#import sql libraries and encryption libraries
from sqlalchemy import create_engine
from sqlalchemy.orm import (sessionmaker, 
                            declarative_base
                            )
import hashlib, binascii, hmac
from dotenv import load_dotenv
import os

load_dotenv()

# defining connection URL
def connect() -> str:
    pg_url = os.getenv("POSTGRESQL_DATABASE_URL")
    if pg_url.startswith("postgresql://"):
        #format url for psyopg reading
        return pg_url.replace("postgresql://", 
                              "postgresql+psycopg://", 
                              1
                              )
    return pg_url


DATABASE_URL = connect()
# create a ping engine to the database
engine = create_engine(DATABASE_URL, pool_pre_ping=True)
#create a local connection session instance 
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
Base = declarative_base()

def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

# Verify password function for authentication
# compare plain password with stored hashed password
# the below code was developed with the the help of AI tools
def verify_password(plain_password: str, stored_hash: str) -> bool:
    try:
        #decode the stored hash
        algo, iter_s, salt_hex, hash_hex = stored_hash.split("$")
        if algo != "pbkdf2_sha256":
            return False
        iterations = int(iter_s)
        salt = binascii.unhexlify(salt_hex)
        expected = binascii.unhexlify(hash_hex)
        # apply encryption method to plain password
        calc = hashlib.pbkdf2_hmac("sha256", plain_password.encode(), salt, iterations)
        #compare the calculated hash with the stored hash
        return hmac.compare_digest(calc, expected)
    
    except Exception:
        return False