251 lines
6.6 KiB
Go
251 lines
6.6 KiB
Go
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// Package reflect implements extensions to the standard reflect lib suitable
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// for implementing marshaling and unmarshaling packages. The main Mapper type
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// allows for Go-compatible named atribute access, including accessing embedded
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// struct attributes and the ability to use functions and struct tags to
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// customize field names.
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//
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package reflectx
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import "sync"
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import (
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"reflect"
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"runtime"
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)
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type fieldMap map[string][]int
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// Mapper is a general purpose mapper of names to struct fields. A Mapper
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// behaves like most marshallers, optionally obeying a field tag for name
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// mapping and a function to provide a basic mapping of fields to names.
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type Mapper struct {
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cache map[reflect.Type]fieldMap
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tagName string
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mapFunc func(string) string
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mutex sync.Mutex
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}
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// NewMapper returns a new mapper which optionally obeys the field tag given
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// by tagName. If tagName is the empty string, it is ignored.
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func NewMapper(tagName string) *Mapper {
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return &Mapper{
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cache: make(map[reflect.Type]fieldMap),
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tagName: tagName,
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}
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}
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// NewMapperFunc returns a new mapper which optionally obeys a field tag and
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// a struct field name mapper func given by f. Tags will take precedence, but
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// for any other field, the mapped name will be f(field.Name)
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func NewMapperFunc(tagName string, f func(string) string) *Mapper {
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return &Mapper{
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cache: make(map[reflect.Type]fieldMap),
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tagName: tagName,
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mapFunc: f,
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}
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}
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// TypeMap returns a mapping of field strings to int slices representing
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// the traversal down the struct to reach the field.
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func (m *Mapper) TypeMap(t reflect.Type) fieldMap {
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m.mutex.Lock()
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mapping, ok := m.cache[t]
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if !ok {
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mapping = getMapping(t, m.tagName, m.mapFunc)
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m.cache[t] = mapping
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}
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m.mutex.Unlock()
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return mapping
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}
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// FieldMap returns the mapper's mapping of field names to reflect values. Panics
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// if v's Kind is not Struct, or v is not Indirectable to a struct kind.
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func (m *Mapper) FieldMap(v reflect.Value) map[string]reflect.Value {
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v = reflect.Indirect(v)
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mustBe(v, reflect.Struct)
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r := map[string]reflect.Value{}
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nm := m.TypeMap(v.Type())
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for tagName, indexes := range nm {
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r[tagName] = FieldByIndexes(v, indexes)
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}
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return r
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}
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// FieldByName returns a field by the its mapped name as a reflect.Value.
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// Panics if v's Kind is not Struct or v is not Indirectable to a struct Kind.
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// Returns zero Value if the name is not found.
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func (m *Mapper) FieldByName(v reflect.Value, name string) reflect.Value {
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v = reflect.Indirect(v)
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mustBe(v, reflect.Struct)
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nm := m.TypeMap(v.Type())
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traversal, ok := nm[name]
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if !ok {
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return *new(reflect.Value)
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}
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return FieldByIndexes(v, traversal)
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}
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// FieldsByName returns a slice of values corresponding to the slice of names
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// for the value. Panics if v's Kind is not Struct or v is not Indirectable
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// to a struct Kind. Returns zero Value for each name not found.
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func (m *Mapper) FieldsByName(v reflect.Value, names []string) []reflect.Value {
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v = reflect.Indirect(v)
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mustBe(v, reflect.Struct)
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nm := m.TypeMap(v.Type())
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vals := make([]reflect.Value, 0, len(names))
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for _, name := range names {
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traversal, ok := nm[name]
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if !ok {
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vals = append(vals, *new(reflect.Value))
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} else {
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vals = append(vals, FieldByIndexes(v, traversal))
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}
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}
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return vals
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}
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// Traversals by name returns a slice of int slices which represent the struct
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// traversals for each mapped name. Panics if t is not a struct or Indirectable
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// to a struct. Returns empty int slice for each name not found.
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func (m *Mapper) TraversalsByName(t reflect.Type, names []string) [][]int {
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t = Deref(t)
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mustBe(t, reflect.Struct)
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nm := m.TypeMap(t)
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r := make([][]int, 0, len(names))
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for _, name := range names {
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traversal, ok := nm[name]
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if !ok {
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r = append(r, []int{})
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} else {
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r = append(r, traversal)
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}
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}
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return r
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}
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// FieldByIndexes returns a value for a particular struct traversal.
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func FieldByIndexes(v reflect.Value, indexes []int) reflect.Value {
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for _, i := range indexes {
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v = reflect.Indirect(v).Field(i)
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// if this is a pointer, it's possible it is nil
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if v.Kind() == reflect.Ptr && v.IsNil() {
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alloc := reflect.New(Deref(v.Type()))
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v.Set(alloc)
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}
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}
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return v
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}
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// FieldByIndexesReadOnly returns a value for a particular struct traversal,
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// but is not concerned with allocating nil pointers because the value is
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// going to be used for reading and not setting.
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func FieldByIndexesReadOnly(v reflect.Value, indexes []int) reflect.Value {
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for _, i := range indexes {
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v = reflect.Indirect(v).Field(i)
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}
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return v
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}
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// Deref is Indirect for reflect.Types
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func Deref(t reflect.Type) reflect.Type {
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if t.Kind() == reflect.Ptr {
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t = t.Elem()
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}
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return t
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}
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// -- helpers & utilities --
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type Kinder interface {
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Kind() reflect.Kind
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}
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// mustBe checks a value against a kind, panicing with a reflect.ValueError
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// if the kind isn't that which is required.
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func mustBe(v Kinder, expected reflect.Kind) {
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k := v.Kind()
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if k != expected {
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panic(&reflect.ValueError{Method: methodName(), Kind: k})
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}
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}
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// methodName is returns the caller of the function calling methodName
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func methodName() string {
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pc, _, _, _ := runtime.Caller(2)
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f := runtime.FuncForPC(pc)
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if f == nil {
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return "unknown method"
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}
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return f.Name()
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}
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type typeQueue struct {
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t reflect.Type
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p []int
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}
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// A copying append that creates a new slice each time.
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func apnd(is []int, i int) []int {
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x := make([]int, len(is)+1)
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for p, n := range is {
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x[p] = n
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}
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x[len(x)-1] = i
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return x
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}
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// getMapping returns a mapping for the t type, using the tagName and the mapFunc
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// to determine the canonical names of fields.
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func getMapping(t reflect.Type, tagName string, mapFunc func(string) string) fieldMap {
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queue := []typeQueue{}
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queue = append(queue, typeQueue{Deref(t), []int{}})
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m := fieldMap{}
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for len(queue) != 0 {
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// pop the first item off of the queue
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tq := queue[0]
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queue = queue[1:]
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// iterate through all of its fields
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for fieldPos := 0; fieldPos < tq.t.NumField(); fieldPos++ {
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f := tq.t.Field(fieldPos)
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name := f.Tag.Get(tagName)
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if len(name) == 0 {
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if mapFunc != nil {
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name = mapFunc(f.Name)
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} else {
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name = f.Name
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}
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}
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// if the name is "-", disabled via a tag, skip it
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if name == "-" {
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continue
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}
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// skip unexported fields
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if len(f.PkgPath) != 0 {
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continue
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}
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// bfs search of anonymous embedded structs
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if f.Anonymous {
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queue = append(queue, typeQueue{Deref(f.Type), apnd(tq.p, fieldPos)})
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continue
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}
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// if the name is shadowed by an earlier identical name in the search, skip it
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if _, ok := m[name]; ok {
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continue
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}
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// add it to the map at the current position
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m[name] = apnd(tq.p, fieldPos)
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}
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}
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return m
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}
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