From c12e02acfa580b3d79b4df3406de533ceeefaf84 Mon Sep 17 00:00:00 2001
From: shermp <14854761+shermp@users.noreply.github.com>
Date: Sun, 9 Jun 2019 22:08:26 +1200
Subject: [PATCH] Implement differencing read function

---
 src/minivhd_internal.h |  1 +
 src/minivhd_io.c       | 51 ++++++++++++++++++++++++++++++++++++++++++
 src/minivhd_manage.c   |  1 +
 3 files changed, 53 insertions(+)

diff --git a/src/minivhd_internal.h b/src/minivhd_internal.h
index 58dc96f..e805d26 100644
--- a/src/minivhd_internal.h
+++ b/src/minivhd_internal.h
@@ -59,6 +59,7 @@ typedef struct MVHDSparseHeader {
 typedef struct MVHDBlock {
     uint8_t* bitmap;
     uint32_t offset;
+    bool bitmap_cached;
 } MVHDBlock;
 
 bool mvhd_is_conectix_str(const void* buffer);
diff --git a/src/minivhd_io.c b/src/minivhd_io.c
index b0e8493..520a5f2 100644
--- a/src/minivhd_io.c
+++ b/src/minivhd_io.c
@@ -3,6 +3,13 @@
 #include "minivhd_internal.h"
 #include "minivhd.h"
 
+/* The following bit array macros adapted from 
+   http://www.mathcs.emory.edu/~cheung/Courses/255/Syllabus/1-C-intro/bit-array.html */
+
+#define VHD_SETBIT(A,k)     ( A[(k/8)] |= (0x80 >> (k%8)) )
+#define VHD_CLEARBIT(A,k)   ( A[(k/8)] &= ~(0x80 >> (k%8)) )
+#define VHD_TESTBIT(A,k)    ( A[(k/8)] & (0x80 >> (k%8)) )
+
 static inline void mvhd_check_sectors(int offset, int num_sectors, int total_sectors, int* transfer_sect, int* trunc_sect) {
     *transfer_sect = num_sectors;
     *trunc_sect = 0;
@@ -11,6 +18,20 @@ static inline void mvhd_check_sectors(int offset, int num_sectors, int total_sec
         *trunc_sect = num_sectors - *transfer_sect;
     }
 }
+
+static void mvhd_read_sect_bitmap(MVHDMeta* vhdm, int blk) {
+    if (!vhdm->block[blk].bitmap_cached) {
+        if (vhdm->block[blk].bitmap == NULL) {
+            vhdm->block[blk].bitmap = calloc(vhdm->bm_sect_count, MVHD_SECTOR_SIZE);
+        }
+        if (vhdm->block[blk].offset != MVHD_SPARSE_BLK) {
+            fseeko64(vhdm->f, vhdm->block[blk].offset * MVHD_SECTOR_SIZE, SEEK_SET);
+            fread(vhdm->block[blk].bitmap, vhdm->bm_sect_count * MVHD_SECTOR_SIZE, 1, vhdm->f);
+        }
+        vhdm->block[blk].bitmap_cached = true;
+    }
+}
+
 int mvhd_fixed_read(MVHDMeta* vhdm, int offset, int num_sectors, void* out_buff) {
     int64_t addr;
     int transfer_sectors, truncated_sectors;
@@ -47,4 +68,34 @@ int mvhd_sparse_read(MVHDMeta* vhdm, int offset, int num_sectors, void* out_buff
         buff += MVHD_SECTOR_SIZE;
     }
     return truncated_sectors;
+}
+
+int mvhd_diff_read(MVHDMeta* vhdm, int offset, int num_sectors, void* out_buff) {
+    int transfer_sectors, truncated_sectors;
+    int total_sectors = vhdm->footer.geom.cyl * vhdm->footer.geom.heads * vhdm->footer.geom.spt;
+    mvhd_check_sectors(offset, num_sectors, total_sectors, &transfer_sectors, &truncated_sectors);
+    uint8_t* buff = (uint8_t*)out_buff;
+    MVHDMeta* curr_vhdm = vhdm;
+    int s, ls, blk, sib;
+    ls = offset + transfer_sectors;
+    for (s = offset; s <= ls; s++) {
+        blk = s / vhdm->sect_per_block;
+        sib = s % vhdm->sect_per_block;
+        while (curr_vhdm->footer.disk_type == MVHD_TYPE_DIFF) {
+            mvhd_read_sect_bitmap(vhdm, blk);
+            if (VHD_TESTBIT(curr_vhdm->block[blk].bitmap, sib)) {
+                curr_vhdm = curr_vhdm->parent;
+            } else { break; }
+        }
+        /* We handle actual sector reading using the fixed or sparse functions,
+           as a differencing VHD is also a sparse VHD */
+        if (curr_vhdm->footer.disk_type == MVHD_TYPE_DIFF || curr_vhdm->footer.disk_type == MVHD_TYPE_DYNAMIC) {
+            mvhd_sparse_read(curr_vhdm, s, 1, buff);
+        } else {
+            mvhd_fixed_read(curr_vhdm, s, 1, buff);
+        }
+        curr_vhdm = vhdm;
+        buff += MVHD_SECTOR_SIZE;
+    }
+    return truncated_sectors;
 }
\ No newline at end of file
diff --git a/src/minivhd_manage.c b/src/minivhd_manage.c
index 09cda17..6558589 100644
--- a/src/minivhd_manage.c
+++ b/src/minivhd_manage.c
@@ -83,6 +83,7 @@ static MVHDBlock* mvhd_read_bat(MVHDMeta *vhdm, MVHDError* err) {
     for (uint32_t i = 0; i < vhdm->sparse.max_bat_ent; i++) {
         fread(&vhdm->block[i].offset, sizeof vhdm->block[i].offset, 1, vhdm->f);
         vhdm->block[i].bitmap = NULL;
+        vhdm->block[i].bitmap_cached = false;
     }
     return vhdm->block;
 }