CGCC Rocket Biking

Question

Something I found while looking through some old files. It seemed like a neat idea for a code golf challenge.

The intro

One of the most popular events at the annual Cyclist's Grand Competition for Charity (CGCC) is the rocket bike parcours. The rules are simple: Inspect the parcours and place a bet on wheather is it possible to reach and exactly stop on the finish line. The drivers who bet that it's possible then have to prove it to win. If they can't prove it, the drivers who bet against it win.

The parcours

The rules for the parcours are as follows:

• A parcours consists of uphills, downhills and flat bits. In the examples, it's modelled as a sequence of /, \ and _ respectively.
• The bike starts with a speed of 0 m/s (i.e. before the first character of the sequence).
• The down-/uphills accelerate/decelerate the bike by 1 m/s.
• On the flat bits the driver must either speed up or slow down manually, again, by 1 m/s. They mustn't do nothing.
• If the bike has a speed of 0 m/s at the finish line (i.e. after the last character of the sequence), the parcours was driven correctly.
• The finish line must be reached.
• The speed must remain >= 0 at all times. If it is/becomes 0 at any point, the next piece of the parcours must accelerate the bike, either through a downhill \ or through a flat bit _ where the driver accelerates.

The task

Given such a parcours, take it as the input for a first program and output whether it's possible or not to drive the parcours correctly according to the rules above. If yes, take the sequence as an input for a second program and output a sequence of instructions for the driver on what to do on the flat bits. In the examples, + and - are used to signalize acceleration and deceleration respectively.

The rules

• This is code-golf. The shortest answer wins.
• Submit one or two program/s that validate the parcours (validator) and generate the instructions for the driver if valid (generator). If two programs are submitted, they are scored seperately and independently from the one program submissions.
• The I/O format may be freely chosen. All characters may be substituted freely.
• The generator program only needs to work on parcours that can be driven correctly.
• The input length is not bounded.

The examples

Input is taken

Input: 
  \_///

Formatted for clarity:
      /
     /
  \_/

Validator output: 
  Falsy (The driver will come to a stop before the last uphill)

Input: 
  \_\__/

Formatted: 
  \_
    \__/

Validator output:
  Truthy

Generator output:
  --+

Input: 
  \_\_\

Formatted:
  \_
    \_
      \

Validator output: 
  Falsy (The driver will finish the parcours with at least 1 m/s)

More examples:

Invalid:

/_/\       (Driver can't go uphill with a speed of 0 at the start)

\/_\\/

/\_\//_\

_\/\///_\\

/__//\/_//_\

/_\\\_\/_/__/\_/_\\\\\_\\/\_\//\\\\/\_///\/__\_\_\/_\\\_\//__//_\__/\\___/_\/_/\_\_/\\\\_\_\_\__\_\\

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

Valid:

_\_/
Generator: +-

\\\_//
Generator: -

\/\_\_
Generator: --

_\\__/\/
Generator: +--

\\_\__\/\/
Generator: ---

\\\__\____\/
Generator: +-----

__\\_/\______/\\\_\/\/_///\\_\//\_\/_\_/__/___/__\\_\_//__\\_/___\_______//\/_//__\/\\\__\/__\__/_\/
Generator: ++++++++++++++++++++++-------------------------

_/\_//\_\\//___\\//\/\/_\\_\/\\/\_/__\_//\/\__\/\//\\/\/\\\/\\\_//_\/\_\\\/__/\__/\_/_\\__/___/\__\_////\___\/\\\_//\\_\__/\_/\//\/_\/__\//\\_//\\/\/_/\\/_/\_\/\_/__//__/\//_/\_\__\_\_/\////__///__/////_\/__/\\/\_\\/////_\//_/\\\/\//\\/\_\_////\\_\/\_\//_/_/_\\__\\\\\_//\\_/__//\_/\_\/\_//__//__\_\//\_///_/////__\\/\\/\//_______//_\//_/_/__/\/\/_\/__/\///_\_/\/////\/\__///__////_/\\\_\/\___\\\//\/\_\/\////\_/\___\\_\_/_\\__\\___/_\_///\____/\\\/\_/\/\\\\/_\/\/\\\/\\__///___/___\\_///\_/\///__/_/____\\/___\___\\/\__//_\/\\//\_//\_____/____/\/\_\/_\__/\\\/__/_/_/\__/_/_\\/_///__\\\\/_\//_/__\//\_\//_\/_\\_\/__/_\_\\_\\//_//_\_/_\/\\\\/__/\/___//__/_/\/_\/\//___\/_//\/___\__/\/_\//__\\_\///\\/_\_/_\\//\\_//\\\\\_///_\_/__/\_/_/_\\_\_/\/_//_\\/__\\/\\__//__\__\\\\_/\____\___\//\_\_//_/_/__//_\_/_///__/_/__\/\///\_\/_\_/_\\//\//\//\_/\\/\/\///\/\/\\_\//\__\\//__\_\_\\_\_//_\/_/__/__\_\\\_//\\/__\_\\_/\\_/\_////\/_\_\\__\/_\//_\_\/_/__\\//__\_\\\\/__/_\_\/_\\__///\/\_\/\_/\\_\\/\/_\\\//\/\//\_\/_/_/\//\\\/\\/\\_/__/_///__\\/\_\_//_//\//___/___\_/\/\__/\/_/\\\_\\/__\//\/__\_\//\__\/\__\//__\_/_\\_/____/\/_\/__\_\\_//_\\_\\__/_\/_/\\/\\__\\/__//_\/\/\_\/_/__/\/_\_////\\/_\_/\\\_///___\_/__/\/\__/\\_//_\_\_/\/\_\_/\\\/___//__///___\/_\\/_\\\\/__\//\__\\_\\//\/\____\/__///___//\__/_\__\__\\\\\/_\\\//\/\\//__/__\\//\\/\\\__\\\_\_//__/_/\__\_\_/\/\\\_\\//___\_/\/\___/_/\/_\\_\____/\///\/\\\\_/\/_\\_\//\/\/_/_\_//__\\\_\____\/\\_//\\\\\//_/_\_/\\_\/___\/__\_\_\\\\//__\\__\_/_/_\_\\__\_/\///__\/\/__/\__\_\\\/_//\_\//__\/\\/\\/__/\/\\\/____/_\__\\_/_\\\//__//_/_\\//\/\/\_/\_/\\_\//\__/\\/\\_\_/_/\_/_/_/\\_\_//__\/\__\\/_\////____/\_/\//\\//\_/\/_/////_\_\/\/\\////\/\\_/////_\___\\\\\/___\///__//\\__//_\\/\/_//\\__\\//\\/__\\/_\/_/_\_/\__//\__\\\\_/\/\__//_\//_\\___/_/_/___\/_/_\\\\__\_____///__//_//_//\///\//_//\\_\_\_/\/__/\\/\_\/\_///__//_//_\\///_/_/_/\\_\__\\/\____\__//////__\//_/_\\/__/\////\_/\__///\\/\//__\/__/_/_//\//_/\/__/\_/_//\\/\\___/_\\_/\/\_/_\__\__\/\/_/\__\///__////\__/\__/_\\_////////__\/_/____\/_\\\\/\__\\\_\///__\/////_\//\//_\__\/\\__\\_/\/\\//__\_/_\/\//\_\/\\/\\/\\_\\///\\\__//_\\_\//\__//_\/\_///\\/\/\_\//\/____\\____/_/\/_/\/__\\\_/__\\/_/_/\/\\___////__\\/////\___\\/_\_//_\\/_/\//_/\__\/___\__\/\//\/__\_\\\_\/_\//\/\//\/\\///\///\/\_\\//\_\\/\\\____\\\/__/////_\_\///_\\\\_\__\_____\\_///\//\_//\/\_\\_\\/\__/\\/\\_//_\//\\/\_\\_/__/_\/\\/\_///\/\\/\/__/\_\/\\_/\////___\\//\\_\/\//\//\___//\/_/_/_/__\//_/\\_\_\/_/__///\_\\_\\\\_/______\/\\__\/__/_\_/__\/\\___\_/__//\_\/_/_\\_//\/\_/\__/\\\_\\///\_/\\_/_\/\\/_\/_\\\/_\\\_/_\///\\/__//_\\\__/___\\_/\\_\__\\\//\_/_///\_//\/\/\_\\/\\/__\_____//\\_/\/\/\///\_\__\______\//_\\\\/_/\\/\\//_\\_/\\__/__\\\/\\\_/_/\__\_\//____\\\\//\\_/\\/\\\_\///_//\_//\\\/\__//////_/_\//\\\\__/___///_/_///\\_/_/\_\_\//\/__\__//\//\/\/\__\__\\/_\_\//\_//\\\///\\\/_\\____\\\/__\\_/\//\/\\\/\_//_/_\\\_\///__//\_/_/\__//___//_\//\/_\//\\///\__\__//\//\/\/\___///_//_\/\\_/_/_\//__//_\_\\//\/\_//_/_\/\/__/////\_/__\//\/_/\__/\_\\\/_\_/__\\_/_/\///_/\_/\/__/_/\/\/\\//\///\/\\/__\\_\\/_/_\_/\\//\_\\_\______\\_\_\////_\_/\\__/_/\\\\//_\/\_\\_/\_\__\_\\\\///_/__/\__\_\/\____\__/_//\/_\\/_//\/_//\/_/_\_//_/_///_\\\_//\_\_/__///_/\_/___\___/\_\_/\\//_\/_\/////\\_\//_//_/___\/\\/_//\__\__/___//\___/_\\\/\\_/_/_\\//\/_\\_//_//_/\__/_/_/\_\/_\\\/__\\___\___/_//\/_///\_/__/__\__\\\__/_//\\/_/_/__\\\\_\\_\/_\_\\/_/_/\/\\__\/_/__/\//__//_\_/\\\\_\/_\_//\__/\\_///_/\\\/\\/\_/\_\/\\\//\_/_\\_\\//\//\\//___///\\\//\__\_//_//\/_/\/\/__\\\//\/____/___/\__/\\_\_///\//__/_//_\\\\_////\__\//\_/_/_/\\_/_/\\___/\\\\_\_\///\_//_\_\\\/\/\_/__\//_\\/_////_\\_/\___\/___/__/_//_\\/_\/__/\\/__///\\/\/_\\__\///\//\\\\_\\_\/_\_\//\\/\\/\\//\_\\\__/\\\__/\_\\\_/_\\_/\\__/\\/__\\/\\\/__/\//_\/__\___\//___/_//\_//_///__/_/\\\_/_/\_/\_/__///_/_\_\_/\__/\\__/\//\__\/////_/_\\\\\/__\\\___\/___//__\\/\\_\\\\/_/__\_/\/_//\/\_/\\/\_\\/\\_\\\\/\_/_/\/_//_/__/\\\_///_/\/_\\///_\/\/__\\_//\\/\__//_\\__/_\//\\_\_\__\\/\/\/__/\/_\\\\__////_\/_//\\//_/\\/\__/_\/\__/\//_/\__//_\_\//\_//____\\\\_\/\/___\/\/\/___/__\///\/_\___//\_/\__\\_/___///\//_/__/___//_\/__\\\_\/_\\//\__/\____/_/\\/__\///_/\__\\/_/_/___\/_\__\/_\\/\/\_/\_\/\__\\\//\\\/\_\____/\/_\_/\\/__\\///_\_\\//__\_/__\//\___/\_\_//_/___\_/\\\/\\_\///\/_\\/_\_/_\/_\\\_\___\_\//_//\_\_\_\/_\\/\//_\\/\\__////\\__\___\/_/\_/\___/_\_\_\__/__\//\/_\\___\/\\_/\\_\\/_\\__/__\_/\_\__/\//____\//_____\__/\//_\////\//\//\\//__/__/\\\/_//__/\\\_\//\_/\\\\/___\__//_/\\/__\_/\__\/_\//__\_/___/__/_\///\/__/\_\\__/__\/\_\/\/_/\/\_//_\_\/\__\/_\\/////__/_\_/\_/\_/_\/\\_\\\//\_\\\/\_//\//_\__//\//\\\/_\/\/\/\_/_/_/___/\\/\_\\/\\_/\__\/\//////\_/\////\\__/__\\\_//\_///_//\/\_/\/_/\/__\_\__\//_/_/_\\\/\/_/\\__/\_\_\_\_//\\\//___\__/\___\_____\_/\\/__\\\\\_\/__/\\/\____/\//___/_//\/__////\\//_\/\_/\/\\\/_/__\\/\////_\/\/_\/\___/__/\___//\////\\\_\\\_/\////___/\\\\/\_/_//_/\_/\_/\\/\/_//____///___\/\_\_\\//_/\\\/\___\/_\/_/__\//__/____\/\_/_\_\/\//\\_//\///\_\_\_/\\//\_\\_/_/_\__\_\__\//\\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
Generator: ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

The generator output is just an example. It needn't be "ordered" as the one seen above (although I assume this to be more practical to implement).

That last test case may or may not be 10000 chars long and my computer may or may not be trying to find a valid parcours of length 500000 at the moment.

Answer 1

sed -E - Validator, 82 61 59 55 48 bytes

:a
s/D(_*)U/\1/
ta
s/_U|D_//
ta
s/__//g
/./cF
cT

Attempt This Online!

Explanation:

First following transformation is applied on the input: \___/ -> ___. This is done repeatedly, till the whole terrain is of following shape: _/_/___\_\_ - rising from start and then falling till end.

Then following substrings are removed: _/ and \_. Now the terrain is of flat shape ____ and it is just checked if the number of _ is even.

sed -E - Generator, 115 107 78 72 bytes

:a
s/D(_*)U/\1/
ta
s/D([^_]*)_/\1M/
s/_([^_]*)U/P\1/
ta
s/_/P/
s/_/M/
ta

Attempt This Online!

Answer 2

Python 3.8 - Validator, 92 88 bytes

lambda c,d={1}:1in[d:={g+1-a*2for g in d if g for a in(f in'/_',f in'/')}for f in c][-1]

Try it online!

Python 3.8 - Generator, 115 113 bytes

lambda c,d={1:''}:[d:={g+1-a*2:d[g]+(f=='_')*'+-'[a]for g in d if g for a in(f in'/_',f in'/')}for f in c][-1][1]

Try it online!

Thanks to Kevin Cruijssen, Steffan, and xnor for -6 bytes.

Answer 3

C++ (gcc), 107 bytes (generator and validator)

g(char*x,int s=0){return*x?*x-47&&g(x+1,s+1)&&(*x=*x-95?32:43)||*x-92&&s&&g(x+1,s-1)&&(*x=*x-95?32:45):!s;}

Try it online!

C++ (gcc), 71 bytes (only validator)

v(char*x,int s=0){return*x?*x-47&&v(x+1,s+1)||*x-92&&s&&v(x+1,s-1):!s;}

Try it online!

Note: in both functions 4 bytes could be saved by replacing the boolean ands and or with bitwise ones, but it would be extremely slow. In the generator, the ands before (*x=*x-95?32:43) and before (*x=*x-95?32:45) must be boolean in any case (otherwise it does not output the correct instructions).

C++ (gcc), 239 bytes (formatter, extra)

#include <cstring>
#include <cstdio>
f(char*x){int*a=new int[strlen(x)+1],i=a[0]=0,m=0,M=0;for(;x[i];++i)a[i+1]=(a[i]+=(x[i]==92))-(x[i]==47),m=a[i]<m?a[i]:m,M=a[i]>M?a[i]:M;for(;m<=M;++m,puts(""))for(i=0;x[i];++i)putchar(a[i]-m?32:x[i]);}

Try it online!

Answer 4

x86-64 machine code, 45 bytes (validator and generator)

31 D2 6A 01 59 AC 88 C4 B0 2B 9E 7B 13 73 07 FF CA 79 05 AA FF C2 FF C2 9E 75 03 83 C1 02 E2 E5 D1 E9 B0 2D F3 AA 88 0F 19 CA 19 C0 C3

Try it online!

Following the standard calling convention for Unix-like systems (from the System V AMD64 ABI), this takes in RSI the address of the input, as a null-terminated byte string; takes in RDI an address at which to place a string, which will be the instructions for the driver if it's possible; and returns in EAX −1 if possible and 0 if impossible.

In assembly:

f:
    xor edx, edx

Set EDX to 0. EDX will keep track of the minimum possible speed at each point.

    push 1; pop rcx

Set ECX to 1. ECX will keep track of the maximum possible speed plus 1 at each point.

repeat:
    lodsb
    mov ah, al

Load a character from the string into AL, advancing the pointer, and then move it into AH.

    mov al, '+'

Set AL to the ASCII code of '+'.

    sahf

Set flags based on the character in AH. Here are the possibilities, with the important bits bolded:

    SZ A P C
NUL 00100000
\   01011100
_   01011111
/   00101111

    jnp done

Jump if PF=0, true here only for the null terminator.

    jnc must_accel

Jump if CF=0, true here only for \.

    dec edx
    jns second_part

(For _ and /) Subtract 1 from EDX (the minimum speed). Jump if the result is nonnegative, in which case we are done with this part. Otherwise...

    stosb

Write + to the output string, advancing the pointer.

    inc edx

Add 1 to EDX. The next instruction adds 1 more, for a net +1.

must_accel:
    inc edx

(For \) Add 1 to EDX.

second_part:
    sahf

Set flags based on the character in AH again (as the inc or dec has overwritten them).

    jnz must_decel

Jump if ZF=0, true here only for /.

    add ecx, 2

(For _ and \) Add 2 to ECX (the maximum speed plus 1); the next instruction will subtract 1, for a net +1.

must_decel:
    loop repeat

(/ jumps here) Subtract 1 from ECX and jump back if the result is nonzero (if the maximum speed did not drop to −1).

done:

If the parcours is possible, the minimum speed is 0, and the maximum speed is 2 times the number of flat pieces that should be decelerated on (because the maximum speed is obtained by accelerating on all of them, and changing one acceleration to a deceleration reduces the final speed by 2).

If it's not possible, either the end of the string has been reached with a nonzero minimum speed, or the loop ended prematurely with the maximum speed becoming −1 from a /, in which case the minimum speed will have changed from 0 to 1 on that /, and thus will also be nonzero.

    shr ecx, 1

Shift ECX (the maximum speed plus 1) right by 1 bit. If the parcours is possible, this produces the number of flat pieces that should be decelerated on.

    mov al, '-'
    rep stosb

Put the character code of - in AL and write it to the output string ECX times, advancing the pointer and counting down ECX.

    mov [rdi], cl

Add the low byte of ECX, which is now 0, to the output string.

    sbb edx, ecx

Subtract ECX+CF from EDX. ECX is still 0. CF is the bit shifted out of ECX previously.

If the parcours is possible, CF will be 1 and EDX 0, so the result will wrap around to all 1s, and CF will be 1 afterwards.

If it's not possible, EDX will be nonzero and CF may be 0 or 1, but either way the result will not wrap around, so CF will be 0 afterwards.

    sbb eax, eax

Subtract EAX+CF from EAX, making it −CF: −1 if possible and 0 if impossible.

    ret

Return.

Answer 5

05AB1E, 32 bytes

®1‚„_\Ik©0¢ãʒ0®r.;ηO¤Ā(ªdP}ZÄ=i,

Combines the Validator and Generator programs into a single program to save bytes.

Input as a list of characters.
Outputs an empty string as falsey, or 1 as truthy. In addition, outputs all possible generator outputs as a list of lists of -1/1 for deceleration/acceleration respectively.

Try it online or verify (almost) all test cases. (The larger test cases are omitted; the more _ are in the input, the slower the program becomes because of the ã.)

Explanation:

®1‚          # Push pair [-1,1]
   „_\       # Push string "_\"
      I      # Push the input character-list
       k     # Get the 0-based index of each character in "_\",
             # or -1 if it isn't present (in case of "/")
        ©    # Store this list in variable `®` (without popping)
         0¢  # Count the amount of 0s
           ã # Get the cartesian product of the [-1,1] with this count
             # (resulting in all possible count-sized list of -1s/1s)
ʒ            # Filter this list by:
             #  (implicitly push the current list of -1s/1s
 0           #  Push a 0
  ®          #  Push list `®`
   r         #  Reverse the order of these three
    .;       #  Replace every 0 in list `®` with the -1s/1s of the current list in order
 ηO¤Ā(ªdP    #  Check two things:
             #    1. The sum results in exactly 0
             #    2. No prefix is negative (so we never have any negative m/s speeds)
 η           #   Get the prefixes of this list
  O          #   Sum each inner prefix-list
   ¤         #   Push the last sum (without popping), which is the sum of the full list
    Ā        #   Check if it's NOT 0 (0 if 0; 1 otherwise)
     (       #   Negate it (0 if 0; -1 otherwise)
      ª      #   Append it to the list
       d     #   Check for each inner integer if it's non-negative (>= 0)
        P    #   Check if this is truthy for all of them
}            # Close the filter
             # (we now have a list of all possible paths if truthy; or [] if falsey)
 Z           # Push the flattened maximum (without popping): 1/-1 for truthy; "" for falsey
  Ä          # Get its absolute value: 1 for truthy; "" for falsey
   =         # Output it with trailing newline (without popping)
    i        # And if it was 1:
     ,       #  Also output the list itself on another line

Answer 6

brainfuck, 124 bytes (validator and generator)

,[------->-[<->---]+<[---[>->+>->>+<[->-]>[.->]<<<<<<[-]]>[->+>+<<]<[-]]>[->->+>+<<<]<,]--[>+<++++++]>>[--<.>]<++>>[--<<.>>]

If the parcour is valid, it just outputs the instructions; otherwise, it prints one or more bytes of value 0x01, followed by some instructions (which are of course meaningless).

Try it online!

Note: it works only if the difference in height is always less or equal to 255, counting from the start.

brainfuck, 103 bytes (generator only)

,[------->-[<->---]+<[---[>->+>-<<<[-]]>[->+>+<<]<[-]]>[->->+<<]<,]--[>+<++++++]>>[--<.>]<++>>[--<<.>>]

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Note: for both programs, the input must be zero terminated. Both programs works only if the total number of + and - of the output is less than 128.

Answer 7

Python, 62 (generator) + 68 (validator) bytes

-1 @Steffan

(
lambda s:((l:=len(s)//2)-(c:=s.count)("A"))*"+"+(l-c("?"))*"-"
,
lambda s,i=1:[min(i:=i+ord(c)%z-1for c in"?"+s)for z in(7,9)]==[0,i]
)

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Takes "AH?" for "\_/" to save a few bytes

Answer 8

Retina 0.8.2, 104 bytes

+`^(((?<-3>-|/)|(\+|\\))*)((?<8-3>)|((?<3>)))_(((?<-3>_|/)|(?<3>_|\\))*)$(?(3)^)
$1$#5$*+$#8$*-$6
[^-+]

Try it online! Link includes faster test cases. Outputs nothing if there are no flat bits or the input is invalid. Explanation:

+`

Repeat until all the instructions have been generated.

^(((?<-3>-|/)|(\+|\\))*)

Start by matching either - or / (removing from capture group 3 captured in a previous iteration), or either + or \ (capturing into capture group 3), repeating as much as possible, into capture group 1.

((?<8-3>)|((?<3>)))

Either remove from capture group 3 and capture into group 8 or make another capture into capture group 3 and also capture group 5. Then match the flat bit that we're going to create an instruction for, depending on which option was taken.

_(((?<-3>_|/)|(?<3>_|\\))*)

Match _ or / (still removing from capture group 3) or _ or \ (still capturing into capture group 3) as much as possible into capture group 6.

$(?(3)^)

The match needs to reach the end of the string and all of the captures in group 3 must eventually have been removed.

$1$#5$*+$#8$*-$6

Replace the flat bit with the desired instruction, leaving the prefix and suffix unchanged.

[^-+]

Remove anything that isn't an instruction.

Validator only for 29 bytes:

^((?<-2>_|/)|(_|\\))*$(?(2)^)

Try it online! Link includes faster test cases. Explanation: Basically just the last part of the expression from the full program above, treated as a logical match/fail result.

Answer 9

Haskell + hgl, 29 bytes

on(ozW ge**)(m<ce)'/''\\'<pxn

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Explanation

• pxn get all the prefixes
• ce '/' count the /s
• ce '\\' count the \s
• ozW ge all the \ counts are pairwise greater than or equal to the / counts.

Reflection

• There should be shorthand for '\\' and "\\" like there is for '\n' and "\n".
• Here we are computing cumulative counts of two different characters. I could see this being useful in the future. We might as well implement a shortcut for it. It can also be implemented to be faster than the way I've done it here which is always a plus.