refactor: split copier.go into smaller files

Split monolithic copier.go (1077 lines) into focused modules:
- copier.go: Core framework (95 lines)
- copy_value.go: Copy functions and type handling (858 lines)
- reflect.go: Reflection utilities (100 lines)
- converter.go: Type converter (38 lines)
- options.go: Configuration (renamed from optiongs.go)

Bug fixes:
- Fixed string to *string copying (empty strings now work)
- Fixed *time.Time to *time.Time copying
- Fixed nil slice handling
- Fixed copyStructToSlice pointer element handling

Tests: Fixed critical data copy issues. Remaining method-mapping
features require additional implementation.

options.go

@@ -0,0 +1,212 @@
+package copier
+
+import "reflect"
+
+const (
+	noDepthLimited = -1
+)
+
+type convertFunc func(any) (any, error)
+
+type options struct {
+	tagName            string                        // 标签名
+	maxDepth           int                           // 最大复制深度
+	ignoreEmpty        bool                          // 复制时忽略空字段
+	caseSensitive      bool                          // 复制时大小写敏感
+	must               bool                          // 只复制具有must标识的字段
+	detectCircularRefs bool                          // 检测循环引用
+	skipFields         []string                      // 忽略字段
+	fieldNameMapping   map[string]string             // 字段名转映射
+	converters         map[converterPair]convertFunc // 根据源和目标类型处理的类型转换器v
+	convertByName      map[string]convertFunc        // 根据名称处理的类型转换器
+	nameConverter      func(string) string           // 字段名转换函数
+	timeFormats        map[reflect.Type]string       // 时间格式
+}
+
+var (
+	defaultOptions = &options{
+		tagName:  defaultTag,
+		maxDepth: noDepthLimited,
+	}
+)
+
+type option func(*options)
+
+type TypeConverter struct {
+	FieldName string
+	SrcType   any
+	DstType   any
+	Fn        func(src any) (dst any, err error)
+}
+
+type converterPair struct {
+	Name    string
+	SrcType reflect.Type
+	DstType reflect.Type
+}
+
+func getOpt(opts ...option) *options {
+	if len(opts) == 0 {
+		return defaultOptions
+	}
+
+	opt := &options{
+		maxDepth: noDepthLimited,
+		tagName:  defaultTag,
+	}
+
+	for _, o := range opts {
+		o(opt)
+	}
+
+	return opt
+}
+
+func (opt *options) NameConvert(name string) string {
+	if opt.nameConverter != nil {
+		name = opt.nameConverter(name)
+	}
+
+	if toname, ok := opt.fieldNameMapping[name]; ok {
+		name = toname
+	}
+
+	return name
+}
+
+func (opt *options) ExceedMaxDepth(depth int) bool {
+	return opt.maxDepth != noDepthLimited && depth > opt.maxDepth
+}
+
+// WithMaxDepth 设置最大复制深度
+func WithMaxDepth(depth int) option {
+	return func(o *options) {
+		o.maxDepth = depth
+	}
+}
+
+// WithIgnoreEmpty 复制时忽略空字段标识
+func WithIgnoreEmpty() option {
+	return func(o *options) {
+		o.ignoreEmpty = true
+	}
+}
+
+// WithCaseInsensitive 复制时大小写不敏感标识
+func WithCaseSensitive() option {
+	return func(o *options) {
+		o.caseSensitive = true
+	}
+}
+
+// WithMust 添加必须复制标识
+func WithMust() option {
+	return func(o *options) {
+		o.must = true
+	}
+}
+
+// WithDetectCircularRefs 添加检测循环引用标识
+func WithDetectCircularRefs() option {
+	return func(o *options) {
+		o.detectCircularRefs = true
+	}
+}
+
+// WithNameConvertByName 添加根据名称处理的类型转换器
+func WithTypeConvertByName(name string, f func(src any) (dst any, err error)) option {
+	return func(o *options) {
+		if o.convertByName == nil {
+			o.convertByName = make(map[string]convertFunc)
+		}
+
+		o.convertByName[name] = f
+	}
+}
+
+// WithConvertByNames 添加根据名称处理的类型转换器
+func WithTypeConvertByNames(nameConvert map[string]func(src any) (dst any, err error)) option {
+	return func(o *options) {
+		if o.convertByName == nil {
+			o.convertByName = make(map[string]convertFunc)
+		}
+
+		for name, nameConvert := range nameConvert {
+			o.convertByName[name] = nameConvert
+		}
+	}
+}
+
+// WithTypeConvert 添加根据源和目标类型处理的类型转换器
+func WithConverters(converters ...TypeConverter) option {
+	return func(o *options) {
+		if len(converters) > 0 && o.converters == nil {
+			o.converters = make(map[converterPair]convertFunc)
+		}
+
+		for i := range converters {
+			pair := converterPair{
+				SrcType: reflect.TypeOf(converters[i].SrcType),
+				DstType: reflect.TypeOf(converters[i].DstType),
+			}
+
+			o.converters[pair] = converters[i].Fn
+		}
+	}
+}
+
+// WithNameMapping 添加字段名映射
+func WithNameMapping(mappings map[string]string) option {
+	return func(o *options) {
+		o.fieldNameMapping = mappings
+	}
+}
+
+// WithNameConverter 添加字段名转换函数
+func WithNameFn(fn func(string) string) option {
+	return func(o *options) {
+		o.nameConverter = fn
+	}
+}
+
+func WithTimeFormat(targetType any, format string) option {
+	return func(o *options) {
+		if o.timeFormats == nil {
+			o.timeFormats = make(map[reflect.Type]string)
+		}
+
+		t := reflect.TypeOf(targetType)
+		if t.Kind() == reflect.Pointer {
+			t = t.Elem()
+		}
+
+		o.timeFormats[t] = format
+	}
+}
+
+func WithStringTimeFormat(format string) option {
+	return WithTimeFormat("", format)
+}
+
+func WithDefaultTimeFormat(format string) option {
+	return func(o *options) {
+		if o.timeFormats == nil {
+			o.timeFormats = make(map[reflect.Type]string)
+		}
+
+		o.timeFormats[reflect.TypeOf(nil)] = format
+	}
+}
+
+func WitSkipFields(fields ...string) option {
+	return func(o *options) {
+		o.skipFields = fields
+	}
+}
+
+// WithTagName 添加标签名
+func WithTagName(tagName string) option {
+	return func(o *options) {
+		o.tagName = tagName
+	}
+}