import random
from datetime import datetime, timedelta
from typing import List

from langchain_core.exceptions import OutputParserException
from langchain_core.output_parsers import BaseOutputParser
from langchain_core.utils import comma_list


def _generate_random_datetime_strings(
    pattern: str,
    n: int = 3,
    start_date: datetime = datetime(1, 1, 1),
    end_date: datetime = datetime.now() + timedelta(days=3650),
) -> List[str]:
    """Generates n random datetime strings conforming to the
    given pattern within the specified date range.

    Pattern should be a string containing the desired format codes.
    start_date and end_date should be datetime objects representing
    the start and end of the date range.
    """
    examples = []
    delta = end_date - start_date
    for i in range(n):
        random_delta = random.uniform(0, delta.total_seconds())
        dt = start_date + timedelta(seconds=random_delta)
        date_string = dt.strftime(pattern)
        examples.append(date_string)
    return examples


class DatetimeOutputParser(BaseOutputParser[datetime]):
    """Parse the output of an LLM call to a datetime."""

    format: str = "%Y-%m-%dT%H:%M:%S.%fZ"
    """The string value that used as the datetime format."""

    def get_format_instructions(self) -> str:
        examples = comma_list(_generate_random_datetime_strings(self.format))
        return (
            f"Write a datetime string that matches the "
            f"following pattern: '{self.format}'.\n\n"
            f"Examples: {examples}\n\n"
            f"Return ONLY this string, no other words!"
        )

    def parse(self, response: str) -> datetime:
        try:
            return datetime.strptime(response.strip(), self.format)
        except ValueError as e:
            raise OutputParserException(
                f"Could not parse datetime string: {response}"
            ) from e

    @property
    def _type(self) -> str:
        return "datetime"