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write chunk format specification

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Quinn
2025-04-15 22:03:22 +02:00
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docs/mc-data-spec.md
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@@ -49,7 +49,47 @@ return (x >> 5) - (x & 0x80000000);
### chunk format specification
Chunks are stored as NBT-formatted data.
The actual structure of this is best illustrated by [the wiki](https://minecraft.wiki/w/Chunk_format#NBT_structure).
#### notable tags
| path | tag type | description |
|:-------------------------------------------|:-------------|-------------------------------------------------------------------------------|
| `DataVersion` | `int` | version of the NBT structure |
| `xPos` | `int` | X chunk position of the chunk from the world origin |
| `zPos` | `int` | Z chunk position of the chunk from the world origin |
| `yPos` | `int` | lowest Y section position in the chunk |
| `InhabitedTime` | `long` | the accumilative number of ticks that players have been present in this chunk |
| `sections` | `list` | list of compounds containing the chunk sections |
| `sections[i].Y` | `byte/int` | the Y block position of this section (can be int in versions ≥`1.18` |
| `sections[i].block_states.palette` | `list` | contains the block states for this particular section |
| `sections[i].block_states.palette[i].Name` | `string` | block [resource location](https://minecraft.wiki/w/Resource_location) |
| `sections[i].block_states.data` | `long array` | [view below](#block-state-data) |
##### block state data
Contains 4096 indices, which are packed in a specific way.
All indices are the same with, though the width is decided by the minimum width required to index the palette. In versions ≥`1.16`, the indices are not packed across multiple elements of the array.
So they have an alignment requirement of 64 bits.
There might be an additional section at the top and or bottom of the world used to store light, so that light travels properly over and under the world limits.
This is an example on how to access individual block info from a single section;
```c
unsigned w = 0;
int32_t tmp = block_states->palette.len;
while (tmp >>= 1) w++; // get the width of the palette length
// get the data for calculating the index (NOTE: this process is only applicable for mc versions <1.16)
unsigned pos = (y << 8) + (z << 4) + x; // acquire the position in the data array according to; 16²•y + 16•z + x
unsigned bit = pos * w; // calc which bit the value starts at.
unsigned sgmt = bit / 64; // calc which segment the data lives in
unsigned offs = bit & 63; // calc the offset within the segment
// calculate the correct index
uint16_t idx;
idx = (block_states->dat[sgmt] >> offs); // acquire the data in the segment
idx |= !((offs + w) > 64) ? 0 : block_states->dat[sgmt+1] << (64-offs); // complete with the data from the other segment, if present
idx &= (1 << w) - 1; // truncate the data to only contain what we desire.
blockdat blk = block_states->palette.dat[idx];
```
### MCR format specification
#### header