import calendar
from datetime import date, timedelta
from decimal import Decimal

from django.db.models import Sum
from django.shortcuts import render
from django.contrib.auth.decorators import login_required
from django.utils import timezone

from app.expenses.models import Category, Expense, FixedExpenseConcept
from app.yearly.models import PlannedExpense

from .models import Income, MonthlyFixedExpense, MonthlyGoal, MonthlyNote


def _offset_month(year, month, offset):
	"""Shift year/month by offset months."""
	total = (year * 12 + month - 1) + offset
	return total // 12, total % 12 + 1


def _get_month_weeks(year, month):
	"""Return list of (start_date, end_date) for each week of the month."""
	first_day = date(year, month, 1)
	last_day = date(year, month, calendar.monthrange(year, month)[1])

	weeks = []
	# Start from the Monday of the first week
	current = first_day - timedelta(days=first_day.weekday())
	while current <= last_day:
		week_start = max(current, first_day)
		week_end = min(current + timedelta(days=6), last_day)
		weeks.append((week_start, week_end))
		current += timedelta(days=7)
	return weeks


def _build_month_end_data(year, month):
	"""Build weekly expense breakdown for month end section."""
	weeks = _get_month_weeks(year, month)

	expenses = Expense.objects.filter(
		created_at__date__year=year,
		created_at__date__month=month,
	).select_related("category", "subcategory")

	# Weekly totals
	weekly_totals = []
	for start, end in weeks:
		week_total = sum(
			e.amount for e in expenses if start <= e.created_at.date() <= end
		)
		weekly_totals.append(week_total)
	monthly_expense_total = sum(weekly_totals)

	# Category breakdown by week
	categories = Category.objects.prefetch_related("subcategories").all()
	category_weeks = []
	for cat in categories:
		cat_expenses = [e for e in expenses if e.category_id == cat.id]
		if not cat_expenses:
			continue
		cat_weekly = []
		for start, end in weeks:
			wk_total = sum(
				e.amount for e in cat_expenses if start <= e.created_at.date() <= end
			)
			cat_weekly.append(wk_total)
		cat_total = sum(cat_weekly)
		category_weeks.append(
			{
				"category": cat,
				"weekly": cat_weekly,
				"total": cat_total,
			}
		)

	# Transpose: rows=weeks, cols=categories
	week_rows = []
	for i in range(len(weeks)):
		row = []
		for cw in category_weeks:
			row.append(cw["weekly"][i])
		week_rows.append({"week_num": i + 1, "values": row})

	return {
		"weeks": weeks,
		"week_count": len(weeks),
		"weekly_totals": weekly_totals,
		"monthly_expense_total": monthly_expense_total,
		"category_weeks": category_weeks,
		"week_rows": week_rows,
	}


@login_required
def month_view(request):
	today = timezone.localdate()
	offset = int(request.GET.get("offset", 0))
	year, month = _offset_month(today.year, today.month, offset)
	current_month = date(year, month, 1)
	is_current_month = offset == 0

	note, _ = MonthlyNote.objects.get_or_create(year=year, month=month)

	# Income
	incomes = Income.objects.filter(date__year=year, date__month=month)
	total = incomes.aggregate(t=Sum("amount"))["t"] or Decimal("0")

	# Clone months for income
	available_income_months = (
		Income.objects.values_list("date__year", "date__month")
		.distinct()
		.order_by("-date__year", "-date__month")
	)
	clone_months = [
		(y, m) for y, m in available_income_months if not (y == year and m == month)
	]

	# Fixed expenses (bulk create missing, then fetch all)
	concepts = FixedExpenseConcept.objects.all()
	existing_ids = set(
		MonthlyFixedExpense.objects.filter(year=year, month=month).values_list(
			"concept_id", flat=True
		)
	)
	to_create = [
		MonthlyFixedExpense(year=year, month=month, concept=c)
		for c in concepts
		if c.id not in existing_ids
	]
	if to_create:
		MonthlyFixedExpense.objects.bulk_create(to_create)
	fe_entries = MonthlyFixedExpense.objects.filter(
		year=year, month=month
	).select_related("concept")
	fe_total = fe_entries.aggregate(t=Sum("amount"))["t"] or Decimal("0")

	# Clone months for fixed expenses
	available_fe_months = (
		MonthlyFixedExpense.objects.filter(amount__gt=0)
		.values_list("year", "month")
		.distinct()
		.order_by("-year", "-month")
	)
	clone_fe_months = [
		(y, m) for y, m in available_fe_months if not (y == year and m == month)
	]

	# Default previous month
	prev_year, prev_month = _offset_month(year, month, -1)

	# Planned expenses for this month
	planned_expenses = PlannedExpense.objects.filter(year=year, month=month)
	planned_total = sum(pe.amount for pe in planned_expenses)

	# Budget (income - fixed - savings - planned)
	budget = total - fe_total - note.savings_target - planned_total

	# Month end data
	month_end = _build_month_end_data(year, month)
	# Include planned expenses in month end total
	month_end["monthly_expense_total"] += planned_total

	context = {
		"note": note,
		"current_month": current_month,
		"incomes": incomes,
		"total": total,
		"year": year,
		"month": month,
		"today": today.isoformat(),
		"clone_months": clone_months,
		"clone_fe_months": clone_fe_months,
		"prev_year": prev_year,
		"prev_month": prev_month,
		"fe_entries": fe_entries,
		"fe_total": fe_total,
		"savings_target": note.savings_target,
		"budget": budget,
		"goals": MonthlyGoal.objects.filter(
			year=year, month=month, kind=MonthlyGoal.GOAL
		),
		"promises": MonthlyGoal.objects.filter(
			year=year, month=month, kind=MonthlyGoal.PROMISE
		),
		"offset": offset,
		"prev_offset": offset - 1,
		"next_offset": offset + 1,
		"is_current_month": is_current_month,
		# Month end
		"month_end": month_end,
		"savings_result": budget - month_end["monthly_expense_total"],
		# Planned expenses
		"planned_expenses": planned_expenses,
		"planned_total": planned_total,
	}
	return render(request, "pages/monthly/month.html", context)