randstring.rn

//  Copyright 2021 Google LLC.
//
// 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.

// TODO: Speed this up with a call to a runtime function.
func randString(len: Uint) {
  result = secret("")
  while result.length() < <u64>len {
    randVal64 = rand64
    result += randVal64.toStringLE()
  }
  result.resize(len)
  return result
}

// Find the smallest mask of all 1's such that mask & n == n.
func findUintMask(n: Uint) {
  mask = n
  dist = 1u32
  while mask & (mask + 1) != 0 {
    mask |= mask >> dist
    dist <<= 1
  }
  return mask
}

func randRange(n: Uint) {
  if n == 0 {
    raise Exception.Internal, "randRange passed 0"
  }
  mask = findUintMask(n)
  do {
    if widthof(n) <= 64 {
      val = mask & !<n>rand64
    } else {
      val = mask & randString((widthof(n) + 7) >> 3).toUintLE(typeof(n))
    }
  } while reveal(val) >= n
  return val
}