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Handle invalid triplets

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"Invalid triplets" are triplets that have failed Hamming 24/18 decoding.

Previously, invalid triplets were converted at load time to either all zero
bits or, in the case of X/26 enhancement triplets, to "dummy" triplets of
reserved mode 11110 with a row address group that hopefully have no effect.
Now the X/26 enhancement triplet list can explicitly store invalid triplets
and will show them as "error decoding triplet".

Invalid triplets in other packets such as X/28/0 will still be zeroed out at
load time.

Since the TTI format has no provision for storing invalid triplets, saving a
page will convert the invalid triplets to reserved mode 11110 as described
above.

The actual bits of invalid triplets are not stored on the assumption that
they are not recoverable. Thus exporting to t42 format will write an invalid
triplet as a Hamming coded result of all zero bits which will still cause a
Hamming decoding failure.
This commit is contained in:
Gavin MacGregor
2025-11-04 17:56:04 +00:00
parent 1b3623d61b
commit 1efa8c196d
8 changed files with 125 additions and 47 deletions
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74
src/qteletextmaker/saveformats.cpp
View File

@@ -160,8 +160,23 @@ QByteArray SaveTTIFormat::format18BitPacket(QByteArray packet)
// TTI stores the triplets 6 bits at a time like we do, without Hamming encoding
// We don't touch the first byte; the caller replaces it with the designation code
// unless it's X/1-X/25 used in (G)POP pages
for (int i=1; i<packet.size(); i++)
for (int i=1; i<packet.size(); i++) {
// Save invalid triplets as address 41, mode 0x1e, data 0
// which hopefully won't do anything when parsed as X/26 enhancements
if ((packet.at(i) & 0xff) == 0xff)
switch (i % 3) {
case 1:
packet[i] = 41;
break;
case 2:
packet[i] = 0x1e;
break;
case 0:
packet[i] = 0;
break;
}
packet[i] = packet.at(i) | 0x40;
}
return packet;
}
@@ -322,26 +337,30 @@ QByteArray SaveT42Format::format4BitPacket(QByteArray packet)
QByteArray SaveT42Format::format18BitPacket(QByteArray packet)
{
for (int c=1; c<packet.size(); c+=3) {
unsigned int D5_D11;
unsigned int D12_D18;
unsigned int P5, P6;
unsigned int Byte_0;
for (int c=1; c<packet.size(); c+=3)
// For invalid packets, save as all zeroes which will fail Hamming 24/18 decoding
if ((packet.at(c) & 0xff) == 0xff)
packet[c] = packet[c+1] = packet[c+2] = 0;
else {
unsigned int D5_D11;
unsigned int D12_D18;
unsigned int P5, P6;
unsigned int Byte_0;
const unsigned int toEncode = packet[c] | (packet[c+1] << 6) | (packet[c+2] << 12);
const unsigned int toEncode = packet[c] | (packet[c+1] << 6) | (packet[c+2] << 12);
Byte_0 = (hamming_24_18_forward[0][(toEncode >> 0) & 0xff] ^ hamming_24_18_forward[1][(toEncode >> 8) & 0xff] ^ hamming_24_18_forward_2[(toEncode >> 16) & 0x03]);
packet[c] = Byte_0;
Byte_0 = (hamming_24_18_forward[0][(toEncode >> 0) & 0xff] ^ hamming_24_18_forward[1][(toEncode >> 8) & 0xff] ^ hamming_24_18_forward_2[(toEncode >> 16) & 0x03]);
packet[c] = Byte_0;
D5_D11 = (toEncode >> 4) & 0x7f;
D12_D18 = (toEncode >> 11) & 0x7f;
D5_D11 = (toEncode >> 4) & 0x7f;
D12_D18 = (toEncode >> 11) & 0x7f;
P5 = 0x80 & ~(hamming_24_18_parities[0][D12_D18] << 2);
packet[c+1] = D5_D11 | P5;
P5 = 0x80 & ~(hamming_24_18_parities[0][D12_D18] << 2);
packet[c+1] = D5_D11 | P5;
P6 = 0x80 & ((hamming_24_18_parities[0][Byte_0] ^ hamming_24_18_parities[0][D5_D11]) << 2);
packet[c+2] = D12_D18 | P6;
}
P6 = 0x80 & ((hamming_24_18_parities[0][Byte_0] ^ hamming_24_18_parities[0][D5_D11]) << 2);
packet[c+2] = D12_D18 | P6;
}
return packet;
}
@@ -440,14 +459,21 @@ bool SaveEP1Format::getWarnings(const PageBase &subPage)
QByteArray SaveEP1Format::format18BitPacket(QByteArray packet)
{
for (int c=1; c<packet.size(); c+=3) {
// Shuffle triplet bits to 6 bit address, 5 bit mode, 7 bit data
packet[c+2] = ((packet.at(c+2) & 0x3f) << 1) | ((packet.at(c+1) & 0x20) >> 5);
packet[c+1] = packet.at(c+1) & 0x1f;
// Address of termination marker is 7f instead of 3f
if (packet.at(c+1) == 0x1f && packet.at(c) == 0x3f)
packet[c] = 0x7f;
}
for (int c=1; c<packet.size(); c+=3)
if ((packet.at(c+1) & 0xff) == 0xff) {
// Save invalid triplets as address 41, mode 0x1e, data 0
// which hopefully won't do anything when parsed as X/26 enhancements
packet[c] = 41;
packet[c+1] = 0x1e;
packet[c+2] = 0;
} else {
// Shuffle triplet bits to 6 bit address, 5 bit mode, 7 bit data
packet[c+2] = ((packet.at(c+2) & 0x3f) << 1) | ((packet.at(c+1) & 0x20) >> 5);
packet[c+1] = packet.at(c+1) & 0x1f;
// Address of termination marker is 7f instead of 3f
if (packet.at(c+1) == 0x1f && packet.at(c) == 0x3f)
packet[c] = 0x7f;
}
return packet;
}