#! /bin/bash

### XXX Chose not to 'set -e' because arithmetic evaluation can cause a non-zero exit.
### XXX Chose not to 'set -u' because unset params ($1, $2, ...) cause failures.
### Solutions to the above are ugly.

declare -a paths
declare -i n=0

read_command() {
    local char

    read -rsn 1 char
    case "$char" in
        q | Q) echo 'CMD_QUIT';;
        h | H) echo 'CMD_MOVE_BACK';;
        l | L) echo 'CMD_MOVE_FORWARD';;
        f | F) echo 'CMD_FAVORITE';;
            *) echo 'CMD_UKNOWN';;
    esac
}

paths_find() {
    local -r root_dir="$1"
    local path

    IFS=$'\n'
    for path in $(find "$root_dir" -type f)
    do
        if file "$path" | grep 'image data' > /dev/null
        then
            (( n++ ))
            # \r jumps to the beginning of the line:
            printf '\rFound %d images.' "$n" >&2
            paths["$(( n - 1 ))"]="$path"
        fi
    done
    unset IFS
    printf '\n' >&2
}

set_wallpaper() {
    local -ri i="$1"
    local -r path="$2"

    printf '%d of %d %s\n' "$(( i + 1 ))" "$n" "$path"
    feh --bg-scale "$path"
}

paths_preview() {
    local -i i=0
    local path="${paths[$i]}"

    set_wallpaper "$i" "$path"

    while :
    do
        if [[ "${paths[$i]}" != "$path" ]]
        then
            path="${paths[$i]}"
            set_wallpaper "$i" "$path"
        fi

        case "$(read_command)" in
            CMD_QUIT)
                exit 0;;
            CMD_MOVE_BACK)
                (( i = i == 0 ? (n - 1) : i - 1));;
            CMD_MOVE_FORWARD)
                (( i = (i + 1) % n ));;
            CMD_FAVORITE)
                wallpaper_fav;;
            CMD_UKNOWN)
                continue;;
        esac
    done
}

main() {
    local root_dir

    case "$1" in
        '') root_dir="$DIR_WALLPAPERS";;
        *) root_dir="$1";;
    esac

    paths_find "$root_dir"
    paths_preview
}

main "$@"