"""Best-effort parser for requirements.txt files"""

import urllib.parse

from pathlib import Path

import sys

import os

import re

# `#` starts a comment only at end of line and after whitespace

COMMENT_RE = re.compile(r'(^|\s+)#.*$')

# Assume URLs start with a scheme; don't look for "egg=" URLs otherwise

URL_START_RE = re.compile(r'^[-_+a-zA-Z0-9]+://')

ENV_VAR_RE = re.compile(r'(?P\$\{(?P<name>[A-Z0-9_]+)\})')

PKGNAME_RE = re.compile(r'^[-_a-zA-Z0-9]+')

# The requirements.txt format evolved rather organically; expect weirdness.

def convert_requirements_txt(lines, path:Path = None):

    """Convert lines of a requirements file to PEP 440-style requirement strs

    This does NOT handle all of requirements.txt features (only pip can do

    that), but tries its best.

    The resulting requirements might not actually be valid (either because

    they're wrong in the file, or because we missed a special case).

    path is the path to the requirements.txt file, used for options like `-r`.

    """

    requirements = []

    lines = combine_logical_lines(lines)

    lines = strip_comments(lines)

    lines = expand_env_vars(lines)

    if path:

        filename = path.name

    else:

        filename = '<requirements file>'

    for line in lines:

        if URL_START_RE.match(line):

            # Handle URLs with "egg=..." fragments

            # see https://pip.pypa.io/en/stable/cli/pip_install/#vcs-support

            parsed_url = urllib.parse.urlparse(line)

            parsed_fragment = urllib.parse.parse_qs(parsed_url.fragment)

            if 'egg' in parsed_fragment:

                # Prepend the package name to the URL.

                match = PKGNAME_RE.match(parsed_fragment['egg'][0])

                if match:

                    pkg_name = match[0]

                    requirements.append(f'{pkg_name}@{line}')

                    continue

            # If that didn't work, pass the line on;

            # the caller will deal with invalid requirements

            requirements.append(line)

        elif line.startswith('-r'):

            recursed_path = line[2:].strip()

            if path:

                recursed_path = path.parent / recursed_path

            recursed_path = Path(recursed_path)

            with recursed_path.open() as f:

                requirements.extend(convert_requirements_txt(f, recursed_path))

        elif line.startswith('-'):

            raise ValueError(f'{filename}: unsupported requirements file option: {line}')

        else:

            requirements.append(line)

    return requirements

def combine_logical_lines(lines):

    """Combine logical lines together (backslash line-continuation)"""

    pieces = []

    for line in lines:

        line = line.rstrip('\n')

        # Whole-line comments *only* are removed before line-contionuation

        if COMMENT_RE.match(line):

            continue

        if line.endswith('\\'):

            pieces.append(line[:-1])

        else:

            # trailing whitespace is only removed from full logical lines

            pieces.append(line.rstrip())

            yield ''.join(pieces)

            pieces = []

    yield ''.join(pieces)

def strip_comments(lines):

    for line in lines:

        line, *rest = COMMENT_RE.split(line, maxsplit=1)

        line = line.strip()

        if line:

            yield line

def expand_env_vars(lines):

    def repl(match):

        value = os.getenv(match['name'])

        if value is None:

            return match['var']

        return value

    for line in lines:

        yield ENV_VAR_RE.sub(repl, line)