Modify VirtualizeUtils.elementsThatOverlapOffsets to use binary search

Summary:
elementsThatOverlapOffsets was quite inefficient as it was doing a linear scan over all items without an early out for each input offset.

The number of items can be large, so we'd want to use binary search here.
Before: O(MN), where M is the # offsets and N is the # items
After: O(MlogN)

As a utility method, elementsThatOverlapOffsets should not be responsible for checking that the offsets are in increasing order

'binary-search' dep added via https://www.internalfb.com/intern/wiki/React_Native/Building_Product_Experiences/Third_Party_Packages_(npm)/

Changelog:
[Internal] - Modify VirtualizeUtils.elementsThatOverlapOffsets to use binary search

Reviewed By: javache

Differential Revision: D36292326

fbshipit-source-id: 5f22eb5533b69e2ebe5c1cb34e4f82605838f0a7

Libraries/Lists/VirtualizeUtils.js

@@ -27,27 +27,36 @@ export function elementsThatOverlapOffsets(
   },
   zoomScale: number = 1,
 ): Array<number> {
-  const out = [];
-  let outLength = 0;
-  for (let ii = 0; ii < itemCount; ii++) {
-    const frame = getFrameMetrics(ii);
-    const trailingOffset = (frame.offset + frame.length) * zoomScale;
-    for (let kk = 0; kk < offsets.length; kk++) {
-      if (out[kk] == null && trailingOffset >= offsets[kk]) {
-        out[kk] = ii;
-        outLength++;
-        if (kk === offsets.length - 1) {
-          invariant(
-            outLength === offsets.length,
-            'bad offsets input, should be in increasing order: %s',
-            JSON.stringify(offsets),
-          );
-          return out;
-        }
+  const result = [];
+  for (let offsetIndex = 0; offsetIndex < offsets.length; offsetIndex++) {
+    const currentOffset = offsets[offsetIndex];
+    let left = 0;
+    let right = itemCount - 1;
+
+    while (left <= right) {
+      // eslint-disable-next-line no-bitwise
+      const mid = left + ((right - left) >>> 1);
+      const frame = getFrameMetrics(mid);
+      const scaledOffsetStart = frame.offset * zoomScale;
+      const scaledOffsetEnd = (frame.offset + frame.length) * zoomScale;
+
+      // We want the first frame that contains the offset, with inclusive bounds. Thus, for the
+      // first frame the scaledOffsetStart is inclusive, while for other frames it is exclusive.
+      if (
+        (mid === 0 && currentOffset < scaledOffsetStart) ||
+        (mid !== 0 && currentOffset <= scaledOffsetStart)
+      ) {
+        right = mid - 1;
+      } else if (currentOffset > scaledOffsetEnd) {
+        left = mid + 1;
+      } else {
+        result[offsetIndex] = mid;
+        break;
       }
     }
   }
-  return out;
+
+  return result;
 }
 
 /**

Libraries/Lists/__tests__/VirtualizeUtils-test.js

@@ -65,8 +65,20 @@ describe('elementsThatOverlapOffsets', function () {
       elementsThatOverlapOffsets(offsets, frames.length, ii => frames[ii], 1),
     ).toEqual([1, 1, 3]);
   });
+  it('handles frame boundaries', function () {
+    const offsets = [0, 100, 200, 300];
+    function getFrameMetrics(index: number) {
+      return {
+        length: 100,
+        offset: 100 * index,
+      };
+    }
+    expect(
+      elementsThatOverlapOffsets(offsets, 100, getFrameMetrics, 1),
+    ).toEqual([0, 0, 1, 2]);
+  });
   it('handles out of bounds', function () {
-    const offsets = [150, 900];
+    const offsets = [-100, 150, 900];
     const frames = [
       {offset: 0, length: 50},
       {offset: 50, length: 150},
@@ -74,17 +86,6 @@ describe('elementsThatOverlapOffsets', function () {
     ];
     expect(
       elementsThatOverlapOffsets(offsets, frames.length, ii => frames[ii], 1),
-    ).toEqual([1]);
-  });
-  it('errors on non-increasing offsets', function () {
-    const offsets = [150, 50];
-    const frames = [
-      {offset: 0, length: 50},
-      {offset: 50, length: 150},
-      {offset: 250, length: 100},
-    ];
-    expect(() => {
-      elementsThatOverlapOffsets(offsets, frames.length, ii => frames[ii], 1);
-    }).toThrowErrorMatchingSnapshot();
+    ).toEqual([undefined, 1]);
   });
 });

Libraries/Lists/__tests__/__snapshots__/VirtualizeUtils-test.js.snap

@@ -1,3 +0,0 @@
-// Jest Snapshot v1, https://goo.gl/fbAQLP
-
-exports[`elementsThatOverlapOffsets errors on non-increasing offsets 1`] = `"bad offsets input, should be in increasing order: [150,50]"`;