import { describe, it, expect } from 'vitest'

/**
 * Unit tests for the virtual scrolling logic in SequenceViewer.vue.
 * Tests the pure computation functions extracted from the component's computed properties.
 *
 * Virtual scrolling parameters:
 * - ITEM_HEIGHT = 60px
 * - BUFFER_SIZE = 5 items
 * - VIRTUAL_SCROLL_THRESHOLD = 200 messages
 */

const ITEM_HEIGHT = 60
const BUFFER_SIZE = 5
const VIRTUAL_SCROLL_THRESHOLD = 200

// --- Pure logic functions (mirror of component computed properties) ---

function useVirtualScroll(messageCount: number): boolean {
  return messageCount > VIRTUAL_SCROLL_THRESHOLD
}

function totalHeight(messageCount: number): number {
  return messageCount * ITEM_HEIGHT
}

function visibleStart(scrollTop: number, messageCount: number): number {
  if (!useVirtualScroll(messageCount)) return 0
  const start = Math.floor(scrollTop / ITEM_HEIGHT) - BUFFER_SIZE
  return Math.max(0, start)
}

function visibleEnd(
  scrollTop: number,
  containerHeight: number,
  messageCount: number
): number {
  if (!useVirtualScroll(messageCount)) return messageCount
  const visibleCount = Math.ceil(containerHeight / ITEM_HEIGHT)
  const end = Math.floor(scrollTop / ITEM_HEIGHT) + visibleCount + BUFFER_SIZE
  return Math.min(messageCount, end)
}

function offsetY(scrollTop: number, messageCount: number): number {
  return visibleStart(scrollTop, messageCount) * ITEM_HEIGHT
}

describe('Virtual Scrolling Logic', () => {
  describe('useVirtualScroll threshold', () => {
    it('should NOT use virtual scroll for 200 messages or fewer', () => {
      expect(useVirtualScroll(0)).toBe(false)
      expect(useVirtualScroll(100)).toBe(false)
      expect(useVirtualScroll(200)).toBe(false)
    })

    it('should use virtual scroll for more than 200 messages', () => {
      expect(useVirtualScroll(201)).toBe(true)
      expect(useVirtualScroll(500)).toBe(true)
      expect(useVirtualScroll(5000)).toBe(true)
    })
  })

  describe('totalHeight', () => {
    it('should compute total height as messageCount * ITEM_HEIGHT', () => {
      expect(totalHeight(0)).toBe(0)
      expect(totalHeight(1)).toBe(60)
      expect(totalHeight(100)).toBe(6000)
      expect(totalHeight(1000)).toBe(60000)
      expect(totalHeight(5000)).toBe(300000)
    })
  })

  describe('visibleStart', () => {
    it('should return 0 when virtual scroll is disabled (≤200 messages)', () => {
      expect(visibleStart(0, 100)).toBe(0)
      expect(visibleStart(1000, 200)).toBe(0)
    })

    it('should return 0 when scrolled to top with virtual scroll enabled', () => {
      expect(visibleStart(0, 500)).toBe(0)
    })

    it('should include buffer items before the visible area', () => {
      // scrollTop = 600 means first visible item is at index 10
      // With buffer of 5, visibleStart = 10 - 5 = 5
      expect(visibleStart(600, 500)).toBe(5)
    })

    it('should clamp to 0 when buffer would go negative', () => {
      // scrollTop = 120 means first visible item is at index 2
      // With buffer of 5, visibleStart = 2 - 5 = -3 → clamped to 0
      expect(visibleStart(120, 500)).toBe(0)
    })

    it('should compute correctly for large scroll positions', () => {
      // scrollTop = 30000 means first visible item is at index 500
      // With buffer of 5, visibleStart = 500 - 5 = 495
      expect(visibleStart(30000, 1000)).toBe(495)
    })
  })

  describe('visibleEnd', () => {
    it('should return messageCount when virtual scroll is disabled', () => {
      expect(visibleEnd(0, 500, 100)).toBe(100)
      expect(visibleEnd(0, 500, 200)).toBe(200)
    })

    it('should compute visible end based on container height and buffer', () => {
      // containerHeight = 500, so visibleCount = ceil(500/60) = 9
      // scrollTop = 0, so first visible = 0
      // end = 0 + 9 + 5 = 14
      expect(visibleEnd(0, 500, 500)).toBe(14)
    })

    it('should clamp to messageCount when end exceeds total', () => {
      // With few messages relative to viewport
      // scrollTop = 0, containerHeight = 500, visibleCount = 9
      // end = 0 + 9 + 5 = 14, but messageCount = 201
      // For scrollTop near the end:
      // scrollTop = 12000 → first visible = 200, end = 200 + 9 + 5 = 214 → clamped to 201
      expect(visibleEnd(12000, 500, 201)).toBe(201)
    })

    it('should compute correctly for mid-scroll positions', () => {
      // scrollTop = 3000, containerHeight = 500
      // first visible = floor(3000/60) = 50
      // visibleCount = ceil(500/60) = 9
      // end = 50 + 9 + 5 = 64
      expect(visibleEnd(3000, 500, 1000)).toBe(64)
    })
  })

  describe('offsetY', () => {
    it('should return 0 when virtual scroll is disabled', () => {
      expect(offsetY(0, 100)).toBe(0)
      expect(offsetY(5000, 200)).toBe(0)
    })

    it('should return 0 when scrolled to top', () => {
      expect(offsetY(0, 500)).toBe(0)
    })

    it('should compute offset based on visibleStart', () => {
      // visibleStart(600, 500) = 5, so offsetY = 5 * 60 = 300
      expect(offsetY(600, 500)).toBe(300)
    })

    it('should compute correctly for large scroll positions', () => {
      // visibleStart(30000, 1000) = 495, so offsetY = 495 * 60 = 29700
      expect(offsetY(30000, 1000)).toBe(29700)
    })
  })

  describe('rendered item count', () => {
    it('should render only a subset of items when virtual scrolling is active', () => {
      const messageCount = 1000
      const containerH = 500

      // For any scroll position, rendered items = visibleEnd - visibleStart
      const scrollPos = 3000
      const start = visibleStart(scrollPos, messageCount)
      const end = visibleEnd(scrollPos, containerH, messageCount)
      const renderedCount = end - start

      // visibleCount = ceil(500/60) = 9, plus 2*buffer = 10 → max ~19 items rendered
      // Way less than the 1000 total messages
      expect(renderedCount).toBeLessThan(30)
      expect(renderedCount).toBeGreaterThan(0)
    })

    it('should render all items when below threshold', () => {
      const messageCount = 150
      const containerH = 500
      const scrollPos = 0

      const start = visibleStart(scrollPos, messageCount)
      const end = visibleEnd(scrollPos, containerH, messageCount)

      expect(start).toBe(0)
      expect(end).toBe(messageCount)
    })

    it('should maintain render performance target (≤100ms render budget)', () => {
      // With 5000 messages, virtual scroll renders only ~19 items max
      // At 60px height, a 500px container shows ~8 items + 2*5 buffer = 18 items
      // 18 items at worst case should render well under 100ms
      const messageCount = 5000
      const containerH = 500

      for (let scrollPos = 0; scrollPos < totalHeight(messageCount); scrollPos += 6000) {
        const start = visibleStart(scrollPos, messageCount)
        const end = visibleEnd(scrollPos, containerH, messageCount)
        const renderedCount = end - start

        // Never render more than visibleCount + 2*buffer items
        const maxRendered = Math.ceil(containerH / ITEM_HEIGHT) + 2 * BUFFER_SIZE
        expect(renderedCount).toBeLessThanOrEqual(maxRendered)
      }
    })
  })
})