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/** @file bitbuffer.h
*
* Implementation of a circular buffer of bits.
*
* @author Daniel Silverstone
*/
#include <stdlib.h>
#include "bitbuffer.h"
#ifndef MIN
#define MIN(a,b) (((a)<(b))?(a):(b))
#endif
struct bitbuffer_s {
int size;
int firstused;
int nextfree;
int bitsused;
BitField bits[0];
};
#define BYTE_FOR_BIT(n) (n>>3)
#define SHIFT_FOR_BIT(n) (n&7)
BitBuffer
bitbuffer_new(const int size)
{
int bytesize = ((size + 7) & ~7) >> 3;
BitBuffer ret = (BitBuffer)(malloc(sizeof(struct bitbuffer_s) + bytesize));
ret->size = size;
ret->firstused = 0;
ret->nextfree = 0;
ret->bitsused = 0;
return ret;
}
void
bitbuffer_free(BitBuffer buf)
{
free(buf);
}
int
bitbuffer_free_space(const BitBuffer buf)
{
return buf->size - buf->bitsused;
}
int
bitbuffer_available_bits(const BitBuffer buf)
{
return buf->bitsused;
}
int
bitbuffer_add_bits(BitBuffer buf, const BitField bits, int count)
{
int bits_to_add = MIN(MIN(8, count), bitbuffer_free_space(buf));
int i;
for (i = 0; i < bits_to_add; ++i) {
BitField subfield = buf->bits[BYTE_FOR_BIT(buf->nextfree)];
if (bits & (1<<SHIFT_FOR_BIT(i)))
subfield |= (1<<SHIFT_FOR_BIT(buf->nextfree));
else
subfield &= ~(1<<SHIFT_FOR_BIT(buf->nextfree));
buf->bits[BYTE_FOR_BIT(buf->nextfree++)] = subfield;
if (buf->nextfree == buf->size)
buf->nextfree = 0;
}
buf->bitsused += bits_to_add;
return bits_to_add;
}
int
bitbuffer_extract_bits(BitBuffer buf, BitField *bits, int count)
{
int bits_to_extract = MIN(MIN(8, count), bitbuffer_available_bits(buf));
int i;
for (i = 0; i < bits_to_extract; ++i) {
BitField subfield = buf->bits[BYTE_FOR_BIT(buf->firstused)];
if (subfield & (1<<SHIFT_FOR_BIT(buf->firstused++)))
*bits |= (1<<SHIFT_FOR_BIT(i));
else
*bits &= ~(1<<SHIFT_FOR_BIT(i));
if (buf->firstused == buf->size)
buf->firstused = 0;
}
buf->bitsused -= bits_to_extract;
return bits_to_extract;
}
#ifdef TEST
#include <stdio.h>
void
xassert(const char* msg, int val, int line, BitBuffer buf, BitField bits)
{
if (val == 0) {
fprintf(stdout, "FAILURE on line %d of condition %s\n", line, msg);
fprintf(stdout, "Buffer at %p: size %d firstused %d nextfree %d\n",
buf, buf->size, buf->firstused, buf->nextfree);
fprintf(stdout, "Bitfield is %02x\n", bits);
exit(1);
}
}
#ifdef assert
#undef assert
#endif
#define assert(X) xassert(#X, X, __LINE__, buf, bits)
int main(int argc, char** argv)
{
BitBuffer buf;
BitField bits = 0xA5;
buf = bitbuffer_new(32);
assert(buf != NULL);
assert(bitbuffer_free_space(buf) == 32);
assert(bitbuffer_available_bits(buf) == 0);
assert(bitbuffer_add_bits(buf, bits, 8) == 8);
assert(bitbuffer_add_bits(buf, bits, 8) == 8);
assert(bitbuffer_add_bits(buf, bits, 8) == 8);
assert(bitbuffer_add_bits(buf, bits, 8) == 8);
assert(bitbuffer_add_bits(buf, bits, 8) == 0);
assert(bitbuffer_free_space(buf) == 0);
assert(bitbuffer_available_bits(buf) == 32);
assert(bitbuffer_extract_bits(buf, &bits, 8) == 8);
assert(bits == 0xA5);
assert(bitbuffer_extract_bits(buf, &bits, 8) == 8);
assert(bits == 0xA5);
assert(bitbuffer_extract_bits(buf, &bits, 8) == 8);
assert(bits == 0xA5);
assert(bitbuffer_extract_bits(buf, &bits, 8) == 8);
assert(bits == 0xA5);
assert(bitbuffer_extract_bits(buf, &bits, 8) == 0);
bits = 0x96;
assert(bitbuffer_add_bits(buf, bits, 8) == 8);
bits = 0xF;
assert(bitbuffer_add_bits(buf, bits, 4) == 4);
bits = 0x96;
assert(bitbuffer_add_bits(buf, bits, 8) == 8);
bits = 0xF;
assert(bitbuffer_add_bits(buf, bits, 4) == 4);
assert(bitbuffer_extract_bits(buf, &bits, 8) == 8);
assert(bits == 0x96);
assert(bitbuffer_extract_bits(buf, &bits, 8) == 8);
assert(bits == 0x6F);
assert(bitbuffer_extract_bits(buf, &bits, 4) == 4);
assert((bits & 0xf) == 0x9);
assert(bitbuffer_extract_bits(buf, &bits, 8) == 4);
assert((bits & 0xf) == 0xF);
return 0;
}
#endif
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