks.go b/decode_hooks.go
index 77578d00..92e6f76f 100644
--- a/decode_hooks.go
+++ b/decode_hooks.go
@@ -1,6 +1,7 @@
 package mapstructure
 
 import (
+	"encoding"
 	"errors"
 	"fmt"
 	"net"
@@ -230,3 +231,26 @@ func RecursiveStructToMapHookFunc() DecodeHookFunc {
 		return f.Interface(), nil
 	}
 }
+
+// TextUnmarshallerHookFunc returns a DecodeHookFunc that applies
+// strings to the UnmarshalText function, when the target type
+// implements the encoding.TextUnmarshaler interface
+func TextUnmarshallerHookFunc() DecodeHookFuncType {
+	return func(
+		f reflect.Type,
+		t reflect.Type,
+		data interface{}) (interface{}, error) {
+		if f.Kind() != reflect.String {
+			return data, nil
+		}
+		result := reflect.New(t).Interface()
+		unmarshaller, ok := result.(encoding.TextUnmarshaler)
+		if !ok {
+			return data, nil
+		}
+		if err := unmarshaller.UnmarshalText([]byte(data.(string))); err != nil {
+			return nil, err
+		}
+		return result, nil
+	}
+}
diff --git a/decode_hooks_test.go b/decode_hooks_test.go
index d6902534..b3165bc9 100644
--- a/decode_hooks_test.go
+++ b/decode_hooks_test.go
@@ -2,6 +2,7 @@ package mapstructure
 
 import (
 	"errors"
+	"math/big"
 	"net"
 	"reflect"
 	"testing"
@@ -419,3 +420,28 @@ func TestStructToMapHookFuncTabled(t *testing.T) {
 
 	}
 }
+
+func TestTextUnmarshallerHookFunc(t *testing.T) {
+	cases := []struct {
+		f, t   reflect.Value
+		result interface{}
+		err    bool
+	}{
+		{reflect.ValueOf("42"), reflect.ValueOf(big.Int{}), big.NewInt(42), false},
+		{reflect.ValueOf("invalid"), reflect.ValueOf(big.Int{}), nil, true},
+		{reflect.ValueOf("5"), reflect.ValueOf("5"), "5", false},
+	}
+
+	for i, tc := range cases {
+		f := TextUnmarshallerHookFunc()
+		actual, err := DecodeHookExec(f, tc.f, tc.t)
+		if tc.err != (err != nil) {
+			t.Fatalf("case %d: expected err %#v", i, tc.err)
+		}
+		if !reflect.DeepEqual(actual, tc.result) {
+			t.Fatalf(
+				"case %d: expected %#v, got %#v",
+				i, tc.result, actual)
+		}
+	}
+}