import { useState } from 'react'
import { ChevronLeft, ChevronRight, Calendar as CalendarIcon, CalendarDays } from 'lucide-react'
import { PRIORITY_CONFIG } from '../utils/api'
import { useLanguage } from '../i18n/LanguageContext'

const DAYS = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat']

function getMonthData(year, month) {
  const firstDay = new Date(year, month, 1).getDay()
  const daysInMonth = new Date(year, month + 1, 0).getDate()
  const prevDays = new Date(year, month, 0).getDate()

  const cells = []
  // Previous month trailing days
  for (let i = firstDay - 1; i >= 0; i--) {
    cells.push({ day: prevDays - i, current: false, date: new Date(year, month - 1, prevDays - i) })
  }
  // Current month
  for (let d = 1; d <= daysInMonth; d++) {
    cells.push({ day: d, current: true, date: new Date(year, month, d) })
  }
  // Next month leading days
  const remaining = 42 - cells.length
  for (let d = 1; d <= remaining; d++) {
    cells.push({ day: d, current: false, date: new Date(year, month + 1, d) })
  }
  return cells
}

function getWeekData(startDate) {
  const cells = []
  const start = new Date(startDate)
  start.setDate(start.getDate() - start.getDay())
  for (let i = 0; i < 7; i++) {
    const d = new Date(start)
    d.setDate(start.getDate() + i)
    cells.push({ day: d.getDate(), current: true, date: d })
  }
  return cells
}

function dateKey(d) {
  return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, '0')}-${String(d.getDate()).padStart(2, '0')}`
}

export default function TaskCalendarView({ tasks, onTaskClick }) {
  const { t } = useLanguage()
  const today = new Date()
  const [year, setYear] = useState(today.getFullYear())
  const [month, setMonth] = useState(today.getMonth())
  const [calView, setCalView] = useState('month')
  const [weekStart, setWeekStart] = useState(() => {
    const d = new Date(); d.setDate(d.getDate() - d.getDay()); return d
  })

  const cells = calView === 'month' ? getMonthData(year, month) : getWeekData(weekStart)
  const todayKey = dateKey(today)

  // Group tasks by due_date
  const tasksByDate = {}
  const unscheduled = []
  for (const task of tasks) {
    const dd = task.due_date || task.dueDate
    if (dd) {
      const key = dd.slice(0, 10) // yyyy-mm-dd
      if (!tasksByDate[key]) tasksByDate[key] = []
      tasksByDate[key].push(task)
    } else {
      unscheduled.push(task)
    }
  }

  const prevMonth = () => {
    if (month === 0) { setMonth(11); setYear(y => y - 1) }
    else setMonth(m => m - 1)
  }
  const nextMonth = () => {
    if (month === 11) { setMonth(0); setYear(y => y + 1) }
    else setMonth(m => m + 1)
  }
  const prevWeek = () => setWeekStart(d => { const n = new Date(d); n.setDate(n.getDate() - 7); return n })
  const nextWeek = () => setWeekStart(d => { const n = new Date(d); n.setDate(n.getDate() + 7); return n })

  const goToday = () => {
    setYear(today.getFullYear()); setMonth(today.getMonth())
    const d = new Date(); d.setDate(d.getDate() - d.getDay()); setWeekStart(d)
  }

  const monthLabel = new Date(year, month).toLocaleString('default', { month: 'long', year: 'numeric' })
  const weekLabel = (() => {
    const start = new Date(weekStart)
    start.setDate(start.getDate() - start.getDay())
    const end = new Date(start); end.setDate(start.getDate() + 6)
    const fmt = (d) => d.toLocaleString('default', { month: 'short', day: 'numeric' })
    return `${fmt(start)} – ${fmt(end)}, ${end.getFullYear()}`
  })()

  const getPillColor = (task) => {
    const p = task.priority || 'medium'
    if (p === 'urgent') return 'bg-red-500 text-white'
    if (p === 'high') return 'bg-orange-400 text-white'
    if (p === 'medium') return 'bg-amber-400 text-amber-900'
    return 'bg-gray-300 text-text-secondary'
  }

  return (
    <div className="flex gap-4">
      {/* Calendar grid */}
      <div className="flex-1">
        {/* Nav */}
        <div className="flex items-center justify-between mb-4">
          <div className="flex items-center gap-2">
            <button onClick={calView === 'month' ? prevMonth : prevWeek} className="p-1.5 rounded-lg hover:bg-surface-tertiary text-text-tertiary hover:text-text-primary transition-colors">
              <ChevronLeft className="w-4 h-4" />
            </button>
            <h3 className="text-sm font-semibold text-text-primary min-w-[180px] text-center">
              {calView === 'month' ? monthLabel : weekLabel}
            </h3>
            <button onClick={calView === 'month' ? nextMonth : nextWeek} className="p-1.5 rounded-lg hover:bg-surface-tertiary text-text-tertiary hover:text-text-primary transition-colors">
              <ChevronRight className="w-4 h-4" />
            </button>
          </div>
          <div className="flex items-center gap-2">
            <div className="flex bg-surface-tertiary rounded-lg p-0.5">
              <button
                onClick={() => setCalView('month')}
                className={`px-2.5 py-1 text-[11px] font-medium rounded-md transition-colors flex items-center gap-1 ${calView === 'month' ? 'bg-surface shadow-sm text-text-primary' : 'text-text-tertiary'}`}
              >
                <CalendarIcon className="w-3 h-3" />
                Month
              </button>
              <button
                onClick={() => setCalView('week')}
                className={`px-2.5 py-1 text-[11px] font-medium rounded-md transition-colors flex items-center gap-1 ${calView === 'week' ? 'bg-surface shadow-sm text-text-primary' : 'text-text-tertiary'}`}
              >
                <CalendarDays className="w-3 h-3" />
                Week
              </button>
            </div>
            <button onClick={goToday} className="px-3 py-1 text-xs font-medium text-brand-primary hover:bg-brand-primary/5 rounded-lg transition-colors">
              {t('tasks.today')}
            </button>
          </div>
        </div>

        {/* Day headers */}
        <div className="grid grid-cols-7 mb-1">
          {DAYS.map(d => (
            <div key={d} className="text-center text-[10px] font-medium text-text-tertiary uppercase py-1">
              {d}
            </div>
          ))}
        </div>

        {/* Cells */}
        <div className="grid grid-cols-7 border-t border-l border-border">
          {cells.map((cell, i) => {
            const key = dateKey(cell.date)
            const isToday = key === todayKey
            const dayTasks = tasksByDate[key] || []
            return (
              <div
                key={i}
                className={`border-r border-b border-border ${calView === 'week' ? 'min-h-[300px]' : 'min-h-[90px]'} p-1 ${
                  cell.current ? 'bg-surface' : 'bg-surface-secondary/50'
                }`}
              >
                <div className={`text-[11px] font-medium mb-0.5 w-6 h-6 flex items-center justify-center rounded-full ${
                  isToday ? 'bg-brand-primary text-white' : cell.current ? 'text-text-primary' : 'text-text-tertiary'
                }`}>
                  {cell.day}
                </div>
                <div className="space-y-0.5">
                  {dayTasks.slice(0, calView === 'week' ? 10 : 3).map(task => (
                    <button
                      key={task._id || task.id}
                      onClick={() => onTaskClick(task)}
                      className={`w-full text-start text-[10px] px-1.5 py-0.5 rounded truncate font-medium hover:opacity-80 transition-opacity ${
                        task.status === 'done' ? 'bg-emerald-100 text-emerald-700 line-through' : getPillColor(task)
                      }`}
                      title={task.title}
                    >
                      {task.title}
                    </button>
                  ))}
                  {dayTasks.length > (calView === 'week' ? 10 : 3) && (
                    <div className="text-[9px] text-text-tertiary text-center font-medium">
                      +{dayTasks.length - (calView === 'week' ? 10 : 3)} more
                    </div>
                  )}
                </div>
              </div>
            )
          })}
        </div>
      </div>

      {/* Unscheduled sidebar */}
      {unscheduled.length > 0 && (
        <div className="w-48 shrink-0">
          <h4 className="text-xs font-semibold text-text-tertiary uppercase mb-2">{t('tasks.unscheduled')}</h4>
          <div className="space-y-1.5 max-h-[500px] overflow-y-auto">
            {unscheduled.map(task => {
              const priority = PRIORITY_CONFIG[task.priority] || PRIORITY_CONFIG.medium
              return (
                <button
                  key={task._id || task.id}
                  onClick={() => onTaskClick(task)}
                  className="w-full text-start bg-surface border border-border rounded-lg p-2 hover:border-brand-primary/30 transition-colors"
                >
                  <div className="flex items-center gap-1.5">
                    <div className={`w-2 h-2 rounded-full ${priority.color} shrink-0`} />
                    <span className={`text-xs font-medium truncate ${task.status === 'done' ? 'text-text-tertiary line-through' : 'text-text-primary'}`}>
                      {task.title}
                    </span>
                  </div>
                </button>
              )
            })}
          </div>
        </div>
      )}
    </div>
  )
}