Compare different Rouge implementations  
Updates - the differences between the following three
(1) https://github.com/bheinzerling/pyrouge (previously popular. The one I trust the most)
(2) https://github.com/google-research/google-research/tree/master/rouge
(3) https://github.com/pltrdy/files2rouge (used in fairseq)
can be explained by two things, stemming and handling multiple sentences.

Stemming: 
(1), (2): default is no stemming. (3): default is with stemming ==> No stemming is the correct default as you did [here](https://github.com/huggingface/datasets/blob/master/metrics/rouge/rouge.py#L84)

Multiple sentences:
(1) `rougeL` splits text using `\n`
(2) `rougeL` ignores `\n`. 
(2) `rougeLsum` splits text using `\n`
(3) `rougeL` splits text using `.`

For (2), `rougeL` and `rougeLsum` are identical if the sequence doesn't contain `\n`. With `\n`, it is `rougeLsum` that matches (1) not `rougeL`. 

Overall, and as far as I understand, for your implementation here https://github.com/huggingface/datasets/blob/master/metrics/rouge/rouge.py#L65 to match the default, you only need to change `rougeL` [here](https://github.com/huggingface/datasets/blob/master/metrics/rouge/rouge.py#L86) to `rougeLsum` to correctly compute metrics for text with newlines.

Tagging @sshleifer who might be interested.
I used RougeL implementation provided in `datasets` [here](https://github.com/huggingface/datasets/blob/master/metrics/rouge/rouge.py) and it gives numbers that match those reported in the pegasus paper but very different from those reported in other papers, [this](https://arxiv.org/pdf/1909.03186.pdf) for example.
Can you make sure the google-research implementation you are using matches the official perl implementation? 
There are a couple of python wrappers around the perl implementation, [this](https://pypi.org/project/pyrouge/) has been commonly used, and [this](https://github.com/pltrdy/files2rouge) is used in fairseq). 
There's also a python reimplementation [here](https://github.com/pltrdy/rouge) but its RougeL numbers are way off. 

Compare different Rouge implementations   
 I used RougeL implementation provided in `datasets` [here](https://github.com/huggingface/datasets/blob/master/metrics/rouge/rouge.py) and it gives numbers that match those reported in the pegasus paper but very different from those reported in other papers, [this](https://arxiv.org/pdf/1909.03186.pdf) for example.
Can you make sure the google-research implementation you are using matches the official perl implementation? 
There are a couple of python wrappers around the perl implementation, [this](https://pypi.org/project/pyrouge/) has been commonly used, and [this](https://github.com/pltrdy/files2rouge) is used in fairseq). 
There's also a python reimplementation [here](https://github.com/pltrdy/rouge) but its RougeL numbers are way off. 
 
 Updates - the differences between the following three
(1) https://github.com/bheinzerling/pyrouge (previously popular. The one I trust the most)
(2) https://github.com/google-research/google-research/tree/master/rouge
(3) https://github.com/pltrdy/files2rouge (used in fairseq)
can be explained by two things, stemming and handling multiple sentences.

Stemming: 
(1), (2): default is no stemming. (3): default is with stemming ==> No stemming is the correct default as you did [here](https://github.com/huggingface/datasets/blob/master/metrics/rouge/rouge.py#L84)

Multiple sentences:
(1) `rougeL` splits text using `\n`
(2) `rougeL` ignores `\n`. 
(2) `rougeLsum` splits text using `\n`
(3) `rougeL` splits text using `.`

For (2), `rougeL` and `rougeLsum` are identical if the sequence doesn't contain `\n`. With `\n`, it is `rougeLsum` that matches (1) not `rougeL`. 

Overall, and as far as I understand, for your implementation here https://github.com/huggingface/datasets/blob/master/metrics/rouge/rouge.py#L65 to match the default, you only need to change `rougeL` [here](https://github.com/huggingface/datasets/blob/master/metrics/rouge/rouge.py#L86) to `rougeLsum` to correctly compute metrics for text with newlines.

Tagging @sshleifer who might be interested.