What is it mean Difficuly & prob when using vanitygen?

I am running this command

$ vanitygen64.exe 1abcdefg
Difficulty: 778316249440
[125.17 Kkey/s][total 39015067414][Prob 4.9%][50% in 46.3d]

What is it mean Difficulty? I understand that higher takes more time, but what is the scale, why it is different from total? Why alphabet is more difficult than only digits vanitygen1234567`?
What is it mean 50% in 46 days, why it not tell me 100%. After 46 days, how much percent will be that it will complete in the next 46 days? If it is same reasons as in Half-Life in nuclear physics I understand it.
The big question. What it is Prob 4.9%? Prob of what?

Aminadav Glickshtein

Posted 2016-07-27T07:11:45.743

Reputation: 250

1General warning: use of vanity addresses is bad. Bad for yourself and bad for others you transact with. Address re-use is bad for privacy and safety. – Jannes – 2016-07-27T12:06:04.223

I know. But it is for a gift. Giving someone BTC on his name, then he can use it once... – Aminadav Glickshtein – 2016-07-27T12:59:25.973

Answers

Difficulty tells you how likely it is that vanitygen will be able to find the address pattern you're looking for on the next guess. A value of 2n is twice as difficult (half as likely) than a value of n.
Because the process is completely random, it's impossible to give you a '100%' ETA. We can only estimate that, on average, you will find your key before some time period half the time (hence why 50% is used). Note that it re-adjusts the value to 75% once it hits that 50% mark (and re-adjusts again after hitting 75%, until the key is eventually found).
That is the likelihood that you would have found your key by now, from the time when you started generating your key.

Klakurka

Posted 2016-07-27T07:11:45.743

Reputation: 108

Thank you. Do you know why finding alphabet is more difficult that finding only digits address: 1abcdef takes more time than 1123456 – Aminadav Glickshtein – 2016-07-28T07:01:53.593