hevc: derive partial merge list

The merge list only needs to be derived up to the merge index.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/hevc_mvs.c

@@ -311,6 +311,7 @@ static void derive_spatial_merge_candidates(HEVCContext *s, int x0, int y0,
                                             int nPbW, int nPbH,
                                             int log2_cb_size,
                                             int singleMCLFlag, int part_idx,
+                                            int merge_idx,
                                             struct MvField mergecandlist[])
 {
     HEVCLocalContext *lc   = &s->HEVClc;
@@ -379,8 +380,12 @@ static void derive_spatial_merge_candidates(HEVCContext *s, int x0, int y0,
         is_available_a1 = 0;
     }
 
-    if (is_available_a1)
-        mergecandlist[nb_merge_cand++] = TAB_MVF_PU(A1);
+    if (is_available_a1) {
+        mergecandlist[0] = TAB_MVF_PU(A1);
+        if (merge_idx == 0)
+            return;
+        nb_merge_cand++;
+    }
 
     // above spatial merge candidate
     is_available_b1 = AVAILABLE(cand_up, B1);
@@ -411,8 +416,12 @@ static void derive_spatial_merge_candidates(HEVCContext *s, int x0, int y0,
     if (is_available_b1 && is_available_b0)
         check_MER = !COMPARE_MV_REFIDX(B0, B1);
 
-    if (is_available_b0 && check_MER)
-        mergecandlist[nb_merge_cand++] = TAB_MVF_PU(B0);
+    if (is_available_b0 && check_MER) {
+        mergecandlist[nb_merge_cand] = TAB_MVF_PU(B0);
+        if (merge_idx == nb_merge_cand)
+            return;
+        nb_merge_cand++;
+    }
 
     // left bottom spatial merge candidate
     check_MER = 1;
@@ -426,8 +435,12 @@ static void derive_spatial_merge_candidates(HEVCContext *s, int x0, int y0,
     if (is_available_a1 && is_available_a0)
         check_MER = !COMPARE_MV_REFIDX(A0, A1);
 
-    if (is_available_a0 && check_MER)
-        mergecandlist[nb_merge_cand++] = TAB_MVF_PU(A0);
+    if (is_available_a0 && check_MER) {
+        mergecandlist[nb_merge_cand] = TAB_MVF_PU(A0);
+        if (merge_idx == nb_merge_cand)
+            return;
+        nb_merge_cand++;
+    }
 
     // above left spatial merge candidate
     check_MER = 1;
@@ -443,8 +456,12 @@ static void derive_spatial_merge_candidates(HEVCContext *s, int x0, int y0,
     if (is_available_b1 && is_available_b2)
         check_MER_1 = !COMPARE_MV_REFIDX(B2, B1);
 
-    if (is_available_b2 && check_MER && check_MER_1 && nb_merge_cand != 4)
-        mergecandlist[nb_merge_cand++] = TAB_MVF_PU(B2);
+    if (is_available_b2 && check_MER && check_MER_1 && nb_merge_cand != 4) {
+        mergecandlist[nb_merge_cand] = TAB_MVF_PU(B2);
+        if (merge_idx == nb_merge_cand)
+            return;
+        nb_merge_cand++;
+    }
 
     // temporal motion vector candidate
     if (s->sh.slice_temporal_mvp_enabled_flag &&
@@ -468,6 +485,8 @@ static void derive_spatial_merge_candidates(HEVCContext *s, int x0, int y0,
                 mergecandlist[nb_merge_cand].mv[1]      = mv_l1_col;
                 mergecandlist[nb_merge_cand].ref_idx[1] = 0;
             }
+            if (merge_idx == nb_merge_cand)
+                return;
             nb_merge_cand++;
         }
     }
@@ -500,6 +519,8 @@ static void derive_spatial_merge_candidates(HEVCContext *s, int x0, int y0,
                 mergecandlist[nb_merge_cand].mv[1].x      = l1_cand.mv[1].x;
                 mergecandlist[nb_merge_cand].mv[1].y      = l1_cand.mv[1].y;
                 mergecandlist[nb_merge_cand].is_intra     = 0;
+                if (merge_idx == nb_merge_cand)
+                    return;
                 nb_merge_cand++;
             }
         }
@@ -517,6 +538,8 @@ static void derive_spatial_merge_candidates(HEVCContext *s, int x0, int y0,
         mergecandlist[nb_merge_cand].ref_idx[0]   = zero_idx < nb_refs ? zero_idx : 0;
         mergecandlist[nb_merge_cand].ref_idx[1]   = zero_idx < nb_refs ? zero_idx : 0;
 
+        if (merge_idx == nb_merge_cand)
+            return;
         nb_merge_cand++;
         zero_idx++;
     }
@@ -547,7 +570,8 @@ void ff_hevc_luma_mv_merge_mode(HEVCContext *s, int x0, int y0, int nPbW,
 
     ff_hevc_set_neighbour_available(s, x0, y0, nPbW, nPbH);
     derive_spatial_merge_candidates(s, x0, y0, nPbW, nPbH, log2_cb_size,
-                                    singleMCLFlag, part_idx, mergecand_list);
+                                    singleMCLFlag, part_idx,
+                                    merge_idx, mergecand_list);
 
     if (mergecand_list[merge_idx].pred_flag[0] == 1 &&
         mergecand_list[merge_idx].pred_flag[1] == 1 &&