counter_simd.nim

proc counter*[T: byte](a: openArray[T]): Counter[T] =
  let mask0 = set1_epi8(0)
  let mask1 = set1_epi8(1)
  let mask2 = set1_epi8(2)
  let mask3 = set1_epi8(3)
  var i = 0
  while i < a.len-avx.width:
    let ymm = loadu_byte(unsafeAddr a[i])
    if 0 < popcnt_u32 movemask_epi8 cmpgt_epi8(ymm, mask3):
      unroll for off in 0..<(avx.width):
        result.flat256[extract_epi8(ymm, off)].inc
    else:
      result.flat256[0] += popcnt_u32 movemask_epi8 cmpeq_epi8(ymm, mask0)
      result.flat256[1] += popcnt_u32 movemask_epi8 cmpeq_epi8(ymm, mask1)
      result.flat256[2] += popcnt_u32 movemask_epi8 cmpeq_epi8(ymm, mask2)
      result.flat256[3] += popcnt_u32 movemask_epi8 cmpeq_epi8(ymm, mask3)
    i += avx.width
  for i in i..<a.len:
    result.flat256[a[i]].inc