#!/usr/bin/env bash
set -euo pipefail

# Creates btrfs subvolume snapshots of /, /home, /opt into /.snapshots/
# Names: root-YYYY-MM-DD, home-YYYY-MM-DD, opt-YYYY-MM-DD
# If a snapshot with the same name exists, it is rotated to -old; existing -old is deleted first.

SNAPDIR="/.snapshots"
DATE="$(date +%F)"

declare -A SRC=(
  [root]="/"
  [home]="/home"
  [opt]="/opt"
)

need_root() {
  if [[ "${EUID:-$(id -u)}" -ne 0 ]]; then
    echo "ERROR: must run as root" >&2
    exit 1
  fi
}

ensure_snapdir() {
  if [[ ! -d "$SNAPDIR" ]]; then
    mkdir -p "$SNAPDIR"
  fi
}

is_btrfs_subvol() {
  local p="$1"
  btrfs subvolume show "$p" >/dev/null 2>&1
}

rotate_if_exists() {
  local base="$1"
  local cur="${SNAPDIR}/${base}-${DATE}"
  local old="${cur}-old"

  if is_btrfs_subvol "$cur"; then
    if is_btrfs_subvol "$old"; then
      btrfs subvolume delete "$old"
    elif [[ -e "$old" ]]; then
      # Path exists but is not a btrfs subvolume (unexpected) — remove it
      rm -rf --one-file-system "$old"
    fi

    # Rename current snapshot to -old
    mv -T "$cur" "$old"
  fi
}

create_snapshot() {
  local base="$1"
  local src="${SRC[$base]}"
  local dst="${SNAPDIR}/${base}-${DATE}"

  # Ensure destination doesn't exist (could happen if mv failed or something created it)
  if is_btrfs_subvol "$dst"; then
    echo "ERROR: destination subvolume already exists: $dst" >&2
    exit 1
  elif [[ -e "$dst" ]]; then
    echo "ERROR: destination path exists and is not a subvolume: $dst" >&2
    exit 1
  fi

  btrfs subvolume snapshot "$src" "$dst"
}

main() {
  need_root
  ensure_snapdir

  # Rotate existing snapshots (if any)
  for base in root home opt; do
    rotate_if_exists "$base"
  done

  # Create new snapshots
  for base in root home opt; do
    create_snapshot "$base"
  done
}

main "$@"