import re
import os

from fastapi import Request
from starlette.middleware.base import BaseHTTPMiddleware
from typing import Dict

class SecurityHeadersMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        response = await call_next(request)
        response.headers.update(set_security_headers())
        return response

def set_security_headers() -> Dict[str, str]:
    """
    Sets security headers based on environment variables.

    This function reads specific environment variables and uses their values
    to set corresponding security headers. The headers that can be set are:
    - cache-control
    - strict-transport-security
    - referrer-policy
    - x-content-type-options
    - x-download-options
    - x-frame-options
    - x-permitted-cross-domain-policies

    Each environment variable is associated with a specific setter function
    that constructs the header. If the environment variable is set, the
    corresponding header is added to the options dictionary.

    Returns:
        dict: A dictionary containing the security headers and their values.
    """
    options = {}
    header_setters = {
        'CACHE_CONTROL': set_cache_control,
        'HSTS': set_hsts,
        'REFERRER_POLICY': set_referrer,
        'XCONTENT_TYPE': set_xcontent_type,
        'XDOWNLOAD_OPTIONS': set_xdownload_options,
        'XFRAME_OPTIONS': set_xframe,
        'XPERMITTED_CROSS_DOMAIN_POLICIES': set_xpermitted_cross_domain_policies,
    }

    for env_var, setter in header_setters.items():
        value = os.environ.get(env_var, None)
        if value:
            header = setter(value)
            if header:
                options.update(header)

    return options

# Set HTTP Strict Transport Security(HSTS) response header
def set_hsts(value: str):
    pattern = r'^max-age=(\d+)(;includeSubDomains)?(;preload)?$'
    match = re.match(pattern, value, re.IGNORECASE)
    if not match:
        return 'max-age=31536000;includeSubDomains'
    return {
        'Strict-Transport-Security': value
    }

# Set X-Frame-Options response header
def set_xframe(value: str):
    pattern = r'^(DENY|SAMEORIGIN)$'
    match = re.match(pattern, value, re.IGNORECASE)
    if not match:
        value = 'DENY'
    return {
        "X-Frame-Options": value
    }

# Set Referrer-Policy response header
def set_referrer(value: str):
    pattern = r'^(no-referrer|no-referrer-when-downgrade|origin|origin-when-cross-origin|same-origin|strict-origin|strict-origin-when-cross-origin|unsafe-url)$'
    match = re.match(pattern, value, re.IGNORECASE)
    if not match:
        value = 'no-referrer'
    return {
        'Referrer-Policy': value
    }

# Set Cache-Control response header
def set_cache_control(value: str):
    pattern = r'^(public|private|no-cache|no-store|must-revalidate|proxy-revalidate|max-age=\d+|s-maxage=\d+|no-transform|immutable)(,\s*(public|private|no-cache|no-store|must-revalidate|proxy-revalidate|max-age=\d+|s-maxage=\d+|no-transform|immutable))*$'
    match = re.match(pattern, value, re.IGNORECASE)
    if not match:
        value = 'no-store, max-age=0'

    return {
        'Cache-Control': value
    }

# Set X-Download-Options response header
def set_xdownload_options(value: str):
    if value != 'noopen':
        value = 'noopen'
    return {
        'X-Download-Options': value
    }

# Set X-Content-Type-Options response header
def set_xcontent_type(value: str):
    if value != 'nosniff':
        value = 'nosniff'
    return {
        'X-Content-Type-Options': value
    }

# Set X-Permitted-Cross-Domain-Policies response header
def set_xpermitted_cross_domain_policies(value: str):
    pattern = r'^(none|master-only|by-content-type|by-ftp-filename)$'
    match = re.match(pattern, value, re.IGNORECASE)
    if not match:
        value = 'none'
    return {
        'X-Permitted-Cross-Domain-Policies': value
    }