var assert = require('nanoassert')
var randombytes_buf = require('.').randombytes_buf
var blake2b = require('blake2b')

module.exports.crypto_kdf_PRIMITIVE = 'blake2b'
module.exports.crypto_kdf_BYTES_MIN = 16
module.exports.crypto_kdf_BYTES_MAX = 64
module.exports.crypto_kdf_CONTEXTBYTES = 8
module.exports.crypto_kdf_KEYBYTES = 64

function STORE64_LE(dest, int) {
  var mul = 1
  var i = 0
  dest[0] = int & 0xFF
  while (++i < 8 && (mul *= 0x100)) {
    dest[i] = (int / mul) & 0xFF
  }
}

module.exports.crypto_kdf_derive_from_key = function crypto_kdf_derive_from_key (subkey, subkey_id, ctx, key) {
  assert(subkey.length >= module.exports.crypto_kdf_BYTES_MIN, 'subkey must be at least crypto_kdf_BYTES_MIN')
  assert(subkey_id >= 0 && subkey_id <= 0x1fffffffffffff, 'subkey_id must be safe integer')
  assert(ctx.length >= module.exports.crypto_kdf_CONTEXTBYTES, 'context must be at least crypto_kdf_CONTEXTBYTES')

  var ctx_padded = new Uint8Array(blake2b.PERSONALBYTES)
  var salt = new Uint8Array(blake2b.SALTBYTES)

  ctx_padded.set(ctx, 0, module.exports.crypto_kdf_CONTEXTBYTES)
  STORE64_LE(salt, subkey_id)

  var outlen = Math.min(subkey.length, module.exports.crypto_kdf_BYTES_MAX)
  blake2b(outlen, key, salt, ctx_padded, true)
    .final(subkey)
}

module.exports.crypto_kdf_keygen = function crypto_kdf_keygen (out) {
  assert(out.length >= module.exports.crypto_kdf_KEYBYTES, 'out.length must be crypto_kdf_KEYBYTES')
  randombytes_buf(out.subarray(0, module.exports.crypto_kdf_KEYBYTES))
}