Don't emit duplicate occurrences (sourcegraph#194)

Fixes sourcegraph#152

Author: olafurpg

snapshots/output/pure-js/src/main.js

@@ -67,7 +67,6 @@
 //    documentation ```ts\nvar a: number\n```
   print_fib(a)
 //^^^^^^^^^ reference pure-js 1.0.0 src/`main.js`/print_fib().
-//          ^ reference pure-js 1.0.0 src/`main.js`/a.
 //          ^ reference pure-js 1.0.0 src/`main.js`/a.
 
   function forever() {

snapshots/output/syntax/src/local.ts

@@ -37,8 +37,6 @@
 //          ^ reference local 8
 //             ^^ reference local 9
 //                 ^^^^^^ reference typescript 4.8.4 lib/`lib.es5.d.ts`/Array#reduce().
-//                 ^^^^^^ reference typescript 4.8.4 lib/`lib.es5.d.ts`/Array#reduce().
-//                 ^^^^^^ reference typescript 4.8.4 lib/`lib.es5.d.ts`/Array#reduce().
 //                         ^^^^^^^^^^^^^ definition local 16
 //                         documentation ```ts\n(parameter) previousValue: string\n```
 //                                        ^^^^^^^^^^^^ definition local 17

snapshots/output/syntax/src/structural-type.ts

@@ -17,7 +17,6 @@
 //         ^^^^^^^ reference typescript 4.8.4 lib/`lib.es2015.symbol.wellknown.d.ts`/Promise#
 //         ^^^^^^^ reference typescript 4.8.4 lib/`lib.es2018.promise.d.ts`/Promise#
 //                 ^^^^^^^ reference typescript 4.8.4 lib/`lib.es2015.promise.d.ts`/PromiseConstructor#resolve().
-//                 ^^^^^^^ reference typescript 4.8.4 lib/`lib.es2015.promise.d.ts`/PromiseConstructor#resolve().
 //                           ^^^^^^ definition syntax 1.0.0 src/`structural-type.ts`/member0:
 //                           documentation ```ts\n(property) member: number\n```
   }

src/FileIndexer.ts

@@ -45,7 +45,7 @@ export class FileIndexer {
     if (symbol.isEmpty()) {
       return
     }
-    this.document.occurrences.push(
+    this.pushOccurrence(
       new scip.scip.Occurrence({
         range: [0, 0, 0],
         symbol: symbol.value,
@@ -111,7 +111,7 @@ export class FileIndexer {
         // Skip empty symbols
         continue
       }
-      this.document.occurrences.push(
+      this.pushOccurrence(
         new scip.scip.Occurrence({
           range,
           symbol: scipSymbol.value,
@@ -153,7 +153,7 @@ export class FileIndexer {
       if (scipSymbol.isEmpty()) {
         continue
       }
-      this.document.occurrences.push(
+      this.pushOccurrence(
         new scip.scip.Occurrence({
           range,
           symbol: scipSymbol.value,
@@ -190,7 +190,7 @@ export class FileIndexer {
       if (scipSymbol.isEmpty()) {
         continue
       }
-      this.document.occurrences.push(
+      this.pushOccurrence(
         new scip.scip.Occurrence({
           range,
           symbol: scipSymbol.value,
@@ -227,6 +227,17 @@ export class FileIndexer {
     )
   }
 
+  private pushOccurrence(occurrence: scip.scip.Occurrence): void {
+    if (this.document.occurrences.length > 0) {
+      const lastOccurrence =
+        this.document.occurrences[this.document.occurrences.length - 1]
+      if (isEqualOccurrence(lastOccurrence, occurrence)) {
+        return
+      }
+    }
+    this.document.occurrences.push(occurrence)
+  }
+
   private relationships(
     declaration: ts.Node,
     declarationSymbol: ScipSymbol
@@ -673,3 +684,26 @@ function scriptElementKind(
   }
   return ts.ScriptElementKind.unknown
 }
+
+function isEqualOccurrence(
+  a: scip.scip.Occurrence,
+  b: scip.scip.Occurrence
+): boolean {
+  return (
+    a.symbol_roles === b.symbol_roles &&
+    a.symbol === b.symbol &&
+    isEqualArray(a.range, b.range)
+  )
+}
+
+function isEqualArray<T>(a: T[], b: T[]): boolean {
+  if (a.length !== b.length) {
+    return false
+  }
+  for (let index = 0; index < a.length; index++) {
+    if (a[index] !== b[index]) {
+      return false
+    }
+  }
+  return true
+}