import sharp from 'sharp'

function dataUrlToBuffer(dataUrl: string): Buffer {
  const match = dataUrl.match(/^data:image\/png;base64,(.+)$/)
  if (!match) {
    throw new Error('Expected PNG data URL')
  }
  return Buffer.from(match[1], 'base64')
}

export type MaskStats = {
  width: number
  height: number
  coverage: number
  mean: number
}

export async function maskStatsFromDataUrl(dataUrl: string): Promise<MaskStats> {
  const buf = dataUrlToBuffer(dataUrl)
  const image = sharp(buf)
  const meta = await image.metadata()
  if (!meta.width || !meta.height) {
    throw new Error('mask missing dimensions')
  }

  const { data } = await image.ensureAlpha().raw().toBuffer({ resolveWithObject: true })
  const pixels = meta.width * meta.height
  let alphaSum = 0
  let solid = 0
  let lumSum = 0
  let solidLum = 0
  let alphaVaries = false

  for (let i = 3; i < data.length; i += 4) {
    const r = data[i - 3]
    const g = data[i - 2]
    const b = data[i - 1]
    const alpha = data[i]
    alphaSum += alpha
    if (alpha >= 128) solid += 1
    const lum = Math.round((r + g + b) / 3)
    lumSum += lum
    if (lum >= 128) solidLum += 1
    if (alpha !== 255) alphaVaries = true
  }

  if (!alphaVaries) {
    return {
      width: meta.width,
      height: meta.height,
      coverage: pixels > 0 ? solidLum / pixels : 0,
      mean: pixels > 0 ? lumSum / (pixels * 255) : 0,
    }
  }

  return {
    width: meta.width,
    height: meta.height,
    coverage: pixels > 0 ? solid / pixels : 0,
    mean: pixels > 0 ? alphaSum / (pixels * 255) : 0,
  }
}