# Package org.apache.datasketches.memory

package org.apache.datasketches.memory

This package provides high performance primitive and primitive array access to direct (native), off-heap memory and memory-mapped file resources, and consistent views into `ByteBuffer`, and on-heap primitive arrays. It can be used as a more comprehensive and flexible replacement for `ByteBuffer`.

In addition, this package provides:

- Two different access APIs: read-only `Memory` and `WritableMemory` for absolute offset access, and read-only `Buffer` and `WritableBuffer` for relative positional access (similar to ByteBuffer).
- Clean separation of Read-only API from Writable API, which makes writable versus read-only resources detectable at compile time.
- The conversion from Writable to read-only is just a cast, so no unnecessary objects are created. For example:

  >          WritableMemory wMem = ...
  >          Memory mem = wMem;

- `AutoCloseable` for the external resources that require it, which enables compile-time checks for non-closed resources.
- Immediate invalidation of all downstream references of an AutoCloseable resource when that resource is closed, either manually or by the JVM. This virtually eliminates the possibility of accidentally writing into the memory space previously owned by a closed resource.
- Improved performance over the prior Memory implementation.
- No external dependencies, which makes it simple to install in virtually any Java environment.

More specifically, this package provides access to four different types of resources using two different access APIs. These resources are contiguous blobs of bytes that provide at least byte-level read and write access. The four resources are:

- Direct (a.k.a. Native) off-heap memory allocated by the user.
- Memory-mapped files, both writable and read-only.
- `ByteBuffers`, both heap-based and direct, writable and read-only.
- Heap-based primitive arrays, which can be accessed as writable or read-only.

The two different access APIs are:

- Memory, WritableMemory: Absolute offset addressing into a resource.
- Buffer, WritableBuffer: Position relative addressing into a resource.

In addition, all combinations of access APIs and backing resources can be accessed via multibyte primitive methods (e.g. getLong(...), getLongArray(...), putLong(...), putLongArray(...)) as either `ByteOrder.BIG_ENDIAN` or `ByteOrder.LITTLE_ENDIAN`.

The resources don't know or care about the access APIs, and the access APIs don't really know or care what resource they are accessing.

An access API is joined with a resource with a static factory method.

Moving back and forth between Memory and Buffer:

>         Memory mem = ...
>         Buffer buf = mem.asBuffer();
>         ...
>         Memory mem2 = buf.asMemory();
>         ...

Hierarchical memory regions can be easily created:

>          WritableMemory wMem = ...
>          WritableMemory wReg = wMem.writableRegion(offset, length); //OR
>          Memory region = wMem.region(offset, length);

Author:
  Lee Rhodes

Class
Description
Buffer
Defines the read-only API for relative positional access to a resource.
BufferPositionInvariantsException
Position operation violation.
DefaultMemoryRequestServer
This example MemoryRequestServer is simple but demonstrates one of many ways to manage continuous requests for larger memory.
Memory
Defines the read-only API for offset access to a resource.
MemoryRequestServer
The MemoryRequestServer is a callback interface to provide a means to request more memory for heap and off-heap WritableMemory resources that are not file-memory-mapped backed resources.
MurmurHash3
The MurmurHash3 is a fast, non-cryptographic, 128-bit hash function that has excellent avalanche and 2-way bit independence properties.
Positional
Defines the relative positional API.
Resource
The base class for Memory and Buffer plus some common static variables and check methods.
WritableBuffer
Defines the writable API for relative positional access to a resource
WritableMemory
Defines the writable API for offset access to a resource.
XxHash
The XxHash is a fast, non-cryptographic, 64-bit hash function that has excellent avalanche and 2-way bit independence properties.