ARROW-6675: [JS] Add scanReverse function to dataFrame and filteredDataframe

js/src/compute/dataframe.ts

@@ -31,6 +31,7 @@ export type NextFunc = (idx: number, batch: RecordBatch) => void;
 
 Table.prototype.countBy = function(this: Table, name: Col | string) { return new DataFrame(this.chunks).countBy(name); };
 Table.prototype.scan = function(this: Table, next: NextFunc, bind?: BindFunc) { return new DataFrame(this.chunks).scan(next, bind); };
+Table.prototype.scanReverse = function(this: Table, next: NextFunc, bind?: BindFunc) { return new DataFrame(this.chunks).scanReverse(next, bind); };
 Table.prototype.filter = function(this: Table, predicate: Predicate): FilteredDataFrame { return new DataFrame(this.chunks).filter(predicate); };
 
 export class DataFrame<T extends { [key: string]: DataType } = any> extends Table<T> {
@@ -49,6 +50,18 @@ export class DataFrame<T extends { [key: string]: DataType } = any> extends Tabl
             }
         }
     }
+    public scanReverse(next: NextFunc, bind?: BindFunc) {
+        const batches = this.chunks, numBatches = batches.length;
+        for (let batchIndex = numBatches; --batchIndex >= 0;) {
+            // load batches
+            const batch = batches[batchIndex];
+            if (bind) { bind(batch); }
+            // yield all indices
+            for (let index = batch.length; --index >= 0;) {
+                next(index, batch);
+            }
+        }
+    }
     public countBy(name: Col | string) {
         const batches = this.chunks, numBatches = batches.length;
         const count_by = typeof name === 'string' ? new Col(name) : name as Col;
@@ -130,6 +143,23 @@ export class FilteredDataFrame<T extends { [key: string]: DataType } = any> exte
             }
         }
     }
+    public scanReverse(next: NextFunc, bind?: BindFunc) {
+        const batches = this._chunks;
+        const numBatches = batches.length;
+        for (let batchIndex = numBatches; --batchIndex >= 0;) {
+            // load batches
+            const batch = batches[batchIndex];
+            // TODO: bind batches lazily
+            // If predicate doesn't match anything in the batch we don't need
+            // to bind the callback
+            if (bind) { bind(batch); }
+            const predicate = this._predicate.bind(batch);
+            // yield all indices
+            for (let index = batch.length; --index >= 0;) {
+                if (predicate(index, batch)) { next(index, batch); }
+            }
+        }
+    }
     public count(): number {
         // inlined version of this:
         // let sum = 0;

js/src/table.ts

@@ -44,6 +44,7 @@ export interface Table<T extends { [key: string]: DataType } = any> {
     clone(chunks?: RecordBatch<T>[], offsets?: Uint32Array): Table<T>;
 
     scan(next: import('./compute/dataframe').NextFunc, bind?: import('./compute/dataframe').BindFunc): void;
+    scanReverse(next: import('./compute/dataframe').NextFunc, bind?: import('./compute/dataframe').BindFunc): void;
     countBy(name: import('./compute/predicate').Col | string): import('./compute/dataframe').CountByResult;
     filter(predicate: import('./compute/predicate').Predicate): import('./compute/dataframe').FilteredDataFrame<T>;
 }

js/test/unit/table-tests.ts

@@ -326,6 +326,24 @@ describe(`Table`, () => {
                     }
                 });
             });
+            describe(`scanReverse()`, () => {
+                test(`yields all values`, () => {
+                    const table = datum.table();
+                    let expected_idx = values.length;
+                    table.scanReverse((idx, batch) => {
+                        const columns = batch.schema.fields.map((_, i) => batch.getChildAt(i)!);
+                        expect(columns.map((c) => c.get(idx))).toEqual(values[--expected_idx]);
+                    });
+                });
+                test(`calls bind function with every batch`, () => {
+                    const table = datum.table();
+                    let bind = jest.fn();
+                    table.scanReverse(() => { }, bind);
+                    for (let batch of table.chunks) {
+                        expect(bind).toHaveBeenCalledWith(batch);
+                    }
+                });
+            });
             test(`count() returns the correct length`, () => {
                 const table = datum.table();
                 const values = datum.values();
@@ -434,6 +452,26 @@ describe(`Table`, () => {
                             }
                         });
                     });
+                    describe(`scanReverse()`, () => {
+                        test(`iterates over expected values in reverse`, () => {
+                            let expected_idx = expected.length;
+                            filtered.scanReverse((idx, batch) => {
+                                const columns = batch.schema.fields.map((_, i) => batch.getChildAt(i)!);
+                                expect(columns.map((c) => c.get(idx))).toEqual(expected[--expected_idx]);
+                            });
+                        });
+                        test(`calls bind function on every batch`, () => {
+                            // Techincally, we only need to call bind on
+                            // batches with data that match the predicate, so
+                            // this test may fail in the future if we change
+                            // that - and that's ok!
+                            let bind = jest.fn();
+                            filtered.scanReverse(() => { }, bind);
+                            for (let batch of table.chunks) {
+                                expect(bind).toHaveBeenCalledWith(batch);
+                            }
+                        });
+                    });
                 });
             }
             test(`countBy on dictionary returns the correct counts`, () => {