import React, { useState, useEffect, useRef } from 'react';
import {
    format,
    addMonths,
    subMonths,
    startOfMonth,
    endOfMonth,
    startOfWeek,
    endOfWeek,
    addDays,
    isSameMonth,
    isSameDay,
    setMonth,
    setYear,
    getYear,
    getMonth
} from 'date-fns';
import { enUS, de } from 'date-fns/locale';

interface DatePickerProps {
    selected: Date;
    onSelect: (date: Date) => void;
    onClose: () => void;
    language?: string;
}

export default function DatePicker({ selected, onSelect, onClose, language = 'en' }: DatePickerProps) {
    const [currentMonth, setCurrentMonth] = useState(new Date(selected));
    const [view, setView] = useState<'calendar' | 'month-year'>('calendar');
    const modalRef = useRef<HTMLDivElement>(null);

    const locale = language === 'de' ? de : enUS;

    // Close on click outside
    useEffect(() => {
        const handleClickOutside = (event: MouseEvent) => {
            if (modalRef.current && !modalRef.current.contains(event.target as Node)) {
                onClose();
            }
        };
        document.addEventListener('mousedown', handleClickOutside);
        return () => document.removeEventListener('mousedown', handleClickOutside);
    }, [onClose]);

    // Calendar Header
    const renderHeader = () => {
        return (
            <div className="datepicker-header">
                <button onClick={() => setCurrentMonth(subMonths(currentMonth, 1))}>
                    <svg width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
                        <polyline points="15 18 9 12 15 6"></polyline>
                    </svg>
                </button>
                <div
                    className="datepicker-current-month"
                    onClick={() => setView(view === 'calendar' ? 'month-year' : 'calendar')}
                >
                    {format(currentMonth, 'MMMM yyyy', { locale })}
                    <svg width="12" height="12" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="3" strokeLinecap="round" strokeLinejoin="round" style={{ marginLeft: '6px', opacity: 0.5 }}>
                        <polyline points="6 9 12 15 18 9"></polyline>
                    </svg>
                </div>
                <button onClick={() => setCurrentMonth(addMonths(currentMonth, 1))}>
                    <svg width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
                        <polyline points="9 18 15 12 9 6"></polyline>
                    </svg>
                </button>
            </div>
        );
    };

    // Days of Week Header
    const renderDays = () => {
        const days = [];
        const startDate = startOfWeek(currentMonth, { weekStartsOn: 1 }); // Monday start

        for (let i = 0; i < 7; i++) {
            days.push(
                <div className="datepicker-day-name" key={i}>
                    {format(addDays(startDate, i), 'EEEEEE', { locale })}
                </div>
            );
        }
        return <div className="datepicker-days-row">{days}</div>;
    };

    // Calendar Cells
    const renderCells = () => {
        const monthStart = startOfMonth(currentMonth);
        const monthEnd = endOfMonth(monthStart);
        const startDate = startOfWeek(monthStart, { weekStartsOn: 1 });
        const endDate = endOfWeek(monthEnd, { weekStartsOn: 1 });

        const dateFormat = 'd';
        const rows = [];
        let days = [];
        let day = startDate;
        let formattedDate = '';

        while (day <= endDate) {
            for (let i = 0; i < 7; i++) {
                formattedDate = format(day, dateFormat);
                const cloneDay = day;

                days.push(
                    <div
                        className={`datepicker-cell ${!isSameMonth(day, monthStart)
                            ? 'disabled'
                            : isSameDay(day, selected)
                                ? 'selected'
                                : ''
                            } ${isSameDay(day, new Date()) ? 'today' : ''}`}
                        key={day.toString()}
                        onClick={() => {
                            onSelect(cloneDay);
                            onClose();
                        }}
                    >
                        <span className="number">{formattedDate}</span>
                    </div>
                );
                day = addDays(day, 1);
            }
            rows.push(
                <div className="datepicker-row" key={day.toString()}>
                    {days}
                </div>
            );
            days = [];
        }
        return <div className="datepicker-body">{rows}</div>;
    };

    // Month/Year Selection View
    const renderMonthYearSelector = () => {
        const years = [];
        const currentYear = getYear(new Date());
        for (let y = currentYear - 5; y <= currentYear + 5; y++) {
            years.push(y);
        }

        const months = Array.from({ length: 12 }, (_, i) => {
            return format(setMonth(new Date(), i), 'MMM', { locale });
        });

        return (
            <div className="datepicker-month-year-view">
                <div className="datepicker-years">
                    {years.map(year => (
                        <div
                            key={year}
                            className={`datepicker-year-option ${getYear(currentMonth) === year ? 'selected' : ''}`}
                            onClick={() => setCurrentMonth(setYear(currentMonth, year))}
                        >
                            {year}
                        </div>
                    ))}
                </div>
                <div className="datepicker-months">
                    {months.map((month, index) => (
                        <div
                            key={month}
                            className={`datepicker-month-option ${getMonth(currentMonth) === index ? 'selected' : ''}`}
                            onClick={() => {
                                setCurrentMonth(setMonth(currentMonth, index));
                                setView('calendar');
                            }}
                        >
                            {month}
                        </div>
                    ))}
                </div>
            </div>
        );
    };

    return (
        <div className="datepicker-modal" ref={modalRef}>
            {renderHeader()}
            {view === 'calendar' ? (
                <>
                    {renderDays()}
                    {renderCells()}
                </>
            ) : (
                renderMonthYearSelector()
            )}
            <style jsx>{`
                .datepicker-modal {
                    position: absolute;
                    top: 100%;
                    right: 0;
                    margin-top: 8px;
                    background: white;
                    border-radius: 8px;
                    border-bottom: 5px solid #00ceb0; /* Weekly Teal/Green */
                    box-shadow: 0 10px 25px rgba(0,0,0,0.1);
                    padding: 20px;
                    z-index: 1000;
                    width: 320px;
                    font-family: var(--font-inter, sans-serif);
                    user-select: none;
                    animation: fadeIn 0.2s ease-out;
                }
                .datepicker-header {
                    display: flex;
                    justify-content: space-between;
                    align-items: center;
                    margin-bottom: 20px;
                }
                .datepicker-header button {
                    background: none;
                    border: none;
                    cursor: pointer;
                    font-size: 1.5rem;
                    color: #999;
                    padding: 0 8px;
                    line-height: 1;
                    font-weight: 300;
                }
                .datepicker-header button:hover {
                    color: #333;
                }
                .datepicker-current-month {
                    font-weight: 800;
                    font-size: 1rem;
                    cursor: pointer;
                    text-transform: uppercase;
                    letter-spacing: 0.05em;
                }
                .datepicker-days-row {
                    display: grid;
                    grid-template-columns: repeat(7, 1fr); /* Force 7 columns */
                    margin-bottom: 12px;
                    justify-items: center; /* Center content */
                }
                .datepicker-day-name {
                    width: 40px;
                    text-align: center;
                    font-size: 0.85rem;
                    color: #000;
                    font-weight: 700;
                }
                .datepicker-body {
                    display: flex; /* Still flex col for rows, OR better yet, just one grid */
                    flex-direction: column;
                    gap: 4px;
                }
                /* Alternative: Use one big grid for body cells? */
                /* But current implementation renders rows. Let's keep rows but make them grids. */
                .datepicker-row {
                    display: grid;
                    grid-template-columns: repeat(7, 1fr);
                    gap: 0; /* Gap handled by padding or just empty space? Current had space-between */
                    justify-items: center;
                }
                /* Actually, space-between in flex meant they spread out. Grid 1fr means equal width. */
                /* This is much better for alignment. */

                .datepicker-cell {
                    width: 40px;
                    height: 40px;
                    display: flex;
                    align-items: center;
                    justify-content: center;
                    cursor: pointer;
                    border-radius: 4px;
                    font-size: 1rem;
                    font-weight: 600;
                    transition: all 0.1s ease;
                }
                .datepicker-cell:hover:not(.disabled) {
                    background-color: #f5f5f5;
                }
                .datepicker-cell.selected {
                    background-color: #f0f0f0;
                    color: #000;
                    font-weight: 900;
                }
                .datepicker-cell.today {
                    color: #00ceb0;
                }
                .datepicker-cell.disabled {
                    color: #e0e0e0;
                    pointer-events: none;
                }
                
                .datepicker-month-year-view {
                    display: flex;
                    flex-direction: column;
                    gap: 10px;
                    height: 280px;
                }
                .datepicker-years {
                    display: flex;
                    overflow-x: auto;
                    gap: 8px;
                    padding-bottom: 8px;
                    border-bottom: 1px solid #eee;
                }
                .datepicker-year-option {
                    padding: 4px 8px;
                    cursor: pointer;
                    border-radius: 4px;
                    font-weight: 500;
                }
                .datepicker-year-option.selected {
                    background: #000;
                    color: white;
                }
                .datepicker-months {
                    display: grid;
                    grid-template-columns: repeat(3, 1fr);
                    gap: 8px;
                }
                .datepicker-month-option {
                    padding: 12px;
                    text-align: center;
                    cursor: pointer;
                    border-radius: 4px;
                    font-weight: 600;
                }
                .datepicker-month-option:hover {
                    background: #f0f0f0;
                }
                .datepicker-month-option.selected {
                    background: #000;
                    color: white;
                }
                
                @keyframes fadeIn {
                    from { opacity: 0; transform: translateY(-5px); }
                    to { opacity: 1; transform: translateY(0); }
                }
            `}</style>
        </div >
    );
}