#!/bin/sh
# Copyright 2023 Huawei Cloud Computing Technology Co., Ltd.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

set -e

check_var_name() {
  local NAME="$1"
  if ! expr match "${NAME}" "[A-Z_a-z][0-9A-Z_a-z]*$" >/dev/null; then
    echo "expand_exec error: invalid variable name '${NAME}'"
    return 1
  fi
  return 0
}

in_list() {
  local NAME="$1"
  local VAR_LIST="$2"
  check_var_name "${NAME}" && expr match "${NAME}" "\(${VAR_LIST}\)$" >/dev/null
}

EXPANDED_VAR=
expand_var() {
  local NAME="$1"
  check_var_name "${NAME}" && EXPANDED_VAR="$(eval "printf \"%s\" \"\${$NAME}\"")"
}

PARSED_ARG=
parse_arg() {
  local ARG="$1"
  local VAR_LIST="$2"
  local RESULT=""
  local VAR_NAME=""
  local PARSE_EXPR=false
  local PARSE_VAR_NAME=false

  while [ ${#ARG} -gt 0 ]; do
    local NEXT="${ARG#?}"
    c="${ARG%"${NEXT}"}"
    ARG="${NEXT}"

    if $PARSE_VAR_NAME; then
      # parse <var> from $(<var>)
      if [ "$c" = ")" ]; then
        # expand var if in cases list
        if in_list "${VAR_NAME}" "${VAR_LIST}" && expand_var "${VAR_NAME}"; then
          RESULT="${RESULT}${EXPANDED_VAR}"
        else
          RESULT="${RESULT}\$(${VAR_NAME})"
        fi
        VAR_NAME=""
        PARSE_VAR_NAME=false
      else
        # accumulate VAR_NAME
        VAR_NAME="${VAR_NAME}$c"
        if [ ${#ARG} -eq 0 ]; then
          # flush unterminated var name
          RESULT="${RESULT}\$(${VAR_NAME}"
        fi
      fi
    else
      # parse single char
      if $PARSE_EXPR; then
        if [ "$c" = "(" ]; then
          # found "$("
          PARSE_VAR_NAME=true
        else
          RESULT="${RESULT}\$$c"
        fi
        PARSE_EXPR=false
      elif [ "$c" = "$" ]; then
        # found "$"
        PARSE_EXPR=true
      else
        # append char
        RESULT="${RESULT}$c"
      fi
    fi
  done

  PARSED_ARG="'"
  while [ ${#RESULT} -gt 0 ]; do
    local NEXT="${RESULT#?}"
    c="${RESULT%"${NEXT}"}"
    RESULT="${NEXT}"
    if [ "$c" = "'" ]; then
      PARSED_ARG="${PARSED_ARG}'\\''"
    else
      PARSED_ARG="${PARSED_ARG}$c"
    fi
  done
  PARSED_ARG="${PARSED_ARG}'"
}

# usage: ./expand_exec [VARS...] -- ARGS...
expand_exec() {
  local VAR_LIST=""
  local VAR_SEP=""
  local EXEC_VEC=""
  local EXEC_SEP=""
  local READ_ARGS=false

  while [ "$#" -ge 1 ]; do
    local ARG="$1";shift
    if $READ_ARGS; then
      parse_arg "${ARG}" "${VAR_LIST}"
      EXEC_VEC="${EXEC_VEC}${EXEC_SEP}${PARSED_ARG}"
      EXEC_SEP=" "
    else
      if [ "${ARG}" = "--" ]; then
        READ_ARGS=true
      elif check_var_name "${ARG}"; then
        VAR_LIST="${VAR_LIST}${VAR_SEP}${ARG}"
        VAR_SEP="\|"
      fi
    fi
  done

  exec /bin/sh -c "${EXEC_VEC}"
}

expand_exec "$@"