diff --git a/rpatch.c b/rpatch.c
index 523c19b..bbe4485 100644
--- a/rpatch.c
+++ b/rpatch.c
@@ -1,14 +1,14 @@
 #include "io_funcs.h"
 #include "rpatch.h"
 
-int
+lp_UMDiffFile
 lp_PatchSet_open(const char *path)
 {
 
 }
 
 int
-lp_PatchSet_covered(lp_PatchSet *ps, u32 offset)
+lp_PatchSet_covered(lp_UMDiffCommand *ps, u32 offset)
 {
 
 }
diff --git a/rpatch.h b/rpatch.h
index e724731..864b971 100644
--- a/rpatch.h
+++ b/rpatch.h
@@ -1,20 +1,6 @@
 #ifndef __RPATCH_H
 #define __RPATCH_H
 
-
-typedef struct {
-    long sector_start;
-    long sector_count;
-
-    /* If zero, block is unchanged */
-    long patch_start;
-
-    /* If smaller than sector_count, repeating data */
-    long patch_len;
-} lp_Patch;
-
-typedef struct {
-
-} lp_PatchSet;
+#include "umdiff/umdiff.h"
 
 #endif //__RPATCH_H
diff --git a/umdiff/Makefile b/umdiff/Makefile
index e69de29..39973c1 100644
--- a/umdiff/Makefile
+++ b/umdiff/Makefile
@@ -0,0 +1,27 @@
+TARGET = umdiff
+C_OBJS = main.o rdiff.o
+OBJS   = $(C_OBJS)
+
+CMAKE := cmake
+
+LIBRSYNC_SOURCE := librsync
+
+ifeq (,$(wildcard /usr/lib*/librsync.*))
+	CFLAGS += -I$(LIBRSYNC_SOURCE)/src
+	OBJS += librsync.a
+else
+	LIBS += -lrsync
+endif
+
+$(TARGET): $(OBJS)
+	$(CC) -o $@ $^ $(CFLAGS) $(LIBS)
+
+librsync:
+	git clone https://github.com/librsync/librsync
+
+librsync.a: $(LIBRSYNC_SOURCE)
+	mkdir -p librsync.build
+	cd librsync.build && $(CMAKE) -DBUILD_SHARED_LIBS=0 $(shell realpath $(LIBRSYNC_SOURCE)) && make
+	mv librsync.build/librsync.a .
+	rm -rf librsync.build
+
diff --git a/umdiff/main.c b/umdiff/main.c
new file mode 100644
index 0000000..8677c17
--- /dev/null
+++ b/umdiff/main.c
@@ -0,0 +1,4 @@
+int main(int argc, char *argv[])
+{
+
+}
diff --git a/umdiff/rdiff.c b/umdiff/rdiff.c
new file mode 100644
index 0000000..e69de29
diff --git a/umdiff/umdiff.h b/umdiff/umdiff.h
new file mode 100644
index 0000000..eee5ed6
--- /dev/null
+++ b/umdiff/umdiff.h
@@ -0,0 +1,77 @@
+#ifndef __UMDIFF_H
+#define __UMDIFF_H
+
+/**
+ * @file       umdiff.h
+ * @author     Karim Vergnes <me@thesola.io>
+ * @copyright  GPLv2
+ * @brief      Definitions for UMDiff file format
+ *
+ * This file contains all required types to read and parse a UMDiff file.
+ */
+
+#define UMDIFF_VERSION 0x00
+
+/**
+ * @brief      Index of commands at 1024 sector interval.
+ *
+ * This list contains the offset of the first command for every 1024 sector
+ * block.
+ *
+ * The value represents an offset to read the file from, in the command list.
+ * If a command exists across a 1024-sector boundary, the offset shall point
+ * to the start of that command.
+ *
+ * Pointers spanning past the end of the disc are undefined and shall not be
+ * used.
+ */
+typedef long lp_UMDiffIndex[2048];
+
+typedef struct 
+__attribute__((packed)) {
+    char magic[7]; /* = 0x7f 'UMDiff' */
+    char version;
+    long cmd_len;
+    long data_start;
+    lp_UMDiffIndex index;
+} lp_UMDiffHeader;
+
+/**
+ * @brief      Definition for a single patch command.
+ *
+ * This is the format for a unit 'patch command'. A patch command is comprised
+ * of five parameters:
+ *
+ * - <tt>sector_start</tt>: Where on the original disc the patch command takes
+ *   effect, in sectors.
+ * - <tt>sector_count</tt>: How many sectors on the original disc the patch
+ *   command spans.
+ * - <tt>patch_start</tt>: Where in the patch source the substitute data is
+ *   found, in sectors.
+ * - <tt>patch_sector_count</tt>: How many sectors on the patch source the
+ *   substitute data occupies. If smaller than <tt>sector_count</tt>, then
+ *   the substitute is a repeating pattern of that length.
+ * - <tt>data_source</tt>: 1 if the substitute data is on the original disc,
+ *   0 if it is in the UMDiff file's data area.
+ */
+typedef struct {
+    long sector_start;
+    long sector_count;
+
+    long patch_start;
+    long patch_sector_count; /* if < sector_count, repeat */
+
+    long data_source; /* 0 = patchfile, 1 = source */
+} lp_UMDiffCommand;
+
+// Commands shall start immediately after the full header
+#define UMDIFF_COMMANDS_START (sizeof lp_UMDiffHeader)
+
+typedef struct {
+    lp_UMDiffHeader *hdr;
+    lp_UMDiffCommand *commands;
+} lp_UMDiffFile;
+
+#endif //__UMDIFF_H
+
+// vim: ft=c.doxygen