126 lines
3.1 KiB
Lua
126 lines
3.1 KiB
Lua
-- Localize globals
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local assert, math_huge, math_frexp, math_floor
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= assert, math.huge, math.frexp, math.floor
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-- Set environment
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local _ENV = {}
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setfenv(1, _ENV)
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-- All little endian
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--+ Reads doubles (f64) or floats (f32)
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--: double reads an f64 if true, f32 otherwise
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function read_float(read_byte, double)
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-- First read the mantissa
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local mantissa = 0
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for _ = 1, double and 6 or 2 do
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mantissa = (mantissa + read_byte()) / 0x100
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end
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-- Second and first byte in big endian: last bit of exponent + 7 bits of mantissa, sign bit + 7 bits of exponent
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local byte_2, byte_1 = read_byte(), read_byte()
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local sign = 1
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if byte_1 >= 0x80 then
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sign = -1
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byte_1 = byte_1 - 0x80
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end
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local exponent = byte_1 * 2
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if byte_2 >= 0x80 then
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exponent = exponent + 1
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byte_2 = byte_2 - 0x80
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end
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mantissa = (mantissa + byte_2) / 0x80
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if exponent == 0xFF then
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if mantissa == 0 then
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return sign * math_huge
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end
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-- Differentiating quiet and signalling nan is not possible in Lua, hence we don't have to do it
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-- HACK ((0/0)^1) yields nan, 0/0 yields -nan
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return sign == 1 and ((0/0)^1) or 0/0
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end
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assert(mantissa < 1)
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if exponent == 0 then
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-- subnormal value
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return sign * 2^-126 * mantissa
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end
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return sign * 2 ^ (exponent - 127) * (1 + mantissa)
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end
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--+ Reads a single floating point number (f32)
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function read_single(read_byte)
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return read_float(read_byte)
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end
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--+ Reads a double (f64)
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function read_double(read_byte)
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return read_float(read_byte, true)
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end
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function read_uint(read_byte, bytes)
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local factor = 1
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local uint = 0
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for _ = 1, bytes do
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uint = uint + read_byte() * factor
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factor = factor * 0x100
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end
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return uint
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end
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function write_uint(write_byte, uint, bytes)
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for _ = 1, bytes do
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write_byte(uint % 0x100)
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uint = math_floor(uint / 0x100)
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end
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assert(uint == 0)
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end
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--: on_write function(double)
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--: double set to true to force f64, false for f32, nil for auto
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function write_float(write_byte, number, on_write, double)
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local sign = 0
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if number < 0 then
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number = -number
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sign = 0x80
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end
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local mantissa, exponent = math_frexp(number)
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exponent = exponent + 127
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if exponent > 1 then
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-- TODO ensure this deals properly with subnormal numbers
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mantissa = mantissa * 2 - 1
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exponent = exponent - 1
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end
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local sign_byte = sign + math_floor(exponent / 2)
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mantissa = mantissa * 0x80
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local exponent_byte = (exponent % 2) * 0x80 + math_floor(mantissa)
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mantissa = mantissa % 1
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local mantissa_bytes = {}
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-- TODO ensure this check is proper
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if double == nil then
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double = mantissa % 2^-23 > 0
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end
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if on_write then
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on_write(double)
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end
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local len = double and 6 or 2
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for index = len, 1, -1 do
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mantissa = mantissa * 0x100
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mantissa_bytes[index] = math_floor(mantissa)
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mantissa = mantissa % 1
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end
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assert(mantissa == 0)
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for index = 1, len do
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write_byte(mantissa_bytes[index])
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end
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write_byte(exponent_byte)
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write_byte(sign_byte)
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end
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function write_single(write_byte, number)
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return write_float(write_byte, number, nil, false)
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end
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function write_double(write_byte, number)
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return write_float(write_byte, number, nil, true)
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end
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-- Export environment
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return _ENV |