[Bug-ddrescue] Reverse mode clarifications

[Florian Sedivy]

Antonio Diaz Diaz wrote:

> Hello Florian.
> 
>> Reverse mode during the copying stage - does it reverse the read operation 
>> itself (thereby forcing sector by sector read), or does it reverse the 
>> direction the current position is moving (thereby respecting the copy block 
>> size)? 
> 
> It reverses the direction the current position is moving, respecting the copy 
> block size and the skip size.

Thats great! So to go into even more detail (and starting a new thread to do 
so):
How do reverse mode, current position and the rescue domain correlate?

Let us for example issue on a 1TB drive the command
ddrescue -R -i 400G -s 200G drive image log

If there is no log, the first position read will be 600GB-64KB (64KB being the 
default copy block size), I assume. 
But where will ddrescue start copying if the current position in the log is
a)100GB
b) 500GB
c) 900GB 

Without the -R reverse mode, the answer would probably be a: 400GB, b: 500GB, 
c: finished. 
What if there is a more complex rescue domain (option -m)?

Greetings, 
Florian