From 724ff531434af943b0a1e212f0e6c8a476bcd87d Mon Sep 17 00:00:00 2001
From: Max Metral <max.metral@sesamecare.com>
Date: Sun, 31 Mar 2024 21:47:38 -0400
Subject: [PATCH] fix(cursor): record completion of a datasource as an empty
 string cursor

---
 src/asDataGenerator.ts  |  5 +++
 src/multiSourcePager.ts |  6 +++-
 src/queuePager.spec.ts  | 78 ++++++++++++++++++++---------------------
 3 files changed, 49 insertions(+), 40 deletions(-)

diff --git a/src/asDataGenerator.ts b/src/asDataGenerator.ts
index 491c850..b886be8 100644
--- a/src/asDataGenerator.ts
+++ b/src/asDataGenerator.ts
@@ -8,6 +8,11 @@ export function asDataGenerator<T extends ResultWithCursor>(
 
   return {
     async *getGenerator(cursor: string | undefined) {
+      if (cursor === '') {
+        // The special empty string cursor indicates we should not fetch any more.
+        return;
+      }
+
       let currentCursor = cursor;
       let continueFetching = true;
 
diff --git a/src/multiSourcePager.ts b/src/multiSourcePager.ts
index 2d83032..6b1acc4 100644
--- a/src/multiSourcePager.ts
+++ b/src/multiSourcePager.ts
@@ -47,6 +47,8 @@ export async function multiSourcePager<Types extends Array<Source>>(
         index,
         key: dataSources[index].sortKey(item.value),
       });
+    } else {
+      cursors[index] = '';
     }
   }));
 
@@ -63,7 +65,9 @@ export async function multiSourcePager<Types extends Array<Source>>(
 
         // Fetch next item from the generator of the data source that the last item came from
         const nextResult = await generators[index].next();
-        if (!nextResult.done) {
+        if (nextResult.done) {
+          cursors[index] = '';
+        } else {
           queue.push({
             result: nextResult.value,
             index,
diff --git a/src/queuePager.spec.ts b/src/queuePager.spec.ts
index 02ca8e6..34846a5 100644
--- a/src/queuePager.spec.ts
+++ b/src/queuePager.spec.ts
@@ -65,45 +65,45 @@ describe('multi source pager', () => {
 
       const p3 = await pager.getNextResults(5);
       expect(p3.results).toMatchInlineSnapshot(`
-      [
-        {
-          "cursor": "WyIyMDIzLTAxLTAzVDAwOjAwOjAwLjAwMFojQyIsIjIwMjMtMDEtMDJUMDA6MDA6MDAuMDAwWiNCQkIiXQ==",
-          "data": "C",
-          "type": "letter",
-        },
-        {
-          "cursor": "WyIyMDIzLTAxLTA0VDAwOjAwOjAwLjAwMFojRCIsIjIwMjMtMDEtMDJUMDA6MDA6MDAuMDAwWiNCQkIiXQ==",
-          "data": "D",
-          "type": "letter",
-        },
-        {
-          "cursor": "WyIyMDIzLTAxLTA1VDAwOjAwOjAwLjAwMFojRSIsIjIwMjMtMDEtMDJUMDA6MDA6MDAuMDAwWiNCQkIiXQ==",
-          "data": "E",
-          "type": "letter",
-        },
-        {
-          "cursor": "WyIyMDIzLTAxLTA1VDAwOjAwOjAwLjAwMFojRSIsIjIwMjMtMDEtMDZUMDA6MDA6MDAuMDAwWiNGRiJd",
-          "data": "FF",
-          "type": "letter",
-        },
-        {
-          "cursor": "WyIyMDIzLTAxLTA1VDAwOjAwOjAwLjAwMFojRSIsIjIwMjMtMDEtMDdUMDA6MDA6MDAuMDAwWiNHRyJd",
-          "data": "GG",
-          "type": "letter",
-        },
-      ]
-    `);
+        [
+          {
+            "cursor": "WyIyMDIzLTAxLTAzVDAwOjAwOjAwLjAwMFojQyIsIjIwMjMtMDEtMDJUMDA6MDA6MDAuMDAwWiNCQkIiXQ==",
+            "data": "C",
+            "type": "letter",
+          },
+          {
+            "cursor": "WyIyMDIzLTAxLTA0VDAwOjAwOjAwLjAwMFojRCIsIjIwMjMtMDEtMDJUMDA6MDA6MDAuMDAwWiNCQkIiXQ==",
+            "data": "D",
+            "type": "letter",
+          },
+          {
+            "cursor": "WyIyMDIzLTAxLTA1VDAwOjAwOjAwLjAwMFojRSIsIjIwMjMtMDEtMDJUMDA6MDA6MDAuMDAwWiNCQkIiXQ==",
+            "data": "E",
+            "type": "letter",
+          },
+          {
+            "cursor": "WyIiLCIyMDIzLTAxLTA2VDAwOjAwOjAwLjAwMFojRkYiXQ==",
+            "data": "FF",
+            "type": "letter",
+          },
+          {
+            "cursor": "WyIiLCIyMDIzLTAxLTA3VDAwOjAwOjAwLjAwMFojR0ciXQ==",
+            "data": "GG",
+            "type": "letter",
+          },
+        ]
+      `);
 
       const p4 = await pager.getNextResults(5);
       expect(p4.results).toMatchInlineSnapshot(`
-      [
-        {
-          "cursor": "WyIyMDIzLTAxLTA1VDAwOjAwOjAwLjAwMFojRSIsIjIwMjMtMDEtMDhUMDA6MDA6MDAuMDAwWiNISCJd",
-          "data": "HH",
-          "type": "letter",
-        },
-      ]
-    `);
+        [
+          {
+            "cursor": "WyIiLCIyMDIzLTAxLTA4VDAwOjAwOjAwLjAwMFojSEgiXQ==",
+            "data": "HH",
+            "type": "letter",
+          },
+        ]
+      `);
     }
   });
 
@@ -137,17 +137,17 @@ describe('multi source pager', () => {
           "type": "letter",
         },
         {
-          "cursor": "WyIyMDIzLTAxLTAzVDAwOjAwOjAwLjAwMFojQyIsIjIwMjMtMDEtMDJUMDA6MDA6MDAuMDAwWiNCQkIiXQ==",
+          "cursor": "WyIyMDIzLTAxLTAzVDAwOjAwOjAwLjAwMFojQyIsIiJd",
           "data": "C",
           "type": "letter",
         },
         {
-          "cursor": "WyIyMDIzLTAxLTA0VDAwOjAwOjAwLjAwMFojRCIsIjIwMjMtMDEtMDJUMDA6MDA6MDAuMDAwWiNCQkIiXQ==",
+          "cursor": "WyIyMDIzLTAxLTA0VDAwOjAwOjAwLjAwMFojRCIsIiJd",
           "data": "D",
           "type": "letter",
         },
         {
-          "cursor": "WyIyMDIzLTAxLTA1VDAwOjAwOjAwLjAwMFojRSIsIjIwMjMtMDEtMDJUMDA6MDA6MDAuMDAwWiNCQkIiXQ==",
+          "cursor": "WyIyMDIzLTAxLTA1VDAwOjAwOjAwLjAwMFojRSIsIiJd",
           "data": "E",
           "type": "letter",
         },