Initial public snapshot

With code from Bill. Thanks Bill!

1 files changed, 99 insertions, 0 deletions

diff --git a/src/bitbuf.h b/src/bitbuf.h
new file mode 100644
index 0000000..8ce5535
--- /dev/null
+++ b/src/bitbuf.h
@@ -0,0 +1,99 @@
+/*
+ * Copyright © 2021 Michael Smith <mikesmiffy128@gmail.com>
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+ * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+ * AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+ * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+ * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+ * OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+ * PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#ifndef INC_BITBUF_H
+#define INC_BITBUF_H
+
+#include <stdbool.h>
+
+#include "intdefs.h"
+
+// NOTE: This code assumes it's running on a little endian machine, because,
+// well, the game runs on a little endian machine.
+// *technically* this could break unit tests in a contrived cross-compile
+// scenario? right now none of the tests care about actual bit values, and we
+// don't cross compile, so this won't matter till later. :)
+
+// handle 8 bytes at a time (COULD do 16 with SSE, but who cares this is fine)
+typedef uvlong bitbuf_cell;
+static const int bitbuf_cell_bits = sizeof(bitbuf_cell) * 8;
+static const int bitbuf_align = _Alignof(bitbuf_cell);
+
+/* A bit buffer, ABI-compatible with bf_write defined in tier1/bitbuf.h */
+struct bitbuf {
+	union {
+		char *buf; /* NOTE: the buffer MUST be aligned as bitbuf_cell! */
+		bitbuf_cell *buf_as_cells;
+	};
+	int sz, nbits, curbit;
+	bool overflow, assert_on_overflow;
+	const char *debugname;
+};
+
+/* Append a value to the bitbuffer, with a specfied length in bits. */
+static inline void bitbuf_appendbits(struct bitbuf *bb, bitbuf_cell x,
+		int nbits) {
+	int idx = bb->curbit / bitbuf_cell_bits;
+	int shift = bb->curbit % bitbuf_cell_bits;
+	// OR into the existing cell (lower bits were already set!)
+	bb->buf_as_cells[idx] |= x << shift;
+	// assign the next cell (that also clears the upper bits for the next OR)
+	// note: if nbits fits in the first cell, this just 0s the next cell, which
+	// is absolutely fine
+	bb->buf_as_cells[idx + 1] = x >> (bitbuf_cell_bits - shift);
+	bb->curbit += nbits;
+}
+
+/* Append a byte to the bitbuffer - same as appendbits(8) but more convenient */
+static inline void bitbuf_appendbyte(struct bitbuf *bb, uchar x) {
+	bitbuf_appendbits(bb, x, 8);
+}
+
+/* Append a sequence of bytes to the bitbuffer, with length given in bytes */
+static inline void bitbuf_appendbuf(struct bitbuf *bb, const char *buf,
+		uint len) {
+	// NOTE! This function takes advantage of the fact that nothing unaligned
+	// is page aligned, so accessing slightly outside the bounds of buf can't
+	// segfault. This is absolutely definitely technically UB, but it's unit
+	// tested and apparently works in practice. If something weird happens
+	// further down the line, sorry!
+	usize unalign = (usize)buf % bitbuf_align;
+	if (unalign) {
+		// round down the pointer
+		bitbuf_cell *p = (bitbuf_cell *)((usize)buf & ~(bitbuf_align - 1));
+		// shift the stored value (if it were big endian, the shift would have
+		// to be the other way, or something)
+		bitbuf_appendbits(bb, *p >> (unalign * 8), (bitbuf_align - unalign) * 8);
+		buf += sizeof(bitbuf_cell) - unalign;
+		len -= unalign;
+	}
+	bitbuf_cell *aligned = (bitbuf_cell *)buf;
+	for (; len > sizeof(bitbuf_cell); len -= sizeof(bitbuf_cell), ++aligned) {
+		bitbuf_appendbits(bb, *aligned, bitbuf_cell_bits);
+	}
+	// unaligned end bytes
+	bitbuf_appendbits(bb, *aligned, len * 8);
+}
+
+/* Clear the bitbuffer to make it ready to append new data */
+static inline void bitbuf_reset(struct bitbuf *bb) {
+	bb->buf[0] = 0; // we have to zero out the lowest cell since it gets ORed
+	bb->curbit = 0;
+}
+
+#endif
+
+// vi: sw=4 ts=4 noet tw=80 cc=80