Replace startsWith with stringContainsCharactersInOrder

src/services/completions.ts

@@ -942,14 +942,38 @@ namespace ts.Completions {
                     }
 
                     const id = getUniqueSymbolId(symbol, typeChecker);
-                    if (!symbolIdMap[id] && startsWith(name.toLowerCase(), tokenTextLowerCase)) {
+                    if (!symbolIdMap[id] && stringContainsCharactersInOrder(name.toLowerCase(), tokenTextLowerCase)) {
                         symbols.push(symbol);
                         symbolToOriginInfoMap[id] = { moduleSymbol, isDefaultExport };
                     }
                 }
             });
         }
 
+        /**
+         * True if you could remove some characters in `a` to get `b`.
+         * E.g., true for "abcdef" and "bdf".
+         * But not true for "abcdef" and "dbf".
+         */
+        function stringContainsCharactersInOrder(str: string, characters: string): boolean {
+            if (characters.length === 0) {
+                return true;
+            }
+
+            let bi = 0;
+            for (let ai = 0; ai < str.length; ai++) {
+                if (str.charCodeAt(ai) === characters.charCodeAt(bi)) {
+                    bi++;
+                    if (bi === characters.length) {
+                        return true;
+                    }
+                }
+            }
+
+            // Did not find all characters in 'b'
+            return false;
+        }
+
         /**
          * Finds the first node that "embraces" the position, so that one may
          * accurately aggregate locals from the closest containing scope.

tests/cases/fourslash/completionsImport_matching.ts

@@ -0,0 +1,22 @@
+/// <reference path="fourslash.ts" />
+
+// @Filename: /a.ts
+// Not included:
+////export function abcde() {}
+////export function dbf() {}
+// Included:
+////export function bdf() {}
+////export function abcdef() {}
+////export function BDF() {}
+
+// @Filename: /b.ts
+////bdf/**/
+
+goTo.marker("");
+
+verify.not.completionListContains("abcde");
+verify.not.completionListContains("dbf");
+
+verify.completionListContains("bdf", "function bdf(): void", "", "function", /*spanIndex*/ undefined, /*hasAction*/ true);
+verify.completionListContains("abcdef", "function abcdef(): void", "", "function", /*spanIndex*/ undefined, /*hasAction*/ true);
+verify.completionListContains("BDF", "function BDF(): void", "", "function", /*spanIndex*/ undefined, /*hasAction*/ true);