#!/usr/bin/env bash
#
# Portimg uses Crux port-like system for creating software deployment images.
# Copyright (C) 2016 Aaron Ball <nullspoon@oper.io>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.
#


#
# Provides *very* basic templating functionality. Interpolates basic jinja
# variable syntax ( "{{ variable }}" ).
#
# Template variables are replaced with matching bash environment variabes. This
# means that template variables can only contain upper case, lower case,
# numbers, underscores. All non-conforming variables are ignored.
#
# @param src  Path to template file to interpolate
# @param dest Destination to copy the interpolated template to
#
function template {
  local src=${1:-}
  local dest=${2:-}

  local _delim    # Regex delimiter (use an uncommon character please)
  local _tmp      # Path to the tmp copy of the original template file
  local _oldifs   # Old value of IFS (for changing it back)
  local _filevars # List of valid variable references found in the template file
  local _varstr   # Template string calling the variable (eg: {{ varname }})
  local _varname  # Name of the actual variable (eg: {{ varname }} == varname)

  # Pick a really weird delimiter to avoid possible regex conflict
  _delim=$'\001'

  # Copy the template to a temp location so we don't work on the original
  _tmp=$(mktemp tmp.template.XXXXXXXX)
  cp -p ${src} ${_tmp}

  # Read all unique template variables specified in the file
  _filevars=("$( grep -o '{{ [_[:alnum:]]\+ }}' ${_tmp} | sort | uniq)")

  _oldifs=${IFS}
  IFS=$'\n'
  for _varstr in ${_filevars[@]}; do
    # Strip '{{ ' and ' }}' from varstr to get variable name
    _varname="${_varstr:3:-3}"

    # Skip any unset but referenced variables
    if [ -z "${!_varname:-}" ]; then
      printf \
        "WARN: Unbound variable '%s' in template '%s'.\n" \
        "${_varname}" \
        "${src}" >&2
      continue
    fi

    # If variable is set, then find and replace
    sed -i "s${_delim}${_varstr}${_delim}${!_varname}${_delim}g" "${_tmp}"
  done
  IFS=${_oldifs}

  # Move the modified version into place, creating any leading dirs if necessary
  install -D "${_tmp}" "${dest}"
  rm -f "${_tmp}"
}