On Saturday, 21 December 2013 at 22:22:09 UTC, Charles Hixson
wrote:
I was planning to ask if there were a better way to do this,
but instead I need to ask what's my mistake?
For some reason, if called with an uninitialized ubyte array,
and an index of 0, it returns a value of 8, even though all the
values in the array are 0.
The error has to be somewhere in the "ret = " statement, but I
sure don't see it.
/** Convert 8 consecutive bytes sliced from a ubyte[] into a
ulong
* @param block The array from which to slice.
* @param n The starting index within the block */
ulong ubytesToUlong(ubyte[] block, int n)
{ ulong ret;
assert (n >= 0);
assert (n + 8 <= block.length);
writefln ("n = %s", n);
writefln ("block[0] = %s", cast(ulong)block[0]);
writefln ("block[1] = %s", cast(ulong)block[1]);
writefln ("block[2] = %s", cast(ulong)block[2]);
writefln ("block[3] = %s", cast(ulong)block[3]);
writefln ("block[4] = %s", cast(ulong)block[4]);
writefln ("block[5] = %s", cast(ulong)block[5]);
writefln ("block[6] = %s", cast(ulong)block[6]);
writefln ("block[7] = %s", cast(ulong)block[7]);
ret = cast(ulong)block[n] * 2^21
+ cast(ulong)block[n+1] * 2^18
+ cast(ulong)block[n+2] * 2^15
+ cast(ulong)block[n+3] * 2^12
+ cast(ulong)block[n+4] * 2^9
+ cast(ulong)block[n+5] * 2^6
+ cast(ulong)block[n+6] * 2^3
+ cast(ulong)block[n+7] * 2^0;
writefln ("ret = %s", ret);
return ret;
}
As pointed out before, you're using '^' which is xor, instead of
^^. Ideally the compiler will optimise your version to be fast,
but you may find you get better performance by doing the bit
manipulations eplicitly:
/**Convert 8 consecutive bytes sliced from a ubyte[]
* to a ulong using bigendian byte ordering.
* @param block The array from which to slice.
* @param n The starting index within the block, default 0*/
ulong ubytesToUlong(ubyte[] block, size_t n = 0)
in
{
assert (n >= 0);
assert (n + 8 <= block.length);
}
body
{
return ((cast(ulong)block[n ]) << 56)
| ((cast(ulong)block[n+1]) << 48)
| ((cast(ulong)block[n+2]) << 40)
| ((cast(ulong)block[n+3]) << 32)
| ((cast(ulong)block[n+4]) << 24)
| ((cast(ulong)block[n+5]) << 16)
| ((cast(ulong)block[n+6]) << 8)
| (cast(ulong)block[n+7]);
}
unittest
{
ubyte[8] a = [0,0,0,0,0,0,0,0];
assert(a[].ubytesToUlong() == 0);
a[7] = 3;
a[6] = 1;
assert(a[].ubytesToUlong() == 259);
}
