run-length-encoding-exercise: added new exercise.

[SimaDovakin]

Added new exercise for #48in24.

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[keiravillekode]

It would be excellent if we had a instructions.append.md that explains the memory addresses the student's solution is expected to use.

If I understand correctly, your example solution uses some memory near address 0 and some memory near address 320, in addition to the memory for the input string.

Should we perhaps be passing in a destination address?

[SimaDovakin]

In instructions.append.md, I mentioned that the buffer for the input string uses bytes 64-319 of linear memory. I use the memory from 0 as the main input memory and the memory from 320 for storing the digits of a number to convert them to a number. But, maybe, there are other solutions that don't require the memory from 320. I think, it is enough to specify the input memory range. Am I right? Maybe I missed something.

[keiravillekode]

My solutions on other low-level tracks would write the digits to the output in reverse order, and then reverse in place.

[SimaDovakin]

I also do it in encode function (actually in $numberToString). But for decode function I store digits in the memory from 320th byte and then consume them to get a single number. I did it because I need to know the number of digits to determine the power of ten for the last digits (123 = 1 * 10^2 + 2 * 10^1 + 3 * 10^0). I have no idea is there a way to do this conversion without storing digits.

[keiravillekode]

(123 = 1 * 10^2 + 2 * 10^1 + 3 * 10^0).

number = 0
for digit in digits:
  number = number * 10 + (digit - '0')

0 * 10 + 1 gives 1
1 * 10 + 2 gives 12
12 * 10 + 3 gives 123

[SimaDovakin]

Cool! Thank you. I'll try to reimplement this part of my solution.

[SimaDovakin]

@keiravillekode Done. I changed the number consuming in decode function. Anything else?

[keiravillekode]

@ErikSchierboom - we can't see the updates until the PR is reopened.

I think we can merge this and we might want to refine or expand instructions.append.md afterwards. A flood of people will attempt this exercise for #48in24 and we want them to have an excellent experience.

In a month or two, we can decide on sample solutions for Dig Deeper.

Here are my two suggestions:

One solution for this exercise would overwrite the input with the output. For the decode case, I would compute the output length and work backwards from the end to the start, returning early as soon as there are no more digits - so decode "abc10d2e" would write 2 'e' characters, then the 10 'd' characters and then return.

Another solution would write the output immediately after the input in memory - assuming instructions.append.md can be phrased such that this is a permissible solution. Perhaps with wording like "There is also sufficient space for the output string, both before and after the input string." This example solution would generate the encode output initially reversed, and reverse in place at the very end. So encode 'abbbbbbbbbbbbc' would write 'cb21a' and then reverse that.

[ErikSchierboom]

This Now has a merge conflict

[SimaDovakin]

I'll resolve it.

[SimaDovakin]

@ErikSchierboom Done