include 'win32a.inc'
  format PE GUI 4.0

section '.text' code readable executable

    mov                 ebx,crc32_table ; base address
    xor                 ecx,ecx ; index used to update table entries
begin:
    mov                 eax,ecx
    and                 edx,0 ; inner loop counter
inner_loop:
    cmp                 dl,8
    jz                  update_table
    test                al,1 ; check for odd number
    jz                  shr_and_loop_back
    shr                 eax,1
    xor                 eax,0EDB88320h ; reversed (lil endian)
    inc                 dl
    jmp                 inner_loop
shr_and_loop_back:
    shr                 eax,1
    inc                 dl
    jmp                 inner_loop
update_table:
    mov                 [ebx+ecx*4],eax
    inc                 ecx
    or                  ch,ch
    jz                  begin
finish: ; as of here, we have full table ... the rest of the code is clear i think
    xor                 edx,edx
    and                 eax,edx
    not                 edx
    mov                 ecx,[input_length]
    mov                 esi,input_data
calc_crc32_checksum:
    lodsb
    xor                 al,dl
    movzx               edi,al
    mov                 eax,[ebx+edi*4]
    shr                 edx,8
    xor                 edx,eax
    loop                calc_crc32_checksum
    not                 edx
    push                10h
    push                crc32_checksum
    push                edx
    call                [itoa]
    add                 esp,0Ch
    push                0
    push                eax
    push                0
    push                0
    call                [MessageBox]
    sub                 eax,eax
    ret
    int3

section '.data' data readable writeable

  crc32_table rd 400h
  input_length dd 4
  input_data db 'fasm',0
  crc32_checksum rb 9


section '.idata' import data readable

  library user32,'user32.dll',\
          ntdll,'ntdll.dll'

  import user32,\
         MessageBox,'MessageBoxA'

  import ntdll,\
         itoa,'_itoa'

section '.reloc' fixups data readable discardable
    

; --- CRC-32c ---
            ; IN: esi = buffer, ecx = length
            ; OUT: edx = crc
crc32_calc: xor         edx, edx
            xor         eax, eax
            xor         ebx, ebx
.next:      lodsb
            mov         bl, dl
            xor         bl, al
            mov         eax, edx
            shr         edx, 8
            xor         edx, dword [crclkp+4*ebx]
            dec         ecx
            jnz         .next
            ret

crclkp:     dd          000000000h, 0F26B8303h, 0E13B70F7h, 01350F3F4h, 0C79A971Fh, 035F1141Ch, 026A1E7E8h, 0D4CA64EBh
            dd          08AD958CFh, 078B2DBCCh, 06BE22838h, 09989AB3Bh, 04D43CFD0h, 0BF284CD3h, 0AC78BF27h, 05E133C24h
            dd          0105EC76Fh, 0E235446Ch, 0F165B798h, 0030E349Bh, 0D7C45070h, 025AFD373h, 036FF2087h, 0C494A384h
            dd          09A879FA0h, 068EC1CA3h, 07BBCEF57h, 089D76C54h, 05D1D08BFh, 0AF768BBCh, 0BC267848h, 04E4DFB4Bh
            dd          020BD8EDEh, 0D2D60DDDh, 0C186FE29h, 033ED7D2Ah, 0E72719C1h, 0154C9AC2h, 0061C6936h, 0F477EA35h
            dd          0AA64D611h, 0580F5512h, 04B5FA6E6h, 0B93425E5h, 06DFE410Eh, 09F95C20Dh, 08CC531F9h, 07EAEB2FAh
            dd          030E349B1h, 0C288CAB2h, 0D1D83946h, 023B3BA45h, 0F779DEAEh, 005125DADh, 01642AE59h, 0E4292D5Ah
            dd          0BA3A117Eh, 04851927Dh, 05B016189h, 0A96AE28Ah, 07DA08661h, 08FCB0562h, 09C9BF696h, 06EF07595h
            dd          0417B1DBCh, 0B3109EBFh, 0A0406D4Bh, 0522BEE48h, 086E18AA3h, 0748A09A0h, 067DAFA54h, 095B17957h
            dd          0CBA24573h, 039C9C670h, 02A993584h, 0D8F2B687h, 00C38D26Ch, 0FE53516Fh, 0ED03A29Bh, 01F682198h
            dd          05125DAD3h, 0A34E59D0h, 0B01EAA24h, 042752927h, 096BF4DCCh, 064D4CECFh, 077843D3Bh, 085EFBE38h
            dd          0DBFC821Ch, 02997011Fh, 03AC7F2EBh, 0C8AC71E8h, 01C661503h, 0EE0D9600h, 0FD5D65F4h, 00F36E6F7h
            dd          061C69362h, 093AD1061h, 080FDE395h, 072966096h, 0A65C047Dh, 05437877Eh, 04767748Ah, 0B50CF789h
            dd          0EB1FCBADh, 0197448AEh, 00A24BB5Ah, 0F84F3859h, 02C855CB2h, 0DEEEDFB1h, 0CDBE2C45h, 03FD5AF46h
            dd          07198540Dh, 083F3D70Eh, 090A324FAh, 062C8A7F9h, 0B602C312h, 044694011h, 05739B3E5h, 0A55230E6h
            dd          0FB410CC2h, 0092A8FC1h, 01A7A7C35h, 0E811FF36h, 03CDB9BDDh, 0CEB018DEh, 0DDE0EB2Ah, 02F8B6829h
            dd          082F63B78h, 0709DB87Bh, 063CD4B8Fh, 091A6C88Ch, 0456CAC67h, 0B7072F64h, 0A457DC90h, 0563C5F93h
            dd          0082F63B7h, 0FA44E0B4h, 0E9141340h, 01B7F9043h, 0CFB5F4A8h, 03DDE77ABh, 02E8E845Fh, 0DCE5075Ch
            dd          092A8FC17h, 060C37F14h, 073938CE0h, 081F80FE3h, 055326B08h, 0A759E80Bh, 0B4091BFFh, 0466298FCh
            dd          01871A4D8h, 0EA1A27DBh, 0F94AD42Fh, 00B21572Ch, 0DFEB33C7h, 02D80B0C4h, 03ED04330h, 0CCBBC033h
            dd          0A24BB5A6h, 0502036A5h, 04370C551h, 0B11B4652h, 065D122B9h, 097BAA1BAh, 084EA524Eh, 07681D14Dh
            dd          02892ED69h, 0DAF96E6Ah, 0C9A99D9Eh, 03BC21E9Dh, 0EF087A76h, 01D63F975h, 00E330A81h, 0FC588982h
            dd          0B21572C9h, 0407EF1CAh, 0532E023Eh, 0A145813Dh, 0758FE5D6h, 087E466D5h, 094B49521h, 066DF1622h
            dd          038CC2A06h, 0CAA7A905h, 0D9F75AF1h, 02B9CD9F2h, 0FF56BD19h, 00D3D3E1Ah, 01E6DCDEEh, 0EC064EEDh
            dd          0C38D26C4h, 031E6A5C7h, 022B65633h, 0D0DDD530h, 00417B1DBh, 0F67C32D8h, 0E52CC12Ch, 01747422Fh
            dd          049547E0Bh, 0BB3FFD08h, 0A86F0EFCh, 05A048DFFh, 08ECEE914h, 07CA56A17h, 06FF599E3h, 09D9E1AE0h
            dd          0D3D3E1ABh, 021B862A8h, 032E8915Ch, 0C083125Fh, 0144976B4h, 0E622F5B7h, 0F5720643h, 007198540h
            dd          0590AB964h, 0AB613A67h, 0B831C993h, 04A5A4A90h, 09E902E7Bh, 06CFBAD78h, 07FAB5E8Ch, 08DC0DD8Fh
            dd          0E330A81Ah, 0115B2B19h, 0020BD8EDh, 0F0605BEEh, 024AA3F05h, 0D6C1BC06h, 0C5914FF2h, 037FACCF1h
            dd          069E9F0D5h, 09B8273D6h, 088D28022h, 07AB90321h, 0AE7367CAh, 05C18E4C9h, 04F48173Dh, 0BD23943Eh
            dd          0F36E6F75h, 00105EC76h, 012551F82h, 0E03E9C81h, 034F4F86Ah, 0C69F7B69h, 0D5CF889Dh, 027A40B9Eh
            dd          079B737BAh, 08BDCB4B9h, 0988C474Dh, 06AE7C44Eh, 0BE2DA0A5h, 04C4623A6h, 05F16D052h, 0AD7D5351h