CRC16 ccitt 1021 - Part 2

[Russell Haley <russ.haley@...>]

Hi,

I have a working C hash function that I turned into a Lua module:

int16_t crc_1021_c(int16_t old_crc, int8_t data)
{
    int16_t crc;
    int16_t x;

    x = ((old_crc>>8) ^ data) & 0xff;
    x ^= x>>4;
    crc = (old_crc << 8) ^ (x << 12) ^ (x << 5) ^ x;

    crc &= 0xffff; //disable this on 16 bit processors
    return crc;
}

I run it in a script like this:

local sf = require 'libsf'

print('Lua Calling C hash:')
local crc = 0xffff
local data = "" 0x00, 0x00, 0x0d, 0x00}
for i = 1, 8,1 do
--~ print(data[i])
    crc = sf.crc1021(crc, data[i])
end

I happened to be looking at the Lua 5.3 Bitwise Operators reference and the C code looked like I could convert it to Lua, but I get a different number when running the same data through the different codes. My attempted Lua conversion of the C hash:

function ccitt_16_c(byte_array)

    local function hash(orig_crc, byte)
        local crc
        local x;

        x = ((orig_crc >> 8) ~ byte) & 0xff;
        x = x ~ (x >> 4);
        crc = (orig_crc << 8) ~ (x << 12) ~ (x << 5) ~ x;

        crc = crc & 0xffff;
        return crc
    end

    local crc = 0xffff
    for i in ipairs(byte_array) do
        crc = hash(crc, byte_array[i])
    end

    return crc & 0xffff
end

I get a much different number from the two code bases even though they are run on the same table. Here is my test script:

test-crc.lua:

local sf = require 'libsf'
local lcrc = require 'crc1021'

print('Lua Calling C hash:')
local crc = 0xffff
local data = "" 0x00, 0x00, 0x0d, 0x00}
for i = 1, 8,1 do
--~ print(data[i])
crc = sf.crc1021(crc, data[i])
end

print(crc)

local pl = string.pack('<I<I',0x000a0000,0x000d0000)
print('C on packed string')
local crc = sf.crc16(pl)
print(crc)

print('Lua Clark Li:')
print('Disabled, my variant didn\'t work')
--~ print(lcrc.ccitt_16_l(data))

print('Lua converted C:')
print(lcrc.ccitt_16_c(data))

Results: 

C:\Users\russh\projects\NLuaSerialScripts> lua .\test-crc.lua
Lua Calling C hash:
-16032.0
C on packed string
-16032.0
Lua Clark Li:
Disabled, my variant didn't work
Lua converted C:
49504

Any ideas why I get a different number? All input greatly appreciated.

Russ