"""Tests for utility functions (backend/utils.py)."""
import pytest
from datetime import datetime, timezone, timedelta
from utils import utc_to_ist, format_ist_timestamp

IST = timezone(timedelta(hours=5, minutes=30))


class TestUtcToIst:
    def test_midnight_utc_becomes_530_ist(self):
        utc = datetime(2024, 1, 1, 0, 0, 0)
        ist = utc_to_ist(utc)
        assert ist.hour == 5
        assert ist.minute == 30

    def test_adds_five_hours_thirty_minutes(self):
        utc = datetime(2024, 6, 15, 10, 0, 0)
        ist = utc_to_ist(utc)
        assert ist.hour == 15
        assert ist.minute == 30

    def test_rolls_over_to_next_day(self):
        utc = datetime(2024, 1, 1, 22, 0, 0)  # 22:00 UTC → 03:30 next day IST
        ist = utc_to_ist(utc)
        assert ist.day == 2
        assert ist.hour == 3
        assert ist.minute == 30

    def test_rolls_over_to_next_month(self):
        utc = datetime(2024, 1, 31, 23, 0, 0)  # Jan 31 → Feb 1 IST
        ist = utc_to_ist(utc)
        assert ist.month == 2
        assert ist.day == 1

    def test_output_has_ist_timezone_offset(self):
        utc = datetime(2024, 1, 1, 12, 0, 0)
        ist = utc_to_ist(utc)
        assert ist.utcoffset() == timedelta(hours=5, minutes=30)

    def test_preserves_seconds(self):
        utc = datetime(2024, 3, 15, 8, 45, 30)
        ist = utc_to_ist(utc)
        assert ist.second == 30

    def test_noon_utc_is_1730_ist(self):
        utc = datetime(2024, 7, 4, 12, 0, 0)
        ist = utc_to_ist(utc)
        assert ist.hour == 17
        assert ist.minute == 30


class TestFormatIstTimestamp:
    def test_converts_z_suffix_timestamp(self):
        result = format_ist_timestamp("2024-01-01T00:00:00Z")
        assert "+05:30" in result

    def test_converts_explicit_utc_offset_timestamp(self):
        result = format_ist_timestamp("2024-01-01T00:00:00+00:00")
        assert "+05:30" in result

    def test_midnight_utc_produces_0530_ist(self):
        result = format_ist_timestamp("2024-01-01T00:00:00Z")
        assert "05:30:00+05:30" in result

    def test_noon_utc_produces_1730_ist(self):
        result = format_ist_timestamp("2024-01-01T12:00:00Z")
        assert "17:30:00+05:30" in result

    def test_returns_iso_format_string(self):
        result = format_ist_timestamp("2024-06-15T08:00:00Z")
        # Should be parseable as ISO datetime
        parsed = datetime.fromisoformat(result)
        assert parsed is not None

    def test_invalid_text_raises_value_error(self):
        with pytest.raises(ValueError):
            format_ist_timestamp("not-a-date")

    def test_invalid_month_raises_value_error(self):
        with pytest.raises(ValueError):
            format_ist_timestamp("2024-13-01T00:00:00Z")

    def test_empty_string_raises_value_error(self):
        with pytest.raises(ValueError):
            format_ist_timestamp("")

    def test_slash_separated_date_raises_value_error(self):
        with pytest.raises(ValueError):
            format_ist_timestamp("2024/01/01T00:00:00")  # Slashes not valid ISO format