From f7b251b05f9962c2a5cb75304e3cf6e875d813a2 Mon Sep 17 00:00:00 2001
From: Brian Hulette <brian.hulette@ccri.com>
Date: Fri, 20 Oct 2017 16:07:58 -0400
Subject: [PATCH 1/2] only load a dictionary once

---
 js/src/reader/arrow.ts | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/js/src/reader/arrow.ts b/js/src/reader/arrow.ts
index 9716c7fb601..b50838dc8d0 100644
--- a/js/src/reader/arrow.ts
+++ b/js/src/reader/arrow.ts
@@ -52,9 +52,12 @@ export function* readBuffers(...bytes: Array<Uint8Array | Buffer | string>) {
         let index = -1, fieldsLength = schema.fieldsLength();
         if (batch.id) {
             while (++index < fieldsLength) {
+                let found: boolean = false;
                 for (let [id, vector] of readDictionaries(schema.fields(index), batch, state, dictionaries)) {
                     dictionaries[id] = dictionaries[id] && dictionaries[id].concat(vector) || vector;
+                    found = true;
                 }
+                if (found) break;
             }
         } else {
             while (++index < fieldsLength) {

From aec2f6dbf11dbdf513c2caaca44907d72b71e5d7 Mon Sep 17 00:00:00 2001
From: Brian Hulette <brian.hulette@ccri.com>
Date: Fri, 20 Oct 2017 17:41:25 -0400
Subject: [PATCH 2/2] readDictionary now returns a single vector or null

---
 js/src/reader/arrow.ts      | 13 +++++++------
 js/src/reader/dictionary.ts | 18 +++++++-----------
 2 files changed, 14 insertions(+), 17 deletions(-)

diff --git a/js/src/reader/arrow.ts b/js/src/reader/arrow.ts
index b50838dc8d0..dbb6acd0e79 100644
--- a/js/src/reader/arrow.ts
+++ b/js/src/reader/arrow.ts
@@ -23,7 +23,7 @@ import { readFile } from './file';
 import { readStream } from './stream';
 import { readVector } from './vector';
 import { Vector } from '../vector/vector';
-import { readDictionaries } from './dictionary';
+import { readDictionary } from './dictionary';
 
 import ByteBuffer = flatbuffers.ByteBuffer;
 export import Schema = Schema_.org.apache.arrow.flatbuf.Schema;
@@ -51,13 +51,14 @@ export function* readBuffers(...bytes: Array<Uint8Array | Buffer | string>) {
         let state = { nodeIndex: 0, bufferIndex: 0 };
         let index = -1, fieldsLength = schema.fieldsLength();
         if (batch.id) {
+            // A dictionary batch only contain a single vector. Traverse each
+            // field and its children until we find one that uses this dictionary
             while (++index < fieldsLength) {
-                let found: boolean = false;
-                for (let [id, vector] of readDictionaries(schema.fields(index), batch, state, dictionaries)) {
-                    dictionaries[id] = dictionaries[id] && dictionaries[id].concat(vector) || vector;
-                    found = true;
+                let vector = readDictionary(schema.fields(index), batch, state, dictionaries);
+                if (vector) {
+                    dictionaries[batch.id] = dictionaries[batch.id] && dictionaries[batch.id].concat(vector) || vector;
+                    break;
                 }
-                if (found) break;
             }
         } else {
             while (++index < fieldsLength) {
diff --git a/js/src/reader/dictionary.ts b/js/src/reader/dictionary.ts
index abf7ac3dfb9..61698e80c00 100644
--- a/js/src/reader/dictionary.ts
+++ b/js/src/reader/dictionary.ts
@@ -21,20 +21,16 @@ import * as Schema_ from '../format/Schema_generated';
 import { IteratorState, Dictionaries } from './arrow';
 import Field = Schema_.org.apache.arrow.flatbuf.Field;
 
-export function* readDictionaries(field: Field | null,
-                                  batch: MessageBatch,
-                                  iterator: IteratorState,
-                                  dictionaries: Dictionaries) {
+export function readDictionary(field: Field | null,
+                               batch: MessageBatch,
+                               iterator: IteratorState,
+                               dictionaries: Dictionaries) {
     let id: string, encoding = field && field.dictionary();
     if (encoding && batch.id === (id = encoding.id().toFloat64().toString())) {
-        yield [id, readVector(field, batch, iterator, null)];
-        return;
+        return readVector(field, batch, iterator, null);
     }
     for (let i = -1, n = field && field.childrenLength() || 0; ++i < n;) {
-        // Since a dictionary batch can only contain a single vector, return early after we find it
-        for (let result of readDictionaries(field.children(i), batch, iterator, dictionaries)) {
-            yield result;
-            return;
-        }
+        let vector = readDictionary(field.children(i), batch, iterator, dictionaries);
+        if (vector) return vector;
     }
 }