block/dmg: factor out block type check

In preparation for adding bzip2 support, split the type check into a
separate function. Make all offsets relative to the begin of a chunk
such that it is easier to recognize the position without having to
add up all offsets. Some comments are added to describe the fields.

There is no functional change.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 1420566495-13284-11-git-send-email-peter@lekensteyn.nl
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/dmg.c

@@ -192,6 +192,18 @@ typedef struct DmgHeaderState {
     uint32_t max_sectors_per_chunk;
 } DmgHeaderState;
 
+static bool dmg_is_known_block_type(uint32_t entry_type)
+{
+    switch (entry_type) {
+    case 0x00000001:    /* uncompressed */
+    case 0x00000002:    /* zeroes */
+    case 0x80000005:    /* zlib */
+        return true;
+    default:
+        return false;
+    }
+}
+
 static int dmg_read_mish_block(BDRVDMGState *s, DmgHeaderState *ds,
                                uint8_t *buffer, uint32_t count)
 {
@@ -231,22 +243,19 @@ static int dmg_read_mish_block(BDRVDMGState *s, DmgHeaderState *ds,
 
     for (i = s->n_chunks; i < s->n_chunks + chunk_count; i++) {
         s->types[i] = buff_read_uint32(buffer, offset);
-        offset += 4;
-        if (s->types[i] != 0x80000005 && s->types[i] != 1 &&
-            s->types[i] != 2) {
+        if (!dmg_is_known_block_type(s->types[i])) {
             chunk_count--;
             i--;
-            offset += 36;
+            offset += 40;
             continue;
         }
-        offset += 4;
 
-        s->sectors[i] = buff_read_uint64(buffer, offset);
+        /* sector number */
+        s->sectors[i] = buff_read_uint64(buffer, offset + 8);
         s->sectors[i] += out_offset;
-        offset += 8;
 
-        s->sectorcounts[i] = buff_read_uint64(buffer, offset);
-        offset += 8;
+        /* sector count */
+        s->sectorcounts[i] = buff_read_uint64(buffer, offset + 0x10);
 
         if (s->sectorcounts[i] > DMG_SECTORCOUNTS_MAX) {
             error_report("sector count %" PRIu64 " for chunk %" PRIu32
@@ -256,12 +265,12 @@ static int dmg_read_mish_block(BDRVDMGState *s, DmgHeaderState *ds,
             goto fail;
         }
 
-        s->offsets[i] = buff_read_uint64(buffer, offset);
+        /* offset in (compressed) data fork */
+        s->offsets[i] = buff_read_uint64(buffer, offset + 0x18);
         s->offsets[i] += in_offset;
-        offset += 8;
 
-        s->lengths[i] = buff_read_uint64(buffer, offset);
-        offset += 8;
+        /* length in (compressed) data fork */
+        s->lengths[i] = buff_read_uint64(buffer, offset + 0x20);
 
         if (s->lengths[i] > DMG_LENGTHS_MAX) {
             error_report("length %" PRIu64 " for chunk %" PRIu32
@@ -273,6 +282,7 @@ static int dmg_read_mish_block(BDRVDMGState *s, DmgHeaderState *ds,
 
         update_max_chunk_size(s, i, &ds->max_compressed_size,
                               &ds->max_sectors_per_chunk);
+        offset += 40;
     }
     s->n_chunks += chunk_count;
     return 0;