(Windows 7) Solitaire practical joke, part II


Now take a look on the first part of the loop:

.text:0000000100036684 loc_100036684:                          ; CODE XREF: SolitaireGame::InitialDeal(void)+C0↓j
.text:0000000100036684                 mov     eax, 4EC4EC4Fh
.text:0000000100036689                 mul     edi
.text:000000010003668B                 mov     r8d, edx
.text:000000010003668E                 shr     r8d, 4          ; unsigned int
.text:0000000100036692                 mov     eax, r8d
.text:0000000100036695                 imul    eax, 52
.text:0000000100036698                 mov     edx, edi
.text:000000010003669A                 sub     edx, eax        ; unsigned int
.text:000000010003669C                 mov     rcx, [rbx+128h] ; this
.text:00000001000366A3                 call    ?CreateCard@CardTable@@IEAAPEAVCard@@II@Z ; CardTable::CreateCard(uint,uint)
.text:00000001000366A8                 mov     rdx, rax        ; struct Card *
.text:00000001000366AB                 mov     rcx, rbx        ; this
.text:00000001000366AE                 call    ?Push@CardStack@@QEAAXPEAVCard@@@Z ; CardStack::Push(Card *)
.text:00000001000366B3                 inc     edi
.text:00000001000366B5                 cmp     edi, 52
.text:00000001000366B8                 jb      short loc_100036684

What is with multiplication by 4EC4EC4Fh? Surely, this is division by multiplication. And what Hex-Rays can say:

  v5 = 0;
    v6 = CardTable::CreateCard(v4[37], v5 % 0x34, v5 / 0x34);
    CardStack::Push((CardStack *)v4, v6);
  while ( v5 < 0x34 );

Somehow, CreateCard() functions takes two arguments: iterator divided by 52 and a remainder of the division operation. Hard to say, why they did so. Solitaire can't allow more than 52 cards, so the last argument is senseless, it's always zero.

But when I patch "cmp edi, 52" instruction at 0x1000366B5 to be "cmp edi, 53", I found that there are now 53 cards. The last one is "two of clubs", because it's numbered as 0th card.

During the last iteration, 0x52 is divided by 0x52, remainder is zero, so 0th card is added twice.

What a frustration, there are two "two of clubs":

This is patched Windows 7 Solitare: https://yurichev.com/blog/solitaire2/Solitaire53.exe.

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Please drop me email about any bug(s) and suggestion(s): dennis(@)yurichev.com.