/* a2freq - convert MIDI note numbers or scientific pitch notations to frequencies */
/* by Antoine Amarilli (2011) */
/* licence: public domain */
/* compilation: cc -lm -o a2freq a2freq.c */

#include<stdio.h>
#include<math.h>
#include<ctype.h>

#define E_ARGS 1
#define E_PARSE 2

#define BAD_ACCID 42


float key2freq(int n)
{
  /* Return frequency of midi note number n, inspired per the "Piano key
   * frequencies" article on wikipedia */

  return 440 * pow(2, ((float) (n - 69))/12);
}


int name2num(char name)
{
  /* Return note number adjustment for upper case scientific pitch
   * notation note name name. Return <0 on error. */

  switch(name)
  {
    case 'C':
      return 0;
      break;
    case 'D':
      return 2;
      break;
    case 'E':
      return 4;
      break;
    case 'F':
      return 5;
      break;
    case 'G':
      return 7;
      break;
    case 'A':
      return 9;
      break;
    case 'B':
      return 11;
      break;
    default:
      return -1;
  }
}


int accid2num(char accid)
{
  /* Return note number adjustment for scientific pitch notation note
   * accidental. Return <0 on error. */

  switch (accid) {
    case 'b':
      return -1;
      break;
    case '#':
      return 1;
      break;
    default:
      return BAD_ACCID;
  }
}


int note2num(int octave, int note)
{
  /* Return MIDI note number for octave and note. */

  return (octave + 1)*12 + (note);
}


float pitch2freq(char* a)
{
  /* Return frequency of scientific pitch notation note a, by converting
   * it to a key number and using key2freq. Return <0 on parse error. */

  int note, octave = 4, accid, num;
  int octavepos = 1; // position where octave indication is expected

  /* parse note name */
  if (!a[0]) return -1;
  note = name2num(toupper(a[0]));
  if (note < 0) return note;
  
  /* parse accidental */
  if (a[octavepos] && !isdigit(a[octavepos])) {
    accid = accid2num(a[octavepos]);
    if (accid == BAD_ACCID) return -1;
    note += accid;
    octavepos++;
  }

  /* parse octave */
  num = !a[octavepos]?note2num(octave, note):note2num(atoi(a + octavepos), note);
  return key2freq(num);
}


float do_one(char* a)
{
  int result;

  if (result = atoi(a))
    return key2freq(result);
  else return pitch2freq(a);
}


int main(int argc, char** argv)
{
  int i;
  double result;

  if (argc <= 1) {
    fprintf(stderr,
        "Usage: %s [NOTE]...\n", argv[0]);
    fprintf(stderr,
        "Convert MIDI note numbers or scientific pitch notations to frequencies.\n");
    fprintf(stderr,
        "NOTE is either a MIDI note number or a key in scientific pitch notation.\n");
    fprintf(stderr,
        "Scientific pitch notation examples: \"A4\", \"C#2\", \"Bb6\"\n");
    return E_ARGS;
  }
      
  for (i=1; i<argc; i++)
  {
    result = do_one(argv[i]);
    if (result >= 0)
      printf("%f\n", result);
    else {
      fprintf(stderr, "a2freq: cannot parse %s\n", argv[i]);
      return E_PARSE;
    }
  }

  return 0;
}