#!/usr/bin/env bash

## rdlink [-d|--debug] <PATH>
##
## License: GNU GPL V3 or later
##  Author: Martin Winkler

# Exit on error. Append "|| true" if you expect an error.
set -o errexit
# Exit on error inside any functions or subshells.
set -o errtrace
# Do not allow use of undefined vars. Use ${VAR:-} to use an undefined VAR
set -o nounset
# Catch the error in case mysqldump fails (but gzip succeeds) in `mysqldump |gzip`
set -o pipefail

LOG_LEVEL="${LOG_LEVEL:-1}" # 7 = debug -> 0 = emergency

function rl::log () {
  local log_level="${1:-${LOG_LEVEL}}"
  shift

  # all remaining arguments are to be printed
  local log_line=""

  while IFS=$'\n' read -r log_line; do
    printf "%s [%9s] %s\n" "$(date -u +"%Y-%m-%d %H:%M:%S UTC")" "${log_level}" "${log_line}" 1>&2
  done <<< "${@:-}"
}
emergency() {                                  rl::log emergency "${@}"; exit 1; }
alarm()     { [[ "${LOG_LEVEL:-0}" -ge 1 ]] && rl::log alert "${@}"; true; }
critical()  { [[ "${LOG_LEVEL:-0}" -ge 2 ]] && rl::log critical "${@}"; true; }
error()     { [[ "${LOG_LEVEL:-0}" -ge 3 ]] && rl::log error "${@}"; true; }
warning()   { [[ "${LOG_LEVEL:-0}" -ge 4 ]] && rl::log warning "${@}"; true; }
notice()    { [[ "${LOG_LEVEL:-0}" -ge 5 ]] && rl::log notice "${@}"; true; }
info()      { [[ "${LOG_LEVEL:-0}" -ge 6 ]] && rl::log info "${@}"; true; }
debug()     { [[ "${LOG_LEVEL:-0}" -ge 7 ]] && rl::log debug "${@}"; true; }
msg()       {                                  echo "$@"; true; }

### Script
rl::rdlink() {
  local subject=
  local work=

  info "Processing: $*"
  info "  with pwd: $(pwd)"

  subject="$(rl::cleanpath "${1:-}")" || true

  # Follow multiple symlinks
  while subject="$(rl::quicklink "${subject}")" ; do
    : # A link was resolved at least once
    info "  rl::rdlink - Link found: $subject"
  done

  # Special cases handling
  {
    # If subject is still a link, after rl::quicklink call(s)
    # current user has no permission to access the link itself
    # (e.g. /proc/**/cwd)
    if [ -L "${subject}" ] ; then
      info "  rl::rdlink exit - can't access link ${subject}"
      printf "\n"
      return 1
    fi
  }

  # Empty output if (dirname $subject) is not a valid path
  if ! work="$(rl::canon "${subject}")" ; then
    info "  rl::rdlink exit - invalid path ${work}"
    printf "\n"
    return 1
  else
    subject="${work}"
  fi

  printf "%s\n" "${subject}"
}

rl::quicklink() {
  subject=
  work=

  info "Quicklink... ${1:-}"

  # Check if current candidate is a symlink
  if ! subject=$(readlink -- "${1:-}"); then
    printf -- "%s\n" "${1:-}"
    return 1
  fi
  info "  rl::quicklink symlink ${1} -> ${subject}"

  # relative symlink target; prepend its parent direcotry
  if [[ "${subject}" != "/"* ]]; then
    work="$(rl::canon "$(dirname -- "${1:-}")")"
    subject="${work}/${subject}"
    info "  rl::quicklink relative link resolved: ${subject}"
  fi

  printf "%s\n" "${subject}"
  return 0
}

rl::canon() {
  local subject=
  local work=
  local bname=
  local run=1
  local start=
  local retval=0

  start="${1:-}"
  info "Canonicalize path... ${start}"

  while (( run )) ; do
    if work="$(cd "${start}" >/dev/null 2>&1 && pwd -P)" ; then
      # Special: `pwd -P` returns with // as root for links starting at /
      # e.g. $(readlink /proc/self/root) == "/"
      #      $(cd /proc/self/root/mnt && pwd -P) == "//mnt"
      subject="$(rl::normalize "${work}")"
      info "  rl::canon valid directory: ${subject}"
      run=0
    elif work="$(cd "$(dirname -- "${start}")" >/dev/null 2>&1 && pwd -P)" ; then
      bname="$(basename -- "${start}")"

      # Special: / produces //
      [[ "${work}" == "/" ]] && work=

      subject="${work}${bname:+"/${bname}"}"
      info "  rl::canon valid parent: ${subject}"

      # Special: Succeed with valid element after second run; see special below
      # e.g. /root/.
      #      * /root is valid but not accessible
      if (( retval )) && [ -e "${subject}" ] ;then
        info "  rl::canon valid element"
        retval=0
      fi
      run=0
    else
      # Special: Some paths may be resolvable after normalization
      # e.g. somedir/..
      #      * base "somedir" does not exist
      #      * but irrelevant because of /..
      #      * resolves to pwd but fails by readlink -f
      work="$(rl::normalize "${start}")"
      if [[ "${work}" != "${start}" ]] ; then
        info "  rl::canon retry with: ${work}"
        start="${work}"
        retval=1
        continue
      fi
      info "  rl::canon invalid path: ${work}"
      subject="${work}"
      run=0 && retval=1
    fi
  done

  printf -- "%s\n" "${subject}"
  return ${retval}
}

rl::cleanpath() {
  local work=
  local rex_tmp=

  info "Cleaning path... ${1:-}"
  work="${1:-}"

  # Remove multiple /
  while [[ "${work:-}" = *"//"* ]]; do
    work="${work/\/\///}"
  done

  info "  rl::cleanpath result: ${work}"
  printf -- "%s\n" "${work}"
}

rl::normalize() {
  local work=

  info "Normalizing path... ${1:-}"
  work="${1:-}"

  # Remove dir/.. sequences.
  local rex_tmp='[^/][^/]*/\.\./*'
  while [[ "${work}" =~ $rex_tmp ]] ; do
    work="${work/"${BASH_REMATCH[0]}"/}"
  done

  # Remove /./ and /.$ sequences.
  rex_tmp='/\.(/|$)'
  while [[ "$work" =~ $rex_tmp ]]; do
    work="${work/${BASH_REMATCH[0]}/${BASH_REMATCH[1]/$//}}"
  done

  # Remove leading ./
  work="${work#./*}"

  # Remove trailing /
  rex_tmp='(.*[^/])/$'
  if [[ "${work}" =~ $rex_tmp ]] ; then
    work="${BASH_REMATCH[1]}"
  fi

  info "  rl::normalize result: ${work}"
  printf -- "%s\n" "${work}"
}

rl::main() {
  local file=
  local arg=

  for arg in "$@"; do
    case "$arg" in
      --)
        shift
        break ;;
      -d|--debug)
        shift
        LOG_LEVEL=7 ;;
      -dd)
        shift
        set -o xtrace ;;
      -*)
        shift ;;
    esac
  done

  info "BASH rdlink"

  for file in "$@"; do
    rl::rdlink "$file"
  done
}

# Provide as function to be called when sourced
rdlink() {
  rl::main "$@"
}

### Check if script is _sourced
### https://stackoverflow.com/a/28776166
_sourced=0
if [ -n "${ZSH_EVAL_CONTEXT:-}" ]; then
  case ${ZSH_EVAL_CONTEXT:-} in *:file) _sourced=1;; esac
elif [ -n "${KSH_VERSION:-}" ]; then
  [ "$(cd "$(dirname -- "$0")" && pwd -P)/$(basename -- "$0")" \
    != "$(cd "$(dirname -- "${.sh.file}")" && pwd -P)/$(basename -- "${.sh.file}")" ] && _sourced=1
elif [ -n "${BASH_VERSION:-}" ]; then
  (return 0 2>/dev/null) && _sourced=1
else # All other shells: examine $0 for known shell binary filenames
  # Detects `sh` and `dash`; add additional shell filenames as needed.
  case ${0##*/} in sh|dash) _sourced=1;; esac
fi
###

if (( ! _sourced )); then
  rl::main "$@"
fi

### Script EOF