From 455151abf746c6fabb9ef1e337592229119f82c0 Mon Sep 17 00:00:00 2001
From: Brendan Robert <brobert@adobe.com>
Date: Wed, 10 Jul 2024 13:34:09 -0500
Subject: [PATCH] #44: Tighter sector order detection, at least for prodos
 volumes.

---
 src/main/java/jace/hardware/FloppyDisk.java | 31 ++++++++++++---------
 1 file changed, 18 insertions(+), 13 deletions(-)

diff --git a/src/main/java/jace/hardware/FloppyDisk.java b/src/main/java/jace/hardware/FloppyDisk.java
index be3c8c3..ca0acf5 100644
--- a/src/main/java/jace/hardware/FloppyDisk.java
+++ b/src/main/java/jace/hardware/FloppyDisk.java
@@ -118,13 +118,13 @@ public class FloppyDisk {
      */
     public FloppyDisk(File diskFile) throws IOException {
         FileInputStream input = new FileInputStream(diskFile);
-        readDisk(input);
+        readDisk(input, diskFile.getName().toUpperCase().endsWith(".PO") ? SectorOrder.PRODOS : SectorOrder.DOS);
         writeProtected = !diskFile.canWrite();
         diskPath = diskFile;
     }
 
     // brendanr: refactored to use input stream
-    public void readDisk(InputStream diskFile) throws IOException {
+    public void readDisk(InputStream diskFile, SectorOrder assumedOrder) throws IOException {
         isNibblizedImage = true;
         volumeNumber = CardDiskII.DEFAULT_VOLUME_NUMBER;
         headerLength = 0;
@@ -154,7 +154,17 @@ public class FloppyDisk {
             }
             
             if (sectorOrder == SectorOrder.UNKNOWN) {
-                sectorOrder = detectSectorOrder(nibbles, SectorOrder.DOS);
+                if (isProdosVolumeBlock(nibbles, 0x0400)) {
+                    System.out.println("Prodos volume block found at 0x0400");
+                    sectorOrder = SectorOrder.PRODOS;
+                } else if (isProdosVolumeBlock(nibbles, 0x0B00)) {
+                    System.out.println("Prodos volume block found at 0x0A00");
+                    sectorOrder = SectorOrder.DOS;
+                }
+            }
+            if (sectorOrder == SectorOrder.UNKNOWN) {
+                System.out.println("Assuming sector order based on file extension");
+                sectorOrder = assumedOrder;
             }
             System.out.println(null == sectorOrder ? "Sector order is null" : "Sector order is " + sectorOrder.name());
             currentSectorOrder = sectorOrder.sectors;
@@ -174,27 +184,22 @@ public class FloppyDisk {
         StateManager.markDirtyValue(currentSectorOrder);
     }
 
-    private SectorOrder detectSectorOrder(byte[] nibbles, SectorOrder defaultOrder) {
-        // Easiest way to tell sector order is see if we can parse a prodos volume at block 2
-        int offset = 0x0400;
-        SectorOrder result = defaultOrder;
+    private boolean isProdosVolumeBlock(byte[] nibbles, int offset) {
         // First two bytes are zero (no previous block)
         if (nibbles[offset] != 0 || nibbles[offset+1] != 0) {
-            return result;
+            return false;
         }
         // Next two bytes are either both zero or at least in the range of 3...280
         int nextBlock = (nibbles[offset+2] & 0x0ff) | (nibbles[offset+3] << 8);
         if (nextBlock == 1 || nextBlock == 2 || nextBlock > 280) {
-            return result;
+            return false;
         }
         // Now check total blocks at offset 0x29
         int totalBlocks = (nibbles[offset+0x29] & 0x0ff) | (nibbles[offset+0x2a] << 8);
         if (totalBlocks != 280) {
-            System.out.println("Prodos detection failed: total blocks is not 280: " + totalBlocks);
-            return result;
+            return false;
         }
-        System.out.println("This disk appears to be Prodos-ordered.");
-        return SectorOrder.PRODOS;
+        return true;
     }
     /*
      * Convert a block-format disk to a 6-by-2 nibblized encoding scheme (raw NIB disk format)