From 03a3457f10a18f8a024b8adc2bbd0deac0fe2443 Mon Sep 17 00:00:00 2001
From: David Garcia <david.garcialopez@koahealth.com>
Date: Mon, 2 Nov 2020 22:23:04 +0100
Subject: [PATCH] fix(itemExtent): Fix rounded issue using
 precisionErrorTolerance (#68199)

---
 .../rendering/sliver_fixed_extent_list.dart   | 14 +++-
 .../sliver_fixed_extent_layout_test.dart      | 82 +++++++++++++++++++
 2 files changed, 95 insertions(+), 1 deletion(-)

diff --git a/packages/flutter/lib/src/rendering/sliver_fixed_extent_list.dart b/packages/flutter/lib/src/rendering/sliver_fixed_extent_list.dart
index 2073226100..9a75f5a3fc 100644
--- a/packages/flutter/lib/src/rendering/sliver_fixed_extent_list.dart
+++ b/packages/flutter/lib/src/rendering/sliver_fixed_extent_list.dart
@@ -85,7 +85,15 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
   /// order, without gaps, starting from layout offset zero.
   @protected
   int getMaxChildIndexForScrollOffset(double scrollOffset, double itemExtent) {
-    return itemExtent > 0.0 ? math.max(0, (scrollOffset / itemExtent).ceil() - 1) : 0;
+    if (itemExtent > 0.0) {
+      final double actual = scrollOffset / itemExtent - 1;
+      final int round = actual.round();
+      if (_isWithinPrecisionErrorTolerance(actual, round)) {
+        return math.max(0, round);
+      }
+      return math.max(0, actual.ceil());
+    }
+    return 0;
   }
 
   /// Called to estimate the total scrollable extents of this object.
@@ -350,3 +358,7 @@ class RenderSliverFixedExtentList extends RenderSliverFixedExtentBoxAdaptor {
     markNeedsLayout();
   }
 }
+
+bool _isWithinPrecisionErrorTolerance(double actual, int round) {
+  return (actual - round).abs() < precisionErrorTolerance;
+}
diff --git a/packages/flutter/test/rendering/sliver_fixed_extent_layout_test.dart b/packages/flutter/test/rendering/sliver_fixed_extent_layout_test.dart
index 8c4fd336dd..67a4c8ea61 100644
--- a/packages/flutter/test/rendering/sliver_fixed_extent_layout_test.dart
+++ b/packages/flutter/test/rendering/sliver_fixed_extent_layout_test.dart
@@ -41,6 +41,74 @@ void main() {
     expect(children[1].attached, false);
     expect(children[2].attached, true);
   });
+
+  group('getMaxChildIndexForScrollOffset', () {
+    // Regression test for https://github.com/flutter/flutter/issues/68182
+
+    const double genericItemExtent = 600.0;
+    const double extraValueToNotHaveRoundingIssues = 0.0000001; // 6 zeros
+    const double extraValueToHaveRoundingIssues = 0.00000001; // 7 zeros
+
+    test('should be 0 when item extent is 0', () {
+      const double offsetValueWhichDoesntCare = 1234;
+      final int actual = testGetMaxChildIndexForScrollOffset(offsetValueWhichDoesntCare, 0);
+      expect(actual, 0);
+    });
+
+    test('should be 0 when offset is 0', () {
+      final int actual = testGetMaxChildIndexForScrollOffset(0, genericItemExtent);
+      expect(actual, 0);
+    });
+
+    test('should be 0 when offset is equal to item extent', () {
+      final int actual = testGetMaxChildIndexForScrollOffset(genericItemExtent, genericItemExtent);
+      expect(actual, 0);
+    });
+
+    test('should be 1 when offset is greater than item extent', () {
+      final int actual = testGetMaxChildIndexForScrollOffset(
+        genericItemExtent + 1, genericItemExtent);
+      expect(actual, 1);
+    });
+
+    test('should be 1 when offset is slightly greater than item extent', () {
+      final int actual = testGetMaxChildIndexForScrollOffset(
+        genericItemExtent + extraValueToNotHaveRoundingIssues, genericItemExtent);
+      expect(actual, 1);
+    });
+
+    test('should be 4 when offset is four times and a half greater than item extent', () {
+      final int actual = testGetMaxChildIndexForScrollOffset(
+        genericItemExtent * 4.5, genericItemExtent);
+      expect(actual, 4);
+    });
+
+    test('should be 5 when offset is 6 times greater than item extent', () {
+      const double anotherGenericItemExtent = 414.0;
+      final int actual = testGetMaxChildIndexForScrollOffset(
+        anotherGenericItemExtent * 6, anotherGenericItemExtent);
+      expect(actual, 5);
+    });
+
+    test('should be 5 when offset is 6 times greater than a specific item extent where the division will return more than 13 zero decimals', () {
+      const double itemExtentSpecificForAProblematicSreenSize = 411.42857142857144;
+      final int actual = testGetMaxChildIndexForScrollOffset(
+          itemExtentSpecificForAProblematicSreenSize * 6 + extraValueToHaveRoundingIssues,
+          itemExtentSpecificForAProblematicSreenSize);
+      expect(actual, 5);
+    });
+
+    test('should be 0 when offset is 0.00000001 times greater than item extent where the divison will return more than 13 zero decimals', () {
+      final int actual = testGetMaxChildIndexForScrollOffset(
+        genericItemExtent + extraValueToHaveRoundingIssues, genericItemExtent);
+      expect(actual, 0);
+    });
+  });
+}
+
+int testGetMaxChildIndexForScrollOffset(double scrollOffset, double itemExtent) {
+  final TestRenderSliverFixedExtentBoxAdaptor renderSliver = TestRenderSliverFixedExtentBoxAdaptor();
+  return renderSliver.getMaxChildIndexForScrollOffset(scrollOffset, itemExtent);
 }
 
 class TestRenderSliverBoxChildManager extends RenderSliverBoxChildManager {
@@ -103,3 +171,17 @@ class TestRenderSliverBoxChildManager extends RenderSliverBoxChildManager {
   @override
   void setDidUnderflow(bool value) { }
 }
+
+class TestRenderSliverFixedExtentBoxAdaptor extends RenderSliverFixedExtentBoxAdaptor {
+  TestRenderSliverFixedExtentBoxAdaptor()
+    :super(childManager: TestRenderSliverBoxChildManager(children: <RenderBox>[]));
+
+  @override
+  // ignore: unnecessary_overrides
+  int getMaxChildIndexForScrollOffset(double scrollOffset, double itemExtent) {
+    return super.getMaxChildIndexForScrollOffset(scrollOffset, itemExtent);
+  }
+
+  @override
+  double get itemExtent => throw UnimplementedError();
+}