package main

import (
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"regexp"
	"strings"
	"time"
)

// modes are feh's background modes, by the word feh_set takes.
var modes = []string{"fill", "center", "max", "scale", "tile"}

// Wallpaper is images and how they are laid on the screens.
type Wallpaper struct {
	Images []string `json:"images"`
	Mode   string   `json:"mode"`
	At     string   `json:"at,omitempty"`
}

func fehbg() string { return filepath.Join(operatorHome(), ".fehbg") }

// sessionRecord is where feh_set notes what it put up, for feh_current: in the runtime directory,
// so it lasts exactly as long as the login, like the wallpaper itself.
func sessionRecord(s Session) string {
	if s.RuntimeDir == "" {
		return ""
	}
	return filepath.Join(s.RuntimeDir, "feh", "current.json")
}

// SetResult is what feh_set answers.
type SetResult struct {
	Wallpaper
	Note string `json:"note"`
}

// Set puts images up as the wallpaper for this session.
func Set(images []string, mode string) (SetResult, error) {
	if mode == "" {
		mode = "fill"
	}
	known := false
	for _, m := range modes {
		known = known || m == mode
	}
	if !known {
		return SetResult{}, fmt.Errorf("mode %q is one of %s", mode, strings.Join(modes, ", "))
	}
	if len(images) == 0 {
		return SetResult{}, errors.New("images is required: at least one image")
	}
	var paths []string
	for _, img := range images {
		p := img
		if strings.HasPrefix(p, "~/") {
			p = filepath.Join(operatorHome(), p[2:])
		}
		if !filepath.IsAbs(p) {
			p = filepath.Join(operatorHome(), p)
		}
		info, err := os.Stat(p)
		if err != nil {
			return SetResult{}, fmt.Errorf("image %s: %w", img, err)
		}
		if info.IsDir() {
			return SetResult{}, fmt.Errorf("image %s is a directory", img)
		}
		paths = append(paths, p)
	}
	s, err := findSession()
	if err != nil {
		return SetResult{}, err
	}
	args := append([]string{"--no-fehbg", "--bg-" + mode}, paths...)
	r, err := s.run(15*time.Second, "", "feh", args...)
	if err != nil {
		return SetResult{}, err
	}
	if r.Code != 0 {
		return SetResult{}, fmt.Errorf("feh: %s", strings.TrimSpace(r.Stderr))
	}
	w := Wallpaper{Images: paths, Mode: mode, At: time.Now().Format(time.RFC3339)}
	if rec := sessionRecord(s); rec != "" {
		if err := os.MkdirAll(filepath.Dir(rec), 0o700); err == nil {
			raw, _ := json.Marshal(w)
			_ = os.WriteFile(rec, raw, 0o600)
		}
	}
	return SetResult{Wallpaper: w, Note: "for this session; the declared wallpaper returns at the next login"}, nil
}

// CurrentResult is what feh_current answers.
type CurrentResult struct {
	Declared *Wallpaper `json:"declared"`
	Session  *Wallpaper `json:"session,omitempty"`
}

var bgMode = regexp.MustCompile(`--bg-(fill|center|max|scale|tile)\b`)

// Current is the declared wallpaper and the one set in this session.
func Current() (CurrentResult, error) {
	var out CurrentResult
	if raw, err := os.ReadFile(fehbg()); err == nil {
		out.Declared = parseFehbg(string(raw), operatorHome())
	}
	if rec := sessionRecord(findEnvironment()); rec != "" {
		if raw, err := os.ReadFile(rec); err == nil {
			var w Wallpaper
			if json.Unmarshal(raw, &w) == nil {
				out.Session = &w
			}
		}
	}
	return out, nil
}

// parseFehbg reads the feh line of a ~/.fehbg: its mode and its images, with $HOME expanded.
func parseFehbg(script, home string) *Wallpaper {
	for _, line := range strings.Split(script, "\n") {
		line = strings.TrimSpace(line)
		if !strings.HasPrefix(line, "feh ") {
			continue
		}
		w := &Wallpaper{Images: []string{}}
		if m := bgMode.FindStringSubmatch(line); m != nil {
			w.Mode = m[1]
		}
		for _, word := range shellWords(line)[1:] {
			if strings.HasPrefix(word, "-") {
				continue
			}
			word = strings.ReplaceAll(strings.ReplaceAll(word, "${HOME}", home), "$HOME", home)
			w.Images = append(w.Images, word)
		}
		return w
	}
	return nil
}

// shellWords splits a simple command line on blanks, honouring single and double quotes.
func shellWords(line string) []string {
	var words []string
	var cur strings.Builder
	var quote byte
	in := false
	for i := 0; i < len(line); i++ {
		c := line[i]
		switch {
		case quote != 0 && c == quote:
			quote = 0
		case quote != 0:
			cur.WriteByte(c)
		case c == '\'' || c == '"':
			quote, in = c, true
		case c == ' ' || c == '\t':
			if in {
				words = append(words, cur.String())
				cur.Reset()
				in = false
			}
		default:
			cur.WriteByte(c)
			in = true
		}
	}
	if in {
		words = append(words, cur.String())
	}
	return words
}