1 files changed, 85 insertions, 0 deletions
diff --git a/vp9/decoder/vp9_dsubexp.c b/vp9/decoder/vp9_dsubexp.c
new file mode 100644
index 000000000..0f204dfd2
--- /dev/null
+++ b/vp9/decoder/vp9_dsubexp.c
@@ -0,0 +1,85 @@
+/*
+  Copyright (c) 2010 The WebM project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "vp9/common/vp9_entropy.h"
+
+#include "vp9/decoder/vp9_dsubexp.h"
+
+static int inv_recenter_nonneg(int v, int m) {
+  if (v > 2 * m)
+    return v;
+
+  return v % 2 ? m - (v + 1) / 2 : m + v / 2;
+}
+
+static int decode_uniform(vp9_reader *r, int n) {
+  int v;
+  const int l = get_unsigned_bits(n);
+  const int m = (1 << l) - n;
+  if (!l)
+    return 0;
+
+  v = vp9_read_literal(r, l - 1);
+  return v < m ?  v : (v << 1) - m + vp9_read_bit(r);
+}
+
+
+static int merge_index(int v, int n, int modulus) {
+  int max1 = (n - 1 - modulus / 2) / modulus + 1;
+  if (v < max1) {
+    v = v * modulus + modulus / 2;
+  } else {
+    int w;
+    v -= max1;
+    w = v;
+    v += (v + modulus - modulus / 2) / modulus;
+    while (v % modulus == modulus / 2 ||
+           w != v - (v + modulus - modulus / 2) / modulus) v++;
+  }
+  return v;
+}
+
+static int inv_remap_prob(int v, int m) {
+  const int n = 255;
+
+  v = merge_index(v, n - 1, MODULUS_PARAM);
+  m--;
+  if ((m << 1) <= n) {
+    return 1 + inv_recenter_nonneg(v + 1, m);
+  } else {
+    return n - inv_recenter_nonneg(v + 1, n - 1 - m);
+  }
+}
+
+static int decode_term_subexp(vp9_reader *r, int k, int num_syms) {
+  int i = 0, mk = 0, word;
+  while (1) {
+    const int b = i ? k + i - 1 : k;
+    const int a = 1 << b;
+    if (num_syms <= mk + 3 * a) {
+      word = decode_uniform(r, num_syms - mk) + mk;
+      break;
+    } else {
+      if (vp9_read_bit(r)) {
+        i++;
+        mk += a;
+      } else {
+        word = vp9_read_literal(r, b) + mk;
+        break;
+      }
+    }
+  }
+  return word;
+}
+
+void vp9_diff_update_prob(vp9_reader *r, vp9_prob* p) {
+  int delp = decode_term_subexp(r, SUBEXP_PARAM, 255);
+  *p = (vp9_prob)inv_remap_prob(delp, *p);
+}