On 4chan, a popular game is get. Every post on the site gets a sequential post ID. Since you can't influence or determine them, people try to guess (at least a part of) their own post number, usually the first few digits. Another version of the game is called dubs, and it's goal is to get repeating digits at the end of the number (i.e 1234555).

Your task, if you wish to accept it, is to write a program that takes a post id as input (standard integer, you can assume below 2^32), and returns how many repeating digits are on the end.


  • Standard loopholes are disallowed.
  • The program can be a function, full program, REPL command, whatever works, really, as long as no external uncounted code/arguments are needed to run it.
  • Input can come from STDIN, function arguments, command line argument, file, whatever suits you.

Test Cases

Input: 14892093
Output: 1

Input: 12344444
Output: 5

Input: 112311
Output: 2

Input: 888888
Output: 6

Input: 135866667 //Post number I got on /pol/ few days ago, rip
Output: 1
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    \$\begingroup\$ Are we allowed to take input as string? \$\endgroup\$ Mar 28, 2017 at 9:11
  • 7
    \$\begingroup\$ @DeadPossum I would assume that's allowed, since you get a string anyway if you read the input from STDIN, command-line argument or file (which are all admissible input methods). \$\endgroup\$ Mar 28, 2017 at 9:19
  • 1
    \$\begingroup\$ Can we assume that the input will be greater than 0? \$\endgroup\$ Mar 28, 2017 at 10:01
  • 2
    \$\begingroup\$ Upvote for the dubs game! Check'em! \$\endgroup\$ Mar 28, 2017 at 12:31
  • 2
    \$\begingroup\$ Disappointed: The question ID on SE does not have dubs. \$\endgroup\$
    – Num Lock
    Mar 30, 2017 at 9:42

63 Answers 63

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J, 14 bytes


Try it online!

It's fortunate that the challenge allows a REPL command. As a verb (function) it's 20 bytes:

3 :'>:i.&1}.~:|.":y'

For arbitrarily large integers, the literal must be suffixed by x.


Applied right to left, all monadic verbs:

  • ": Format - Stringifies its argument
  • |. Reverse
  • ~: Nub sieve - Produces a boolean array of 1's each time a new item is introduced, 0 otherwise
  • }. Behead - Removes first item of array
  • i.&1 Index of - Gets index of first 1. i. is actually a dyadic verb, but &1 ties right argument to 1.
  • >: Increment

Common Lisp, 72 chars

(lambda(i)(do((n i(floor n 10))(c 1(1+ c)))((/= 0(mod(mod n 100)11))c)))


(lambda (i)
  (do ((n i (floor n 10))
       (c 1 (1+ c)))
      ((/= 0 (mod (mod n 100) 11)) c)))


(/= 0 (mod (mod n 100) 11))

This checks to see if the last two digits of n are different- if n%100 is not divisible by 11.

(do ((n i (floor n 10))
     (c 1 (1+ c)))
    (... c))

do is a powerful generic looping construct in lisp. Its first argument is a list describing the looping variables. Each element is a list of (variable initial-value step-form) where step-form is evaluated each time through the loop to update that variable. The second argument is a test to perform to end the loop (the test I covered above) and a list of values to return after looping (in this case just c, which has been incrementing each time though the loop).

(lambda (i) ...) Wraps the operation in an anonymous function that takes the number as an integer.


x86-16 machine code, IBM PC DOS, 22 21 20 bytes


00000000: d1ee 8a1c 8d38 8a05 fdf3 aef6 d191 052f  .....8........./
00000010: 0ecd 10c3                                ....

Build and test DUBS.COM with xxd -R.

Unassembled listing:

D1 EE       SHR  SI, 1              ; SI to command line tail (80H) 
8A 1C       MOV  BL, BYTE PTR[SI]   ; BX = input string length 
8D 38       LEA  DI, [BX][SI]       ; DI to end of string 
8A 05       MOV  AL, BYTE PTR[DI]   ; AL = last char
FD          STD                     ; set LODS to decrement 
F3 AE       REPZ SCASB              ; scan [DI] backwards until AL doesn't match
F6 D1       NOT  CL                 ; negate CL to get number of matches + 1
91          XCHG AX, CX             ; AL = CL 
05 2F0E     ADD  AX, 0E2FH          ; ASCII convert and adjust, BIOS tty function
CD 10       INT  10H                ; write number to console 
C3          RET                     ; return to DOS

A standalone PC DOS executable program. Input via command line, output number to console.


This takes advantage of a few "features" of PC DOS register startup values; specifically BH = 0, CX = 00FFH and SI = 100H. REPZ decrements CX on every compare operation, so starting at 00FFH will yield a negative count of reps. It is off-by-one because REPZ decrements before comparison, so the first non-match still gets counted. That number is ones-complimented and adjusted by 1 to get the count of matched digits.

Also, since CH = 0 it's possible to combine ADD AL, '0'-1 and MOV AH, 0EH into a single instruction ADD AX, 0E2FH to save 1 byte, since AH will be 0. Old-school SIMD for you!


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