Improve performance of multiple calls to H5Sget_select_elem_pointlist (…

…#270) (#277)

* Cache the pointer to the next point to process after the last call to
H5S__get_select_elem_pointlist.  This allows the normal process of
iterating over the points in batches to be much more efficient, as the
library does not need to traverse the entirety of the preceding points
every time the funciton is re-entered.

* Update RELEASE.txt for point selection iteration performance fix.

release_docs/RELEASE.txt

@@ -316,6 +316,15 @@ New Features
 
     Library
     -------
+    - Improved performance of H5Sget_select_elem_pointlist
+
+      Modified library to cache the point after the last block of points
+      retrieved by H5Sget_select_elem_pointlist, so a subsequent call to the
+      same function to retrieve the next block of points from the list can
+      proceed immediately without needing to iterate over the point list.
+
+      (NAF - 2021/01/19)
+
     - Add S3 and HDFS VFDs to HDF5 maintenance
 
       Fix windows requirements and java tests. Windows requires CMake 3.13.

src/H5Spkg.h

@@ -95,6 +95,11 @@ struct H5S_pnt_node_t {
 /* Information about point selection list */
 typedef struct {
     H5S_pnt_node_t *head; /* Pointer to head of point list */
+
+    hsize_t last_idx;     /* Index of the point after the last returned from H5S__get_select_elem_pointlist() */
+    H5S_pnt_node_t *last_idx_pnt; /* Point after the last returned from H5S__get_select_elem_pointlist().
+        * If we ever add a way to remove points or add points in the middle of
+        * the pointlist we will need to invalidate these fields. */
 } H5S_pnt_list_t;
 
 /* Information about new-style hyperslab spans */

src/H5Spoint.c

@@ -618,6 +618,10 @@ H5S_point_copy(H5S_t *dst, const H5S_t *src, hbool_t H5_ATTR_UNUSED share_select
         curr = curr->next;
     } /* end while */
 
+    /* Clear cached iteration point */
+    dst->select.sel_info.pnt_lst->last_idx = 0;
+    dst->select.sel_info.pnt_lst->last_idx_pnt = NULL;
+
 done:
     if (ret_value < 0 && dst->select.sel_info.pnt_lst) {
         /* Traverse the (incomplete?) dst list, freeing all memory */
@@ -956,6 +960,7 @@ H5S_point_deserialize(H5S_t *space, const uint8_t *buf)
 static herr_t
 H5S_get_select_elem_pointlist(H5S_t *space, hsize_t startpoint, hsize_t numpoints, hsize_t *buf)
 {
+    const hsize_t   endpoint = startpoint + numpoints; /* Index of last point in iteration */
     H5S_pnt_node_t *node; /* Point node */
     unsigned        rank; /* Dataspace rank */
 
@@ -967,14 +972,20 @@ H5S_get_select_elem_pointlist(H5S_t *space, hsize_t startpoint, hsize_t numpoint
     /* Get the dataspace extent rank */
     rank = space->extent.rank;
 
-    /* Get the head of the point list */
-    node = space->select.sel_info.pnt_lst->head;
+    /* Check for cached point at the correct index */
+    if(space->select.sel_info.pnt_lst->last_idx_pnt
+            && startpoint == space->select.sel_info.pnt_lst->last_idx)
+        node = space->select.sel_info.pnt_lst->last_idx_pnt;
+    else {
+        /* Get the head of the point list */
+        node = space->select.sel_info.pnt_lst->head;
 
-    /* Iterate to the first point to return */
-    while (node != NULL && startpoint > 0) {
-        startpoint--;
-        node = node->next;
-    } /* end while */
+        /* Iterate to the first point to return */
+        while (node != NULL && startpoint > 0) {
+            startpoint--;
+            node = node->next;
+        } /* end while */
+    } /* end else */
 
     /* Iterate through the node, copying each point's information */
     while (node != NULL && numpoints > 0) {
@@ -984,6 +995,10 @@ H5S_get_select_elem_pointlist(H5S_t *space, hsize_t startpoint, hsize_t numpoint
         node = node->next;
     } /* end while */
 
+    /* Cached next point in iteration */
+    space->select.sel_info.pnt_lst->last_idx = endpoint;
+    space->select.sel_info.pnt_lst->last_idx_pnt = node;
+
     FUNC_LEAVE_NOAPI(SUCCEED)
 } /* H5S_get_select_elem_pointlist() */
 
@@ -1494,6 +1509,10 @@ H5S_point_project_simple(const H5S_t *base_space, H5S_t *new_space, hsize_t *off
         } /* end while */
     }     /* end else */
 
+    /* Clear cached iteration point */
+    new_space->select.sel_info.pnt_lst->last_idx = 0;
+    new_space->select.sel_info.pnt_lst->last_idx_pnt = NULL;
+
     /* Number of elements selected will be the same */
     new_space->select.num_elem = base_space->select.num_elem;