Convert between balanced bases!

Question

Balanced bases:

Balanced bases are essentially the same as normal bases, except that digits can be positive or negative, while in normal bases digits can only be positive.

From here on, balanced bases of base b may be represented as balb - so balanced base 4 = bal4.

In this challenge's definition, the range of digits in a balanced base of base b is from -(k - 1) to b - k, where

k = ceil(b/2)

Examples of the range of digits in various balanced bases:

bal10:
  k = ceil(10/2) = 5
  range = -(5 - 1) to 10 - 5 = -4 to 5
        = -4, -3, -2, -1, 0, 1, 2, 3, 4, 5
bal5:
  k = ceil(5/2) = 3
  range = -(3 - 1) to 5 - 3 = -2 to 2
        = -2, -1, 0, 1, 2

Representations of numbers in balanced bases is basically the same as normal bases. For example, the representation of the number 27 (base 10) to bal4 (balanced base 4) is 2 -1 -1, because

  2 -1 -1 (bal4)
= 2 * 4^2 + -1 * 4 + -1 * 1
= 32 + (-4) + (-1)
= 27 (base 10)

Task:

Your task is, given three inputs:

• the number to be converted (n)
  • this input can be flexible, see "I/O Flexibility"
• the base which n is currently in (b)
• the base which n is to be converted to (c)

Where 2 < b, c < 1,000.

Return the number in balanced base c representation of n. The output can also be flexible.

The program/function must determine the length of n from the input itself.

I/O Flexibility:

Your input n and output can be represented in these ways:

• your language's definition of an array
• a string, with any character as a separator (e.g. spaces, commas)

Examples:

Note that these use a Python array as n and the output. You may use whatever fits your language, as long as it fits within "I/O Flexibility"'s definition.

[2, -1, -1] 4 7 = [1, -3, -1]
[1, 2, 3, 4] 9 5 = [1, 2, 2, -1, 2]
[10, -9, 10] 20 5 = [1, 1, 1, -2, 1, 0]

This is code-golf, so shortest code in bytes wins!

Answer 1

Mathematica, 85 bytes

#~FromDigits~#2~IntegerDigits~#3//.{p___,a_:0,b_,q___}/;b>⌊#3/2⌋:>{p,a+1,b-#3,q}&

Explanation

#~FromDigits~#2

Convert #1 (1 is implied--input 1, a list of digits) into an integer base #2 (input 2).

... ~IntegerDigits~#3

Convert the resulting integer into base #3 (input 3), creating a list of digits.

... //.{p___,a_:0,b_,q___}/;b>⌊#3/2⌋:>{p,a+1,b-#3,q}

Repeatedly replace the list of digits; if a digit is greater than floor(#3/2), then subtract #3 from it and add 1 to the digit to the left. If there is nothing on the left, insert a 0 and add 1.

Answer 2

Perl 6, 121 bytes

->\n,\b,\c{sub f{sum [R,](@^n)Z*($^b X**0..*)}
first {f(b,n)==f c,$_},map {[$_-($_>floor c/2)*c for .base(c).comb]},0..*}

Slow brute-force solution.

How it works:

• map {[ .base(c).comb]}, 0..* -- Generate the lazy infinite sequence of natural numbers in base c, with each number represented as an array of digits.
• $_ - ($_ > floor c/2) * c -- Transform it by subtracting c from each digit that is greater than floor(c / 2).
• first { f(b, n) == f(c, $_) }, ... -- Get the first array of that sequence which when interpreted as a base c number, equals the input array n interpreted as a base b number.
• sub f { sum [R,](@^n) Z* ($^b X** 0..*) } -- Helper function that turns an array @^n into a number in base $^b, by taking the sum of the products yielded by zipping the reversed array with the sequence of powers of the base.

Answer 3

JavaScript (ES6), 89 bytes

(n,b,c,g=(n,d=n%c,e=d+d<c)=>[...(n=n/c+!e|0)?g(n):[],e?d:d-c])=>g(n.reduce((r,d)=>r*b+d))

100 bytes works for negative values of n.

(n,b,c,g=(n,d=(n%c+c)%c)=>[...(n-=d,n/=c,d+d<c||(d-=c,++n),n?g(n):[]),d])=>g(n.reduce((r,d)=>r*b+d))

Answer 4

Mathematica, 118 114 bytes

IntegerDigits[#3~FromDigits~#2,k=⌊#/2⌋;#]//.{{a_,x___}/;a>k:>{1,a-#,x},{x___,a_,b_,y___}/;b>k:>{x,a+1,b-#,y}}&

⌊ and ⌋ are the 3-byte characters U+230A and U+230B, respectively. Converts #3 to base 10 from base #2, then converts to base # (so the argument order is reversed from the examples). If any digit is greater than the maximum allowed digit k=⌊#/2⌋, decrement that digit by # and increment the next digit up (may need to prepend 1). Keep doing this until all the digits are less than k.