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.

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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

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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';

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

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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

Answer 3

Okay, so there are a few things you want to handle.

First, you want to be able to loop around in the array. That's easy: use the modulus operator, which in php is %.

function transpose($chord, $increment) {
  $map = array('A', 'A#', 'B', 'C', 'C#', 'D', 'D#', 'E', 'F', 'F#', 'G', 'G#');

  // Get the index of the given chord
  $index = array_search($chord, $map);
  if($index === false)
    return false;

  // Get the transposed index and chord
  $transpose_index = ($index + $increment) % count($map);
  if($transpose_index < 0)
    $transpose_index += count($map);
  return $map[$transpose_index];
}

Second, you want to be able to strip out the actual chords that matter to you. You can do this using a regular expression (RegEx):

function transposeFull($chords, $increment) {

  // This RegEx looks for one character (optionally followed by a sharp).
  //     .\#?
  // This RegEx looks for an optional series of characters which are not /
  //     [^\/]*
  // Put them together to get a RegEx that looks for an expanded chord
  //     (.\#?)([^\/]*)
  // Then, do it again, but add a / first, and make it optional.
  //     (\/(.\#?)([^\/]*))?
  $regex = '%(.\#?)([^\/]*)(\/(.\#?)([^\/]*))?%';

  // Note that the () allow us to pull out the matches.
  // $matches[0] is always the full thing.
  // $matches[i] is the ith match 
  //   (so $matches[3] is the whole optional second chord; which is not useful)
  $matches = array();
  preg_match($regex, $chords, $matches);

  // Then, we get any parts that were matched and transpose them.
  $chord1 = (count($matches) >= 2) ? transpose($matches[1], $increment) : false;
  $expanded1 = (count($matches) >= 2) ? $matches[2] : '';
  $chord2 = (count($matches) >= 5) ? transpose($matches[4], $increment) : false;
  $expanded2 = (count($matches) >= 6) ? $matches[5] : '';

  // Finally, put it back together.
  $chords = '';
  if($chord1 !== false)
    $chords .= $chord1.$expanded1;
  if($chord2 !== false)
    $chords .= '/'.$chord2.$expanded2;

  return $chords;
}