Fix T48167: Mesh select random factor is not accurate

Selecting random percentage would not reliably guarantee the precise
percentage of selected elements now randomize an index array instead
and use the precise number of elements from this array.

Note that this change has only been made to edit-mesh
and should be applied to all random selection operators.

1 files changed, 31 insertions, 10 deletions

diff --git a/source/blender/editors/mesh/editmesh_select.c b/source/blender/editors/mesh/editmesh_select.c
index f5d91f29f74..0aeaee57548 100644
--- a/source/blender/editors/mesh/editmesh_select.c
+++ b/source/blender/editors/mesh/editmesh_select.c
@@ -4632,35 +4632,56 @@ static int edbm_select_random_exec(bContext *C, wmOperator *op)
       seed_iter += BLI_ghashutil_strhash_p(obedit->id.name);
     }
 
-    RNG *rng = BLI_rng_new_srandom(seed_iter);
-
     if (em->selectmode & SCE_SELECT_VERTEX) {
+      int elem_map_len = 0;
+      BMVert **elem_map = MEM_mallocN(sizeof(*elem_map) * em->bm->totvert, __func__);
       BMVert *eve;
       BM_ITER_MESH (eve, &iter, em->bm, BM_VERTS_OF_MESH) {
-        if (!BM_elem_flag_test(eve, BM_ELEM_HIDDEN) && BLI_rng_get_float(rng) < randfac) {
-          BM_vert_select_set(em->bm, eve, select);
+        if (!BM_elem_flag_test(eve, BM_ELEM_HIDDEN)) {
+          elem_map[elem_map_len++] = eve;
         }
       }
+
+      BLI_array_randomize(elem_map, sizeof(*elem_map), elem_map_len, seed);
+      const int count_select = elem_map_len * randfac;
+      for (int i = 0; i < count_select; i++) {
+        BM_vert_select_set(em->bm, elem_map[i], select);
+      }
+      MEM_freeN(elem_map);
     }
     else if (em->selectmode & SCE_SELECT_EDGE) {
+      int elem_map_len = 0;
+      BMEdge **elem_map = MEM_mallocN(sizeof(*elem_map) * em->bm->totedge, __func__);
       BMEdge *eed;
       BM_ITER_MESH (eed, &iter, em->bm, BM_EDGES_OF_MESH) {
-        if (!BM_elem_flag_test(eed, BM_ELEM_HIDDEN) && BLI_rng_get_float(rng) < randfac) {
-          BM_edge_select_set(em->bm, eed, select);
+        if (!BM_elem_flag_test(eed, BM_ELEM_HIDDEN)) {
+          elem_map[elem_map_len++] = eed;
         }
       }
+      BLI_array_randomize(elem_map, sizeof(*elem_map), elem_map_len, seed);
+      const int count_select = elem_map_len * randfac;
+      for (int i = 0; i < count_select; i++) {
+        BM_edge_select_set(em->bm, elem_map[i], select);
+      }
+      MEM_freeN(elem_map);
     }
     else {
+      int elem_map_len = 0;
+      BMFace **elem_map = MEM_mallocN(sizeof(*elem_map) * em->bm->totface, __func__);
       BMFace *efa;
       BM_ITER_MESH (efa, &iter, em->bm, BM_FACES_OF_MESH) {
-        if (!BM_elem_flag_test(efa, BM_ELEM_HIDDEN) && BLI_rng_get_float(rng) < randfac) {
-          BM_face_select_set(em->bm, efa, select);
+        if (!BM_elem_flag_test(efa, BM_ELEM_HIDDEN)) {
+          elem_map[elem_map_len++] = efa;
         }
       }
+      BLI_array_randomize(elem_map, sizeof(*elem_map), elem_map_len, seed);
+      const int count_select = elem_map_len * randfac;
+      for (int i = 0; i < count_select; i++) {
+        BM_face_select_set(em->bm, elem_map[i], select);
+      }
+      MEM_freeN(elem_map);
     }
 
-    BLI_rng_free(rng);
-
     if (select) {
       /* was EDBM_select_flush, but it over select in edge/face mode */
       EDBM_selectmode_flush(em);