3=C, 5 = E. If > 26 then 27=AA, 28=AB, 29=AC

number_to_alphabet.pl

#!/usr/bin/env perl

use v5.12;
use strict;
use warnings;

use Test::More;

my %tests = (
    1   => "A",
    1.0 => "A",
    2   => "B",
    25  => "Y",
    26  => "Z",
    27  => "AA",
    52  => "AZ",
    54  => "BB",
    78  => "BZ",
    (26**2 + 26) => "ZZ",
    (26**2 + 26 + 1) => "AAA",
    (26**3 + 26**2 + 26) => "ZZZ",
    (26**3 + 26**2 + 26 + 1) => "AAAA",
);

for my $have (keys %tests) {
    my $want = $tests{$have};
    is to_letters($have), $want, "to_letters($have) -> $want";
}

for my $negative (0, -1, -100) {
    ok !eval { to_letters($negative) }, "negative input $negative";
    like $@, qr{^0 and less cannot be converted to letters};
}

for my $decimal (1.1, 2.001) {
    ok !eval { to_letters($decimal) }, "decimal input $decimal";
    like $@, qr{^Decimals cannot be converted to letters};
}

done_testing;

use Carp;

sub to_letters {
    my $number = shift;
    croak "0 and less cannot be converted to letters" if $number <= 0;
    croak "Decimals cannot be converted to letters" if int $number != $number;

    state $BASE = 26;
    state $CHAR_BASE = 65;

    my @letters;
    while( $number > 0 ) {
        $number--;  # A is 1, not 0
        my $shift = $number % $BASE;
        unshift @letters, chr( $CHAR_BASE + $shift );
        $number = int($number / $BASE);
    }

    return join '', @letters;
}