<template>
  <div class="panel">
    <div class="panel-header">
      <div class="panel-title-row">
        <h2 class="panel-title">
          Selected items
        </h2>
        <span v-if="total" class="item-count">{{ total }}</span>
      </div>
      <div class="search-wrap">
        <span class="search-icon" aria-hidden="true">
          <svg width="13" height="13" viewBox="0 0 13 13" fill="none">
            <circle cx="5.5" cy="5.5" r="4" stroke="currentColor" stroke-width="1.4" />
            <path d="M8.5 8.5L11.5 11.5" stroke="currentColor" stroke-width="1.4" stroke-linecap="round" />
          </svg>
        </span>
        <input
          class="search-input"
          type="text"
          placeholder="Search..."
          :value="search"
          @input="onSearchInput"
        >
      </div>
    </div>

    <div ref="panelBody" class="panel-body" @scroll="onScroll">
      <div :style="{ height: topSpacer + 'px' }" aria-hidden="true" />

      <VueDraggableNext
        v-model="visibleSlice"
        tag="ul"
        class="item-list"
        handle=".drag-handle"
        @start="onDragStarted"
        @end="onDragEndFull"
      >
        <li
          v-for="(item, vIdx) in visibleSlice"
          :key="item.id"
          class="item-row"
          :style="{ '--i': Math.min(vIdx, 14) }"
        >
          <span class="drag-handle" title="Drag to reorder" aria-hidden="true">
            <svg width="8" height="12" viewBox="0 0 8 12" fill="currentColor">
              <circle cx="2" cy="2" r="1.2" />
              <circle cx="6" cy="2" r="1.2" />
              <circle cx="2" cy="6" r="1.2" />
              <circle cx="6" cy="6" r="1.2" />
              <circle cx="2" cy="10" r="1.2" />
              <circle cx="6" cy="10" r="1.2" />
            </svg>
          </span>
          <span class="item-id">#{{ item.id }}</span>
          <span class="item-value">{{ item.value }}</span>
          <button class="action-btn" title="Remove from selected" @click="emit('deselect', item.id!)">
            <svg width="14" height="14" viewBox="0 0 14 14" fill="none" aria-hidden="true">
              <path d="M11.5 7H2.5M5.5 3.5L2 7L5.5 10.5" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" />
            </svg>
          </button>
        </li>
      </VueDraggableNext>

      <div :style="{ height: bottomSpacer + 'px' }" aria-hidden="true" />

      <div v-if="loading" class="status-row">
        <span class="status-dots">
          <span /><span /><span />
        </span>
      </div>
      <div v-if="hasMore && !loading" ref="sentinel" class="sentinel" />
      <div v-if="!loading && !hasMore && items.length === 0" class="empty-state">
        <span class="empty-label">No items selected</span>
      </div>
    </div>
  </div>
</template>

<script setup lang="ts">
import type { Item } from '~/services/api'
import { computed, onMounted, onUnmounted, ref, watch } from 'vue'
import { VueDraggableNext } from 'vue-draggable-next'

const ITEM_HEIGHT = 47
const OVERSCAN = 5

const props = defineProps<{
  items: Item[]
  total: number | string
  loading: boolean
  hasMore: boolean
  search: string
}>()

const emit = defineEmits<{
  (e: 'update:search', val: string): void
  (e: 'loadMore'): void
  (e: 'deselect', id: number): void
  (e: 'reorder', id: number, afterId: number | null): void
}>()

// ── Local items (source of truth for drag ordering) ──────────────────────────

const localItems = ref([...props.items])

watch(() => props.items, (val) => {
  localItems.value = [...val]
}, { deep: true })

// ── Virtual scroll ───────────────────────────────────────────────────────────

const panelBody = ref<HTMLElement | null>(null)
const scrollTop = ref(0)
const containerHeight = ref(500)

function onScroll() {
  scrollTop.value = panelBody.value?.scrollTop ?? 0
}

const startIdx = computed(() =>
  Math.max(0, Math.floor(scrollTop.value / ITEM_HEIGHT) - OVERSCAN),
)
const endIdx = computed(() =>
  Math.min(
    localItems.value.length,
    Math.ceil((scrollTop.value + containerHeight.value) / ITEM_HEIGHT) + OVERSCAN,
  ),
)

// Writable computed: VueDraggableNext reads/writes the visible slice.
// The setter splices changes back into localItems so the full list stays in sync.
const visibleSlice = computed({
  get: () => localItems.value.slice(startIdx.value, endIdx.value),
  set: (newVal: Item[]) => {
    localItems.value.splice(startIdx.value, endIdx.value - startIdx.value, ...newVal)
  },
})

const topSpacer = computed(() => startIdx.value * ITEM_HEIGHT)
const bottomSpacer = computed(() => (localItems.value.length - endIdx.value) * ITEM_HEIGHT)

let resizeObserver: ResizeObserver | null = null

onMounted(() => {
  containerHeight.value = panelBody.value?.clientHeight ?? 500
  resizeObserver = new ResizeObserver((entries) => {
    const entry = entries[0]
    if (entry)
      containerHeight.value = entry.contentRect.height
  })
  if (panelBody.value)
    resizeObserver.observe(panelBody.value)
})

// ── Infinite scroll sentinel ─────────────────────────────────────────────────

const sentinel = ref<HTMLElement | null>(null)
let observer: IntersectionObserver | null = null

watch(sentinel, (el) => {
  if (observer) {
    observer.disconnect()
    observer = null
  }
  if (el) {
    observer = new IntersectionObserver((entries) => {
      if (entries[0]?.isIntersecting && props.hasMore && !props.loading) {
        emit('loadMore')
      }
    }, { threshold: 0.1 })
    observer.observe(el)
  }
})

// ── Auto-scroll during drag ──────────────────────────────────────────────────

const SCROLL_ZONE = 80
const SCROLL_MAX_SPEED = 12

let isDragging = false
let scrollSpeed = 0
let scrollRafId: number | null = null

function scrollLoop() {
  if (!isDragging || !panelBody.value) {
    scrollRafId = null
    return
  }
  if (scrollSpeed !== 0) {
    panelBody.value.scrollTop += scrollSpeed
  }
  scrollRafId = requestAnimationFrame(scrollLoop)
}

function onPointerMoveDrag(e: PointerEvent) {
  if (!isDragging || !panelBody.value)
    return
  const rect = panelBody.value.getBoundingClientRect()
  const y = e.clientY - rect.top
  if (y >= 0 && y < SCROLL_ZONE) {
    scrollSpeed = -((SCROLL_ZONE - y) / SCROLL_ZONE) * SCROLL_MAX_SPEED
  }
  else if (y > rect.height - SCROLL_ZONE && y <= rect.height) {
    scrollSpeed = ((y - (rect.height - SCROLL_ZONE)) / SCROLL_ZONE) * SCROLL_MAX_SPEED
  }
  else {
    scrollSpeed = 0
  }
}

function onDragStarted() {
  isDragging = true
  scrollSpeed = 0
  document.addEventListener('pointermove', onPointerMoveDrag)
  scrollRafId = requestAnimationFrame(scrollLoop)
}

function onDragEndFull(event: { oldIndex: number, newIndex: number }): void {
  isDragging = false
  scrollSpeed = 0
  document.removeEventListener('pointermove', onPointerMoveDrag)
  if (scrollRafId !== null) {
    cancelAnimationFrame(scrollRafId)
    scrollRafId = null
  }

  // The visibleSlice setter has already applied the reorder to localItems.
  // event indices are relative to the visible slice (startIdx offset).
  const actualNewIdx = startIdx.value + event.newIndex
  const moved = localItems.value[actualNewIdx]
  if (!moved)
    return
  const after = actualNewIdx > 0 ? localItems.value[actualNewIdx - 1] : null
  emit('reorder', moved.id!, after?.id ?? null)
}

onUnmounted(() => {
  observer?.disconnect()
  resizeObserver?.disconnect()
  document.removeEventListener('pointermove', onPointerMoveDrag)
  if (scrollRafId !== null)
    cancelAnimationFrame(scrollRafId)
})

// ── Search ───────────────────────────────────────────────────────────────────

let debounceTimer: ReturnType<typeof setTimeout> | null = null

function onSearchInput(e: Event) {
  const val = (e.target as HTMLInputElement).value
  if (debounceTimer)
    clearTimeout(debounceTimer)
  debounceTimer = setTimeout(() => {
    emit('update:search', val)
  }, 300)
}
</script>

<style scoped lang="scss">
.panel {
  display: flex;
  flex-direction: column;
  height: 100%;
  background: var(--canvas);
}

/* ── Header ── */
.panel-header {
  height: 108px;
  padding: 18px 20px 16px;
  border-bottom: 1px solid var(--border);
  background: var(--surface);
  flex-shrink: 0;
}

.panel-title-row {
  display: flex;
  align-items: center;
  gap: 8px;
  margin-bottom: 12px;
}

.panel-title {
  margin: 0;
  font-size: 0.72rem;
  font-weight: 500;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: var(--text-secondary);
  font-family: var(--font-sans);
}

.item-count {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-width: 18px;
  height: 18px;
  padding: 0 5px;
  background: var(--surface-subtle);
  border: 1px solid var(--border);
  border-radius: 9999px;
  font-family: var(--font-mono);
  font-size: 0.65rem;
  color: var(--text-secondary);
}

.search-wrap {
  position: relative;
  display: flex;
  align-items: center;
}

.search-icon {
  position: absolute;
  left: 10px;
  display: flex;
  align-items: center;
  color: var(--text-muted);
  pointer-events: none;
}

.search-input {
  width: 100%;
  padding: 7px 12px 7px 30px;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--surface-subtle);
  font-family: var(--font-sans);
  font-size: 0.825rem;
  color: var(--text-primary);
  outline: none;
  transition:
    border-color 150ms ease,
    background 150ms ease;

  &::placeholder {
    color: var(--text-muted);
  }

  &:focus {
    border-color: var(--text-primary);
    background: var(--surface);
  }
}

/* ── Body & list ── */
.panel-body {
  flex: 1;
  overflow-y: auto;

  &::-webkit-scrollbar {
    width: 3px;
  }
  &::-webkit-scrollbar-track {
    background: transparent;
  }
  &::-webkit-scrollbar-thumb {
    background: var(--border);
    border-radius: 2px;
  }
}

.item-list {
  list-style: none;
  margin: 0;
  padding: 0;
}

@keyframes item-in {
  from {
    opacity: 0;
    transform: translateY(6px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

.item-row {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 9px 12px 9px 14px;
  border-bottom: 1px solid var(--border);
  animation: item-in 350ms cubic-bezier(0.16, 1, 0.3, 1) both;
  animation-delay: calc(var(--i, 0) * 25ms);
  transition: background 120ms ease;

  &:hover {
    background: var(--surface-subtle);

    .action-btn {
      opacity: 1;
    }
    .drag-handle {
      color: var(--text-secondary);
    }
  }
}

.drag-handle {
  cursor: grab;
  color: var(--text-muted);
  display: flex;
  align-items: center;
  flex-shrink: 0;
  padding: 6px;
  transition: color 120ms ease;
  user-select: none;

  &:active {
    cursor: grabbing;
  }
}

.item-id {
  font-family: var(--font-mono);
  font-size: 0.72rem;
  color: var(--text-muted);
  flex-shrink: 0;
  min-width: 44px;
}

.item-value {
  flex: 1;
  font-family: var(--font-mono);
  font-size: 0.8rem;
  color: var(--text-primary);
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
}

.action-btn {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 28px;
  height: 28px;
  background: none;
  border: none;
  border-radius: var(--radius-sm);
  color: var(--text-secondary);
  cursor: pointer;
  opacity: 0;
  flex-shrink: 0;
  transition:
    color 120ms ease,
    background 120ms ease,
    opacity 120ms ease;

  &:hover {
    color: var(--text-primary);
    background: var(--border);
  }
}

/* ── States ── */
.status-row {
  display: flex;
  justify-content: center;
  padding: 24px;
}

.status-dots {
  display: flex;
  gap: 5px;
  align-items: center;

  span {
    width: 4px;
    height: 4px;
    border-radius: 50%;
    background: var(--text-muted);
    animation: dot-blink 1.2s ease-in-out infinite;

    &:nth-child(2) {
      animation-delay: 0.2s;
    }
    &:nth-child(3) {
      animation-delay: 0.4s;
    }
  }
}

@keyframes dot-blink {
  0%,
  80%,
  100% {
    opacity: 0.25;
  }
  40% {
    opacity: 1;
  }
}

.empty-state {
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 56px 24px;
}

.empty-label {
  font-size: 0.8rem;
  color: var(--text-muted);
  letter-spacing: 0.02em;
}

.sentinel {
  height: 1px;
}
</style>
