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/*
 * Copyright (C) 2022 Square, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.squareup.wire.schema.internal

import java.nio.charset.Charset
import okio.BufferedSource
import okio.ByteString.Companion.decodeHex

private val UNICODE_BOMS = okio.Options.of(
  "efbbbf".decodeHex(), // UTF-8
  "feff".decodeHex(), // UTF-16BE
  "fffe0000".decodeHex(), // UTF-32LE
  "fffe".decodeHex(), // UTF-16LE
  "0000feff".decodeHex(), // UTF-32BE
)

internal fun BufferedSource.readBomAsCharset(default: Charset = Charsets.UTF_8): Charset {
  return when (select(UNICODE_BOMS)) {
    0 -> Charsets.UTF_8
    1 -> Charsets.UTF_16BE
    2 -> Charsets.UTF_32LE
    3 -> Charsets.UTF_16LE
    4 -> Charsets.UTF_32BE
    -1 -> default
    else -> throw AssertionError()
  }
}