local char, byte, floor, band, rshift = string.char, string.byte, math.floor, bit32.band, bit32.arshift


local PREC = 10
local PREC_POW = 2 ^ PREC
local PREC_POW_HALF = 2 ^ (PREC - 1)
local STRENGTH_MIN = 2 ^ (PREC - 8 + 1)

local function make_predictor()
    local charge, strength, previous_bit = 0, 0, false

    return function(current_bit)
        local target = current_bit and 127 or -128

        local next_charge = charge + floor((strength * (target - charge) + PREC_POW_HALF) / PREC_POW)
        if next_charge == charge and next_charge ~= target then
            next_charge = next_charge + (current_bit and 1 or -1)
        end

        local z = current_bit == previous_bit and PREC_POW - 1 or 0
        local next_strength = strength
        if next_strength ~= z then next_strength = next_strength + (current_bit == previous_bit and 1 or -1) end
        if next_strength < STRENGTH_MIN then next_strength = STRENGTH_MIN end

        charge, strength, previous_bit = next_charge, next_strength, current_bit
        return charge
    end
end

local function make_dec()
    local predictor = make_predictor()
    local low_pass_charge = 0
    local previous_charge, previous_bit = 0, false

    return function (input)

        local output, output_n = {}, 0
        for i = 1, #input do
            local input_byte = byte(input, i)
            for _ = 1, 8 do
                local current_bit = band(input_byte, 1) ~= 0
                local charge = predictor(current_bit)

                local antijerk = charge
                if current_bit ~= previous_bit then
                    antijerk = floor((charge + previous_charge + 1) / 2)
                end

                previous_charge, previous_bit = charge, current_bit

                low_pass_charge = low_pass_charge + floor(((antijerk - low_pass_charge) * 140 + 0x80) / 256)

                output_n = output_n + 1
                output[output_n] = low_pass_charge

                input_byte = rshift(input_byte, 1)
            end
        end

        return output
    end
end



local function make_truebit_dec(max_amp)
    max_amp = max_amp or 127

    return function (input)
        local output, output_n = {}, 0
        for i = 1, #input do
            local input_byte = byte(input, i)
            for _ = 1, 8 do
                local bit_is_1 = band(input_byte, 1) ~= 0
                output_n = output_n + 1
                output[output_n] = bit_is_1 and max_amp or -max_amp
                input_byte = rshift(input_byte, 1)
            end
        end
        return output
    end
end

return {
    make_dec = make_dec,
    make_truebit_dec = make_truebit_dec
}