On Mon, 5 Nov 2018 at 01:26, Gerald Pfeifer gerald@pfeifer.com wrote:
Hi Henri, hi everyone,
earlier this year via
commit 2f6fbdec8caee311e2fb85b2ddc9fbd8a9afaea5 Author: H. Verbeet hverbeet@gmail.com Date: Sat May 24 10:33:36 2008 +0200
d3d8: Test our texop implementation.we go the following function in dlls/d3d8/tests/visual.c:
static BOOL color_match(D3DCOLOR c1, D3DCOLOR c2, BYTE max_diff) { if (abs((c1 & 0xff) - (c2 & 0xff)) > max_diff) return FALSE; c1 >>= 8; c2 >>= 8; if (abs((c1 & 0xff) - (c2 & 0xff)) > max_diff) return FALSE; c1 >>= 8; c2 >>= 8; if (abs((c1 & 0xff) - (c2 & 0xff)) > max_diff) return FALSE; c1 >>= 8; c2 >>= 8; if (abs((c1 & 0xff) - (c2 & 0xff)) > max_diff) return FALSE; return TRUE; }
Intuitively it's clear what this does, however there's a little challenge: c1 and c2 are unsigned integers (D3DCOLOR being DWORD being unsigned long), and so are c1 & 0xff and c2 & 0xff.
Substraction of two unsigned integer types is done in unsigned math, so we may be seeing wrapping, but never a negative number as a result -- and abs() is a noop.
As Ken pointed out, the code has been in the tree a little longer than this year. As for the issue you point out, it's true that the function assumes sizeof(DWORD) == sizeof(int), and a two's complement system. I.e., in two's complement 0u - 1u == ~0u == -1, etc. I don't think those are unreasonable assumptions. Perhaps the function has room for improvement, but I'd prefer to not simply sprinkle some casts on top.