#!/usr/bin/env bash
gversion="0"
pictureuri="picture-uri"

validate_config() {
    if [ "$XDG_CURRENT_DESKTOP" != "GNOME" ]; then
        quit "Error: GNOME is not detected. Please install GNOME."
    fi

    if [ -z "$WALLPAPER_DIR" ]; then
        quit "Error: The wallpaper directory environment variable (WALLPAPER_DIR) has not been set."
    fi

    if [ ! -e "$WALLPAPER_DIR" ]; then
        quit "Error: The wallpaper directory \"$WALLPAPER_DIR\" does not exist."
    fi

    if [ ! -d "$WALLPAPER_DIR" ]; then
        quit "Error: The wallpaper directory \"$WALLPAPER_DIR\" is not a directory."
    fi

    if [ ! -r "$WALLPAPER_DIR"  ]; then
        quit "Error: The wallpaper directory \"$WALLPAPER_DIR\" is not accessible. "
    fi
}
prepare() {
    get_gnome_version
    get_gnome_color_scheme
    set_gnome_color_scheme
    get_wallpaper
    validate_wallpaper
}
get_gnome_version() {
    if [ -r "/usr/share/gnome/gnome-version.xml" ]; then
        gversion="$(grep -i "platform" /usr/share/gnome/gnome-version.xml | tr -d " <platform>" | tr -d "/")"
    elif [ -e "/usr/bin/gnome-shell" ] && [ -x "/usr/bin/gnome-shell" ]; then
       gversion="$(gnome-shell --version | cut -d' ' -f3 | cut -d'.' -f1)"
    fi
}
get_gnome_color_scheme() {
    if [ "$gversion" -ge 42 ]; then
        style="$(gsettings get org.gnome.desktop.interface color-scheme)"
    fi
}
set_gnome_color_scheme() {
    if [ "$style" = "'prefer-dark'" ]; then
        pictureuri="picture-uri-dark"
    fi
}
get_wallpaper() {
    selection="$(find "$WALLPAPER_DIR" -type f -name "*.jpg" -o -name "*.png" | shuf -n1)"
}
validate_wallpaper() {
    if [ ! -f "$selection" ]; then
        quit "Error: The wallpaper \"$WALLPAPER_DIR\" is not a file."
    fi

    if [ ! -r "$selection" ]; then
        quit "Error: The wallpaper \"$WALLPAPER_DIR\" is not accessible."
    fi
}
set_wallpaper() {
    gsettings set org.gnome.desktop.background "$pictureuri" "file://$selection"
}
quit() {
    if [[ "$1" = 0 ]]
    then
            exit 0
    else
            printf "%b\n" "$1"
            exit 1
    fi
}

validate_config
prepare
set_wallpaper
quit 0