47 lines
1.8 KiB
Markdown
47 lines
1.8 KiB
Markdown
# BTree
|
|
|
|
This package contains a small library for creating and reading a static b-tree in as implicit pointer-less datastructure.
|
|
Both binary indices (i.e. sets) are supported, as well as arbitrary multiple-of-keysize key-value mappings where the data is
|
|
interlaced with the keys in the leaf nodes. This is a fairly low-level datastructure.
|
|
|
|
The b-trees are specified through a [BTreeContext](src/main/java/nu/marginalia/btree/model/BTreeContext.java)
|
|
which contains information about the data and index layout.
|
|
|
|
The b-trees are written through a [BTreeWriter](src/main/java/nu/marginalia/btree/BTreeWriter.java) and
|
|
read with a [BTreeReader](src/main/java/nu/marginalia/btree/BTreeReader.java).
|
|
|
|
## Demo
|
|
|
|
```java
|
|
BTreeContext ctx = new BTreeContext(
|
|
4, // num layers max
|
|
1, // entry size, 1 = the leaf node has just just the key
|
|
BTreeBlockSize.BS_4096); // page size
|
|
|
|
// Allocate a memory area to work in, see the array library for how to do this with files
|
|
LongArray array = LongArray.allocate(8192);
|
|
|
|
// Write a btree at offset 123 in the area
|
|
long[] items = new long[400];
|
|
BTreeWriter writer = new BTreeWriter(array, ctx);
|
|
final int offsetInFile = 123;
|
|
|
|
long btreeSize = writer.write(offsetInFile, items.length, slice -> {
|
|
// here we *must* write items.length * entry.size words in slice
|
|
// these items must be sorted!!
|
|
|
|
for (int i = 0; i < items.length; i++) {
|
|
slice.set(i, items[i]);
|
|
}
|
|
});
|
|
|
|
// Read the BTree
|
|
|
|
BTreeReader reader = new BTreeReader(array, ctx, offsetInFile);
|
|
reader.findEntry(items[0]);
|
|
```
|
|
|
|
## Useful Resources
|
|
|
|
Youtube: [Abdul Bari, 10.2 B Trees and B+ Trees. How they are useful in Databases](https://www.youtube.com/watch?v=aZjYr87r1b8). This isn't exactly the design implemented in this library, but very well presented and a good refresher.
|