#!/bin/sh

# Determine the next wallpaper by round robin from the cloud
# and install it as the background files referenced in
# ~/.config/gnome-settings/{misc,shell-extensions}.ini
#
# Candidates:
#  - Pairs => Both "<name>-d.jpg" and "<name>-l.jpg" exist
#  - Singles => Any other "*.jpg" not following the -d/-l scheme,
#               used as dark and light at the same time

set -eu


SOURCE_DIR="$HOME/Cloud/Null-Tech/Backgrounds"
TARGET_DIR="${XDG_DATA_HOME:-$HOME/.local/share}/backgrounds"
STATE_FILE="${XDG_STATE_HOME:-$HOME/.local/state}/current-background.txt"


[ -d "$SOURCE_DIR" ] || { echo "No such folder: $SOURCE_DIR" >&2; exit 1; }


# One candidate per line, tab-separated: <dark-file> <light-file>
candidates=$(
    for jpg in "$SOURCE_DIR"/*.jpg; do
        [ -e "$jpg" ] || continue  # because the glob does not expand if empty

        case "$jpg" in

            *-d.jpg)  # Complete pairs only
                stem="${jpg%-d.jpg}"
                [ -f "$stem-l.jpg" ] && printf '%s\t%s\n' "$jpg" "$stem-l.jpg"
                ;;

            *-l.jpg)  # Already handled via its "-d" counterpart above
                ;;

            *)  # A single file to be used for both modes
                printf '%s\t%s\n' "$jpg" "$jpg"
                ;;

        esac
    done
)


if [ -z "$candidates" ]; then
    echo "No usable *.jpg files in: $SOURCE_DIR" >&2
    exit 1
fi


echo "Found $(printf '%s\n' "$candidates" | wc -l) candidate(s)"


# The glob expands alphabetically => "$candidates" is already sorted
# => Pick the first candidate sorting strictly after the last-used one,
#    falling back to the first (e.g., for start, wrap-around, or missing state file)
last=$(cat "$STATE_FILE" 2>/dev/null || true)
chosen=$(printf '%s\n' "$candidates" | awk -F '\t' -v last="$last" '$1 > last { print; exit }')
[ -n "$chosen" ] || chosen=$(printf '%s\n' "$candidates" | head -n 1)


dark=$(printf '%s' "$chosen" | cut -f 1)
light=$(printf '%s' "$chosen" | cut -f 2)
echo "Chosen: $(basename "$dark")"


mkdir -p "$TARGET_DIR" "$(dirname "$STATE_FILE")"
cp -- "$dark" "$TARGET_DIR/dark.jpg"
cp -- "$light" "$TARGET_DIR/light.jpg"
printf '%s\n' "$dark" > "$STATE_FILE"


echo "Installed as: $TARGET_DIR/{dark,light}.jpg"