const MEBIBYTE = 1024 * 1024

export const MAX_LOGICAL_UPLOAD_BYTES = 10 * 1024 * MEBIBYTE
export const MAX_UPLOAD_CHUNK_BYTES = 20 * MEBIBYTE
export const MAX_UPLOAD_CHUNKS = Math.ceil(MAX_LOGICAL_UPLOAD_BYTES / (10 * MEBIBYTE))

const LEGACY_BROWSER_CHUNK_SIZES = [10 * MEBIBYTE, 20 * MEBIBYTE]

/**
 * Resolve and validate the physical chunk size used for one upload session.
 * Older already-open browser pages did not send chunkSize to init; only the two
 * historically supported 10/20 MiB layouts are inferred for those sessions.
 */
export function resolveUploadChunkSize(
  fileSize: number,
  totalChunks: number,
  rawChunkSize?: unknown,
): number | null {
  if (
    !Number.isSafeInteger(fileSize)
    || fileSize <= 0
    || fileSize > MAX_LOGICAL_UPLOAD_BYTES
    || !Number.isInteger(totalChunks)
    || totalChunks <= 0
    || totalChunks > MAX_UPLOAD_CHUNKS
  ) {
    return null
  }

  const hasExplicitChunkSize = rawChunkSize !== undefined
    && rawChunkSize !== null
    && String(rawChunkSize).trim() !== ''

  if (hasExplicitChunkSize) {
    const chunkSize = Number(rawChunkSize)
    if (
      !Number.isSafeInteger(chunkSize)
      || chunkSize <= 0
      || chunkSize > MAX_UPLOAD_CHUNK_BYTES
      || Math.ceil(fileSize / chunkSize) !== totalChunks
    ) {
      return null
    }
    return chunkSize
  }

  return LEGACY_BROWSER_CHUNK_SIZES.find(
    (chunkSize) => Math.ceil(fileSize / chunkSize) === totalChunks,
  ) || null
}

export function getExpectedUploadChunkBytes(input: {
  fileSize: number
  totalChunks: number
  chunkSize: number
  chunkIndex: number
}): number | null {
  const { fileSize, totalChunks, chunkSize, chunkIndex } = input
  if (
    !Number.isInteger(chunkIndex)
    || chunkIndex < 0
    || chunkIndex >= totalChunks
    || resolveUploadChunkSize(fileSize, totalChunks, chunkSize) !== chunkSize
  ) {
    return null
  }

  const expectedBytes = chunkIndex === totalChunks - 1
    ? fileSize - chunkSize * (totalChunks - 1)
    : chunkSize

  return Number.isSafeInteger(expectedBytes)
    && expectedBytes > 0
    && expectedBytes <= MAX_UPLOAD_CHUNK_BYTES
    ? expectedBytes
    : null
}