bits.mbt

// This file is based on the Go implementation found here:
// https://cs.opensource.google/go/go/+/refs/tags/go1.23.1:src/math/bits/bits.go
// which has the copyright notice:
// Copyright 2017 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// reverse8 returns the value of x with its bits in reversed order.
///|
fn reverse8(x : Byte) -> Byte {
  return rev8tab[x.to_int()].to_byte()
}

// reverse16 returns the value of x with its bits in reversed order.
///|
fn reverse16(x : UInt) -> UInt {
  return rev8tab[((x & 0xff00) >> 8).reinterpret_as_int()] |
    (rev8tab[(x & 0xff).reinterpret_as_int()] << 8)
}

test "reverse16 works as expected" {
  let got = reverse16(32768)
  inspect!(got, content="1")
}

///|
let rev8tab : Array[UInt] = [
  0, 128, 64, 192, 32, 160, 96, 224, 16, 144, 80, 208, 48, 176, 112, 240, 8, 136,
  72, 200, 40, 168, 104, 232, 24, 152, 88, 216, 56, 184, 120, 248, 4, 132, 68, 196,
  36, 164, 100, 228, 20, 148, 84, 212, 52, 180, 116, 244, 12, 140, 76, 204, 44, 172,
  108, 236, 28, 156, 92, 220, 60, 188, 124, 252, 2, 130, 66, 194, 34, 162, 98, 226,
  18, 146, 82, 210, 50, 178, 114, 242, 10, 138, 74, 202, 42, 170, 106, 234, 26, 154,
  90, 218, 58, 186, 122, 250, 6, 134, 70, 198, 38, 166, 102, 230, 22, 150, 86, 214,
  54, 182, 118, 246, 14, 142, 78, 206, 46, 174, 110, 238, 30, 158, 94, 222, 62, 190,
  126, 254, 1, 129, 65, 193, 33, 161, 97, 225, 17, 145, 81, 209, 49, 177, 113, 241,
  9, 137, 73, 201, 41, 169, 105, 233, 25, 153, 89, 217, 57, 185, 121, 249, 5, 133,
  69, 197, 37, 165, 101, 229, 21, 149, 85, 213, 53, 181, 117, 245, 13, 141, 77, 205,
  45, 173, 109, 237, 29, 157, 93, 221, 61, 189, 125, 253, 3, 131, 67, 195, 35, 163,
  99, 227, 19, 147, 83, 211, 51, 179, 115, 243, 11, 139, 75, 203, 43, 171, 107, 235,
  27, 155, 91, 219, 59, 187, 123, 251, 7, 135, 71, 199, 39, 167, 103, 231, 23, 151,
  87, 215, 55, 183, 119, 247, 15, 143, 79, 207, 47, 175, 111, 239, 31, 159, 95, 223,
  63, 191, 127, 255,
]