/**
 * 无依赖 PNG 像素差异对比（8 位、非隔行、RGB/RGBA）。
 *
 * 用途：验证 CSS 重构（如 token 化）是否改变了实际渲染。
 *
 * 用法：node scripts/pngdiff.mjs <before.png> <after.png> [channelThreshold]
 */

import { readFileSync } from 'node:fs'
import { inflateSync } from 'node:zlib'

function decodePng(buf) {
  if (buf.readUInt32BE(0) !== 0x89504e47) throw new Error('不是 PNG')
  let pos = 8
  let width = 0
  let height = 0
  let bitDepth = 0
  let colorType = 0
  let interlace = 0
  const idat = []

  while (pos < buf.length) {
    const len = buf.readUInt32BE(pos)
    const type = buf.toString('ascii', pos + 4, pos + 8)
    const data = buf.subarray(pos + 8, pos + 8 + len)
    if (type === 'IHDR') {
      width = data.readUInt32BE(0)
      height = data.readUInt32BE(4)
      bitDepth = data[8]
      colorType = data[9]
      interlace = data[12]
    } else if (type === 'IDAT') {
      idat.push(data)
    } else if (type === 'IEND') {
      break
    }
    pos += 12 + len
  }

  if (bitDepth !== 8) throw new Error(`仅支持 8 位深度，实际 ${bitDepth}`)
  if (interlace !== 0) throw new Error('不支持隔行 PNG')
  const channels = colorType === 6 ? 4 : colorType === 2 ? 3 : 0
  if (!channels) throw new Error(`不支持的颜色类型 ${colorType}`)

  const raw = inflateSync(Buffer.concat(idat))
  const stride = width * channels
  const pixels = Buffer.alloc(height * stride)

  for (let y = 0; y < height; y++) {
    const filter = raw[y * (stride + 1)]
    const line = raw.subarray(y * (stride + 1) + 1, y * (stride + 1) + 1 + stride)
    const prev = y > 0 ? pixels.subarray((y - 1) * stride, y * stride) : null
    const out = pixels.subarray(y * stride, (y + 1) * stride)
    for (let i = 0; i < stride; i++) {
      const a = i >= channels ? out[i - channels] : 0
      const b = prev ? prev[i] : 0
      const c = prev && i >= channels ? prev[i - channels] : 0
      let v = line[i]
      switch (filter) {
        case 0:
          break
        case 1:
          v = (v + a) & 0xff
          break
        case 2:
          v = (v + b) & 0xff
          break
        case 3:
          v = (v + ((a + b) >> 1)) & 0xff
          break
        case 4: {
          const p = a + b - c
          const pa = Math.abs(p - a)
          const pb = Math.abs(p - b)
          const pc = Math.abs(p - c)
          const pr = pa <= pb && pa <= pc ? a : pb <= pc ? b : c
          v = (v + pr) & 0xff
          break
        }
        default:
          throw new Error(`未知过滤器 ${filter}`)
      }
      out[i] = v
    }
  }
  return { width, height, channels, pixels }
}

const [, , beforePath, afterPath, thresholdArg] = process.argv
const threshold = Number(thresholdArg ?? 0)

const a = decodePng(readFileSync(beforePath))
const b = decodePng(readFileSync(afterPath))

if (a.width !== b.width || a.height !== b.height) {
  console.log(`尺寸不同：${a.width}x${a.height} vs ${b.width}x${b.height}`)
  const minW = Math.min(a.width, b.width)
  const minH = Math.min(a.height, b.height)
  console.log(`仅比较重叠区域 ${minW}x${minH}`)
}

const w = Math.min(a.width, b.width)
const h = Math.min(a.height, b.height)
let changed = 0
let maxDelta = 0
let sumDelta = 0
const perChannel = [0, 0, 0, 0]

for (let y = 0; y < h; y++) {
  for (let x = 0; x < w; x++) {
    let diff = false
    for (let c = 0; c < 3; c++) {
      const ia = y * a.width * a.channels + x * a.channels + c
      const ib = y * b.width * b.channels + x * b.channels + c
      const d = Math.abs(a.pixels[ia] - b.pixels[ib])
      if (d > threshold) diff = true
      if (d > 0) perChannel[c] += 1
      if (d > maxDelta) maxDelta = d
      sumDelta += d
    }
    if (diff) changed++
  }
}

const total = w * h
console.log(`总像素      ${total}`)
console.log(`变化像素    ${changed}  (${((changed / total) * 100).toFixed(3)}%)`)
console.log(`最大通道差  ${maxDelta}`)
console.log(`平均通道差  ${(sumDelta / (total * 3)).toFixed(4)}`)
console.log(`通道变化数  R=${perChannel[0]} G=${perChannel[1]} B=${perChannel[2]}`)