#!/usr/bin/awk -f
#
# Copyright (c) 2015, Eduardo A. Bustamante López
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are met:
#     * Redistributions of source code must retain the above copyright
#       notice, this list of conditions and the following disclaimer.
#     * Redistributions in binary form must reproduce the above copyright
#       notice, this list of conditions and the following disclaimer in the
#       documentation and/or other materials provided with the distribution.
#     * Neither the name of the <organization> nor the
#       names of its contributors may be used to endorse or promote products
#       derived from this software without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
# WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
# DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT HOLDER> BE LIABLE FOR ANY
# DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
# (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
# SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#
function emit_error_message() {
  printf "[%s]: %s: %s\n", NR, ERROR, ERROR_CAUSE;
}

function _emit_character(c) {
  printf "%s", c
}

function _push_to_variable_name(c) {
  CURRENT_VAR_NAME = CURRENT_VAR_NAME c
}

function _emit_variable(    f,r,l,s) {
  if("@" == substr(CURRENT_VAR_NAME, 1, 1)) {
    f = ENVIRON[CURRENT_VAR_NAME]

    s = ""
    while(0 < (r = getline l < f)) {
      printf "%s%s", s, l; s = "\n"
    }

    close(f)
  } else {
    printf "%s", ENVIRON[CURRENT_VAR_NAME]
  }
  CURRENT_VAR_NAME = ""
}

function _string_fsm(c) {
  if(c == "$") {
    return "dollar";
  } else
  if(c == "\\") {
    return "escape";
  }

  _emit_character(c);

  return "string";
}

function _escape_fsm(c) {
  _emit_character(c);

  return "string";
}

function _dollar_fsm(c) {
  if("{" == c) {
    return "left-brace";
  } else if(index(VAR_NAME, c) || "@" == c) {
    _push_to_variable_name(c);

    return "variable";
  }

  ERROR = "invalid variable name character"
  ERROR_CAUSE = c

  return "error";
}

function _variable_fsm(c) {
  if(index(VAR_NAME, c)) {
    _push_to_variable_name(c);

    return "variable";
  }

  _emit_variable();

  return _string_fsm(c);
}

function _left_brace_fsm(c) {
  if(index(VAR_NAME, c) || "@" == c) {
    _push_to_variable_name(c);

    return "variable-brace";
  }

  ERROR = "invalid variable name character"
  ERROR_CAUSE = c

  return "error";
}

function _right_brace_fsm(c) {
  return _string_fsm(c);
}

function _variable_brace_fsm(c) {
  if("}" == c) {
    _emit_variable();
  
    return "right-brace";
  } else
  if(index(VAR_NAME, c)) {
    _push_to_variable_name(c);

    return "variable-brace";
  }

  ERROR = "missing closing right brace"
  ERROR_CAUSE = c

  return "error";
}

function fsm(state, c) {
  if(state == "string") {
    return _string_fsm(c);
  } else
  if(state == "escape") {
    return _escape_fsm(c);
  } else
  if(state == "dollar") {
    return _dollar_fsm(c);
  } else
  if(state == "variable") {
    return _variable_fsm(c);
  } else
  if(state == "left-brace") {
    return _left_brace_fsm(c);
  } else
  if(state == "right-brace") {
    return _right_brace_fsm(c);
  } else
  if(state == "variable-brace") {
    return _variable_brace_fsm(c);
  } else
  if(state == "error") {
    emit_error_message();
    exit 1;
  }
}

BEGIN {
  VAR_NAME = "abcdefghijklmnopqrstuvwxyz"
  VAR_NAME = VAR_NAME toupper(VAR_NAME)
  VAR_NAME = VAR_NAME "_-"

  CURRENT_VAR = ""

  state = "string"
}

{
  n = length;
  for(i = 1; i <= (n + 1); i++) {
    if(i == n + 1) {
      c = "\n";
    } else {
      c = substr($0, i, 1);
    }

    state = fsm(state, c);
  }
}

END {
  fsm(state, "");
}