Package it.unimi.dsi.util

Class ImmutableExternalPrefixMap

java.lang.Object
it.unimi.dsi.fastutil.objects.AbstractObject2LongFunction<CharSequence>
it.unimi.dsi.util.AbstractPrefixMap
it.unimi.dsi.util.ImmutableExternalPrefixMap
All Implemented Interfaces:
Function<CharSequence,​Long>, Object2LongFunction<CharSequence>, PrefixMap<MutableString>, StringMap<MutableString>, Serializable, Function<CharSequence,​Long>, ToLongFunction<CharSequence>
public class ImmutableExternalPrefixMap
extends AbstractPrefixMap
implements Serializable
An immutable prefix map mostly stored in external memory.

Instances of this class write on a dump stream the string in the domain of the prefix map. More precisely, the dump stream is formed by blocks, and each block (with user-definable length, possibly the size of a basic disk I/O operation) is filled as much as possible with strings front coded and compressed with a HuTuckerCodec. Each block starts with the length of the first string in unary, followed by the encoding of the string. Then, for each string we write in unary the length of the common prefix (in characters) with the previous string, the length of the remaining suffix (in characters) and finally the encoded suffix. Note that if the encoding of a string is longer than a block, the string will occupy more than one block.

We keep track using an ImmutableBinaryTrie of the strings at the start of each block in Hu–Tucker coding: thus, we are able to retrieve the interval corresponding to a given prefix by calling getApproximatedInterval() and scanning at most two blocks.

Self-contained or non-self-contained

There are two kinds of external prefix maps: self-contained and non-self-contained. In the first case, you get a serialised object that you can load at any time. The dump stream is serialised with the object and expanded at each deserialisation in the Java temporary directory. If you deserialise a map several times, you will get correspondingly many copies of the dump stream in the temporary directory. The dump streams are deleted when the JVM exits. This mechanism is not very efficient, but since this class implements several interfaces it is essential that clients can make things work in a standard way.

Alternatively, you can give at creation time a filename for the dump stream. The resulting non-self-contained external prefix map can be serialised, but after deserialisation you need to set the dump stream filename or even directly the dump stream (for instance, to an input bit stream wrapping a byte array where the dump stream has been loaded). You can deserialise many copies of an external prefix map, letting all copies share the same dump stream.

This data structure is not synchronised, and concurrent reads may cause problems because of clashes in the usage of the underlying input bit stream. It would not be a good idea in any case to open a new stream for each caller, as that would certainly lead to disk thrashing.

The main method of this class helps in building large external prefix maps.

Since:
0.9.3
Author:
Sebastiano Vigna
See Also:
Serialized Form