From 9562dd19ce8244264d1eb32f97f3fbcc550a74c5 Mon Sep 17 00:00:00 2001
From: Peter Evans <peter@modul8r.com>
Date: Sat, 3 Mar 2018 21:29:19 -0600
Subject: [PATCH] Rewrite dd to use phases and half-track steps

Phases will yet be the death of me. THE DEATH OF ME.
---
 include/apple2.dd.h | 10 +++++++
 src/apple2.dd.c     | 66 ++++++++++++++++++++++++---------------------
 tests/apple2.dd.c   | 19 ++++++++-----
 3 files changed, 57 insertions(+), 38 deletions(-)

diff --git a/include/apple2.dd.h b/include/apple2.dd.h
index ed799e7..bb74d90 100644
--- a/include/apple2.dd.h
+++ b/include/apple2.dd.h
@@ -61,6 +61,16 @@ enum apple2_dd_mode {
  */
 #define MAX_SECTOR_POS 4095
 
+/*
+ * These are the possible phases that can energize the cogs in a disk
+ * drive motor. We represent them as bits, as more than one phase can be
+ * on at the same time.
+ */
+#define DD_PHASE1 0x1
+#define DD_PHASE2 0x2
+#define DD_PHASE3 0x4
+#define DD_PHASE4 0x8
+
 struct apple2dd {
     /*
      * Inside the disk drive there is a stepper motor, and it's
diff --git a/src/apple2.dd.c b/src/apple2.dd.c
index 5525da8..214bf9c 100644
--- a/src/apple2.dd.c
+++ b/src/apple2.dd.c
@@ -13,30 +13,7 @@
 #include "apple2.dec.h"
 #include "apple2.enc.h"
 #include "apple2.h"
-
-// This is a small sort of state machine, which I adapted from AppleInPC
-// (https://github.com/sosaria7/appleinpc) because I could not honestly
-// think of a cleaner way of representing the step transitions that are
-// possible. All credit to that project.
-static int stepper_fsm[][8] =
-{
-	{  0,  0,  0,  0,  0,  0,  0,  0 },	// 0000
-	{  0, -1, -2, -3,  0,  3,  2,  1 },	// 1000
-	{  2,  1,  0, -1, -2, -3,  0,  3 },	// 0100
-	{  1,  0, -1, -2, -3,  0,  3,  2 },	// 1100
-	{  0,  3,  2,  1,  0, -1, -2, -3 },	// 0010
-	{  0, -1,  0,  1,  0, -1,  0,  1 },	// 1010
-	{  3,  2,  1,  0, -1, -2, -3,  0 },	// 0110
-	{  2,  1,  0, -1, -2, -3,  0,  3 },	// 1110
-	{ -2, -3,  0,  3,  2,  1,  0, -1 },	// 0001
-	{ -1, -2, -3,  0,  3,  2,  1,  0 },	// 1001
-	{  0,  1,  0, -1,  0,  1,  0, -1 },	// 0101
-	{  0, -1, -2, -3,  0,  3,  2,  1 },	// 1101
-	{ -3,  0,  3,  2,  1,  0, -1, -2 },	// 0011
-	{ -2, -3,  0,  3,  2,  1,  0, -1 },	// 1011
-	{  0,  3,  2,  1,  0, -1, -2, -3 },	// 0111
-	{  0,  0,  0,  0,  0,  0,  0,  0 }	// 1111
-};
+#include "vm_di.h"
 
 /*
  * Create a new disk drive. We do not create a memory segment for the
@@ -257,17 +234,44 @@ apple2_dd_decode(apple2dd *drive)
 void
 apple2_dd_phaser(apple2dd *drive)
 {
-    int phase = drive->phase_state;
-    int last = drive->last_phase;
+    int next = drive->phase_state;
+    int prev = drive->last_phase;
+    int step = 0;
 
     if (!drive->online) {
         return;
     }
 
-    // Look up the number of steps to move according to the current
-    // phase and the current track position.
-    apple2_dd_step(drive, 
-                   stepper_fsm[phase][drive->track_pos & 0x7]);
+    // If PHASE1 is on and PHASE4 was previously on, then we want to
+    // mimic an inward step
+    if ((next & DD_PHASE1) && (prev & DD_PHASE4)) {
+        next = DD_PHASE4 << 1;
+    }
+
+    // If, however, PHASE4 is on and previously PHASE1 was on, then we
+    // want to mimic an outward step
+    if ((next & DD_PHASE4) && (prev & DD_PHASE1)) {
+        next = DD_PHASE1 >> 1;
+    }
+
+    // If an adjacent phase is on, add an inward or outward step. If
+    // _both_ adjacent phases are on, the step will count as zero (or no
+    // step).
+    if (next & (prev << 1)) {
+        step++;
+    }
+
+    if (next & (prev >> 1)) {
+        step--;
+    }
+
+    // If the opposite cog is also on, then our accounting is for
+    // naught; nullify the step movement.
+    if ((next & (prev << 2)) || (next & (prev >> 2))) {
+        step = 0;
+    }
+
+    apple2_dd_step(drive, step);
 
     // Recall our trickery above with the phase variable? Because of it,
     // we have to save the phase_state field into last_phase, and not
@@ -288,7 +292,7 @@ apple2_dd_position(apple2dd *drive)
         return 0;
     }
 
-    int track_offset = (drive->track_pos / 2) * ENC_ETRACK;
+    int track_offset = (drive->track_pos / 4) * ENC_ETRACK;
     return track_offset + drive->sector_pos;
 }
 
diff --git a/tests/apple2.dd.c b/tests/apple2.dd.c
index ef8a343..ef44aa0 100644
--- a/tests/apple2.dd.c
+++ b/tests/apple2.dd.c
@@ -203,25 +203,30 @@ Test(apple2_dd, decode)
 
 Test(apple2_dd, phaser)
 {
+    // The drive must be online for any change to matter
+    drive->online = true;
+
     // Test going backwards
     drive->track_pos = 3;
     drive->phase_state = 1;
-    drive->last_phase = 0;
-    drive->online = true;
+    drive->last_phase = 2;
 
     apple2_dd_phaser(drive);
 
-    cr_assert_eq(drive->track_pos, 0);
+    cr_assert_eq(drive->track_pos, 2);
     cr_assert_eq(drive->last_phase, 1);
 
     // Forwards
-    drive->phase_state = 9;
+    drive->phase_state = 2;
     drive->track_pos = 5;
-    drive->last_phase = 8;
+    drive->last_phase = 1;
+    apple2_dd_phaser(drive);
+    cr_assert_eq(drive->track_pos, 6);
+    cr_assert_eq(drive->last_phase, 2);
+    drive->phase_state = 4;
     apple2_dd_phaser(drive);
-
     cr_assert_eq(drive->track_pos, 7);
-    cr_assert_eq(drive->last_phase, 9);
+    cr_assert_eq(drive->last_phase, 4);
 }
 
 Test(apple2_dd, switch_phase)