[MRG] raise error if mass mismatch in emd2 (#386)

* Two lines added in the function emd2 to ensure that the distributions have the same mass (same as it already was in the function emd).

* The same mass test has been moved inside the function f(b) to be compatible with emd2 with multiple b.

* Test added.
The function test_emd_dimension_and_mass_mismatch (in test/test_ot.py) has been modified to check for mass mismatch with emd2.

* Add PR in releases.md

* Merge and add PR in releases.md

* Add name in contributors.md

* Correction contribution in contributors.md

* Move test on mass outside of functions f(b)

* Update doc of emd and emd2

Co-authored-by: Camille Le Coz <clecoz@camelot.ipsl.polytechnique.fr>
Co-authored-by: Rémi Flamary <remi.flamary@gmail.com>

4 files changed, 14 insertions, 0 deletions

diff --git a/CONTRIBUTORS.md b/CONTRIBUTORS.md
index 0909b14..c535c09 100644
--- a/CONTRIBUTORS.md
+++ b/CONTRIBUTORS.md
@@ -38,6 +38,7 @@ The contributors to this library are:
 * [Nathan Cassereau](https://github.com/ncassereau-idris) (Backends)
 * [Cédric Vincent-Cuaz](https://github.com/cedricvincentcuaz) (Graph Dictionary Learning)
 * [Eloi Tanguy](https://github.com/eloitanguy) (Generalized Wasserstein Barycenters)
+* [Camille Le Coz](https://www.linkedin.com/in/camille-le-coz-8593b91a1/) (EMD2 debug)
 
 ## Acknowledgments
 
diff --git a/RELEASES.md b/RELEASES.md
index b384617..78a7d9e 100644
--- a/RELEASES.md
+++ b/RELEASES.md
@@ -17,6 +17,7 @@
 - Fixed an issue where pointers would overflow in the EMD solver, returning an
 incomplete transport plan above a certain size (slightly above 46k, its square being
 roughly 2^31) (PR #381)
+- Error raised when mass mismatch in emd2 (PR #386)
 
 
 ## 0.8.2
diff --git a/ot/lp/__init__.py b/ot/lp/__init__.py
index 572781d..17411d0 100644
--- a/ot/lp/__init__.py
+++ b/ot/lp/__init__.py
@@ -230,6 +230,8 @@ def emd(a, b, M, numItermax=100000, log=False, center_dual=True, numThreads=1):
         If this behaviour is unwanted, please make sure to provide a
         floating point input.
 
+    .. note:: An error will be raised if the vectors :math:`\mathbf{a}` and :math:`\mathbf{b}` do not sum to the same value.
+
     Uses the algorithm proposed in :ref:`[1] <references-emd>`.
 
     Parameters
@@ -389,6 +391,8 @@ def emd2(a, b, M, processes=1,
         If this behaviour is unwanted, please make sure to provide a
         floating point input.
 
+    .. note:: An error will be raised if the vectors :math:`\mathbf{a}` and :math:`\mathbf{b}` do not sum to the same value.
+
     Uses the algorithm proposed in :ref:`[1] <references-emd2>`.
 
     Parameters
@@ -481,6 +485,11 @@ def emd2(a, b, M, processes=1,
     assert (a.shape[0] == M.shape[0] and b.shape[0] == M.shape[1]), \
         "Dimension mismatch, check dimensions of M with a and b"
 
+    # ensure that same mass
+    np.testing.assert_almost_equal(a.sum(0),
+                                   b.sum(0,keepdims=True), err_msg='a and b vector must have the same sum')
+    b = b * a.sum(0) / b.sum(0,keepdims=True)
+
     asel = a != 0
 
     numThreads = check_number_threads(numThreads)
diff --git a/test/test_ot.py b/test/test_ot.py
index ba3ef6a..9a4e175 100644
--- a/test/test_ot.py
+++ b/test/test_ot.py
@@ -29,9 +29,12 @@ def test_emd_dimension_and_mass_mismatch():
 
     np.testing.assert_raises(AssertionError, ot.emd2, a, a, M)
 
+    # test emd and emd2 for mass mismatch
+    a = ot.utils.unif(n_samples)
     b = a.copy()
     a[0] = 100
     np.testing.assert_raises(AssertionError, ot.emd, a, b, M)
+    np.testing.assert_raises(AssertionError, ot.emd2, a, b, M)
 
 
 def test_emd_backends(nx):