Compiler metadata

[dungba88]

Description

Make FSTCompiler.compile() to only return the FSTMetadata. Depending on whether the DataOutput used implements FSTReader or not, the returned FST might be unreadable. Thus having this method only return the FSTMetadata means we will never return an unusable FST.

The

Now to create the FST there will be 2 ways:

1. If a FSTReader DataOutput is used, we can create the FST with:

var fstMetadata = fstCompiler.compile();
var fst = FST.fromFSTReader(fstMetadata, fstCompiler.getFSTReader());

2. If a non-FSTReader DataOutput is used, we need to first create the corresponding DataInput, then call:

var fstMetadata = fstCompiler.compile();
if (fstMetadata == null) {
    return null; // FST is empty
}

// Create the DataInput
var dataInput = ...

var fst = new FST<>(fstMetadata, dataInput);

Note that this PR depends on 2 others to be merged first:

• Allow FST builder to use different writer (#12543)  apache/lucene#12624
• Remove FST constructors with DataInput for metadata apache/lucene#12803

FST.fromFSTReader will also handle the case where fstMetadata is null, in which case it would return null. It is essentially a sugar syntactic utility to make the migration easier. So we will change this code:

var fst = fstCompiler.compile();

to this:

var fst = FST.fromFSTReader(fstCompiler.compile(), fstCompiler.getFSTReader());

instead of:

var fstMetadata = fstCompiler.compile();
if (fstMetadata == null) {
    return;
}
var fst = new FST<>(fstMetadata, fstCompiler.getFSTReader());

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[github-actions]

This PR has not had activity in the past 2 weeks, labeling it as stale. If the PR is waiting for review, notify the [email protected] list. Thank you for your contribution!