import pytest
from datetime import datetime, timedelta
from src.calculation_target_reliability.utils import generate_down_periods

def test_generate_down_periods_count():
    start = datetime(2025, 1, 1)
    end = datetime(2025, 1, 31)
    # Test fixed number of periods
    periods = generate_down_periods(start, end, num_periods=5)
    # It attempts to generate 5, but might be fewer due to overlaps
    assert len(periods) <= 5
    
    # Check they are within range
    for p_start, p_end in periods:
        assert p_start >= start
        assert p_end <= end
        assert p_start < p_end

def test_generate_down_periods_no_overlap():
    start = datetime(2025, 1, 1)
    end = datetime(2025, 1, 31)
    periods = generate_down_periods(start, end, num_periods=10)
    
    # Sort and check gaps
    for i in range(len(periods) - 1):
        assert periods[i][1] <= periods[i+1][0]

def test_generate_down_periods_too_small_range():
    start = datetime(2025, 1, 1)
    end = datetime(2025, 1, 2)
    # Requesting 5 days duration in 1 day range
    periods = generate_down_periods(start, end, num_periods=1, min_duration=5)
    assert len(periods) == 0