#!/usr/bin/env bash

die() {
  printf "invalid roll: $1\n" "${@:2}" >&2
  exit 1
}

debug() {
  if [[ -v opts[debug] ]]; then
    printf "DEBUG: $1\n" "${@:2}"
  fi
}

verbose() {
  if [[ -v opts[verbose] ]]; then
    printf "$1\n" "${@:2}"
  fi
}

is_integer() {
  if (( $# )); then
    [[ $1 =~ ^[0-9]+$ ]]
  else
    return 1
  fi
}

roll_die() {
  if (( $# )); then
    printf %d "$(( (RANDOM % $1) + 1 ))"
  else
    printf 0
  fi
}

declare -A opts

while (( $# )); do
  case $1 in
  -v)
    opts[verbose]=1
    ;;
  -d)
    opts[debug]=1
    ;;
  -x)
    set -x
    ;;
  -*)
    die "unknown option '%s'" "$1"
    ;;
  *)
    break
  esac

  shift
done

if (( $# )); then
  IFS=d read -r quantity rest <<< "$1"

  if [[ -z $quantity ]]; then
    quantity=1
  elif ! is_integer "$quantity"; then
    die "dice quantity is not a number"
  fi

  debug "quantity of dice is %d" "$quantity"

  while (( ${#rest} )) && [[ ${rest:0:1} != @(+|-) ]]; do
    sides+=${rest:0:1} rest=${rest:1}
  done

  if ! is_integer "$sides"; then
    die "not a good number of die sides"
  fi

  debug "die has %d side(s)" "$sides"
  total_mod=0

  while (( ${#rest} )); do
    mod_type=${rest:0:1} rest=${rest:1} next_mod=

    while [[ -n $rest && ${rest:0:1} != @(+|-) ]]; do
      next_mod+=${rest:0:1} rest=${rest:1}
    done

    if ! is_integer "$next_mod"; then
      die "'%s' is not a proper modifier" "$next_mod"
    elif [[ $mod_type = + ]]; then
      (( total_mod += next_mod ))
      debug "adding %d to modifier for a total of %d" "$next_mod" "$total_mod"
    else
      (( total_mod -= next_mod ))
      debug "subtracting %d from modifier for a total of %d" "$next_mod" "$total_mod"
    fi
  done

  score=0 count=0

  while (( quantity-- )); do
    roll=$(roll_die "$sides")
    (( score += roll ))
    debug "rolled a %d. score is now %d" "$roll" "$score"
    verbose "roll %d: %d" "$(( count++ ))" "$roll"
  done

  verbose "score before modifier: %d" "$score"
  (( score += total_mod ))
  verbose "score after modifier: %d" "$score"
  debug "score total after modifier is %d" "$score"

  if [[ ! -v opts[verbose] ]]; then
    printf '%d\n' "$score"
  fi
fi