On Saturday, 13 December 2014 at 11:20:21 UTC, Manolo 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).
Sorry, lack or accuraccy: the maximum value represented was a 24
bit unsigned integer, but the data length was 32 bit for this
value.
The thing is that the format included various fields, but because
of the memory price the algo saved space when values where less
than 0X7F, because only one byte was needed. Nowadays such as
format would allow for example always 4 bytes to describes the
data length:
nowadays:
data len "L" | data
0 1 2 3 | 4 ... L-1
so "nowadays", we can afford 4 bytes to say that a field length
is 1
olddays:
variable len VL | data
1 to ? | VL ... VL-1
"olddays", they used only one byte to say that a field length is
1.
