#!/usr/bin/env python3

import os
import re
import argparse

IFS = (" ", "\t", "\n")


class EOFException(Exception):
    pass


class Argument():
    def __init__(self, value: str, allow_expansion=True):
        pass

    def expand():
        pass


class Pipeline():
    def __init__(self, instructions: list=[]):
        self.instructions = instructions

    def execute(self):
        for instruction in self.instructions:
            instruction.execute()


class Instruction():
    def __init__(self, tokens: list=[]):
        self.prog = tokens[0]
        self.args = tokens[1:]


    def execute(self):
        cpid = os.fork()
        if cpid == 0:
            os.execvp(self.prog, [self.prog] + self.args)
        else:
            os.waitpid(cpid, 0)


class PipelineParser():
    def read_escaped(self, string_iterator):
        return next(string_iterator)

    def read_literal(self, string_iterator):
        literal = ""
        while True:
            c = next(string_iterator)
            if c != "'":
                literal += c
            else:
                return literal

    def read_quoted(self, string_iterator):
        # TODO handle substitutions
        quoted = ""
        while True:
            c = next(string_iterator)
            if c == "\\":
                quoted += self.read_escaped(string_iterator)
            elif c != '"':
                quoted += c
            else:
                return quoted


    def get_next_token(self, string_iterator):
        token = None
        while True:
            try:
                c = next(string_iterator)
            except StopIteration:
                break

            # Skip leading whitespaces
            if token is None and c in IFS:
                continue
            elif token is None:
                token = ""

            if c == "'":
                token += self.read_literal(string_iterator)
            elif c == "\\":
                token += self.read_escaped(string_iterator)
            elif c == "\"":
                token += self.read_quoted(string_iterator)
            elif c in IFS:
                return token
            else:
                token += c

        return token

    def tokenize(self, string_iterator):
        while True:
            token = self.get_next_token(string_iterator)
            if token is None:
                break
            yield token

    def parse(self, line):
        tokens = list(self.tokenize(iter(line)))
        return Pipeline([Instruction(tokens)])


def read_next_pipeline(fh):
    # TODO Support multiple pipelines per line
    # TODO Support instructions spawning multiple lines

    parser = PipelineParser()

    while True:
        line = fh.readline()
        if line == "":
            raise EOFException()

        line = line.strip()
        # We ignore empty lines and lines which start with #
        if not line.startswith("#") and not line == "":
            break

    return parser.parse(line)


def main(arguments):
    for script in arguments["<script>"]:
        with open(script, mode="r", encoding="utf-8") as fh:
            while True:
                try:
                    pipeline = read_next_pipeline(fh)
                    pipeline.execute()

                except EOFException:
                    break

if __name__ == "__main__":
    parser = argparse.ArgumentParser(description="Command interpreter written in Python.")
    parser.add_argument("<script>", action="store", nargs="+", help="Script to run")
    args = parser.parse_args()
    main(vars(args))