On 12/13/2014 03:20 AM, Manolo via Digitalmars-d-learn wrote:
On Saturday, 13 December 2014 at 10:09:27 UTC, Charles Hixson via
Digitalmars-d-learn wrote:
Is there a standard way to do this? The code below is untested, as I
haven't yet written the x7to8 routine, and came up with a better way
to do what this was to accomplish, but it feels as if this should be
somewhere in the standard library, if I could only find it.
/** Repack the data from an array of ubytes into an array of ubytes of
* which only the last 7 are significant. The high bit will be set only
* if the byte would otherwise be zero. */
byte[] x8to7 (ubyte[] bin)
{
ubyte[] bout;
// bit masks: 0 => 0xfe = 11111110, 0x00 = 00000000
// 1 => 0x7f = 01111111, 0x00 = 00000000
// 2 => 0x3f = 00111111, 0x80 = 10000000
// 3 => 0x1f = 00011111, 0xc0 = 11000000
// 4 => 0x0f = 00001111, 0xe0 = 11100000
// 5 => 0x07 = 00000111, 0xf0 = 11110000
// 6 => 0x03 = 00000011, 0xf8 = 11111000
// 7 => 0x01 = 00000001, 0xfc = 11111100
if (bin.length < 1) return bout;
int fByte, fBit;
while (fByte < bin.length)
{ if (fByte + 1 == bin.length && fBit > 1) break;
ubyte b;
switch (fBit)
{ case 0:
b = bin[fByte] / 2;
break;
case 1:
b = bin[fByte] & 0x7f;
break;
case 2:
ubyte b1 = (bin[fByte] & 0x3f) << 1;
ubyte b2 = (bin[fByte + 1] & 0x80) >>> 7;
b ~= (b1 | b2);
break;
case 3:
ubyte b1 = (bin[fByte] & 0x1f) << 2;
ubyte b2 = (bin[fByte + 1] & 0xc0) >>> 6;
b ~= (b1 | b2);
break;
case 4:
ubyte b1 = (bin[fByte] & 0x0f) << 3;
ubyte b2 = (bin[fByte + 1] & 0xe0) >>> 5;
b ~= (b1 | b2);
break;
case 5:
ubyte b1 = (bin[fByte] & 0x07) << 4;
ubyte b2 = (bin[fByte + 1] & 0xf0) >>> 4;
b ~= (b1 | b2);
break;
case 6:
ubyte b1 = (bin[fByte] & 0x03) << 5;
ubyte b2 = (bin[fByte + 1] & 0xf8) >>> 3;
b ~= (b1 | b2);
break;
case 7:
ubyte b1 = (bin[fByte] & 0x01) << 6;
ubyte b2 = (bin[fByte + 1] & 0xfc) >>> 2;
b ~= (b1 | b2);
break;
default:
assert (false, "This path should never be taken");
} // switch (fBit)
if (b == 0) bout ~= 0x80;
else bout ~= b;
fBit = fBit + 7;
if (fBit > 7)
{ fByte++;
fBit -= 7;
}
}
}
Are you trying to make a "kind-of" Variable-Length quantity encoder ?
eg:
0b10101110: last bit not set, integrate 0b10101110 and stop reading.
0b10011001: last bit set, integrate 0b10011000 and continue to next byte.
http://en.wikipedia.org/wiki/Variable-length_quantity
except that this algo limits the length to 24 bits. It was used a lot
with
MIDI, at a time when hardware memory was costly (eg inside hardware
synthesizer or workstations).
What I was trying to do was pack things into 7 bits so I could recode
0's as 128. I finally thought clearly about it and realized that I only
needed to use one particular byte value (I chose 127) to duplicate so I
could repack things with a string of 0's replaced by 127 followed by the
length (up to 126) of zeros, and for 127 itself I'd just emit 127
twice. This was to pack binary data into a string that C routines
wouldn't think had ended partway through. (If I get more than 127 zeros
in a row, I just have more than one packing code.)
