Add default tag (#183)

**Summary of Changes**

this is the implementation for the functionality described in #182 

1. add support for an optional `default` tag to allow setting values
when decoding in case no value is provided
2. limiting the `default` tag scope to only primitive types and their
pointers as well

cache.go

@@ -197,6 +197,7 @@ func (c *cache) createField(field reflect.StructField, parentAlias string) *fiel
 		isSliceOfStructs: isSlice && isStruct,
 		isAnonymous:      field.Anonymous,
 		isRequired:       options.Contains("required"),
+		defaultValue:     options.getDefaultOptionValue(),
 	}
 }
 
@@ -246,8 +247,9 @@ type fieldInfo struct {
 	// isSliceOfStructs indicates if the field type is a slice of structs.
 	isSliceOfStructs bool
 	// isAnonymous indicates whether the field is embedded in the struct.
-	isAnonymous bool
-	isRequired  bool
+	isAnonymous  bool
+	isRequired   bool
+	defaultValue string
 }
 
 func (f *fieldInfo) paths(prefix string) []string {
@@ -303,3 +305,13 @@ func (o tagOptions) Contains(option string) bool {
 	}
 	return false
 }
+
+func (o tagOptions) getDefaultOptionValue() string {
+	for _, s := range o {
+		if strings.HasPrefix(s, "default:") {
+			return strings.Split(s, ":")[1]
+		}
+	}
+
+	return ""
+}

converter.go

@@ -143,3 +143,80 @@ func convertUint64(value string) reflect.Value {
 	}
 	return invalidValue
 }
+
+func convertPointer(k reflect.Kind, value string) reflect.Value {
+	switch k {
+	case boolType:
+		if v := convertBool(value); v.IsValid() {
+			converted := v.Bool()
+			return reflect.ValueOf(&converted)
+		}
+	case float32Type:
+		if v := convertFloat32(value); v.IsValid() {
+			converted := float32(v.Float())
+			return reflect.ValueOf(&converted)
+		}
+	case float64Type:
+		if v := convertFloat64(value); v.IsValid() {
+			converted := float64(v.Float())
+			return reflect.ValueOf(&converted)
+		}
+	case intType:
+		if v := convertInt(value); v.IsValid() {
+			converted := int(v.Int())
+			return reflect.ValueOf(&converted)
+		}
+	case int8Type:
+		if v := convertInt8(value); v.IsValid() {
+			converted := int8(v.Int())
+			return reflect.ValueOf(&converted)
+		}
+	case int16Type:
+		if v := convertInt16(value); v.IsValid() {
+			converted := int16(v.Int())
+			return reflect.ValueOf(&converted)
+		}
+	case int32Type:
+		if v := convertInt32(value); v.IsValid() {
+			converted := int32(v.Int())
+			return reflect.ValueOf(&converted)
+		}
+	case int64Type:
+		if v := convertInt64(value); v.IsValid() {
+			converted := int64(v.Int())
+			return reflect.ValueOf(&converted)
+		}
+	case stringType:
+		if v := convertString(value); v.IsValid() {
+			converted := v.String()
+			return reflect.ValueOf(&converted)
+		}
+	case uintType:
+		if v := convertUint(value); v.IsValid() {
+			converted := uint(v.Uint())
+			return reflect.ValueOf(&converted)
+		}
+	case uint8Type:
+		if v := convertUint8(value); v.IsValid() {
+			converted := uint8(v.Uint())
+			return reflect.ValueOf(&converted)
+		}
+	case uint16Type:
+		if v := convertUint16(value); v.IsValid() {
+			converted := uint16(v.Uint())
+			return reflect.ValueOf(&converted)
+		}
+	case uint32Type:
+		if v := convertUint32(value); v.IsValid() {
+			converted := uint32(v.Uint())
+			return reflect.ValueOf(&converted)
+		}
+	case uint64Type:
+		if v := convertUint64(value); v.IsValid() {
+			converted := uint64(v.Uint())
+			return reflect.ValueOf(&converted)
+		}
+	}
+
+	return invalidValue
+}

decoder.go

@@ -84,13 +84,84 @@ func (d *Decoder) Decode(dst interface{}, src map[string][]string) error {
 			errors[path] = UnknownKeyError{Key: path}
 		}
 	}
+	errors.merge(d.setDefaults(t, v))
 	errors.merge(d.checkRequired(t, src))
 	if len(errors) > 0 {
 		return errors
 	}
 	return nil
 }
 
+//setDefaults sets the default values when the `default` tag is specified,
+//default is supported on basic/primitive types and their pointers,
+//nested structs can also have default tags
+func (d *Decoder) setDefaults(t reflect.Type, v reflect.Value) MultiError {
+	struc := d.cache.get(t)
+	if struc == nil {
+		// unexpect, cache.get never return nil
+		return MultiError{"default-" + t.Name(): errors.New("cache fail")}
+	}
+
+	errs := MultiError{}
+
+	for _, f := range struc.fields {
+		vCurrent := v.FieldByName(f.name)
+
+		if vCurrent.Type().Kind() == reflect.Struct && f.defaultValue == "" {
+			errs.merge(d.setDefaults(vCurrent.Type(), vCurrent))
+		} else if isPointerToStruct(vCurrent) && f.defaultValue == "" {
+			errs.merge(d.setDefaults(vCurrent.Elem().Type(), vCurrent.Elem()))
+		}
+
+		if f.defaultValue != "" && f.isRequired {
+			errs.merge(MultiError{"default-" + f.name: errors.New("required fields cannot have a default value")})
+		} else if f.defaultValue != "" && vCurrent.IsZero() && !f.isRequired {
+			if f.typ.Kind() == reflect.Struct {
+				errs.merge(MultiError{"default-" + f.name: errors.New("default option is supported only on: bool, float variants, string, unit variants types or their corresponding pointers or slices")})
+			} else if f.typ.Kind() == reflect.Slice {
+				vals := strings.Split(f.defaultValue, "|")
+
+				//check if slice has one of the supported types for defaults
+				if _, ok := builtinConverters[f.typ.Elem().Kind()]; !ok {
+					errs.merge(MultiError{"default-" + f.name: errors.New("default option is supported only on: bool, float variants, string, unit variants types or their corresponding pointers or slices")})
+					continue
+				}
+
+				defaultSlice := reflect.MakeSlice(f.typ, 0, cap(vals))
+				for _, val := range vals {
+					//this check is to handle if the wrong value is provided
+					if convertedVal := builtinConverters[f.typ.Elem().Kind()](val); convertedVal.IsValid() {
+						defaultSlice = reflect.Append(defaultSlice, convertedVal)
+					}
+				}
+				vCurrent.Set(defaultSlice)
+			} else if f.typ.Kind() == reflect.Ptr {
+				t1 := f.typ.Elem()
+
+				if t1.Kind() == reflect.Struct || t1.Kind() == reflect.Slice {
+					errs.merge(MultiError{"default-" + f.name: errors.New("default option is supported only on: bool, float variants, string, unit variants types or their corresponding pointers or slices")})
+				}
+
+				//this check is to handle if the wrong value is provided
+				if convertedVal := convertPointer(t1.Kind(), f.defaultValue); convertedVal.IsValid() {
+					vCurrent.Set(convertedVal)
+				}
+			} else {
+				//this check is to handle if the wrong value is provided
+				if convertedVal := builtinConverters[f.typ.Kind()](f.defaultValue); convertedVal.IsValid() {
+					vCurrent.Set(builtinConverters[f.typ.Kind()](f.defaultValue))
+				}
+			}
+		}
+	}
+
+	return errs
+}
+
+func isPointerToStruct(v reflect.Value) bool {
+	return !v.IsZero() && v.Type().Kind() == reflect.Ptr && v.Elem().Type().Kind() == reflect.Struct
+}
+
 // checkRequired checks whether required fields are empty
 //
 // check type t recursively if t has struct fields.