Add get_lower_levels_ctx{|_eob}

This is to separate the logic of getting lower level coeff ctx
for eob and non-eob position

BUG=aomedia:1369

Change-Id: Id1d85d7fbb67e57c58479e00b633d794381132ea

av1/common/txb_common.h

@@ -509,6 +509,21 @@ static INLINE int get_nz_map_ctx_from_stats(
     return ctx + map[AOMMIN(idx, 2)];
   }
 }
+static INLINE int get_lower_levels_ctx_eob(int bwl, int height, int scan_idx) {
+  if (scan_idx == 0) return 0;
+  if (scan_idx <= (height << bwl) / 8) return 1;
+  if (scan_idx <= (height << bwl) / 4) return 2;
+  return 3;
+}
+
+static INLINE int get_lower_levels_ctx(const uint8_t *levels, int coeff_idx,
+                                       int bwl, TX_SIZE tx_size,
+                                       TX_TYPE tx_type) {
+  const TX_CLASS tx_class = tx_type_to_class[tx_type];
+  const int stats =
+      get_nz_mag(levels + get_padded_idx(coeff_idx, bwl), bwl, tx_class);
+  return get_nz_map_ctx_from_stats(stats, coeff_idx, bwl, tx_size, tx_class);
+}
 
 static INLINE int get_nz_map_ctx(const uint8_t *const levels,
                                  const int coeff_idx, const int bwl,

av1/decoder/decodetxb.c

@@ -196,12 +196,11 @@ uint8_t av1_read_coeffs_txb(const AV1_COMMON *const cm, MACROBLOCKD *const xd,
   *eob = rec_eob_pos(eob_pt, eob_extra);
 
   {
-    // Read the last non-zero coefficient
+    // Read the non-zero coefficient with scan index eob-1
     // TODO(angiebird): Put this into a function
     const int c = *eob - 1;
     const int pos = scan[c];
-    const int coeff_ctx =
-        get_nz_map_ctx(levels, pos, bwl, height, c, 1, tx_size, tx_type);
+    const int coeff_ctx = get_lower_levels_ctx_eob(bwl, height, c);
     const int nsymbs = 3;
     aom_cdf_prob *cdf =
         ec_ctx->coeff_base_eob_cdf[txs_ctx][plane_type][coeff_ctx];
@@ -228,7 +227,7 @@ uint8_t av1_read_coeffs_txb(const AV1_COMMON *const cm, MACROBLOCKD *const xd,
     const int c = *eob - 1 - i;
     const int pos = scan[c];
     const int coeff_ctx =
-        get_nz_map_ctx(levels, pos, bwl, height, c, 0, tx_size, tx_type);
+        get_lower_levels_ctx(levels, pos, bwl, tx_size, tx_type);
     aom_cdf_prob *cdf = ec_ctx->coeff_base_cdf[txs_ctx][plane_type][coeff_ctx];
     const int nsymbs = 4;
     int level = aom_read_symbol(r, cdf, nsymbs, ACCT_STR);