81 lines
2.5 KiB
Markdown
81 lines
2.5 KiB
Markdown
# Array Library
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The array library offers easy allocation of large [memory mapped files](https://en.wikipedia.org/wiki/Memory-mapped_file)
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with less performance overhead than the traditional `buffers[i].get(j)`-style constructions
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java often leads to given its suffocating 2 Gb ByteBuffer size limitation.
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It accomplishes this by delegating block oerations down to the appropriate page. If the operation
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crosses a page boundary, it is not delegated and a bit slower.
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It's a very C++-style library that does unidiomatic things with interface default
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functions to get diamond inheritance.
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## Quick demo:
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```java
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var array = LongArray.mmapForWriting(Path.of("/tmp/test"), 1<<16);
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array.transformEach(50, 1000, (pos, val) -> Long.hashCode(pos));
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array.quickSort(50, 1000);
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if (array.binarySearch(array.get(100), 50, 1000) >= 0) {
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System.out.println("Nevermind, I found it!");
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}
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array.range(50, 1000).fill(0, 950, 1);
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array.forEach(0, 100, (pos, val) -> {
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System.out.println(pos + ":" + val);
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});
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```
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## Query Buffers
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The classes [IntQueryBuffer](src/main/java/nu/marginalia/array/buffer/IntQueryBuffer.java)
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and [LongQueryBuffer](src/main/java/nu/marginalia/array/buffer/LongQueryBuffer.java) are used
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heavily in the search engine's query processing.
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They are dual-pointer buffers that offer tools for filtering data.
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```java
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LongQueryBuffer buffer = new LongQueryBuffer(1000);
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// later ...
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// Prepare the buffer for filling
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buffer.reset();
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fillBufferSomehow(buffer);
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// length is updated and data is set
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// read pointer and write pointer is now at 0
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// A typical filtering operation may look like this:
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while (buffer.hasMore()) { // read < end
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if (someCondition(buffer.currentValue())) {
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// copy the value pointed to by the read
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// pointer to the read pointer, and
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// advance both
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buffer.retainAndAdvance();
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}
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else {
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// advance the read pointer
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buffer.rejectAndAdvance();
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}
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}
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// set end to the write pointer, and
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// resets the read and write pointers
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buffer.finalizeFiltering();
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// ... after this we can filter again, or
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// consume the data
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```
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Especially noteworthy are the operations `retain()` and `reject()` in
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[IntArraySearch](src/main/java/nu/marginalia/array/algo/IntArraySearch.java) and [LongArraySearch](src/main/java/nu/marginalia/array/algo/LongArraySearch.java).
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They keep or remove all items in the buffer that exist in the referenced range of the array,
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which must be sorted.
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These are used to offer an intersection operation for the B-Tree with sub-linear run time.
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