#!/bin/bash

readonly PROGRAM_NAME=$(basename "$0")
readonly PROGRAM_LOC=$(readlink -m "$(dirname $0)")
readonly ARGS="$@"
readonly ARG_COUNT="$#"

LAST_SHA=0

usage () {
  echo "usage: $PROGRAM_NAME [DIRECTORY] [WAIT_TIME]"
}

error () {
  local readonly message=$1; shift

  echo "$PROGRAM_NAME: $message"
  usage
  exit 1
}

current_sha () {
  ls -lR "$DIRECTORY" \
    | sha1sum
}

sha_changed? () {
  [[ "$(current_sha)" != "$LAST_SHA" ]]
}

update_sha () {
  LAST_SHA=$(current_sha)
}

alert_update () {
  now=$(date +"%Y-%m-%d %I:%M:%S")
  echo "[$now] Directory Updated"
}

is_dir? () {
  local readonly directory=$1; shift

  [[ -d $directory ]]
}

is_int? () {
  local readonly number=$1; shift

  [[ $number =~ ^[0-9]+$ ]]
}

is_empty? () {
  local var=$1; shift

  [[ -z $var ]]
}

check_arguments () {
  if is_empty? "$DIRECTORY" || is_empty? "$WAIT_TIME"; then
    error "missing operand"
  fi

  if ! is_dir? "$DIRECTORY"; then
    error "$DIRECTORY is not a directory"
  fi

  if ! is_int? "$WAIT_TIME"; then
    error "$WAIT_TIME is not an integer"
  fi
}

run () {
  while true; do
    if sha_changed?; then
      update_sha
      alert_update
    fi

    sleep "$WAIT_TIME"
  done
}

main () {
  local readonly d=$1; shift
  local readonly w=$1; shift

  readonly DIRECTORY=${d:-$(pwd)}
  readonly WAIT_TIME=${w:-15}

  check_arguments
  update_sha

  echo "--------------------------------------------------------------------------------"
  echo "+ Monitoring $DIRECTORY at interval $WAIT_TIME seconds"
  echo "--------------------------------------------------------------------------------"
  run
}

main $ARGS