Orudge: 4 revisions

2011-06-13T17:46:05Z

4 revisions

← Older revision	Revision as of 17:46, 13 June 2011
(No difference)

Orudge: Reverted edits by Apedizivif (Talk) to last version by MizardX

2010-12-07T17:35:44Z

Reverted edits by Apedizivif (Talk) to last version by MizardX

New page

Scenario files (.SC5) and save game files (.SV5) in Locomotion uses the same encodings methods as Rollercoaster Tycoon 2 (.SC6/.SV6). There are three kinds of encoding; Run-length encoding, String encoding, and Scramble.

=== Run-length Encoding ===

There are two types of sequences, selected by the value of the first byte.

{| style="border: 1px solid; margin: 0 auto; border-spacing: 0"
! First byte (''N'')
! Meaning
|- style="background-color: #EEE"
| style="padding: 0.15em 1em" | <code>0x00 ≤ ''N'' ≤ 0x7F</code>
| style="padding: 0.15em 1em" | Copy the next ''N''+1 bytes as they are.
|-
| style="padding: 0.15em 1em" | <code>0x80 ≤ ''N'' ≤ 0xFF</code>
| style="padding: 0.15em 1em" | Repeat the next 1 byte 257-''N'' times.
|}

As an example, concider the encoded byte sequence <code>00 57 FD 65 01 20 48 FE 61 00 21</code>.

{| style="border: 1px solid; margin: 0 auto; border-spacing: 0"
! Encoded
! Decoded
! Comment
|- style="background-color: #EEE"
| style="padding: 0.15em 1em;" | <code>00 57</code>
| style="padding: 0.15em 1em;" | <code>57</code>
| style="padding: 0.15em 1em;" | Copy 1 byte.
|-
| style="padding: 0.15em 1em;" | <code>FD 65</code>
| style="padding: 0.15em 1em;" | <code>65 65 65 65</code>
| style="padding: 0.15em 1em;" | Repeat the byte 4 times.
|- style="background-color: #EEE"
| style="padding: 0.15em 1em;" | <code>01 20 48</code>
| style="padding: 0.15em 1em;" | <code>20 48</code>
| style="padding: 0.15em 1em;" | Copy 2 bytes.
|-
| style="padding: 0.15em 1em;" | <code>FE 61</code>
| style="padding: 0.15em 1em;" | <code>61 61 61</code>
| style="padding: 0.15em 1em;" | Repeat the byte 3 times.
|- style="background-color: #EEE"
| style="padding: 0.15em 1em;" | <code>00 21</code>
| style="padding: 0.15em 1em;" | <code>21</code>
| style="padding: 0.15em 1em;" | Copy 1 byte.
|}

To encode data into RLE, you need to look for sequences of bytes with the same value, or with pairwise distinct values. Let's concider the string "Weeee Haaa!" again. Below I have separated the bytes into groups of distinct and constant values.

{| style="border: 1px solid; margin: 0 auto; border-spacing: 0"
| style="padding: 0.15em 1em;" | <code>57 </code>
| style="padding: 0.15em 1em;" | <code>65 65 65 65 </code>
| style="padding: 0.15em 1em;" | <code>20 48 </code>
| style="padding: 0.15em 1em;" | <code>61 61 61 </code>
| style="padding: 0.15em 1em;" | <code>21 </code>
|}

You can see how this maps to the values in the table before.

=== String encoding ===

For the encoding type <code>0x02</code>, after you have decoded the RLE, you still are left with one layer of encoding. String encoding is a simple variant of LZ compression. As with RLE, there is one prefix byte, but it's bits is split into two parts.

{| style="border: 1px solid; margin: 0 auto; border-spacing: 0"
! High 5 bits (''O'')
! Low 3 bits (''N'')
! Meaning
|- style="background-color: #EEE"
| style="padding: 0.15em 1em" | <code>11111b</code>
| style="padding: 0.15em 1em" | <code>111b</code>
| style="padding: 0.15em 1em" | Copy the next byte of input to the output.
|-
| style="padding: 0.15em 1em" | <code>*</code>
| style="padding: 0.15em 1em" | <code>*</code>
| style="padding: 0.15em 1em" | Repeat ''N''+1 bytes of output, starting ''O''-32 bytes from current position.
|}

It might be good to note that the bytes that are to be copied can span past the current output position. If you copy one byte at the time, the output will get longer, so it is not an error.

As an example, let's go with "Weeee Haaa!" again, since it has much redundancy. It can be encoded as <code>FF 57 FF 65 FA FF 20 FF 48 FF 61 F9 FF 21</code>.

{| style="border: 1px solid; margin: 0 auto; border-spacing: 0"
! Encoded
! Decoded
! Comment
|- style="background-color: #EEE"
| style="padding: 0.15em 1em;" | <code>FF 57</code>
| style="padding: 0.15em 1em;" | <code>57</code>
| style="padding: 0.15em 1em;" | Add 1 byte.
|-
| style="padding: 0.15em 1em;" | <code>FF 6B</code>
| style="padding: 0.15em 1em;" | <code>6B</code>
| style="padding: 0.15em 1em;" | Add 1 byte.
|- style="background-color: #EEE"
| style="padding: 0.15em 1em;" | <code>FB</code>
| style="padding: 0.15em 1em;" | <code>6B 6B 6B</code>
| style="padding: 0.15em 1em;" | <code>FB = 11111 010</code>. Repeat 3 bytes, starting from the last.
|-
| style="padding: 0.15em 1em;" | <code>FF 20</code>
| style="padding: 0.15em 1em;" | <code>20</code>
| style="padding: 0.15em 1em;" | Add 1 byte.
|- style="background-color: #EEE"
| style="padding: 0.15em 1em;" | <code>FF 48</code>
| style="padding: 0.15em 1em;" | <code>48</code>
| style="padding: 0.15em 1em;" | Add 1 byte.
|-
| style="padding: 0.15em 1em;" | <code>FF 61</code>
| style="padding: 0.15em 1em;" | <code>61</code>
| style="padding: 0.15em 1em;" | Add 1 byte.
|- style="background-color: #EEE"
| style="padding: 0.15em 1em;" | <code>F9</code>
| style="padding: 0.15em 1em;" | <code>61 61</code>
| style="padding: 0.15em 1em;" | <code>F9 = 11111 001</code>. Repeat 2 bytes, starting from the last.
|}

The encoding process for this method is much slower than for RLE, as the encoder have to search for (or possibly cache) possible byte sequences to repeat. Since repetition only requires one byte, it is always better to use repetition whenever it is possible. One thing to be careful about, is to not encode a sequence of 8 or more equal bytes as FF (repeat 8 bytes, starting from the last), since that is the code for copying one byte from the input to the output.

=== Scramble ===

The only purpose of this method is to make the content harder to read. The bits of each byte is rotated an odd number of bits, depending on the bytes position within the string.

{| style="border: 1px solid; margin: 0 auto; border-spacing: 0"
! Position
! Encoded bits
! Decoded bits
|- style="background-color: #EEE"
| style="padding: 0.15em 1em;" | <code>+4*N+0</code>
| style="padding: 0.15em 1em;" | <code>AAAAAAA B</code>
| style="padding: 0.15em 1em;" | <code>B AAAAAAA</code>
|-
| style="padding: 0.15em 1em;" | <code>+4*N+1</code>
| style="padding: 0.15em 1em;" | <code>AAAAA BBB</code>
| style="padding: 0.15em 1em;" | <code>BBB AAAAA</code>
|- style="background-color: #EEE"
| style="padding: 0.15em 1em;" | <code>+4*N+2</code>
| style="padding: 0.15em 1em;" | <code>AAA BBBBB</code>
| style="padding: 0.15em 1em;" | <code>BBBBB AAA</code>
|-
| style="padding: 0.15em 1em;" | <code>+4*N+3</code>
| style="padding: 0.15em 1em;" | <code>A BBBBBBB</code>
| style="padding: 0.15em 1em;" | <code>BBBBBBB A</code>
|}

As an example, let's go with "Weeee Haaa!" again.

{| style="border: 1px solid; margin: 0 auto; border-spacing: 0"
! Encoded
| style="padding: 0.15em 1em;" | <code>AE</code>
| style="padding: 0.15em 1em;" | <code>2B</code>
| style="padding: 0.15em 1em;" | <code>AC</code>
| style="padding: 0.15em 1em;" | <code>B2</code>
| style="padding: 0.15em 1em;" | <code>CA</code>
| style="padding: 0.15em 1em;" | <code>01</code>
| style="padding: 0.15em 1em;" | <code>09</code>
| style="padding: 0.15em 1em;" | <code>B0</code>
| style="padding: 0.15em 1em;" | <code>C2</code>
| style="padding: 0.15em 1em;" | <code>0B</code>
| style="padding: 0.15em 1em;" | <code>24</code>
|- style="background-color: #EEE"
! Rotation
| style="padding: 0.15em 1em;" | 1
| style="padding: 0.15em 1em;" | 3
| style="padding: 0.15em 1em;" | 5
| style="padding: 0.15em 1em;" | 7
| style="padding: 0.15em 1em;" | 1
| style="padding: 0.15em 1em;" | 3
| style="padding: 0.15em 1em;" | 5
| style="padding: 0.15em 1em;" | 7
| style="padding: 0.15em 1em;" | 1
| style="padding: 0.15em 1em;" | 3
| style="padding: 0.15em 1em;" | 5
|-
! Decoded
| style="padding: 0.15em 1em;" | <code>57</code>
| style="padding: 0.15em 1em;" | <code>65</code>
| style="padding: 0.15em 1em;" | <code>65</code>
| style="padding: 0.15em 1em;" | <code>65</code>
| style="padding: 0.15em 1em;" | <code>65</code>
| style="padding: 0.15em 1em;" | <code>20</code>
| style="padding: 0.15em 1em;" | <code>48</code>
| style="padding: 0.15em 1em;" | <code>61</code>
| style="padding: 0.15em 1em;" | <code>61</code>
| style="padding: 0.15em 1em;" | <code>61</code>
| style="padding: 0.15em 1em;" | <code>21</code>
|}

[[Category:ScenarioFiles]]

ScenarioFiles:Encoding - Revision history

Orudge: 4 revisions

Orudge: Reverted edits by Apedizivif (Talk) to last version by MizardX