<template>
  <div class="u-tabs" role="tablist" :aria-label="ariaLabel">
    <!-- tabs -->
    <button
      v-for="(t, i) in tabsArr"
      :key="(t.value ?? t.label ?? i) as any"
      class="u-tab"
      :class="{ active: i === currentIndex }"
      role="tab"
      :aria-selected="i === currentIndex"
      :tabindex="i === currentIndex ? 0 : -1"
      @click="setActive(i)"
      @keydown.left.prevent="focusPrev"
      @keydown.right.prevent="focusNext"
      :ref="(el) => assignTabRef(el, i)"
    >
      {{ t.label }}
    </button>

    <!-- 活动下划线 -->
    <span
      class="u-underline"
      :style="{ width: underline.w + 'px', transform: `translateX(${underline.x}px)` }"
      aria-hidden="true"
    />
  </div>

  <!-- 面板（可选） -->
  <div v-if="usePanels" class="u-panels">
    <div
      v-for="(t, i) in tabsArr"
      :key="(t.value ?? t.label ?? i) as any"
      v-show="i === currentIndex"
      role="tabpanel"
      class="u-panel"
    >
      <slot :name="t.slot || 'panel'" :tab="t" :index="i">
        {{ t.content ?? '' }}
      </slot>
    </div>
  </div>
</template>

<script setup lang="ts">
import {
  ref, watch, onMounted, nextTick, computed, onBeforeUnmount,
  type ComponentPublicInstance
} from 'vue'

type TabItem = Readonly<{
  label: string
  value?: string | number
  slot?: string
  content?: string
}>

interface Props {
  tabs: ReadonlyArray<TabItem>
  modelValue?: string | number | null
  defaultIndex?: number
  usePanels?: boolean
  ariaLabel?: string
}

const props = withDefaults(defineProps<Props>(), {
  modelValue: null,
  defaultIndex: 0,
  usePanels: false,
  ariaLabel: ''
})

const emit = defineEmits<{
  (e: 'update:modelValue', v: string | number): void
  (e: 'change', payload: { index: number; tab: TabItem }): void
}>()

const tabsArr = computed(() => props.tabs ?? [])

/** 用数组存每个 tab 的原生按钮元素 */
const tabRefs = ref<Array<HTMLButtonElement | null>>([])

/** 兼容 Vue 的 VNodeRef 签名 */
function assignTabRef(
  el: Element | ComponentPublicInstance | null,
  i: number
) {
  // 这里只会接收到原生 <button>，安全断言为 HTMLButtonElement
  tabRefs.value[i] = (el as HTMLButtonElement) ?? null
}

const currentIndex = ref(0)

function valueToIndex(v: string | number | null) {
  return tabsArr.value.findIndex(t => (t?.value ?? t?.label) === v)
}

watch(
  () => props.modelValue,
  (v) => {
    if (v == null) return
    const idx = valueToIndex(v)
    if (idx >= 0) {
      currentIndex.value = idx
      nextTick(updateUnderline)
    }
  },
  { immediate: true }
)

onMounted(() => {
  if (props.modelValue == null) {
    currentIndex.value = Math.min(
      props.defaultIndex ?? 0,
      Math.max(tabsArr.value.length - 1, 0)
    )
  }
  nextTick(() => {
    updateUnderline()
    window.addEventListener('resize', handleResize)
  })
})

onBeforeUnmount(() => {
  window.removeEventListener('resize', handleResize)
})

/** 下划线位置与宽度 */
const underline = ref({ x: 0, w: 0 })
function updateUnderline() {
  const el = tabRefs.value[currentIndex.value]
  if (!el) return
  const parent = el.parentElement
  if (!parent) return
  const parentRect = parent.getBoundingClientRect()
  const rect = el.getBoundingClientRect()
  underline.value = { x: rect.left - parentRect.left, w: rect.width }
}
function handleResize() {
  updateUnderline()
}

function setActive(i: number) {
  if (i < 0 || i >= tabsArr.value.length) return
  const tab = tabsArr.value[i]!
  currentIndex.value = i
  updateUnderline()
  const v = (tab.value ?? tab.label) as string | number
  emit('update:modelValue', v)
  emit('change', { index: i, tab })
}

function focusPrev() {
  if (!tabsArr.value.length) return
  const i = (currentIndex.value - 1 + tabsArr.value.length) % tabsArr.value.length
  setActive(i)
  tabRefs.value[i]?.focus?.()
}
function focusNext() {
  if (!tabsArr.value.length) return
  const i = (currentIndex.value + 1) % tabsArr.value.length
  setActive(i)
  tabRefs.value[i]?.focus?.()
}

watch(currentIndex, () => nextTick(updateUnderline))
</script>

<style scoped>
.u-tabs {
  --tabs-gap: 48px;
  --tabs-color: #4b5563; /* 灰700 */
  --tabs-active: #0f172a; /* 石板900 */
  --tabs-underline: #0f172a;
  --tabs-font: 16px;

  position: relative;
  display: flex;
  align-items: center;
  gap: var(--tabs-gap);
  padding-inline: 8px;
}

.u-tab {
  position: relative;
  appearance: none;
  border: 0;
  background: transparent;
  padding: 8px 0;
  font-size: var(--tabs-font);
  color: var(--tabs-color);
  cursor: pointer;
  transition: color 0.18s ease, transform 0.18s ease;
  outline: none;
}
.u-tab:hover {
  color: var(--tabs-active);
}
.u-tab.active {
  color: var(--tabs-active);
  font-weight: 700;
}
.u-tab:focus-visible {
  outline: 2px solid rgba(99, 102, 241, 0.35);
  outline-offset: 2px;
  border-radius: 6px;
}

/* 下划线 */
.u-underline {
  position: absolute;
  bottom: 0;
  height: 3px;
  left: 0;
  background: var(--tabs-underline);
  border-radius: 3px;
  transition:
    transform 0.22s cubic-bezier(0.22, 1, 0.36, 1),
    width 0.22s cubic-bezier(0.22, 1, 0.36, 1);
  pointer-events: none;
}

/* 面板（可选） */
.u-panels {
  margin-top: 12px;
}
.u-panel {
  padding: 8px 0;
}

/* 小屏收紧间距 */
@media (max-width: 640px) {
  .u-tabs {
    --tabs-gap: 28px;
    --tabs-font: 15px;
  }
  .u-underline {
    height: 2px;
  }
}
</style>
