Add simplistic bitstream writer implementation

1 files changed, 59 insertions, 0 deletions

diff --git a/bitstream.c b/bitstream.c
new file mode 100644
index 0000000..1f3242e
--- /dev/null
+++ b/bitstream.c
@@ -0,0 +1,59 @@
+/*
+ * Copyright (C) 2023 Mikhail Burakov. This file is part of streamer.
+ *
+ * streamer is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * streamer 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with streamer.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#include "bitstream.h"
+
+void BitstreamAppend(struct Bitstream* bitstream, size_t size, uint32_t bits) {
+  uint8_t* ptr = (uint8_t*)bitstream->data + bitstream->size / 8;
+  size_t vacant_bits = 8 - bitstream->size % 8;
+  *ptr &= ~0 << vacant_bits;
+
+  if (vacant_bits >= size) {
+    *ptr |= bits << (vacant_bits - size);
+    bitstream->size += size;
+    return;
+  }
+
+  *ptr |= bits >> (size - vacant_bits);
+  bitstream->size += vacant_bits;
+  BitstreamAppend(bitstream, size - vacant_bits, bits);
+}
+
+void BitstreamAppendUE(struct Bitstream* bitstream, uint32_t bits) {
+  size_t size = 0;
+  uint32_t dummy = ++bits;
+  while (dummy) {
+    dummy >>= 1;
+    size++;
+  }
+  BitstreamAppend(bitstream, size - 1, 0);
+  BitstreamAppend(bitstream, size, bits);
+}
+
+void BitstreamAppendSE(struct Bitstream* bitstream, int32_t bits) {
+  BitstreamAppendUE(
+      bitstream, bits <= 0 ? (uint32_t)(-2 * bits) : (uint32_t)(2 * bits - 1));
+}
+
+void BitstreamByteAlign(struct Bitstream* bitstream) {
+  uint8_t* ptr = (uint8_t*)bitstream->data + bitstream->size / 8;
+  size_t vacant_bits = 8 - bitstream->size % 8;
+  if (vacant_bits == 8) return;
+
+  *ptr &= ~0 << vacant_bits;
+  bitstream->size += vacant_bits;
+}