Introduce filterStrict option

FEATURE: The new `filterStrict` option can be used to turn off fuzzy
matching of completions.

https://discuss.codemirror.net/t/search-adjustment-for-autocomplete/7975

src/config.ts

@@ -72,6 +72,11 @@ export interface CompletionConfig {
   /// match score. Defaults to using
   /// [`localeCompare`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/localeCompare).
   compareCompletions?: (a: Completion, b: Completion) => number
+  /// When set to true (the default is false), turn off fuzzy matching
+  /// of completions and only show those that start with the text the
+  /// user typed. Only takes effect for results where
+  /// [`filter`](#autocomplete.CompletionResult.filter) isn't false.
+  filterStrict?: boolean
   /// By default, commands relating to an open completion only take
   /// effect 75 milliseconds after the completion opened, so that key
   /// presses made before the user is aware of the tooltip don't go to
@@ -100,6 +105,7 @@ export const completionConfig = Facet.define<CompletionConfig, Required<Completi
       icons: true,
       addToOptions: [],
       positionInfo: defaultPositionInfo as any,
+      filterStrict: false,
       compareCompletions: (a, b) => a.label.localeCompare(b.label),
       interactionDelay: 75,
       updateSyncTime: 100
@@ -109,7 +115,8 @@ export const completionConfig = Facet.define<CompletionConfig, Required<Completi
       icons: (a, b) => a && b,
       tooltipClass: (a, b) => c => joinClass(a(c), b(c)),
       optionClass: (a, b) => c => joinClass(a(c), b(c)),
-      addToOptions: (a, b) => a.concat(b)
+      addToOptions: (a, b) => a.concat(b),
+      filterStrict: (a, b) => a || b,
     })
   }
 })

src/filter.ts

@@ -143,3 +143,24 @@ export class FuzzyMatcher {
     return this.ret(score - word.length, result)
   }
 }
+
+
+export class StrictMatcher {
+  matched: readonly number[] = []
+  score: number = 0
+  folded: string
+
+  constructor(readonly pattern: string) {
+    this.folded = pattern.toLowerCase()
+  }
+
+  match(word: string): {score: number, matched: readonly number[]} | null {
+    if (word.length < this.pattern.length) return null
+    let start = word.slice(0, this.pattern.length)
+    let match = start == this.pattern ? 0 : start.toLowerCase() == this.folded ? Penalty.CaseFold : null
+    if (match == null) return null
+    this.matched = [0, start.length]
+    this.score = match + (word.length == this.pattern.length ? 0 : Penalty.NotFull)
+    return this
+  }
+}

src/state.ts

@@ -4,7 +4,7 @@ import {Option, CompletionSource, CompletionResult, cur, asSource,
         Completion, ensureAnchor, CompletionContext, CompletionSection,
         startCompletionEffect, closeCompletionEffect,
         insertCompletionText, pickedCompletion} from "./completion"
-import {FuzzyMatcher} from "./filter"
+import {FuzzyMatcher, StrictMatcher} from "./filter"
 import {completionTooltip} from "./tooltip"
 import {CompletionConfig, completionConfig} from "./config"
 
@@ -28,14 +28,16 @@ function sortOptions(active: readonly ActiveSource[], state: EditorState) {
     }
   }
 
+  let conf = state.facet(completionConfig)
   for (let a of active) if (a.hasResult()) {
     let getMatch = a.result.getMatch
     if (a.result.filter === false) {
       for (let option of a.result.options) {
         addOption(new Option(option, a.source, getMatch ? getMatch(option) : [], 1e9 - options.length))
       }
     } else {
-      let matcher = new FuzzyMatcher(state.sliceDoc(a.from, a.to)), match
+      let pattern = state.sliceDoc(a.from, a.to), match
+      let matcher = conf.filterStrict ? new StrictMatcher(pattern) : new FuzzyMatcher(pattern)
       for (let option of a.result.options) if (match = matcher.match(option.label)) {
         let matched = !option.displayLabel ? match.matched : getMatch ? getMatch(option, match.matched) : []
         addOption(new Option(option, a.source, matched, match.score + (option.boost || 0)))
@@ -57,7 +59,7 @@ function sortOptions(active: readonly ActiveSource[], state: EditorState) {
   }
 
   let result = [], prev = null
-  let compare = state.facet(completionConfig).compareCompletions
+  let compare = conf.compareCompletions
   for (let opt of options.sort((a, b) => (b.score - a.score) || compare(a.completion, b.completion))) {
     let cur = opt.completion
     if (!prev || prev.label != cur.label || prev.detail != cur.detail ||