From fc3c460b3d92b5e5fecd6b524eb4370c6d3e009c Mon Sep 17 00:00:00 2001
From: Arjun <arjun.sn@synup.com>
Date: Sun, 10 May 2020 21:35:18 +0530
Subject: [PATCH] perf: optimise measurements calculation (#17)

* optimise measurements calculation

* remove additinal const declaration
---
 src/index.js | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/src/index.js b/src/index.js
index d4b8931a..1f8c1a22 100644
--- a/src/index.js
+++ b/src/index.js
@@ -47,21 +47,23 @@ export function useVirtual({
     mountedRef.current = true
   }, [estimateSize, size])
 
-  const measurements = React.useMemo(() => {
+  const { measurements, reversedMeasurements } = React.useMemo(() => {
     const measurements = []
-    for (let i = 0; i < size; i++) {
+    const reversedMeasurements = []
+
+    for (let i = 0, j = size - 1; i < size; i++, j--) {
       const start = measurements[i - 1]?.end || 0
       const size = measuredCache[i] || estimateSize(i)
       const end = start + size
+      const bounds = { index: i, start, size, end }
       measurements[i] = {
-        index: i,
-        start,
-        size,
-        end,
+        ...bounds,
+      }
+      reversedMeasurements[j] = {
+        ...bounds,
       }
     }
-
-    return measurements
+    return { measurements, reversedMeasurements }
   }, [estimateSize, measuredCache, size])
 
   const totalSize = measurements[size - 1]?.end || 0
@@ -72,9 +74,9 @@ export function useVirtual({
   )
   let end = React.useMemo(
     () =>
-      [...measurements]
-        .reverse()
-        .find(rowStat => rowStat.start <= scrollOffsetPlusOuterSize),
+      reversedMeasurements.find(
+        rowStat => rowStat.start <= scrollOffsetPlusOuterSize
+      ),
     [measurements, scrollOffsetPlusOuterSize]
   )