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Avoid overflow when calculating reverse indices #467

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Avoid overflow when calculating reverse indices #467

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cdda7ef
Fix overflow issue for invalid final indices.
charlesmchen-signal Jul 26, 2018
24c5882
Clarify finalIndices overflow changes, add asserts.
michaelkirk Aug 2, 2018
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26 changes: 16 additions & 10 deletions YapDatabase/Extensions/View/Utilities/YapDatabaseViewChange.m
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Original file line number Diff line number Diff line change
Expand Up @@ -2153,24 +2153,30 @@ + (void)postProcessAndFilterRowChanges:(NSMutableArray *)rowChanges
// 1 -> 2
// 2 -> 1
// 3 -> 0
//
// Basically, we find the midpoint, and then move each index to the other side of the midpoint,
// but we keep its distance from the midpoint the same.

if ([reverse containsObject:rowChange->originalGroup])
{
NSUInteger count = [originalMappings visibleCountForGroup:rowChange->originalGroup];
double mid = (count - 1) / 2.0;

rowChange->originalIndex = (NSUInteger)(mid - (rowChange->originalIndex - mid));
NSUInteger forwardIndex = rowChange->originalIndex;
if (count < forwardIndex + 1) {
// Calculating the reverse index would overflow, so we skip it.
NSAssert(NO, @"original index is too large");
} else {
rowChange->originalIndex = count - (forwardIndex + 1);
}
}

if ([reverse containsObject:rowChange->finalGroup])
{
NSUInteger count = [finalMappings visibleCountForGroup:rowChange->finalGroup];
double mid = (count - 1) / 2.0;

rowChange->finalIndex = (NSUInteger)(mid - (rowChange->finalIndex - mid));
NSUInteger forwardIndex = rowChange->finalIndex;
if (count < forwardIndex + 1) {
// This can happen when deleting an item.
// Calculating the reverse index would overflow, so we skip it.
// When deleting a row, the finalIndex is irrelevant anyway.
NSAssert(rowChange->type == YapDatabaseViewChangeDelete, @"final index is too large");
} else {
rowChange->finalIndex = count - (forwardIndex + 1);
}
}
}

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