feature: Add map field name to convert structs dynamically instead of individually with a tag.

Author: Jimmy Spagnola

mapstructure.go

@@ -316,6 +316,10 @@ type DecoderConfig struct {
 	// DecodeNil, if set to true, will cause the DecodeHook (if present) to run
 	// even if the input is nil. This can be used to provide default values.
 	DecodeNil bool
+
+	// MapFieldName is the function used to convert the struct field name to the map's key name.
+	// This can be used to support snake casing, etc., without explicitly mapping each name.
+	MapFieldName func(string) string
 }
 
 // A Decoder takes a raw interface value and turns it into structured
@@ -452,6 +456,12 @@ func NewDecoder(config *DecoderConfig) (*Decoder, error) {
 		config.MatchName = strings.EqualFold
 	}
 
+	if config.MapFieldName == nil {
+		config.MapFieldName = func(s string) string {
+			return s
+		}
+	}
+
 	result := &Decoder{
 		config: config,
 	}
@@ -1068,7 +1078,7 @@ func (d *Decoder) decodeMapFromStruct(name string, dataVal reflect.Value, val re
 		}
 
 		tagValue := f.Tag.Get(d.config.TagName)
-		keyName := f.Name
+		keyName := d.config.MapFieldName(f.Name)
 
 		if tagValue == "" && d.config.IgnoreUntaggedFields {
 			continue

mapstructure_test.go

@@ -3681,3 +3681,43 @@ func uintPtr(v uint) *uint        { return &v }
 func boolPtr(v bool) *bool        { return &v }
 func floatPtr(v float64) *float64 { return &v }
 func interfacePtr(v any) *any     { return &v }
+
+type TestMapFieldName struct {
+	HostName string
+	Username string
+}
+
+func TestDecoder_MapFieldName(t *testing.T) {
+	var structKeys map[string]any
+
+	decoder, err := NewDecoder(&DecoderConfig{
+		ErrorUnused: true, // Enable error on unused keys
+		Result:      &structKeys,
+		MapFieldName: func(s string) string {
+			if s == "HostName" {
+				return "host_name"
+			}
+			return s
+		},
+	})
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	var input TestMapFieldName
+
+	err = decoder.Decode(&input)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	_, ok := structKeys["host_name"]
+	if !ok {
+		t.Fatal("expected host_name to exist")
+	}
+
+	_, ok = structKeys["Username"]
+	if !ok {
+		t.Fatal("expected Username to exist")
+	}
+}