/**
 * Client-side normalisation for face photos headed to a Hikvision face library.
 *
 * The terminal is strict in ways a raw camera roll photo never satisfies, so the browser
 * does the work before upload:
 *   - JPEG only. Phone photos are usually HEIC, which the terminal rejects outright.
 *   - Small. The ~3MB figure in the Hikvision docs is the ISAPI transport ceiling; the
 *     face *library* caps far lower, so an oversized photo uploads happily and then fails
 *     enrolment -- the worst failure mode, because it looks like it worked.
 *   - Portrait. Terminals expect a 3:4 face frame; a square crop tends to cut the chin.
 *
 * Canvas does all of this, so there is no image dependency in this project.
 */

/** Target frame. 3:4 portrait is what the face library expects. */
export const FACE_WIDTH = 480
export const FACE_HEIGHT = 640

/** Face-library budget, well under the ISAPI transport limit. */
export const FACE_TARGET_BYTES = 150 * 1024
export const FACE_MAX_BYTES = 200 * 1024

/** Guard on the *input* file, applied before decoding so a huge file can't exhaust memory. */
export const INPUT_MAX_BYTES = 3 * 1024 * 1024

/** Quality ladder: start high, step down only while over budget. */
const QUALITY_STEPS = [0.85, 0.75, 0.65]

export interface NormalizedFacePhoto {
  /** Bare base64 JPEG, no data: prefix — what the API expects. */
  base64: string
  /** data: URL for previewing in an <img>. */
  previewUrl: string
  bytes: number
  width: number
  height: number
}

export class FacePhotoError extends Error {}

/**
 * Decodes any browser-readable image, cover-crops it to 3:4, resizes to 480x640 and
 * encodes JPEG under the face-library budget.
 */
export async function normalizeFacePhoto(
  source: Blob | HTMLVideoElement
): Promise<NormalizedFacePhoto> {
  if (source instanceof Blob) {
    if (source.size > INPUT_MAX_BYTES) {
      throw new FacePhotoError(
        `That image is ${Math.round(source.size / 1024 / 1024)} MB. Choose one under 3 MB.`
      )
    }
    if (source.size === 0) {
      throw new FacePhotoError("That file is empty.")
    }
  }

  const bitmap = await toBitmap(source)

  try {
    const canvas = document.createElement("canvas")
    canvas.width = FACE_WIDTH
    canvas.height = FACE_HEIGHT

    const ctx = canvas.getContext("2d")
    if (!ctx) throw new FacePhotoError("This browser cannot process images.")

    // Cover-crop: fill the 3:4 frame from the centre rather than squashing the face.
    const scale = Math.max(FACE_WIDTH / bitmap.width, FACE_HEIGHT / bitmap.height)
    const drawWidth = bitmap.width * scale
    const drawHeight = bitmap.height * scale

    ctx.imageSmoothingQuality = "high"
    // A face library needs an opaque background; a transparent PNG would otherwise
    // flatten to black and wreck the exposure.
    ctx.fillStyle = "#ffffff"
    ctx.fillRect(0, 0, FACE_WIDTH, FACE_HEIGHT)
    ctx.drawImage(
      bitmap,
      (FACE_WIDTH - drawWidth) / 2,
      (FACE_HEIGHT - drawHeight) / 2,
      drawWidth,
      drawHeight
    )

    let blob: Blob | null = null
    for (const quality of QUALITY_STEPS) {
      blob = await canvasToBlob(canvas, quality)
      if (blob && blob.size <= FACE_TARGET_BYTES) break
    }

    if (!blob) throw new FacePhotoError("Could not process that image. Try another photo.")

    if (blob.size > FACE_MAX_BYTES) {
      throw new FacePhotoError(
        "Could not compress that photo enough for the access terminal. Try a simpler " +
          "background or better lighting."
      )
    }

    const dataUrl = await blobToDataUrl(blob)
    return {
      base64: dataUrl.slice(dataUrl.indexOf(",") + 1),
      previewUrl: dataUrl,
      bytes: blob.size,
      width: FACE_WIDTH,
      height: FACE_HEIGHT,
    }
  } finally {
    if ("close" in bitmap && typeof bitmap.close === "function") bitmap.close()
  }
}

async function toBitmap(source: Blob | HTMLVideoElement): Promise<ImageBitmap> {
  try {
    // createImageBitmap decodes HEIC on Safari and every other format the browser knows.
    return await createImageBitmap(source as ImageBitmapSource)
  } catch {
    throw new FacePhotoError(
      "That file isn't a readable image. Use a JPEG or PNG photo, or take one with the camera."
    )
  }
}

function canvasToBlob(canvas: HTMLCanvasElement, quality: number): Promise<Blob | null> {
  return new Promise((resolve) => canvas.toBlob(resolve, "image/jpeg", quality))
}

function blobToDataUrl(blob: Blob): Promise<string> {
  return new Promise((resolve, reject) => {
    const reader = new FileReader()
    reader.onload = () => resolve(String(reader.result))
    reader.onerror = () => reject(new FacePhotoError("Could not read the processed image."))
    reader.readAsDataURL(blob)
  })
}