#!/usr/bin/env python3
import sys,re,os
from argparse import ArgumentParser
from argparse import RawTextHelpFormatter

def setup_options():
    parser = ArgumentParser(
        description="""Template Filler

=== Template example: ===
  Hello [[name]]!
[[letter]]

== Value file/Value pair example: ==
[[name]]=John
[[letter]]=@letter.txt
    """,
        formatter_class = RawTextHelpFormatter
    )
    parser.add_argument(
        "-e",
        action = "store_true",
        dest = "env",
        default = False,
        help = "Use the environment to replace ${env} style variables."
    )
    parser.add_argument(
        "-f",
        action = "store",
        dest = "file",
        help = "File name to read keys/values."
    )
    parser.add_argument(
        "-p",
        action = "append",
        dest = "values",
        default = [],
        help = "key=value pairs. This option may be issued several times."
    )
    parser.add_argument(
        'template',
        action = "store",
        nargs = '?',
        help = "Template file to be filled. If not defined, stdin used."
    )
    options = parser.parse_args()
    return options


def parse_file(filename):
    pairs = []
    with open(filename, "r") as reader:
        for i, l in enumerate(reader):
            l = l.rstrip("\n\r")
            if len(l) == 0:
                continue
            tokens = l.split('=', 1)
            if len(tokens) != 2:
                print("File %s:%i key=value pair '%s' does not parse"%(
                    filename, i+1, l,
                ))
                sys.exit(1)
            pairs.append( (tokens[0], tokens[1]) )
    return pairs

def parse_arguments(args):
    pairs = []
    for p in args:
        tokens = p.split('=', 1)
        if len(tokens) != 2:
            print("Argument key=value pair '%s' does not parse"%(p,))
            sys.exit(1)
        pairs.append( (tokens[0], tokens[1]) )
    return pairs


if __name__ == "__main__":
    options = setup_options();
    pairs = []
    if options.file != None:
        pairs.extend(parse_file(options.file))
    pairs.extend(parse_arguments(options.values))
    if options.template == None:
        in_reader = sys.stdin
    else:
        in_reader = open(options.template, 'rb')
    for l in in_reader:
        for p in pairs:
            value = p[1]
            if len(value) > 0:
                if value[0] == "@":
                    value = open(value[1:], 'rt').read()
                elif value[0:2] == "\\@":
                    value = value[1:]
            l = l.replace(p[0], value)
        if options.env:
            var_list = [m.group(0) for m in re.finditer('\${[^ ]+}', l)]
            for v in var_list:
                l = l.replace(v, os.environ.get(v[:-1][2:], ""))
        sys.stdout.write(l)