import csv
import os
from datetime import datetime


class Database:
    def __init__(self, path):
        """A database of all umbrellas and their current state.

        Currently, the data are represented in a csv file.

        Explanation of data types:
        - id        | a unique numerical identity of unsigned integer type.
        - uint      | whatever fits the regex /[0-9]+/. despite carrying a numerical value, it is
                    | internally represented as a string.
        - string    | any UTF-8 string.
        - date      | an ISO 8601 date of format "YYYY-MM-DDThh:mm:ss". default timezone is UTC+8.

        The status of an umbrella consists of:
        - serial        | (id) unique identifier for the umbrella.
        - alias         | (string) future compatibility. a human readable (preferably cute) name
                        | for a particular umbrella
        - status        | (string) one of ("available", "lent", "withheld", "maintenance",
                        | "withheld", "unknown")
                        | available     : is in service on the stand
                        | lent          : is in temporary possession of a user
                        | maintenance   : is on the stand but not in service
                        | withheld      : is not on the stand but rather in the possession of an
                        |                 administrator
                        | unknown       : none of the above
                        | NOTE: the values above are subject to changes.
        - tenant_name   | (string) the person in temporary possession of the umbrella.
        - tenant_id     | (uint) student or faculty ID.
        - tenant_phone  | (uint) phone number via which to contact tenant when the lease is due.
        - tenant_email  | (string) future compatibility. empty for the time being.
        - lent_at       | (date) if status is "lent", lent_at is the submission time of the user's
                        | jForm answer sheet. is blank otherwise.
        """
        self.path = path

    def _lock(self) -> int:
        """Create a lockfile for database.

        If the database is called "db.csv", the lockfile, located in the same
        directory, would be called "db.csv.lock".

        Returns 1 if the lockfile is successfully created, 0 if the lockfile
        already exists (strong hint of unrecovered crash or potential race
        condition - caller of this method should abort whatever it is doing)
        """
        try:
            f = open(self.path + ".lock", "x")
            f.close()
            return 1
        except FileExistsError:
            return 0

    def _unlock(self) -> None:
        """Remove lockfile created by _lock.

        If there is no lockfile, simply ignore.
        """
        try:
            os.remove(self.path + ".lock")
        except FileNotFoundError:
            pass

    def _read(self) -> list:
        """Deserialize database."""
        # Create database if it does not yet exist
        try:
            f = open(self.path, "x")
            f.close()
        except FileExistsError:
            pass

        with open(self.path) as f:
            reader = csv.reader(f)
            umbrellas = [
                {
                    "serial": int(row[0]),
                    "alias": row[1],
                    "status": row[2],
                    "tenant_name": row[3],
                    "tenant_id": row[4],
                    "tenant_phone": row[5],
                    "tenant_email": row[6],
                    "lent_at": datetime.fromisoformat(row[7]) if row[7] else None,
                }
                for row in reader
            ]
            f.close()

            return umbrellas

    def _write(self, umbrellas: list) -> None:
        """Serialize database. When a failure occurs, abort and recover data."""
        # make backup in memory
        with open(self.path) as f:
            backup = f.read()
            f.close()

        # wait until database is locked for this write
        while not self._lock():
            continue

        f = open(self.path, "w")
        try:
            writer = csv.writer(f)
            writer.writerows(
                [
                    [
                        umb["serial"],
                        umb["alias"],
                        umb["status"],
                        umb["tenant_name"],
                        umb["tenant_id"],
                        umb["tenant_phone"],
                        umb["tenant_email"],
                        umb["lent_at"].isoformat(timespec="seconds")
                        if umb["lent_at"]
                        else "",
                    ]
                    for umb in umbrellas
                ]
            )
        except:
            # failure occurred on write
            # abort write, and write back contents as they were before
            # TODO: pass on exception and keep log
            f.close()
            f = open(self.path, "w")
            f.write(backup)
            f.close()

        self._unlock()

    def _update(self, update: dict) -> list:
        """Update status of one umbrella, and return the entire updated database."""
        umbrellas = self._read()
        for idx, umb in enumerate(umbrellas):
            if umb["serial"] == update["serial"]:
                umbrellas[idx] = update
                self._write(umbrellas)
                return umbrellas

    def _find_by_serial(self, serial: int) -> dict:
        """Given a serial number, returns an umbrella with such serial.
        If there is no such umbrella, returns None.
        """
        umbrellas = self._read()
        for umb in umbrellas:
            if umb["serial"] == serial:
                return umb

    def read(self) -> list:
        """Currently an alias for `_read()`."""
        return self._read()

    def take_away(
        self, serial, tenant_name, tenant_id, tenant_phone="", tenant_email=""
    ) -> None:
        """When a user has borrowed an umbrella."""
        umb = self._find_by_serial(serial)
        if umb is None:
            raise ValueError(f"No umbrella with serial {serial} found.")
        elif umb["status"] != "available":
            raise ValueError(f"Umbrella with serial {serial} is inavailable.")
        umb["status"] = "lent"
        umb["tenant_name"] = tenant_name
        umb["tenant_id"] = tenant_id
        umb["tenant_phone"] = tenant_phone
        umb["tenant_email"] = tenant_email
        umb["lent_at"] = datetime.now()
        self._update(umb)

    def give_back(self, serial) -> None:
        """When a user has returned an umbrella."""
        umb = self._find_by_serial(serial)
        if umb is None:
            raise ValueError(f"No umbrella with serial {serial} found.")
        elif umb["status"] != "lent":
            raise ValueError(f"Umbrella with serial {serial} is not lent out.")
        umb["status"] = "available"
        for key in ["tenant_name", "tenant_id", "tenant_phone", "tenant_email"]:
            umb[key] = ""
        umb["lent_at"] = None
        self._update(umb)