diff --git a/source/cli/dzipper/main.d b/source/cli/dzipper/main.d
index 4d70dc0..b7ae0ff 100644
--- a/source/cli/dzipper/main.d
+++ b/source/cli/dzipper/main.d
@@ -42,45 +42,55 @@ private int run(in Opts opts)
     verbose = opts.verbose,
     zipFile = opts.zipFile,
     prependFile = opts.prependFile;
+    File tempFile;
 
-    auto mfile = new MmFile(zipFile);
-    writefln("file length: %d", mfile.length);
-    if (mfile.length < 22)
+    // start memory-mapped zip file scope
     {
-        stderr.cwriteln("<yellow>Not a zip file (too short).</yellow>");
-        return 1;
-    }
-    auto eocd_index = findEocdIn(mfile);
-    if (eocd_index.isNull)
-    {
-        stderr.cwriteln("<yellow>Unable to locate zip metadata (EOCD).</yellow>");
-        return 2;
-    }
-    if (verbose)
-    {
-        writeln("Found EOCD at offset ", eocd_index, ".");
-    }
-    auto eocd = parseEocd(cast(ubyte[]) mfile[eocd_index.get .. $]);
+        auto mfile = new MmFile(zipFile);
+        writefln("file length: %d", mfile.length);
+        if (mfile.length < 22)
+        {
+            stderr.cwriteln("<yellow>Not a zip file (too short).</yellow>");
+            return 1;
+        }
+        auto eocd_index = findEocdIn(mfile);
+        if (eocd_index.isNull)
+        {
+            stderr.cwriteln("<yellow>Unable to locate zip metadata (EOCD).</yellow>");
+            return 2;
+        }
+        if (verbose)
+        {
+            writeln("Found EOCD at offset ", eocd_index, ".");
+        }
+        auto eocd = parseEocd(cast(ubyte[]) mfile[eocd_index.get .. $]);
 
-    if (verbose)
-    {
-        writeln(eocd);
-    }
+        if (verbose)
+        {
+            writeln(eocd);
+        }
 
-    cwriteln("<green>File appears to be a zip file.</green>");
+        cwriteln("<green>File appears to be a zip file.</green>");
 
-    if (eocd.totalCentralDirectoriesCount == 0)
-    {
-        cwriteln("<yellow>Warning: empty zip file.</yellow>");
-    }
+        if (eocd.totalCentralDirectoriesCount == 0)
+        {
+            cwriteln("<yellow>Warning: empty zip file.</yellow>");
+        }
 
-    if (prependFile.length == 0)
-    {
-        mfile.printArchiveMetadata(eocd, verbose);
+        if (prependFile.length == 0)
+        {
+            mfile.printArchiveMetadata(eocd, verbose);
+        }
+        else
+        {
+            tempFile = mfile.prependFileToArchive(prependFile, eocd, verbose);
+        }
     }
-    else
+
+    // the memory file has been closed now, so we can move the tempFile into the zip archive.
+    if (tempFile.isOpen)
     {
-        mfile.prependFileToArchive(prependFile, zipFile, eocd, verbose);
+        tempFile.name = zipFile;
     }
 
     return 0;
diff --git a/source/lib/dzipper/model.d b/source/lib/dzipper/model.d
index acb2929..06ddd85 100644
--- a/source/lib/dzipper/model.d
+++ b/source/lib/dzipper/model.d
@@ -5,8 +5,22 @@ import std.format : FormatSpec;
 import std.array : appender;
 import std.conv : to;
 import std.range : put;
+import std.bitmanip : nativeToLittleEndian;
 import std.datetime.systime : SysTime;
 
+const(uint) EOCD_SIGNATURE_UINT = 0x06054b50;
+const(uint) CD_SIGNATURE_UINT = 0x02014b50;
+const(uint) LOCAL_FILE_SIGNATURE_UINT = 0x04034b50;
+
+/** The End of Central Directory Signature. */
+immutable(ubyte[]) EOCD_SIGNATURE = nativeToLittleEndian(EOCD_SIGNATURE_UINT)[0 .. $];
+
+/** The Central Directory Signature. */
+immutable(ubyte[]) CD_SIGNATURE = nativeToLittleEndian(CD_SIGNATURE_UINT)[0 .. $];
+
+/** The Local File header Signature. */
+immutable(ubyte[]) LOCAL_FILE_SIGNATURE = nativeToLittleEndian(LOCAL_FILE_SIGNATURE_UINT)[0 .. $];
+
 private mixin template StructToString(S)
 {
     void toString(scope void delegate(const(char)[]) sink,
@@ -71,6 +85,10 @@ struct EndOfCentralDirectory
     ushort commentLength;
     ubyte[] comment;
     mixin StructToString!EndOfCentralDirectory;
+
+    size_t length() const {
+        return 22 + comment.length;
+    }
 }
 
 private mixin template FileInformation()
@@ -103,7 +121,7 @@ struct CentralDirectory
 
     /// The length of the CD in bytes (notice that the struct
     /// does not include the CD signature).
-    size_t length()
+    size_t length() const
     {
         return 46 + fileName.length + extraField.length + comment.length;
     }
@@ -117,7 +135,7 @@ struct LocalFileHeader
 
     /// The length of the header in bytes (notice that the struct
     /// does not include the local file header signature).
-    size_t length()
+    size_t length() const
     {
         return 30 + fileName.length + extraField.length;
     }
diff --git a/source/lib/dzipper/package.d b/source/lib/dzipper/package.d
index d0fd610..050fe7a 100644
--- a/source/lib/dzipper/package.d
+++ b/source/lib/dzipper/package.d
@@ -2,3 +2,4 @@ module dzipper;
 
 public import dzipper.model;
 public import dzipper.parser;
+public import dzipper.process;
diff --git a/source/lib/dzipper/parser.d b/source/lib/dzipper/parser.d
index 53ba785..1f3bc7f 100644
--- a/source/lib/dzipper/parser.d
+++ b/source/lib/dzipper/parser.d
@@ -3,7 +3,7 @@ module dzipper.parser;
 import std.typecons : Nullable;
 import std.algorithm.searching : find;
 import std.exception : enforce, basicExceptionCtors;
-import std.bitmanip : nativeToLittleEndian, littleEndianToNative, peek, Endian;
+import std.bitmanip : littleEndianToNative, peek, Endian;
 import std.range : retro, tail, take, slide;
 import std.conv : to;
 import std.string : assumeUTF;
@@ -12,15 +12,6 @@ import std.datetime.systime : DosFileTimeToSysTime, SysTime;
 import dzipper.model;
 import std.string;
 
-/** The End of Central Directory Signature. */
-immutable(ubyte[]) EOCD_SIGNATURE = nativeToLittleEndian!uint(0x06054b50)[0 .. $];
-
-/** The Central Directory Signature. */
-immutable(ubyte[]) CD_SIGNATURE = nativeToLittleEndian!uint(0x02014b50)[0 .. $];
-
-/** The Local File header Signature. */
-immutable(ubyte[]) LOCAL_FILE_SIGNATURE = nativeToLittleEndian!uint(0x04034b50)[0 .. $];
-
 /// Reason why a Zip Archive's metadata couldn't be parsed.
 enum ZipParseError
 {
@@ -209,6 +200,13 @@ LocalFileHeader parseLocalFileHeader(in ubyte[] bytes) @safe
 // the EOCD can only appear in the last 65536 + 22 bytes
 private enum maxEocdLen = 65_535 + 22;
 
+/**
+ * Find the End of Central Directory (EOCD).
+ *
+ * Params:
+ *   source = a source of bytes (must support slice operator, e.g. MmFile).
+ * Returns: index of the End of Central Directory if it can be found, null otherwise.
+ */
 Nullable!size_t findEocdIn(S, size_t windowLen = 56)(ref S source)
 {
     auto bytes = cast(ubyte[])(source.length > maxEocdLen ? source[$ - maxEocdLen .. $] : source[]);
diff --git a/source/lib/dzipper/process.d b/source/lib/dzipper/process.d
index f237606..46c6a40 100644
--- a/source/lib/dzipper/process.d
+++ b/source/lib/dzipper/process.d
@@ -1,6 +1,13 @@
 module dzipper.process;
 
 import std.stdio : File, writeln;
+import std.range : iota;
+import std.file : remove;
+import std.bitmanip : append, Endian;
+import std.array : appender;
+import std.typecons : Nullable;
+import std.conv : to;
+import std.algorithm.comparison : min;
 
 import dzipper.model, dzipper.parser;
 
@@ -14,8 +21,6 @@ void printArchiveMetadata(B)(ref B bytes, in EndOfCentralDirectory eocd, bool ve
 private void checkCentralDirectories(B)(ref B bytes,
     in EndOfCentralDirectory eocd, bool verbose)
 {
-    import std.range : iota;
-
     uint offset = eocd.startOfCentralDirectory;
     foreach (i; iota(0, eocd.diskCentralDirectoriesCount))
     {
@@ -33,9 +38,182 @@ private void checkCentralDirectories(B)(ref B bytes,
     }
 }
 
-void prependFileToArchive(B)(ref B bytes, string prependFile, string zipFile, in EndOfCentralDirectory eocd, bool verbose)
+/// Prepend the contents of a file into the zip archive.
+///
+/// This function works by first copying the contents of `prependFile` into a temp file,
+/// then writing the zip archive's contents into the temp file while shifting the zip metadata
+/// entries' offsets as necessary.
+///
+/// Params:
+///   bytes = zip archive source of bytes
+///   prependFile = file to prepend
+///   eocd = end of central directory structure
+///   verbose = whether to log verbose output
+/// Returns: the temp file the output is written to.
+File prependFileToArchive(B)(ref B bytes, string prependFile, in EndOfCentralDirectory eocd, bool verbose)
 {
-    // auto prepFile = File(prependFile);
-    writeln("not able to prepend file yet!");
+    auto outfile = File.tmpfile;
+    File(prependFile).copyFile(outfile);
+    long archiveStart = cast(long) outfile.tell();
+
+    uint offset = eocd.startOfCentralDirectory;
+    Nullable!long zipStart;
+
+    // first, write all local file headers and the file contents
+    foreach (i; iota(0, eocd.diskCentralDirectoriesCount))
+    {
+        auto cd = parseCd(cast(ubyte[]) bytes[offset .. $]);
+        auto lfh = parseLocalFileHeader(cast(ubyte[]) bytes[cd.startOfLocalFileHeader .. $]);
+        zipStart = cast(long)(zipStart.isNull
+                ? cd.startOfLocalFileHeader
+                : min(cd.startOfLocalFileHeader, zipStart.get));
+
+        if (verbose)
+            writeln("Adding entry: ", lfh.fileName);
+        auto lfhEnd = cd.startOfLocalFileHeader + lfh.length;
+        outfile.rawWrite(bytes[cd.startOfLocalFileHeader .. lfhEnd]);
+        outfile.rawWrite(bytes[lfhEnd .. lfhEnd + lfh.uncompressedSize]);
+        offset += cd.length;
+    }
+
+    if (zipStart.isNull)
+        return outfile;
 
+    const long shift = archiveStart - zipStart.get;
+    offset = eocd.startOfCentralDirectory;
+
+    if (verbose)
+        writeln("Shifting zip archive offsets by ", shift);
+
+    // now, write all central directories
+    foreach (i; iota(0, eocd.diskCentralDirectoriesCount))
+    {
+        auto cd = parseCd(cast(ubyte[]) bytes[offset .. $]);
+        cd.startOfLocalFileHeader = to!uint(cd.startOfLocalFileHeader + shift);
+        outfile.rawWrite(cd.toBytes);
+        offset += cd.length;
+    }
+
+    // write the end-of-central-directory
+    auto shiftEocd = eocd;
+    shiftEocd.startOfCentralDirectory = to!uint(eocd.startOfCentralDirectory + shift);
+    outfile.rawWrite(shiftEocd.toBytes);
+
+    return outfile;
+}
+
+private void appends(T, R)(R range, immutable T value)
+{
+    append!(T, Endian.littleEndian, R)(range, value);
+}
+
+/// Create a byte array representing a Central Directory.
+///
+/// The bytes are returned as they would appear in a zip archive,
+/// i.e. using little endian representation.
+/// 
+/// Params:
+///   cd = the central directory
+/// Returns: byte array in little endian
+ubyte[] toBytes(in CentralDirectory cd)
+{
+    import std.datetime : SysTimeToDosFileTime;
+
+    auto bytes = new ubyte[cd.length];
+    auto ap = appender(&bytes);
+    ap.appends(CD_SIGNATURE_UINT);
+    ap.appends(cd.versionMadeBy);
+    ap.appends(cd.versionRequired);
+    ap.appends(cd.generalPurposeBitFlag);
+    ap.appends(cd.compressionMethod);
+    const dosTime = SysTimeToDosFileTime(cd.lastModificationDateTime);
+    // time is on the lo bytes
+    ap.appends(cast(ushort)(dosTime & 0xFFFF));
+    // date is on the hi bytes
+    ap.appends(cast(ushort)((dosTime >> 16) & 0xFFFF));
+    ap.appends(cd.crc32);
+    ap.appends(cd.compressedSize);
+    ap.appends(cd.uncompressedSize);
+    ap.appends(cd.fileNameLength);
+    ap.appends(cd.extraFieldLength);
+    ap.appends(cd.commentLength);
+    ap.appends(cd.diskNumber);
+    ap.appends(cd.internalFileAttributes);
+    ap.appends(cd.externalFileAttributes);
+    ap.appends(cd.startOfLocalFileHeader);
+    ap.put(cast(const(ubyte)[])(cd.fileName));
+    ap.put(cd.extraField);
+    ap.put(cd.comment);
+    return bytes;
+}
+
+/// Create a byte array representing a End of Central Directory structure.
+///
+/// The bytes are returned as they would appear in a zip archive,
+/// i.e. using little endian representation.
+/// 
+/// Params:
+///   cd = the end of central directory
+/// Returns: byte array in little endian
+ubyte[] toBytes(in EndOfCentralDirectory eocd)
+{
+    auto bytes = new ubyte[eocd.length];
+    auto ap = appender(&bytes);
+    ap.appends(EOCD_SIGNATURE_UINT);
+    ap.appends(eocd.diskNumber);
+    ap.appends(eocd.centralDirectoryDiskNumber);
+    ap.appends(eocd.diskCentralDirectoriesCount);
+    ap.appends(eocd.totalCentralDirectoriesCount);
+    ap.appends(eocd.centralDirectorySize);
+    ap.appends(eocd.startOfCentralDirectory);
+    ap.appends(eocd.commentLength);
+    ap.put(eocd.comment);
+    return bytes;
+}
+
+private void copyFile(scope ref File from, scope ref File to)
+{
+    ubyte[4096] buf;
+    ubyte[] data;
+    do
+    {
+        data = from.rawRead(buf);
+        to.rawWrite(data);
+    }
+    while (data.length > 0);
+}
+
+version (unittest)
+{
+    import tested;
+    import dshould;
+
+    @name("can copy file contents")
+    unittest
+    {
+        scope (exit)
+            "temp__".remove;
+        scope (exit)
+            "temp__2".remove;
+
+        // write some file
+        {
+            auto temp = File("temp__", "wb");
+
+            temp.rawWrite([1, 2, 3]);
+        }
+
+        // copy to other file then add more stuff to it
+        {
+            auto from = File("temp__", "rb");
+            auto to = File("temp__2", "wb");
+            from.copyFile(to);
+            to.rawWrite([4, 5]);
+        }
+
+        auto res = File("temp__2");
+        ubyte[6] buf;
+        auto bytes = res.rawRead(buf);
+        bytes.should.equal([1, 2, 3, 4, 5]);
+    }
 }