package csvutil

import (
	"bytes"
	"encoding/csv"
	"io"
	"reflect"
)

const defaultTag = "csv"

var (
	_bytes = reflect.TypeOf(([]byte)(nil))
	_error = reflect.TypeOf((*error)(nil)).Elem()
)

// Unmarshal parses the CSV-encoded data and stores the result in the slice or
// the array pointed to by v. If v is nil or not a pointer to a struct slice or
// struct array, Unmarshal returns an InvalidUnmarshalError.
//
// Unmarshal uses the std encoding/csv.Reader for parsing and csvutil.Decoder
// for populating the struct elements in the provided slice. For exact decoding
// rules look at the Decoder's documentation.
//
// The first line in data is treated as a header. Decoder will use it to map
// csv columns to struct's fields.
//
// In case of success the provided slice will be reinitialized and its content
// fully replaced with decoded data.
func Unmarshal(data []byte, v any) error {
	val := reflect.ValueOf(v)

	if val.Kind() != reflect.Ptr || val.IsNil() {
		return &InvalidUnmarshalError{Type: reflect.TypeOf(v)}
	}

	switch val.Type().Elem().Kind() {
	case reflect.Slice, reflect.Array:
	default:
		return &InvalidUnmarshalError{Type: val.Type()}
	}

	typ := val.Type().Elem()

	if walkType(typ.Elem()).Kind() != reflect.Struct {
		return &InvalidUnmarshalError{Type: val.Type()}
	}

	dec, err := NewDecoder(newCSVReader(bytes.NewReader(data)))
	if err == io.EOF {
		return nil
	} else if err != nil {
		return err
	}

	// for the array just call decodeArray directly; for slice values call the
	// optimized code for better performance.

	if typ.Kind() == reflect.Array {
		return dec.decodeArray(val.Elem())
	}

	c := countRecords(data)
	slice := reflect.MakeSlice(typ, c, c)

	var i int
	for ; ; i++ {
		// just in case countRecords counts it wrong.
		if i >= c && i >= slice.Len() {
			slice = reflect.Append(slice, reflect.New(typ.Elem()).Elem())
		}

		if err := dec.Decode(slice.Index(i).Addr().Interface()); err == io.EOF {
			break
		} else if err != nil {
			return err
		}
	}

	val.Elem().Set(slice.Slice3(0, i, i))
	return nil
}

// Marshal returns the CSV encoding of slice or array v. If v is not a slice or
// elements are not structs then Marshal returns InvalidMarshalError.
//
// Marshal uses the std encoding/csv.Writer with its default settings for csv
// encoding.
//
// Marshal will always encode the CSV header even for the empty slice.
//
// For the exact encoding rules look at Encoder.Encode method.
func Marshal(v any) ([]byte, error) {
	val := walkValue(reflect.ValueOf(v))

	if !val.IsValid() {
		return nil, &InvalidMarshalError{}
	}

	switch val.Kind() {
	case reflect.Array, reflect.Slice:
	default:
		return nil, &InvalidMarshalError{Type: reflect.ValueOf(v).Type()}
	}

	typ := walkType(val.Type().Elem())
	if typ.Kind() != reflect.Struct {
		return nil, &InvalidMarshalError{Type: reflect.ValueOf(v).Type()}
	}

	var buf bytes.Buffer
	w := csv.NewWriter(&buf)
	enc := NewEncoder(w)

	if err := enc.encodeHeader(typ); err != nil {
		return nil, err
	}

	if err := enc.encodeArray(val); err != nil {
		return nil, err
	}

	w.Flush()
	if err := w.Error(); err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}

func countRecords(s []byte) (n int) {
	var prev byte
	inQuote := false
	for {
		if len(s) == 0 && prev != '"' {
			return n
		}

		i := bytes.IndexAny(s, "\n\"")
		if i == -1 {
			return n + 1
		}

		switch s[i] {
		case '\n':
			if !inQuote && (i > 0 || prev == '"') {
				n++
			}
		case '"':
			inQuote = !inQuote
		}

		prev = s[i]
		s = s[i+1:]
	}
}

// Header scans the provided struct type, struct slice or struct array and generates a CSV header for it.
//
// Field names are written in the same order as struct fields are defined.
// Embedded struct's fields are treated as if they were part of the outer struct.
// Fields that are embedded types and that are tagged are treated like any
// other field.
//
// Unexported fields and fields with tag "-" are ignored.
//
// Tagged fields have the priority over non tagged fields with the same name.
//
// Following the Go visibility rules if there are multiple fields with the same
// name (tagged or not tagged) on the same level and choice between them is
// ambiguous, then all these fields will be ignored.
//
// It is a good practice to call Header once for each type. The suitable place
// for calling it is init function. Look at Decoder.DecodingDataWithNoHeader
// example.
//
// If tag is left empty the default "csv" will be used.
//
// Header will return UnsupportedTypeError if the provided value is nil, is
// not a struct, a struct slice or a struct array.
func Header(v any, tag string) ([]string, error) {
	typ, err := valueType(v)
	if err != nil {
		return nil, err
	}

	if tag == "" {
		tag = defaultTag
	}

	fields := cachedFields(typeKey{tag, typ})
	h := make([]string, len(fields))
	for i, f := range fields {
		h[i] = f.name
	}
	return h, nil
}

func valueType(v any) (reflect.Type, error) {
	val := reflect.ValueOf(v)
	if !val.IsValid() {
		return nil, &UnsupportedTypeError{}
	}

loop:
	for {
		switch val.Kind() {
		case reflect.Ptr, reflect.Interface:
			el := val.Elem()
			if !el.IsValid() {
				break loop
			}
			val = el
		default:
			break loop
		}
	}

	typ := walkType(val.Type())
	switch typ.Kind() {
	case reflect.Struct:
		return typ, nil
	case reflect.Slice, reflect.Array:
		if eTyp := walkType(typ.Elem()); eTyp.Kind() == reflect.Struct {
			return eTyp, nil
		}
	}
	return nil, &UnsupportedTypeError{Type: typ}
}

func newCSVReader(r io.Reader) *csv.Reader {
	rr := csv.NewReader(r)
	rr.ReuseRecord = true
	return rr
}