[gsharp-devel] MIDI conversion

[Christophe Rhodes]

Hi,

On the off-chance that someone else will either find this useful, or
will comment on it: I had need of conversion routines between MIDI
format 0 and format 1 formats today.  Attached is a patch to midi.lisp
(which works standalone) to implement these conversions, via the
function (SETF MIDIFILE-FORMAT).  I have yet to encounter a format 2
file in the wild, so I haven't implemented anything related to that.

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I eventually settled on (SETF MIDIFILE-FORMAT) as the interface as the
least bad of three options.  They were:
  * COERCE-TO-FORMAT midifile format
    (not clear whether this would preserve the identity of MIDIFILE)
  * CHANGE-CLASS midifile class
    (exposing FORMAT{0,1,2}-MIDIFILE to the world feels ugly)
  * (SETF MIDIFILE-FORMAT)
    (possibly surprising amount of work for a SETF function; possibly
    disguises the fact that the conversion is lossy)

In any case, the basic logic is there to support any of these three
interfaces, and probably the perfect solution that I haven't thought
of.

Also in this patch is a bugfix allowing midifiles in flat keys to be
written.

Cheers,

Christophe
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