How to transpose music chords with PHP?

Question

I was wondering how would one create a function, in PHP, which is used for transposing some music chords.

I will try to explain how it works in music theory. I hope I don't forget something. If there are some mistakes, please help me to correct it.

---

1. The simple chords.

The simple chords are almost as simple as an alphabet and it goes like this:

C, C#, D, D#, E, F, F#, G, G#, A, A# B

From B it loops all over again to C. Therefore, If the original chord is E and we want to transpose +1, the resulting chord is F. If we transpose +4, the resulting chord is G#.

2. Expanded chords.

They work almost like the simple chords, but contain a few more characters, which can safely be ignored when transposing. For example:

Cmi, C#7, Dsus7, Emi, Fsus4, F#mi, G ...

So again, as with the simple chords, if we transpose Dsus7 + 3 = Fsus7

3. Non-root bass tone.

A problem arises when the bass plays a different tone than the chord root tone. This is marked by a slash after the chord and also needs to be transposed. Examples:

C/G, Dmi/A, F#sus7/A#

As with examples 1 and 2, everything is the same, but the part after the slash needs transpose too, therefore:

C/G + 5 = F/C

F#sus7/A# + 1 = Gsus7/B

---

So basically, imagine you have a PHP variable called chord and the transpose value transpose. What code would transpose the chord?

Examples:

var chord = 'F#sus7/C#';
var transpose = 3; // remember this value also may be negative, like "-4"
... code here ...
var result; // expected result = 'Asus7/E';

---

I have found an existed question on StackOverflow, at here. They talk about algorithm for chord-progressions.

---

How do I transpose music chords with PHP, by increasing or decreasing by semitones?

Answer 1

A quick solution:

<?php

// produces the expected result
echo transpose("F#sus7/C#",3);

function transpose($chord,$transpose)
{
    // the chords
    $chords = array("C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B");

    $result = "";

    // get root tone
    $root_arr = explode("/",$chord);
    $root = strtoupper($root_arr[0]);

    // the chord is the first character and a # if there is one
    $root = $root[0].((strpos($root, "#") !== false)?"#":"");

    // get any extra info
    $root_extra_info = str_replace("#","",substr($root_arr[0],1)); // assuming that extra info does not have any #

    // find the index on chords array
    $root_index = array_search($root,$chords);
    // transpose the values and modulo by 12 so we always point to existing indexes in our array
    $root_transpose_index = floor(($root_index + $transpose) % 12);

    if ($root_transpose_index < 0)
    {
        $root_transpose_index += 12;
    }

    $result.= $chords[$root_transpose_index].$root_extra_info;

    if(count($root_arr)>1)
    {
        // get the non root tone
        $non_root = $root_arr[1];
        // the chord is the first character and a # if there is one
        $non_root = strtoupper($non_root[0]).((strpos($non_root, "#") !== false)?"#":"");
        // get any extra info
        $non_root_extra_info = str_replace("#","",substr($root_arr[1],1)); // assuming that extra info does not have any #

        // find the index on chords array
        $non_root_index = array_search($non_root,$chords);
        // transpose the values and modulo by 12 so we always point to existing indexes in our array
        $non_root_transpose_index = floor(($non_root_index + $transpose) % 12);

        if ($non_root_transpose_index < 0)
        {
            $non_root_transpose_index += 12;
        }

        $result.= "/".$chords[$non_root_transpose_index].$non_root_extra_info;
    }

    return $result;
}

https://3v4l.org/Cd9Pg

lots of room for improvement in code, i just tried to code it to be easy to understand.

Answer 2

Here my regex idea with preg_replace_callback (use of anonymous function requires PHP 5.3).

function transpose($str, $t=0)
{
  // the chords
  $chords = ["C","C#","D","D#","E","F","F#","G","G#","A","A#","B"];

  // set transpose, return if none
  $t = (int)$t % 12 + 12; if($t % 12 == 0) return $str;

  // regex with callback
  return preg_replace_callback('~[A-G]#?~', function($m) use (&$chords, &$t) {
    return $chords[(array_search($m[0], $chords) + $t) % 12];
  }, $str);
}

Demo at eval.in   (for testing the regex pattern [A-G]#? see regex101)

echo transpose("Cmi, C#7, Dsus7, Emi, Fsus4, F#mi, G C/G, Dmi/A, F#sus7/A#", -3);

Ami, A#7, Bsus7, C#mi, Dsus4, D#mi, E A/E, Bmi/F#, D#sus7/G