package httpx

import (
	"bytes"
	"errors"
	"io"
	"mime/multipart"
	"net/http"
	"net/textproto"

	apierrors "git.yuntuo.hk/k2safe/go-omni/errors"
)

// MultipartReader streams request parts and retains parsing failures, so a
// partially consumed or invalid upload cannot later complete successfully.
// It belongs to a single request and must be read sequentially.
type MultipartReader struct {
	reader  *multipart.Reader
	input   *multipartInput
	current *MultipartPart
	err     error
	done    bool
}

// NextPart closes the preceding part and returns the next part. Like the
// standard library, it decodes quoted-printable content; NextRawPart does not.
// Only a valid final boundary yields io.EOF; malformed input yields an error
// with status 400. Body limits remain detectable as *http.MaxBytesError.
func (r *MultipartReader) NextPart() (*MultipartPart, error) { return r.next(false) }

func (r *MultipartReader) NextRawPart() (*MultipartPart, error) { return r.next(true) }

func (r *MultipartReader) next(raw bool) (*MultipartPart, error) {
	if r.err != nil {
		return nil, r.err
	}
	if r.done {
		return nil, io.EOF
	}
	if r.current != nil {
		if err := r.current.Close(); err != nil {
			return nil, err
		}
	}
	var part *multipart.Part
	var err error
	if raw {
		part, err = r.reader.NextRawPart()
	} else {
		part, err = r.reader.NextPart()
	}
	if err == io.EOF {
		// The standard parser also returns bare EOF for truncated MIME
		// headers. Require an actual closing delimiter before treating EOF
		// as successful completion.
		if !r.input.ended() {
			return nil, r.record(io.ErrUnexpectedEOF)
		}
		r.done = true
		return nil, io.EOF
	}
	if err != nil {
		return nil, r.record(err)
	}
	r.current = &MultipartPart{Header: part.Header, part: part, owner: r}
	return r.current, nil
}

func (r *MultipartReader) record(err error) error {
	if err == nil || err == io.EOF {
		return err
	}
	if r.err == nil {
		var limit *http.MaxBytesError
		if errors.As(err, &limit) {
			r.err = err
		} else {
			r.err = apierrors.New(400, "INVALID_MULTIPART", "invalid multipart request body").WithCause(err)
		}
	}
	return r.err
}

// MultipartPart is a streaming request part. Close drains unread content and
// reports read/parse errors; it never buffers the whole file or creates a file.
type MultipartPart struct {
	Header textproto.MIMEHeader
	part   *multipart.Part
	owner  *MultipartReader
	closed bool
	err    error
}

func (p *MultipartPart) FormName() string { return p.part.FormName() }
func (p *MultipartPart) FileName() string { return p.part.FileName() }

func (p *MultipartPart) Read(buf []byte) (int, error) {
	if p.closed {
		if p.err != nil {
			return 0, p.err
		}
		return 0, io.EOF
	}
	n, err := p.part.Read(buf)
	return n, p.owner.record(err)
}

func (p *MultipartPart) Close() error {
	if !p.closed {
		_, p.err = io.Copy(io.Discard, p)
		p.closed = true
	}
	return p.err
}

// multipartInput observes closing-delimiter lines without buffering file
// content or changing bytes. The MIME parser still validates their syntax and
// placement. Keep only a prefix offset, including across reader boundaries.
type multipartInput struct {
	io.Reader
	boundary             string
	matched              int
	carriage, final, eof bool
}

func (r *multipartInput) Read(p []byte) (int, error) {
	n, err := r.Reader.Read(p)
	r.observe(p[:n])
	if err == io.EOF {
		r.eof = true
	}
	return n, err
}

func (r *multipartInput) ended() bool {
	return r.final || (r.eof && r.matched == len(r.boundary) && !r.carriage)
}

func (r *multipartInput) observe(data []byte) {
	for len(data) > 0 && !r.final {
		if r.matched < 0 {
			newline := bytes.IndexByte(data, '\n')
			if newline < 0 {
				return
			}
			data, r.matched, r.carriage = data[newline+1:], 0, false
			continue
		}
		c := data[0]
		data = data[1:]
		if r.matched < len(r.boundary) {
			if c == r.boundary[r.matched] {
				r.matched++
				continue
			}
		} else {
			switch {
			case c == '\n':
				r.final = true
				continue
			case !r.carriage && (c == ' ' || c == '\t'):
				continue
			case !r.carriage && c == '\r':
				r.carriage = true
				continue
			}
		}
		r.matched, r.carriage = -1, false
		if c == '\n' {
			r.matched = 0
		}
	}
}