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author | Masaru Nomura <massa.nomura@gmail.com> | 2016-12-25 02:06:25 +0900 |
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committer | Masaru Nomura <massa.nomura@gmail.com> | 2016-12-28 12:23:04 +0900 |
commit | 23040fe8da1dfd0595f7904211b8e391dceb78a6 (patch) | |
tree | ca1017a87fc786a035964868794230516474afad /META-INF | |
parent | 23548c4301e48cb69f05cd01ceba418dc9c36d6c (diff) | |
download | scala-23040fe8da1dfd0595f7904211b8e391dceb78a6.tar.gz scala-23040fe8da1dfd0595f7904211b8e391dceb78a6.tar.bz2 scala-23040fe8da1dfd0595f7904211b8e391dceb78a6.zip |
[Backport] Modify ArrayBuilder and WrappedArrayBuilder to be reusable
As they're reusable in 2.12.x with this change[1], it'd be useful
to make them reusable in 2.11.x.
[1] https://github.com/scala/scala/commit/6eaae1b969b68ed3dc65a40613a8168b09246256
With this change, not only are they reusable but also we can avoid
mutation of previously created arrays.
Behaviour(Problem):
Actual behaviour before this modification is as follows;
<ArrayBuilder>
```
scala> import scala.collection.mutable.ArrayBuilder
import scala.collection.mutable.ArrayBuilder
scala> val builder = new ArrayBuilder.ofInt
builder: scala.collection.mutable.ArrayBuilder.ofInt = ArrayBuilder.ofInt
scala> builder ++= Vector.range(1, 17)
res0: builder.type = ArrayBuilder.ofInt
scala> val arr = builder.result()
arr: Array[Int] = Array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)
scala> builder.clear()
scala> builder += 100
res2: builder.type = ArrayBuilder.ofInt
scala> val arr2 = builder.result()
arr2: Array[Int] = Array(100)
scala> arr
res3: Array[Int] = Array(100, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)
// arr should be Array(1, .., 16) but was unexpectedly mutated by `+=` operation
```
`arr` was mutated as follows;
1. `result` & `clear`
- `arr = elems`
- `size = 0`
2. `+=(100)`
- `ensureSize(0 + 1)`
=> `capacity < size || capacity == 0` is `false` as `capacity == 16`
and `size == 1`
- `elems(0) = 100` this is where `arr(0) = 100` was done because we
did not reallocate a new array for `elems` when calling `ensureSize`,
which should have happened.
- `size = 1`
3. `result`
- `mkArray(1)` gives us `arr2 = Array(100)`
<WrappedArrayBuilder>
We can observe almost the same mutation behaviour of ArrayBuilder.
```
scala> import scala.collection.mutable.WrappedArray
import scala.collection.mutable.WrappedArray
scala> import scala.collection.mutable.WrappedArrayBuilder
import scala.collection.mutable.WrappedArrayBuilder
scala> import scala.reflect.ClassTag
import scala.reflect.ClassTag
scala> val builder = new WrappedArrayBuilder(ClassTag.Int)
builder: scala.collection.mutable.WrappedArrayBuilder[Int] = scala.collection.mutable.WrappedArrayBuilder@56cbfb61
scala> builder ++= Vector.range(1, 17)
res0: builder.type = scala.collection.mutable.WrappedArrayBuilder@56cbfb61
scala> builder.result()
res1: scala.collection.mutable.WrappedArray[Int] = WrappedArray(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)
scala> builder.clear()
scala> builder += 100
res3: builder.type = scala.collection.mutable.WrappedArrayBuilder@56cbfb61
scala> res1
res4: scala.collection.mutable.WrappedArray[Int] = WrappedArray(100, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)
scala> builder.result()
res5: scala.collection.mutable.WrappedArray[Int] = WrappedArray(100)
```
Solution:
We should reset `capacity` to `0` when calling `result` so that
`ensureSize(1)` calls `resize(16)`, which satisfies the property of Builder
reusability. Besides mutation of previously created arrays does not happen.
Diffstat (limited to 'META-INF')
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