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

if [ -z "$HABITS" ]; then
  echo "Define a HABITS variable with a list of habits"
fi

if [ -z "$METRICS" ]; then
  echo "Define a METRICS variable with a list of habits"
fi

HABITS_FILE="${HABITS_FILE:-$HOME/.local/share/os/habits.md}"

usage() {
  cat <<'EOF'
Usage: habits <command>

Commands:
  open       Open the habits file in $EDITOR
  new        Add a new day to the habits file
  left  Print today's habits that are not done yet
EOF
}

ensure_file() {
  mkdir -p "$(dirname "$HABITS_FILE")"
  touch "$HABITS_FILE"
}

date_add_days() {
  local day="$1" offset="$2" date_arg epoch

  date_arg="$day $offset days"
  if date -d "$date_arg" +%F >/dev/null 2>&1; then
    date -d "$date_arg" +%F
    return
  fi

  # BSD/macOS date. Parse at noon to avoid midnight DST edges.
  epoch="$(date -j -f '%F %H:%M:%S' "$day 12:00:00" +%s)"
  epoch=$((epoch + offset * 86400))
  date -r "$epoch" +%F
}

open_habits() {
  ensure_file
  "${EDITOR:-vi}" "$HABITS_FILE"
}

print_day() {
  local day="$1"

  printf '# %s\n\n' "$day"
  printf '## Habits\n\n'
  while IFS= read -r habit; do
    [ -n "$habit" ] && printf -- '- [ ] %s\n' "$habit"
  done <<< "$HABITS"
  printf '\n## Metrics \n\n'
  while IFS= read -r metric; do
    [ -n "$metric" ] && printf -- '- %s\n' "$metric:"
  done <<< "$METRICS"
  printf '\n'
}

new_day() {
  ensure_file

  local today latest start day tmp added
  today="$(date +%F)"
  latest="$(grep -E '^# [0-9]{4}-[0-9]{2}-[0-9]{2}$' "$HABITS_FILE" | sed 's/^# //' | sort | tail -n 1 || true)"

  if [ -n "$latest" ] && [[ "$latest" > "$today" ]]; then
    echo "Latest habits entry is in the future: $latest" >&2
    exit 1
  fi

  if [ "$latest" = "$today" ]; then
    echo "Habits entries already exist through $today: $HABITS_FILE"
    return
  fi

  if [ -n "$latest" ]; then
    start="$(date_add_days "$latest" 1)"
  else
    start="$today"
  fi

  tmp="$(mktemp)"
  added=0

  {
    day="$today"
    while [[ "$day" > "$start" || "$day" = "$start" ]]; do
      print_day "$day"
      added=$((added + 1))
      day="$(date_add_days "$day" -1)"
    done
    cat "$HABITS_FILE"
  } > "$tmp"

  mv "$tmp" "$HABITS_FILE"
  echo "Added $added habits entr$( [ "$added" -eq 1 ] && printf 'y' || printf 'ies' ) through $today to $HABITS_FILE"
}

remaining_habits() {
  ensure_file

  local date
  date="$(date +%F)"

  awk -v date="$date" '
    $0 == "# " date { in_day = 1; next }
    in_day && /^# / { exit }
    in_day && /^## Habits[[:space:]]*$/ { in_habits = 1; next }
    in_day && in_habits && /^## / { exit }
    in_day && in_habits && /^- \[ \] / {
      sub(/^- \[ \] /, "")
      print
    }
  ' "$HABITS_FILE"
}

case "${1:-}" in
  open)
    open_habits
    ;;
  new)
    new_day
    open_habits
    ;;
  left)
    remaining_habits
    ;;
  -h|--help|help)
    usage
    ;;
  *)
    usage >&2
    exit 1
    ;;
esac