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Fix rounding of negative numbers (#825) 

Summary:
`YGRoundValueToPixelGrid` currently rounds negative numbers incorrectly. For example:

```
YGRoundValueToPixelGrid(-2.2, 1.0, /* ceil */ false, /* floor */ true) = -2.0
```

However, that operation is supposed to take the floor of the number so the result should acutally be `-3.0`.

There's a detailed comment in `YGRoundValueToPixelGrid` about the fix and why it works.

A symptom that manifested because of this bug is that text nodes could get smaller and smaller on each layout pass. For details see https://github.com/facebook/yoga/issues/824.

Fixes #824

Adam Comella
Microsoft Corp.
Pull Request resolved: https://github.com/facebook/yoga/pull/825

Reviewed By: priteshrnandgaonkar

Differential Revision: D10282064

Pulled By: shergin

fbshipit-source-id: 16ca966e6cb0cfc88b1dbf4ba31e7b1dbe1f2049
This commit is contained in:
Adam Comella 2018-10-11 18:17:14 -07:00 committed by Facebook Github Bot
parent 6b5b4758ff
commit af8c69ecf7
1 changed files with 21 additions and 2 deletions
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23
ReactCommon/yoga/yoga/Yoga.cpp
View File

@ -1,11 +1,10 @@
/*
* Copyright (c) Facebook, Inc. and its affiliates.
* Copyright (c) 2018-present, Facebook, Inc.
*
* This source code is licensed under the MIT license found in the LICENSE
* file in the root directory of this source tree.
*
*/
#include "Yoga.h"
#include <float.h>
#include <string.h>
@ -3608,7 +3607,27 @@ float YGRoundValueToPixelGrid(
const bool forceCeil,
const bool forceFloor) {
float scaledValue = value * pointScaleFactor;
// We want to calculate `fractial` such that `floor(scaledValue) = scaledValue
// - fractial`.
float fractial = fmodf(scaledValue, 1.0f);
if (fractial < 0) {
// This branch is for handling negative numbers for `value`.
//
// Regarding `floor` and `ceil`. Note that for a number x, `floor(x) <= x <=
// ceil(x)` even for negative numbers. Here are a couple of examples:
// - x = 2.2: floor( 2.2) = 2, ceil( 2.2) = 3
// - x = -2.2: floor(-2.2) = -3, ceil(-2.2) = -2
//
// Regarding `fmodf`. For fractional negative numbers, `fmodf` returns a
// negative number. For example, `fmodf(-2.2) = -0.2`. However, we want
// `fractial` to be the number such that subtracting it from `value` will
// give us `floor(value)`. In the case of negative numbers, adding 1 to
// `fmodf(value)` gives us this. Let's continue the example from above:
// - fractial = fmodf(-2.2) = -0.2
// - Add 1 to the fraction: fractial2 = fractial + 1 = -0.2 + 1 = 0.8
// - Finding the `floor`: -2.2 - fractial2 = -2.2 - 0.8 = -3
++fractial;
}
if (YGFloatsEqual(fractial, 0)) {
// First we check if the value is already rounded
scaledValue = scaledValue - fractial;