faster uint8,int8,uint16,int16 countmap

fast float16.jl

const FLOAT16_UNIQS = reinterpret.(Float16, UInt16.(0:2^16-1));

function countmap1(x::Vector{Float16})
  arr = zeros(Int, 65536)

  for xi in x
    @inbounds arr[reinterpret(UInt16, xi)+1] += 1
  end

  res = Dict{Float16, Int}()
  for (i,arr1) in zip(FLOAT16_UNIQS, arr)
    isnan(i) || arr1 == 0 || @inbounds res[i] = arr1
  end
  res
end

faster_count_map_small_int.jl

function toindex(x::Int16)
  Int(x) + 32769
end

function toindex(x::Int8)
  Int(x) + 129
end

function toindex(x::T) where T <: Union{UInt8, UInt16}
  Int(x) + 1
end

function countmap1(x::Vector{T}) where T <: Union{UInt8,UInt16, Int8, Int16}
  arr = zeros(Int, 2^(sizeof(T)*8))

  for xi in x
    @inbounds arr[toindex(xi)] += 1
  end

  res = Dict{T, Int}()
  for (i,arr1) in zip(typemin(T):typemax(T),arr)
    arr1 == 0 || @inbounds res[i] = arr1
  end
  res
end

using BenchmarkTools, StatsBase
x_uint8 = rand(UInt8, 100_000_000);
@btime countmap1($x_uint8);
@btime countmap($x_uint8);

using BenchmarkTools, StatsBase
x_int8 = rand(Int8, 100_000_000);
@btime countmap1($x_int8);
@btime countmap($x_int8);

using BenchmarkTools, StatsBase
x_int16 = rand(Int16, 100_000_000);
@btime countmap1($x_int16);
@btime countmap($x_int16);

using BenchmarkTools, StatsBase
x_uint16 = rand(UInt16, 100_000_000);
@btime countmap1($x_uint16);
@btime countmap($x_uint16);