From 3d1eae53e8efd24f9331bfc973c0951981512d7c Mon Sep 17 00:00:00 2001
From: James McKenzie <root@ka-ata-killa.panaceas.james.local>
Date: Mon, 16 Jan 2023 08:14:09 +0000
Subject: first cut

---
 midi.c | 247 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 247 insertions(+)
 create mode 100644 midi.c

(limited to 'midi.c')

diff --git a/midi.c b/midi.c
new file mode 100644
index 0000000..72c2161
--- /dev/null
+++ b/midi.c
@@ -0,0 +1,247 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <byteswap.h>
+#include <string.h>
+#include <math.h>
+
+
+#define MAX_CH 4
+#define MAX_TICK 100000
+#define SAMPLE_RATE   44100
+#define SAMPLES_PER_TICK   50
+
+#define MAX_SAMPLE (SAMPLES_PER_TICK*MAX_TICK)
+
+uint8_t active_note[MAX_CH][MAX_TICK];
+
+double periods[128];
+
+
+double
+freq (int n)
+{
+  return 440. * pow (2., ((double) (n - 69)) / 12.);
+}
+
+
+void
+calc_freqs (void)
+{
+  unsigned i;
+
+  for (i = 0; i < 128; ++i)
+    periods[i] = ((double) SAMPLE_RATE) / freq (i);
+}
+
+
+
+
+void
+on (unsigned t, unsigned ch, unsigned n)
+{
+  memset (&active_note[ch][t], n, MAX_TICK - t);
+}
+
+void
+off (unsigned t, unsigned ch, unsigned n)
+{
+  memset (&active_note[ch][t], 0, MAX_TICK - t);
+}
+
+
+void
+mtrk (FILE *f, long pos, long end)
+{
+
+  uint32_t t = 0, v, m, dt;
+  uint8_t k;
+
+  end += pos;
+
+
+  while (pos < end) {
+
+    fseek (f, pos, SEEK_SET);
+
+    dt = 0;
+
+    do {
+      fread (&v, 1, 1, f);
+      pos++;
+      dt <<= 7;
+      dt |= v & 0x7f;
+    } while (v & 0x80);
+
+
+    t += dt;
+
+    fread (&v, 1, 1, f);
+
+    if (v & 0x80) {
+      m = v;
+      pos++;
+    } else
+      fseek (f, pos, SEEK_SET);
+
+
+    switch (m & 0xf0) {
+    case 0x80:
+      fread (&k, 1, 1, f);
+      fread (&v, 1, 1, f);
+      pos += 2;
+      off (t, m & 0xf, k);
+      //printf ("%d: OFF %d %d %d\n", t,m &0xf, k, v);
+      break;
+
+    case 0x90:
+      fread (&k, 1, 1, f);
+      fread (&v, 1, 1, f);
+      pos += 2;
+      //printf ("%d: ON %d %d %d\n", t, m&0xf,k, v);
+      on (t, m & 0xf, k);
+      break;
+
+    case 0xf0:
+      switch (m) {
+      case 0xff:
+        fread (&v, 1, 1, f);
+        pos++;
+
+        switch (v) {
+        case 0x3:
+          fread (&k, 1, 1, f);
+          pos++;
+          pos += k;
+          break;
+
+        case 0x54:
+          pos += 6;
+          break;
+
+        case 0x51:
+          pos += 4;
+          break;
+
+        case 0x58:
+          pos += 5;
+          break;
+
+        case 0x2f:
+          pos++;
+          break;
+
+        default:
+          printf ("?2 FF %x\n", v);
+          exit (0);
+          break;
+        }
+
+        break;
+
+      default:
+        printf ("?1 %x\n", m);
+        exit (0);
+      }
+
+      break;
+
+    default:
+      printf ("?0 %x\n", m);
+      exit (0);
+
+
+    }
+  }
+}
+
+
+
+
+
+int
+parse_chunk (FILE *f)
+{
+  char fcc[5] = { 0 };
+  uint32_t len;
+  long o;
+
+  if (fread (fcc, 4, 1, f) != 1)
+    return -1;
+
+  if (fread (&len, 4, 1, f) != 1)
+    return -1;
+
+  len = bswap_32 (len);
+
+  o = ftell (f);
+
+  printf ("%s for %d bytes at %d\n", fcc, (int) len, (int) o);
+
+  if (!strcmp (fcc, "MTrk"))
+    mtrk (f, o, len);
+
+  fseek (f, o + len, SEEK_SET);
+
+  return 0;
+}
+
+
+void
+play (FILE *f)
+{
+  unsigned s, c, t;
+  int8_t v;
+  double d;
+
+
+  calc_freqs();
+
+
+  for (s = 0; s < MAX_SAMPLE; ++s) {
+    t = s / SAMPLES_PER_TICK;
+    v = 0;
+
+    for (c = 0; c < MAX_CH; ++c) {
+      if (active_note[c][t]) {
+        d = ((double) s) / periods[active_note[c][t]];
+        d = fmod (d, 1.0);
+
+        if (d < 0.5)
+          v--;
+        else
+          v++;
+      }
+    }
+
+
+    v += (s % 3) * 2;
+    v -= 2;
+
+    if (v < 0) v = -120;
+    else v = 120;
+
+    fwrite (&v, 1, 1, f);
+  }
+}
+
+int
+main (int argc, char *argv[])
+{
+  FILE *f;
+
+  f = fopen ("fugue21.mid", "rb");
+
+  while (!parse_chunk (f));
+
+  fclose (f);
+
+  f = fopen ("fugue21.raw", "wb");
+  play (f);
+  fclose (f);
+
+
+
+  return 0;
+}
-- 
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