/*
 * ugly-but-mostly-portable packed-int-saving routines
 * determines endianness at run-time
 */

#include <stdio.h>
#include <assert.h>
#include "psave.h"

/* indices to halves of a short */
static int low;
static int high;

/* set indices depending on endianness */
void set_end(void)
{
  low = 1;
  low = *((unsigned char *)&low);
  high = 1 - low;
}

size_t writebyte(FILE *f, unsigned char byte)
{
  return fwrite(&byte, 1, 1, f);
}

size_t readbyte(FILE *f, unsigned char *bytep)
{
  return fread(bytep, 1, 1, f);
}

typedef union {
    unsigned short s;
    unsigned char c[2];
} buf;


/* write at most 8 bits */
int writebits(FILE *f, unsigned t, int b)
{
  static buf u = {0};
  static int used = 0;

  assert(b <= 8);
  assert(sizeof(short) == 2);

  if (!b)  /* zero means flush buffer */
    b = 8 - used;

  u.s |= (t << used);
  used += b;

  if (used>=8) {
    if (writebyte(f, u.c[high]) != 1)
      return 1;
    u.s >>= 8;
    used -= 8;
  }
  return 0;
}

int psave(unsigned a[], int nelems, int bits, FILE *f)
{
  int b, i;
  unsigned t;

  set_end();
  
  for (i=0; i<nelems; i++) {
    t = a[i];
    b = bits;
    for (b=bits; b>0; t >>= 8, b -= 8) {
      if (writebits(f, t & 0xff, b>8 ? 8 : b))
	return i-1;
    }
  }
  writebits(f, 0, 0);
  return i;
}

/* read at most 8 bits */
unsigned readbits(FILE *f, int b)
{
  static buf u;
  static int start = 8;
  static int end = 8;
  unsigned t;

  assert(b <= 8);
  assert(sizeof(short) == 2);

  if (end-start < b) {
    u.s >>= end-8;
    start -= end-8;
    readbyte(f, &u.c[low]);
    end=16;
  }
  t = (u.s >> start) & ~(~0 << b);
  start += b;

  return t;
}

int prestore(unsigned a[], int maxnelems, int bits,   FILE *f)
{
  int b, k, i;
  unsigned t,s;

  set_end();

  for (i=0; i<maxnelems; i++) {
    t = 0;
    s = 0;
    b = bits;
    for (b=bits; b>0; s += 8, b -= 8) {
      k = readbits(f, b>8 ? 8 : b);
      t |= k << s;
    }
    a[i] = t;
  }
  return i;
}