From 1f4a2629d2e568b65bffa3c860be34edd41be494 Mon Sep 17 00:00:00 2001
From: Gregory Oschwald <goschwald@maxmind.com>
Date: Mon, 31 Jul 2023 19:24:26 -0700
Subject: [PATCH] Zero reflect.Value between uses. Closes #145.

---
 decoder.go         |  8 +++---
 reader_test.go     | 61 ++++++++++++++++++++++++++++++++++++++++++++++
 set_zero_120.go    | 10 ++++++++
 set_zero_pre120.go | 10 ++++++++
 4 files changed, 86 insertions(+), 3 deletions(-)
 create mode 100644 set_zero_120.go
 create mode 100644 set_zero_pre120.go

diff --git a/decoder.go b/decoder.go
index 0b8708d..dd0f9ba 100644
--- a/decoder.go
+++ b/decoder.go
@@ -596,18 +596,20 @@ func (d *decoder) decodeMap(
 	mapType := result.Type()
 	keyValue := reflect.New(mapType.Key()).Elem()
 	elemType := mapType.Elem()
-	elemKind := elemType.Kind()
 	var elemValue reflect.Value
 	for i := uint(0); i < size; i++ {
 		var key []byte
 		var err error
 		key, offset, err = d.decodeKey(offset)
-
 		if err != nil {
 			return 0, err
 		}
 
-		if !elemValue.IsValid() || elemKind == reflect.Interface {
+		if elemValue.IsValid() {
+			// After 1.20 is the minimum supported version, this can just be
+			// elemValue.SetZero()
+			reflectSetZero(elemValue)
+		} else {
 			elemValue = reflect.New(elemType).Elem()
 		}
 
diff --git a/reader_test.go b/reader_test.go
index 2ce1ab3..f40541e 100644
--- a/reader_test.go
+++ b/reader_test.go
@@ -454,6 +454,67 @@ func TestComplexStructWithNestingAndPointer(t *testing.T) {
 	assert.NoError(t, reader.Close())
 }
 
+// See GitHub #115.
+func TestNestedMapDecode(t *testing.T) {
+	db, err := Open(testFile("GeoIP2-Country-Test.mmdb"))
+	require.NoError(t, err)
+
+	var r map[string]map[string]any
+
+	require.NoError(t, db.Lookup(net.ParseIP("89.160.20.128"), &r))
+
+	assert.Equal(
+		t,
+		map[string]map[string]any{
+			"continent": {
+				"code":       "EU",
+				"geoname_id": uint64(6255148),
+				"names": map[string]any{
+					"de":    "Europa",
+					"en":    "Europe",
+					"es":    "Europa",
+					"fr":    "Europe",
+					"ja":    "ヨーロッパ",
+					"pt-BR": "Europa",
+					"ru":    "Европа",
+					"zh-CN": "欧洲",
+				},
+			},
+			"country": {
+				"geoname_id":           uint64(2661886),
+				"is_in_european_union": true,
+				"iso_code":             "SE",
+				"names": map[string]any{
+					"de":    "Schweden",
+					"en":    "Sweden",
+					"es":    "Suecia",
+					"fr":    "Suède",
+					"ja":    "スウェーデン王国",
+					"pt-BR": "Suécia",
+					"ru":    "Швеция",
+					"zh-CN": "瑞典",
+				},
+			},
+			"registered_country": {
+				"geoname_id":           uint64(2921044),
+				"is_in_european_union": true,
+				"iso_code":             "DE",
+				"names": map[string]any{
+					"de":    "Deutschland",
+					"en":    "Germany",
+					"es":    "Alemania",
+					"fr":    "Allemagne",
+					"ja":    "ドイツ連邦共和国",
+					"pt-BR": "Alemanha",
+					"ru":    "Германия",
+					"zh-CN": "德国",
+				},
+			},
+		},
+		r,
+	)
+}
+
 func TestNestedOffsetDecode(t *testing.T) {
 	db, err := Open(testFile("GeoIP2-City-Test.mmdb"))
 	require.NoError(t, err)
diff --git a/set_zero_120.go b/set_zero_120.go
new file mode 100644
index 0000000..33b9dff
--- /dev/null
+++ b/set_zero_120.go
@@ -0,0 +1,10 @@
+//go:build go1.20
+// +build go1.20
+
+package maxminddb
+
+import "reflect"
+
+func reflectSetZero(v reflect.Value) {
+	v.SetZero()
+}
diff --git a/set_zero_pre120.go b/set_zero_pre120.go
new file mode 100644
index 0000000..6639de7
--- /dev/null
+++ b/set_zero_pre120.go
@@ -0,0 +1,10 @@
+//go:build !go1.20
+// +build !go1.20
+
+package maxminddb
+
+import "reflect"
+
+func reflectSetZero(v reflect.Value) {
+	v.Set(reflect.Zero(v.Type()))
+}