package services

import (
	"context"
	"fmt"
	"time"

	"github.com/breakpilot/school-service/internal/models"
)

// RolloverSchoolYear advances every tt_class for this user by one grade
// level, removes graduating classes (grade > 13), and updates the
// cal_school_config school-year dates. Operates as a single transaction.
//
// Stammdaten (teachers, subjects, rooms, periods) bleiben unveraendert —
// es aendern sich nur Klassen-Stufen.
func (s *CalendarService) RolloverSchoolYear(ctx context.Context, userID string, req *models.SchoolYearRolloverRequest) (*models.SchoolYearRolloverResult, error) {
	newStart, newEnd := defaultSchoolYearDates(req)

	tx, err := s.db.Begin(ctx)
	if err != nil {
		return nil, err
	}
	defer tx.Rollback(ctx)

	// 1. Remove the graduating cohort first so they don't get bumped to 14.
	gradRes, err := tx.Exec(ctx, `
		DELETE FROM tt_class WHERE created_by_user_id = $1 AND grade_level >= 13
	`, userID)
	if err != nil {
		return nil, fmt.Errorf("delete graduating: %w", err)
	}

	// 2. Promote everyone else.
	promRes, err := tx.Exec(ctx, `
		UPDATE tt_class SET grade_level = grade_level + 1
		WHERE created_by_user_id = $1
	`, userID)
	if err != nil {
		return nil, fmt.Errorf("promote classes: %w", err)
	}

	// 3. Update the school-year dates in the config (creates a row if the
	// user never picked a Bundesland — but that's an edge case; in normal
	// flow the wizard has run before rollover).
	_, err = tx.Exec(ctx, `
		UPDATE cal_school_config
		SET school_year_start = $1::date,
		    school_year_end = $2::date,
		    updated_at = NOW()
		WHERE user_id = $3
	`, newStart, newEnd, userID)
	if err != nil {
		return nil, fmt.Errorf("update config: %w", err)
	}

	if err := tx.Commit(ctx); err != nil {
		return nil, err
	}

	return &models.SchoolYearRolloverResult{
		ClassesPromoted:  int(promRes.RowsAffected()),
		ClassesGraduated: int(gradRes.RowsAffected()),
		NewYearStart:     newStart,
		NewYearEnd:       newEnd,
	}, nil
}

// defaultSchoolYearDates returns the dates from the request if both set,
// otherwise the next school year starting Aug 1 of "this year or next"
// and ending Jul 31 the year after.
func defaultSchoolYearDates(req *models.SchoolYearRolloverRequest) (string, string) {
	if req != nil && req.NewYearStart != nil && req.NewYearEnd != nil {
		return *req.NewYearStart, *req.NewYearEnd
	}
	now := time.Now()
	startYear := now.Year()
	// If we're past August, the "new" year refers to the next calendar year.
	if int(now.Month()) >= 8 {
		startYear++
	}
	start := time.Date(startYear, 8, 1, 0, 0, 0, 0, time.UTC)
	end := time.Date(startYear+1, 7, 31, 0, 0, 0, 0, time.UTC)
	return start.Format("2006-01-02"), end.Format("2006-01-02")
}