#!/bin/bash

if [ $# -eq 0 ]; then
  mail=$(cat)
else
  mail=$(cat "$1")
fi

header=$(
  printf '%s\n' "${mail}" | \
    sed '/^$/q'
)

if ! printf '%s\n' "${header}" | \
  grep -q '^From expiry@letsencrypt\.org' || \
  ! printf '%s\n' "${header}" | \
  grep -qxF 'From: Let'"'"'s Encrypt Expiry Bot <expiry@letsencrypt.org>'; then
  # This mail is not from letsencrypt's Expiry Bot - ignore
  printf '%s\n' "${mail}"
  exit
fi

body=$(
  printf '%s\n' "${mail}" | \
    sed '1,/^$/ d'
)

claimed_expire_date=$(
  date -d"$(
    printf '%s\n' "${body}" | \
      sed '
        s/^.*Your certificate (or certificates) for the names listed below will expire in [0-9]\+ days\? (on \([^)]\+\))\..*$/\1/
        t
        d
      '
  )" '+%s' 2>/dev/null
)

domains=$(
  printf '%s\n' "${body}" | \
    sed -n '
      / for details\.$/,/^For any questions or support, / {
        /^$/,/^$/ {
          /^$/! p
        }
      }
    '
)

if [ -z "${domains}" ] || [ -z "${claimed_expire_date}" ]; then
  printf '%s\n' \
    "${header}" | \
    sed '
      s/^Subject:\s\+/\0PARSE ERROR --- /
    '
  printf '\n'
  printf '%s FAILED TO PARSE THIS MESSAGE!\n\n' \
    "$0"
  printf '%s\n' \
    "${body}"
  exit
fi

regex=''

for domain in ${domains}; do
  domain_regex=$(
    printf '%s\n' "${domain}" | \
      sed 's/[.]/\\\0/g'
  )
  real_expire_date=$(
    date -d"$(
      openssl s_client -showcerts -servername "${domain}" -connect "${domain}:443" </dev/null 2>/dev/null | \
        openssl x509 -noout -dates 2>/dev/null | \
        sed '
          s/^notAfter=//
          t
          d
        '
    )" '+%s' 2>/dev/null
  )
  if [ -z "${real_expire_date}" ]; then
    regex="${regex};s@^\\s*${domain_regex}\\s*@\0 (EXPIRATION UNKNOWN)@"
    continue
  fi
  if [ ${claimed_expire_date} -ge ${real_expire_date} ]; then
    regex="${regex};s@^\\s*${domain_regex}\\s*@\0 (EXPIRATION CONFIRMED)@"
    continue
  fi
  regex="${regex};s@^\\s*${domain_regex}\\s*@\0 (expiration actually $(((real_expire_date-claimed_expire_date)/24/60/60)) days later)@"
done

printf '%s\n\n' \
  "${header}"
printf '%s\n' \
  "${body}" | \
  sed "${regex}"