Add search option to apply term boosting

Term boosting (giving greater or lower importance to specific query
terms) was previously not supported. It was technically possible by
using the `boostDocument` search option (as shown here: #268) but cumbersome and error prone.

This commit adds a new search option, `boostTerm`, which makes it a lot
easier to apply term boosting. The option is a function that is invoked
with each search term, and is expected to return a numeric boost factor.

src/MiniSearch.test.js

@@ -1150,6 +1150,22 @@ describe('MiniSearch', () => {
       expect(results[0].score).toBeCloseTo(resultsWithoutBoost[0].score * boostFactor)
     })
 
+    it('boosts terms by calling boostTerm with normalized query term, term index in the query, and array of all query terms', () => {
+      const query = 'Commedia nova'
+      const boostFactors = {
+        commedia: 1.5,
+        nova: 1.1
+      }
+      const boostTerm = jest.fn((term, i, terms) => boostFactors[term])
+      const resultsWithoutBoost = ms.search(query)
+      const results = ms.search(query, { boostTerm })
+      console.log(results)
+      expect(boostTerm).toHaveBeenCalledWith('commedia', 0, ['commedia', 'nova'])
+      expect(boostTerm).toHaveBeenCalledWith('nova', 1, ['commedia', 'nova'])
+      expect(results[0].score).toBeCloseTo(resultsWithoutBoost[0].score * boostFactors.commedia)
+      expect(results[1].score).toBeCloseTo(resultsWithoutBoost[1].score * boostFactors.nova)
+    })
+
     it('skips document if boostDocument returns a falsy value', () => {
       const query = 'vita'
       const boostDocument = jest.fn((id, term) => id === 3 ? null : 1)

src/MiniSearch.ts

@@ -32,6 +32,17 @@ export type SearchOptions = {
    */
   boost?: { [fieldName: string]: number },
 
+  /**
+   * Function to calculate a boost factor for each term.
+   *
+   * This function, if provided, is called for each query term (as split by
+   * `tokenize` and processed by `processTerm`). It is expected to return a
+   * numeric boost factor for the term. A factor lower than 1 reduces the
+   * importance of the term, a factor greater than 1 increases it. A factor of
+   * exactly 1 is neutral, and does not affect the term's importance.
+   */
+  boostTerm?: (term: string, i: number, terms: string[]) => number,
+
   /**
    * Relative weights to assign to prefix search results and fuzzy search
    * results. Exact matches are assigned a weight of 1.
@@ -480,7 +491,8 @@ export type AutoVacuumOptions = VacuumOptions & VacuumConditions
 type QuerySpec = {
   prefix: boolean,
   fuzzy: number | boolean,
-  term: string
+  term: string,
+  termBoost: number
 }
 
 type DocumentTermFreqs = Map<number, number>
@@ -1685,7 +1697,7 @@ export default class MiniSearch<T = any> {
     const { fuzzy: fuzzyWeight, prefix: prefixWeight } = { ...defaultSearchOptions.weights, ...weights }
 
     const data = this._index.get(query.term)
-    const results = this.termResults(query.term, query.term, 1, data, boosts, boostDocument, bm25params)
+    const results = this.termResults(query.term, query.term, 1, query.termBoost, data, boosts, boostDocument, bm25params)
 
     let prefixMatches
     let fuzzyMatches
@@ -1715,7 +1727,7 @@ export default class MiniSearch<T = any> {
         // account for the fact that prefix matches stay more relevant than
         // fuzzy matches for longer distances.
         const weight = prefixWeight * term.length / (term.length + 0.3 * distance)
-        this.termResults(query.term, term, weight, data, boosts, boostDocument, bm25params, results)
+        this.termResults(query.term, term, weight, query.termBoost, data, boosts, boostDocument, bm25params, results)
       }
     }
 
@@ -1727,7 +1739,7 @@ export default class MiniSearch<T = any> {
         // Weight gradually approaches 0 as distance goes to infinity, with the
         // weight for the hypothetical distance 0 being equal to fuzzyWeight.
         const weight = fuzzyWeight * term.length / (term.length + distance)
-        this.termResults(query.term, term, weight, data, boosts, boostDocument, bm25params, results)
+        this.termResults(query.term, term, weight, query.termBoost, data, boosts, boostDocument, bm25params, results)
       }
     }
 
@@ -1826,6 +1838,7 @@ export default class MiniSearch<T = any> {
     sourceTerm: string,
     derivedTerm: string,
     termWeight: number,
+    termBoost: number,
     fieldTermData: FieldTermData | undefined,
     fieldBoosts: { [field: string]: number },
     boostDocumentFn: ((id: any, term: string, storedFields?: Record<string, unknown>) => number) | undefined,
@@ -1864,7 +1877,7 @@ export default class MiniSearch<T = any> {
         // present. This is currently not supported, and may require further
         // analysis to see if it is a valid use case.
         const rawScore = calcBM25Score(termFreq, matchingFields, this._documentCount, fieldLength, avgFieldLength, bm25params)
-        const weightedScore = termWeight * fieldBoost * docBoost * rawScore
+        const weightedScore = termWeight * termBoost * fieldBoost * docBoost * rawScore
 
         const result = results.get(docId)
         if (result) {
@@ -2118,7 +2131,10 @@ const termToQuerySpec = (options: SearchOptions) => (term: string, i: number, te
   const prefix = (typeof options.prefix === 'function')
     ? options.prefix(term, i, terms)
     : (options.prefix === true)
-  return { term, fuzzy, prefix }
+  const termBoost = (typeof options.boostTerm === 'function')
+    ? options.boostTerm(term, i, terms)
+    : 1
+  return { term, fuzzy, prefix, termBoost }
 }
 
 const defaultOptions = {