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/*
* 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
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