package server

import (
	"encoding/csv"
	"fmt"
	"io"
	"net/http"
	"strings"
)

// exportFormats maps ?format= values to file extension and content type.
var exportFormats = map[string]struct{ ext, contentType string }{
	"csv": {"csv", "text/csv; charset=utf-8"},
	"txt": {"txt", "text/plain; charset=utf-8"},
}

// handleExport downloads a list's titles, one per line, with nothing else:
// no ranks, header row or metadata. Line N is place #N, so an empty place
// is a blank line, and importing the file puts everything back where it was.
func (s *Server) handleExport(w http.ResponseWriter, r *http.Request) {
	format := r.URL.Query().Get("format")
	if format == "" {
		format = "txt"
	}
	f, ok := exportFormats[format]
	if !ok {
		http.Error(w, `format must be "csv" or "txt"`, http.StatusBadRequest)
		return
	}
	l, ok := s.visibleList(w, r)
	if !ok {
		return
	}
	items, err := s.store.Items(r.Context(), l.ID, 0, 0)
	if err != nil {
		s.serverError(w, r, err)
		return
	}
	var titles []string
	for _, it := range items {
		for len(titles) < it.Rank-1 {
			titles = append(titles, "")
		}
		titles = append(titles, it.Title)
	}
	w.Header().Set("Content-Type", f.contentType)
	w.Header().Set("Content-Disposition", fmt.Sprintf(`attachment; filename="%s.%s"`, l.Slug, f.ext))
	w.Header().Set("Cache-Control", "no-store")
	if err := writeTitles(w, format, titles); err != nil {
		s.log.Warn("export failed", "list", l.Slug, "err", err)
	}
}

// flatten keeps one title per line even if a title contains a newline.
var flatten = strings.NewReplacer("\r\n", " ", "\n", " ", "\r", " ")

func writeTitles(w io.Writer, format string, titles []string) error {
	if format == "csv" {
		// The BOM makes Excel read accented titles (Les Misérables) as UTF-8.
		if _, err := io.WriteString(w, "\xef\xbb\xbf"); err != nil {
			return err
		}
		cw := csv.NewWriter(w)
		cw.UseCRLF = true
		for _, t := range titles {
			if err := cw.Write([]string{flatten.Replace(t)}); err != nil {
				return err
			}
		}
		cw.Flush()
		return cw.Error()
	}
	for _, t := range titles {
		if _, err := io.WriteString(w, flatten.Replace(t)+"\n"); err != nil {
			return err
		}
	}
	return nil
}