[LUCENE-2933] Two-stage state expansion for the FST: distance-from-root and child-count criteria.

• Type: Improvement
• Status: Closed
• Priority: Trivial
• Resolution: Fixed
• Fix Version/s: 4.0-ALPHA
• Component/s: core/index
• Assignee: Dawid Weiss
• Reporter: Dawid Weiss
• Created: 21/Feb/11 08:49
• Updated: 28/Aug/22 12:41
• Resolved: 23/Feb/11 10:17

Description

In the current implementation FST states are expanded into a binary search on labels (from a vector of transitions) when the child count of a state exceeds a given predefined threshold (NUM_ARCS_FIXED_ARRAY). This threshold affects automaton size and traversal speed (as it turns out when benchmarked). For some degenerate data sets, close-to-the-root nodes could have a small number of children (below the threshold) and yet be traversed on every single seek.

A fix of this is to introduce two control thresholds:
EXPAND state if (distance-from-root <= MIN_DISTANCE || children-count >= NUM_ARCS_FIXED_ARRAY)

My plan is to create a data set that will prove this first and then to implement the workaround above.