ytatichno opened a new pull request, #652:
URL: https://github.com/apache/commons-collections/pull/652
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According to
https://issues.apache.org/jira/projects/COLLECTIONS/issues/COLLECTIONS-878
Total summary:
- Precompute initial capacity for new HashMap in MapUtils.invertMap to avoid
resize during following insertion.
- Implementation same as JDK's HashMap static factory method.
```
// from HashMap (since 19)
static int calculateHashMapCapacity(int numMappings) {
return (int) Math.ceil(numMappings / (double) DEFAULT_LOAD_FACTOR);
}
```
Why `0.75d`:
- It is DEFAULT_LOAD_FACTOR casted to double
- load factor is being casted to double in HashMap.calculateHashMapCapacity
- For very big maps we rarely can encounter lack of float precision and have
extra resize.
We have extra resize for 50% of Maps. In example:
Let's inspect for maps with size on [4, 16].
- On size 4 will be allocated map with size 4 and when the 4th element will
be inserted the HashMap will be resized.
- On sizes 5, 6 will be allocated map with size rounded to 8 and there will
not be any resizes.
- On sizes 7, 8 will be allocated map with size rounded to 8 and there will
be resize when the 7th element will be inserted.
- On sizes 9, 10, 11, 12 will be allocated map with size rounded to 16 and
everything will be ok
- On sizes 13, 14, 15, 16 will be allocated map with size rounded to 16 and
there will be resize on 13th element insertion.
To sum up:
For each power of two we have segment from 50% to 100%. Because 50% is
previous power of two. And for each initial size from 50% to 75%
(DEFAULT_LOAD_FACTOR) everything ok, but for sizes from 75% to 100% we have
extra resize. So we have 50% of probability to have extra resize.
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