dca: Add math helpers.

They will be used by the integer core decoder.

libavcodec/dcamath.h

+/*
+ * This file is part of Libav.
+ *
+ * Libav is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * Libav is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libavutil/common.h"
+
+
+// clip a signed integer into the (-2^23), (2^23-1) range
+static inline int dca_clip23(int a)
+{
+    return av_clip_intp2(a, 23);
+}
+
+static inline int32_t dca_norm(int64_t a, int bits)
+{
+    if (bits > 0)
+        return (int32_t)((a + (INT64_C(1) << (bits - 1))) >> bits);
+    else
+        return (int32_t)a;
+}
+
+static inline int64_t dca_round(int64_t a, int bits)
+{
+    if (bits > 0)
+        return (a + (INT64_C(1) << (bits - 1))) & ~((INT64_C(1) << bits) - 1);
+    else
+        return a;
+}