KMeans returns less than k clusters of k input points

.travis.yml

@@ -14,6 +14,12 @@ addons:
 
 before_install:
   - sudo sh -c "echo /usr/lib/libeatmydata/libeatmydata.so >> /etc/ld.so.preload"
+  - sudo service postgresql stop 9.6
+  - sudo find /etc -user postgres | xargs sudo chown travis
+  - sudo find /var -user postgres | xargs sudo chown travis
+  - sudo usermod -u 1000 postgres
+  - sudo adduser travis ssl-cert
+  - sudo service postgresql start 9.6
   - sudo add-apt-repository --yes ppa:ubuntugis/ppa
   - sudo add-apt-repository --yes ppa:ubuntugis/ubuntugis-unstable
   - sudo apt-get update -qq

NEWS

@@ -29,6 +29,8 @@ PostGIS 2.5.0
   - #3097, Really allow MULTILINESTRING blades in ST_Split() (Paul Ramsey)
   - #3942, geojson: Do not include private header for json-c >= 0.13 (Björn Esser)
   - #3954, ST_GeometricMedian now supports point weights (Darafei Praliaskouski)
+  - #3965, ST_ClusterKMeans used to lose some clusters on initialization
+           (Darafei Praliaskouski)
 
 
 PostGIS 2.4.0

liblwgeom/lwkmeans.c

@@ -201,6 +201,7 @@ lwgeom_cluster_2d_kmeans(const LWGEOM **geoms, int ngeoms, int k)
 			if (seen[j] == closest)
 			{
 				closest = (closest + 1) % config.num_objs;
+				j = 0;
 			}
 			else
 			{

regress/lwgeom_regress.sql

@@ -169,3 +169,10 @@ SELECT 'ST_Angle_empty' , St_Angle(p1,p2,p3,p4)
 SELECT 'ST_Angle_2_lines', St_Angle(l1,l2)
 	FROM ST_GeomFromtext('LINESTRING(0 1,0 0)') AS l1
 	, ST_GeomFromtext('LINESTRING(1 0, 2 0)') AS l2 ;
+
+--- ST_ClusterKMeans
+select '#3965', count(distinct cid), count(*) from (
+	with points as (select ST_MakePoint(x,y) geom from generate_series(1,5) x, generate_series(1,5) y)
+  select ST_ClusterKMeans(geom, 25) over () as cid, geom
+  from points) z;
+

regress/lwgeom_regress_expected

@@ -32,3 +32,4 @@ ST_Angle_3_pts|1.5707963267949
 ERROR:  Operation on mixed SRID geometries
 ERROR:  Empty geometry
 ST_Angle_2_lines|4.71238898038469
+#3965|25|25