BQN, 19 bytes^SBCS

' '+32⊸<⊸×+⌜˜⌽⊸⌊↕59

Run online!

This uses the whitespace allowance, padding by four spaces on all sides to make the arithmetic work better. It can be adjusted by replacing ↕59 with 4+↕51 at the cost of two characters.

BQN has no built-in idea of the alphabet, but it has a nice system for Unicode character arithmetic. This solution depends on the length 26 of the alphabet, and the distance 33 ('A'-' ') from space to the start. Fortunately these are close together. We will build a matrix of numbers that goes from 0, for ' ', at the corners to 33+26-1, or 58, at the center, then bring all numbers below 33 to 0. At the middle of each edge we get 29, leaving 4 spaces before 'A'. Before plays a big role here.

' '+32⊸<⊸×+⌜˜⌽⊸⌊↕59
                ↕59   # Range            0...58
             ⌽        # Reverse:         58...0
              ⊸⌊      #  Before Minimum: 0…29…0
          +⌜          # Addition table
            ˜         #  with itself
    32⊸<              # More than 32
        ⊸×            #  times the original
' '+                  # Add to space

Another 21-character "exact" solution is (+⌜˜⊏-∘≠↑'A'+⍷)⌽⊸⌊↕51. Now 'A' is used as the basis and spaces come from fill elements in Take (↑), so it will work for any starting character.

posted over 3 years ago

CC BY-SA 4.0

Marshall Lochbaum‭

681 reputation 0 20 68 9

Brain-Flak, 250 bytes

((((()()()){}){}){}()){(({})[()])}{}(<>)<>{({}<>)<>}<>{(({})[()])}([]){{}([(({}))](((()()()){}){}){}()){({}[()])<>((((()()()()){}){}){})<>}{}<>(((((()()()()){}){}){}){}())<>{({}[()])<>(({})())<>}{}{({}[()])<>(({})[()])<>}{}<>((()()()()()){})<>([])}<>

Try it online!

Readable, commented version:

# Push 25
((((()()()){}){}){}())

# Push range(25, 1)
{
    (({})[()])
}
{}

# Push a 0 on to the alternate stack and return to main stack
(<>)<>

# Reverse values onto other stack
{
    ({}<>)
    <>
}
<>

# Push range(25, 0)
{
    (({})[()])
}

# Push the stack height.
([])

# At this point, the stack has 0, 1, 2... 23, 24, 25, 24, 23... 2, 1, 0
# Go through each number of these numbers, and call it N
{
    # Pop the stack height counter
    {}

    # Push (25 - N), and underneath it push N an additional time
    ([(({}))](((()()()){}){}){}())

    # That many times...
    {
        # Decrement
        ({}[()])

        <>

        # Push a space onto the ascii-art stack
        ((((()()()()){}){}){})

        <>
    }
    {}

    # on the ascii-art stack, push the letter A
    <>
    (((((()()()()){}){}){}){}())
    <>

    # N times...
    {
        # Decrement N
        ({}[()])

        # Push "The last character + 1"
        <>
        (({})())
        <>
    }
    {}

    # N times...
    {
        # Decrement N
        ({}[()])

        # Push "The last character - 1"
        <>
        (({})[()])
        <>
    }
    {}

    <>
    # Push a newline
    ((()()()()()){})
    <>

    ([])
}<>

posted over 3 years ago

CC0

DJMcMayhem‭

271 reputation 0 6 27 4

V (vim), 21 bytes

¬AZ¬YA|òlÄ26|xx>>YGpH

Try it online!

Explanation:

¬AZ                     " Insert every character from A to Z
   ¬YA                  " Insert every character from Y to A
      |                 " Go back to the beginning of this line
       ò                " While True...
        l               "   Move to the right (if there is only one character
                        "   on this line, break the loop)
         Ä              "   Duplicate this line upwards
          26|           "   Go to the 26th character on this line
             xx         "   Delete two characters
               >>       "   Indent this line with one space
                 Y      "   Yank this line
                  G     "   Go to the last line
                   p    "   Paste the line that was just yanked downwards
                    H   "   Go back to the first line
                        " (Implicit) End loop

Hexdump:

00000000: ac41 5aac 5941 7cf2 6cc4 3236 7c78 783e  .AZ.YA|.l.26|xx>
00000010: 3e59 4770 48                             >YGpH

posted over 3 years ago

CC0

3y ago

DJMcMayhem‭

271 reputation 0 6 27 4

JavaScript (Node.js), 78 bytes

(m=(a,b=26,c=a(--b))=>b?[c,...m(a,b),c]:[c])=>m(a=>m(b=>(c=25-a-b)<0?32:c+65))

Try it online!

posted over 3 years ago

CC0

Hakerh400‭

1591 reputation 4 55 164 63

Stax, 12 bytes

ç ∩<▬%▌ê▓jFo

Run and debug it

Same method as the Canvas answer.

posted over 3 years ago

CC BY-SA 4.0

Razetime‭

2860 reputation 42 65 321 83

Canvas, ^{10 7} 6 bytes

Ｚ［］／┼┼

Try it here!

Creates the top portion, centers it, and mirrors it.

-3 bytes from dzaima's hint.

-1, found the 6 byter!

Explanation

Z[]/++
Z          push the alphabet
 []        get all prefixes
   /       diagonalize  by padding each line with one less than the previous
    ++     Quad palindromize, without reflecting letters

posted over 3 years ago

CC BY-SA 4.0

3y ago

Razetime‭

2860 reputation 42 65 321 83

Japt -R, 14 13 bytes

;Bãê kb'A û ê

Test it

Takes all substrings of the alphabet, palindromizes them, and then takes only the ones starting with A and centers and mirrors that too.

-1 byte thanks to @Shaggy

posted over 3 years ago

CC BY-SA 4.0

3y ago

Quintec‭

2306 reputation 17 46 242 48

Vyxal C, 8 bytes

kA¦ømƛøm

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Explanation:

kA        # Push the alphabet
  ¦       # Prefixes
   øm     # Palindromize list without duplicating middle
     ƛ    # For each:
      øm  #   Palindromize without duplicating middle
          # 'C' flag - Center and print, separated by newlines

posted over 3 years ago

CC BY-SA 4.0

Aaron Miller‭

181 reputation 0 5 18 2

Jelly, 13 bytes

ØAUƤz⁶ZUŒBŒḄY

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How it works

ØAUƤz⁶ZUŒBŒḄY - Main link. Takes no arguments
ØA            - Yield the uppercase alphabet
   Ƥ          - Over each prefix:
  U           -   Reverse it
    z⁶        - Zip, padding with spaces
      Z       - Transpose
       U      - Reverse each
        ŒB    - Bounce each
          ŒḄ  - Bounce the entire string
            Y - Join by newlines

posted over 3 years ago

CC BY-SA 4.0

3y ago

caird coinheringaahing‭

516 reputation 3 13 61 12

Python 3, 99 bytes

r=[*range(26)]
a="ABCDEFGHIJKLMNOPQRSTUVWXYZ"
for i in r+r[-2::-1]:print(" "*(26-i)+a[:i]+a[i::-1])

Try it online!

posted about 3 years ago

CC BY-SA 4.0

3y ago

celtschk‭

1354 reputation 28 47 175 73

Japt -R, 11 bytes

;Bå+ mê ê û

Test it

;Bå+ mê ê û
;B              :Uppercase alphabet
  å+            :Cumulatively reduce by concatenation
     m          :Map
      ê         :  Palindromise
        ê       :Palindromise
          û     :Centre pad each with spaces to length of the longest
                :Implicit  output joined with newlines

posted over 3 years ago

CC BY-SA 4.0

Shaggy‭

2111 reputation 4 115 214 166