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Clean up position encoding

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We can simply add 0x800800800 to the encoding, then use bit masking.

This works because adding 0x800 maps -2048:2047 to 0x000:0xFFF.
And 0x800800800 is (0x800 << 24 + 0x800 << 12 + 0x800) for x,y,z.

After bitmasking, -0x800 restores the original value range.
This commit is contained in:
Erich Schubert 2025-02-25 23:11:43 +01:00 • committed by sfan5
parent c439d784ac
commit 1f3cf59c7f
1 changed files with 11 additions and 36 deletions
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47
src/database/database.cpp
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@ -7,48 +7,23 @@
/****************
* Black magic! *
****************
* The position hashing is very messed up.
* It's a lot more complicated than it looks.
* The position encoding is a bit messed up because negative
* values were not taken into account.
* But this also maps 0,0,0 to 0, which is nice, and we mostly
* need forward encoding in Luanti.
*/
static inline s16 unsigned_to_signed(u16 i, u16 max_positive)
{
if (i < max_positive) {
return i;
}
return i - (max_positive * 2);
}
// Modulo of a negative number does not work consistently in C
static inline s64 pythonmodulo(s64 i, s16 mod)
{
if (i >= 0) {
return i % mod;
}
return mod - ((-i) % mod);
}
s64 MapDatabase::getBlockAsInteger(const v3s16 &pos)
{
return (u64) pos.Z * 0x1000000 +
(u64) pos.Y * 0x1000 +
(u64) pos.X;
return ((s64) pos.Z << 24) + ((s64) pos.Y << 12) + pos.X;
}
v3s16 MapDatabase::getIntegerAsBlock(s64 i)
{
v3s16 pos;
pos.X = unsigned_to_signed(pythonmodulo(i, 4096), 2048);
i = (i - pos.X) / 4096;
pos.Y = unsigned_to_signed(pythonmodulo(i, 4096), 2048);
i = (i - pos.Y) / 4096;
pos.Z = unsigned_to_signed(pythonmodulo(i, 4096), 2048);
return pos;
// Offset so that all negative coordinates become non-negative
i = i + 0x800800800;
// Which is now easier to decode using simple bit masks:
return { (s16)( (i & 0xFFF) - 0x800),
(s16)(((i >> 12) & 0xFFF) - 0x800),
(s16)(((i >> 24) & 0xFFF) - 0x800) };
}