The Apache Lucene/Solr suggesters are important to Sease : we explored the topic in the past[1] and we strongly believe the autocomplete feature to be vital for a lot of search applications.
This blog post explores in details the current status of the Lucene BlendedInfixSuggester, some bugs of the most recent version ( with the solution attached) and some possible improvements.

BlendedInfixSuggester

The BlendedInfixSuggester is an extension of the AnalyzingInfixSuggester with the additional functionality to weight prefix matches of your query across the matched documents.
It scores higher if a hit is closer to the start of the suggestion.
N.B. at the current stage only the first term in your query will affect the suggestion score

[
      {
        "id":"44",
        "title":"Video gaming: the history"},
      {
        "id":"11",
        "title":"Nowadays Video games are a phenomenal economic business"},
      {
        "id":"55",
        "title":"The new generation of PC and Console Video games"},
      {
        "id":"33",
        "title":"Video games: multiplayer gaming"}]

Let’s explore the score of each Suggestion given various Blender Types :

The final score of the suggestion will be :

long score = (long) (weight * coefficient)

N.B. the reason I highlighted the data type is because it’s directly affecting the first bug we discuss.

Suggestion Score Approximation

The optional weightField parameter is extremely important for the Blended Infix Suggester.
It assigns the value of the suggestion weight ( extracted from the mentioned field).
e.g.
The suggestion may come from the product name field, but the suggestion weight depends on how profitable the product suggested is.

<str name=”field”>productName</str>
<str name=”weightField”>profit</str>

So far, so good, but unfortunately there are two problems with that.

Bug 1 – WeightField Not Defined -> Zero suggestion score

How To Reproduce It : Don’t define any weightField in the suggester config
Effect : the suggestion ranking is lost, all the suggestions have 0 score, position of the match doesn’t matter anymore
The weightField is not a mandatory configuration for the BlendedInfixSuggester.
Your use case could not involve any weight for your suggestions and you are just interested in the positional scoring (the main reason the BlendedInfixSuggester exists in the first place).
Unfortunately, this is not possible at the moment :
If the weightField is not defined, each suggestion will have a weight of 0.
This is because the weight associated to each document in the document dictionary is a long. If the field to extract the weight from, is not defined (null), the weight returned will just be 0.
This doesn’t allow to differentiate when a weight should be 0 ( value extracted from the field) or null ( no value at all ).
A solution has been proposed here[3].

Bug 2 – Bad Approximation Of Suggestion Score For Small Weights

There is a misleading data type casting in the score calculation for the suggestion :

long score = (long) (weight * coefficient)

This apparently innocent cast, actually brings very nasty effects if the weight associated to a suggestion is unitary or small enough.

Weight =1
Video gaming: the history
1-0.1*position = 0.9 * 1 =cast= 0
1/(1+position) = 1/2 = 0.5 * 1 =cast= 0
1/(1+position)^2 = 1/4 = 0.25 * 1 =cast= 0

Weight =2
Video gaming: the history
1-0.1*position = 0.9 * 2=cast= 1
1/(1+position) = 1/2 = 0.5 * 2=cast= 1
1/(1+position)^2 = 1/4 = 0.25 * 2=cast= 0

Basically you risk to lose the ranking of your suggestions reducing the score to only few possible values : 0 or 1 ( in edge cases)

A solution has been proposed here[3]

Multi Term Matches Handling

It is quite common to have multiple terms in the autocomplete query, so your suggester should be able to manage multiple matches in the suggestion accordingly.

Given a simple corpus (composed just by the following suggestions) and the query :
“Mini Bar Frid” 

You see these suggestions:

• 1000 | Mini Bar something Fridge
• 1000 | Mini Bar something else Fridge
• 1000 | Mini Bar Fridge something
• 1000 | Mini Bar Fridge something else
• 1000 | Mini something Bar Fridge

This is because at the moment, the first matching term wins all ( and the other positions are ignored).
This brings a lot of possible ties (1000), that should be broken to give the user a nice and intuitive ranking.

But intuitively I would expect in the results something like (note that allTermsRequired=true and the schema weight field always returns 1000)

• Mini Bar Fridge something
• Mini Bar Fridge something else
• Mini Bar something Fridge
• Mini Bar something else Fridge
• Mini something Bar Fridge

Let’s see a proposed Solution [4] :

Positional Coefficient

Instead of taking into account just the first term position in the suggestion, it’s possible to use all the matching positions from the matched terms [“mini”,”bar”,”fridge”].
Each position match will affect the score with :

• How much the matched term position is distant from the ideal position match
  • Query : Mini Bar Fri, Ideal Positions : [0,1,2]
  • Suggestion 1 : Mini Bar something Fridge, Matched Positions:[0,1,3]
  • Suggestion 2 : Mini Bar something else Fridge, Matched Positions:[0,1,4]
  • Suggestion 2 will be penalised as “Fri” match happens farer (4 > 3) from the ideal position 2
• Earlier the mis-position happened, stronger the penalty for the score to pay
  • Query : Mini Bar Fri, Ideal Positions : [0,1,2]
  • Suggestion 1 : Mini Bar something Fridge, Matched Positions:[0,1,3]
  • Suggestion 2 : Mini something Bar Fridge, Matched Positions:[0,2,3]
  • Suggestion 2 will be additionally penalised as the first mis-match in positions Bar happens closer to the beginning of the suggestion 

Considering only the discountinue position proved useful :

Query1: Bar some
Query2: some
Suggestion : Mini Bar something Fridge
Query 1 Suggestion Matched Terms positions : [1,2]
Query 2 Suggestion Matched Terms positions : [2]

If we compare the suggestion score for both these queries, it would seem unfair to penalise the first one just because it matches 2 terms ( consecutive) while the second query has just one match ( positioned worst than the first match in query1)

Introducing this advanced positional coefficient calculus helped in improving the overall behavior for the experimental test created.
The results obtained were quite promising :

Query : Mini Bar Fri
100 |Mini Bar Fridge something
100 |Mini Bar Fridge something else
100 |Mini Bar Fridge a a a a a a a a a a a a a a a a a a a a a a
26 |Mini Bar something Fridge
22 |Mini Bar something else Fridge
17 |Mini something Bar Fridge
8 |something Mini Bar Fridge
7 |something else Mini Bar Fridge

There is still a tie for the exact prefix matches, but let’s see if we can finalise that improvement as well .

Token Count Coefficient

Let’s focus on the first three ranking suggestions we just saw :

Query : Mini Bar Fri
100 |Mini Bar Fridge something
100 |Mini Bar Fridge something else
100 |Mini Bar Fridge a a a a a a a a a a a a a a a a a a a a a a

Intuitively we want this order to break the ties.
Closer the number of matched terms with the total number of terms for the suggestion, the better.
Ideally we want our top scoring suggestion to just have the matched terms if possible.
We also don’t want to bring strong inconsistencies for the other suggestions, we should ideally only affect the ties.
This is achievable calculating an additional coefficient, dependant on the term counts :
Token Count Coefficient = matched terms count / total terms count

Then we can scale this value accordingly :
90% of the final score will derive from the positional coefficient
10% of the final score will derive from the token count coefficient

Query : Mini Bar Fri
90 * 1.0 + 10*3/4 = 97|Mini Bar Fridge something
90 * 1.0 + 10*3/5 = 96|Mini Bar Fridge something else
90 * 1.0 + 10*3/25 = 91|Mini Bar Fridge a a a a a a a a a a a a a a a a a a a a a a

It will require some additional tuning but the overall idea should bring a better ranking function to the BlendedInfix when multiple terms matches are involved!
If you have any suggestion, feel free to leave a comment below!
Code is available in the Github Pull Request attached to the Lucene Jira issue[4]

[1] Solr Autocomplete
[2] Blended Infix Suggester Solr Wiki
[3] LUCENE-8343
[4] LUCENE-8347