diff --git a/src/reader/data.zig b/src/reader/data.zig
index ce0826f..84b7565 100644
--- a/src/reader/data.zig
+++ b/src/reader/data.zig
@@ -7,6 +7,8 @@ const FragEntry = @import("../fragment.zig").FragEntry;
 const BlockSize = @import("../inode/file.zig").BlockSize;
 const Compression = @import("../superblock.zig").Compression;
 
+const CompletionMap = std.ArrayHashMap(usize, []u8);
+
 const DataReaderError = error{
     EOF,
     InvalidIndex,
@@ -69,9 +71,22 @@ pub fn DataReader(comptime T: type) type {
 
         pub fn writeTo(self: Self, wrt: anytype) !void {
             comptime std.debug.assert(std.meta.hasFn(@TypeOf(wrt), "write") or std.meta.hasFn(@TypeOf(wrt), "pwrite"));
+            var wg: std.Thread.WaitGroup = .{};
+            wg.startMany(self.numBlocks());
+            var map: CompletionMap = .init(self.alloc);
+            defer map.deinit();
+            var mut: std.Thread.Mutex = .{};
+            var cond: std.Thread.Condition = .{};
+            std.Thread.spawn(.{ .allocator = self.alloc }, writeThread, .{ self, wrt, &map, &mut, &cond, null, null });
+            for (0..self.numBlocks()) |i| {}
+            wg.wait();
         }
         pub fn writeToNoBlock(self: Self, wrt: anytype, comptime finish: anytype, finish_args: anytype) !void {
             comptime std.debug.assert(std.meta.hasFn(@TypeOf(wrt), "write") or std.meta.hasFn(@TypeOf(wrt), "pwrite"));
+            _ = self;
+            _ = finish;
+            _ = finish_args;
+            return error{TODO}.TODO;
         }
 
         fn numBlocks(self: Self) usize {
@@ -79,9 +94,12 @@ pub fn DataReader(comptime T: type) type {
             if (self.frag != null) out += 1;
             return out;
         }
-
+        /// Returns the decompressed data block at the given idx.
+        /// If the block is sparse (filled with 0s), a zero length slice is returned.
         fn blockAt(self: Self, idx: usize) ![]u8 {
-            if (idx >= self.sizes.len) return DataReaderError.InvalidIndex;
+            if (idx >= self.numBlocks()) return DataReaderError.InvalidIndex;
+            const size = self.sizes[idx];
+            if (size.size == 0) return &[0]u8{};
             const block = try self.alloc.alloc(u8, blk: {
                 if (idx == self.numBlocks() - 1) break :blk self.file_size % self.block_size;
                 break :blk self.block_size;
@@ -90,7 +108,7 @@ pub fn DataReader(comptime T: type) type {
                 @memcpy(block, self.frag.?);
                 return;
             }
-            if (self.sizes[idx].uncompressed) {
+            if (size.uncompressed) {
                 _ = try self.rdr.pread(block, self.offsets[idx]);
                 return;
             }
@@ -102,5 +120,60 @@ pub fn DataReader(comptime T: type) type {
             );
             return block;
         }
+
+        fn writeThread(
+            self: Self,
+            wrt: anytype,
+            map: *CompletionMap,
+            mut: *std.Thread.Mutex,
+            cond: *std.Thread.Condition,
+            comptime finish: anytype,
+            finish_args: anytype,
+        ) void {
+            var cur_idx: usize = 0;
+            mut.lock();
+            defer mut.unlock();
+            while (cur_idx < self.numBlocks()) {
+                cond.wait(mut);
+                if (comptime std.meta.hasFn(@TypeOf(wrt), "pwrite")) {
+                    for (map.keys()) |k| {
+                        const blk = map.fetchSwapRemove(k).?.value;
+                        defer self.alloc.free(blk);
+                        if (blk.len > 0) {
+                            _ = wrt.pwrite(map.fetchSwapRemove(k).?.value, self.block_size * k) catch |err| {
+                                std.debug.print("ERROR: {}\n", .{err});
+                                //TODO: handle properly.
+                            };
+                        } else {
+                            _ = wrt.pwrite(&[1]u8{0}, (self.block_size * (k + 1)) - 1) catch |err| {
+                                std.debug.print("ERROR: {}\n", .{err});
+                                //TODO: handle properly.
+                            };
+                        }
+                        cur_idx += 1;
+                    }
+                    continue;
+                }
+                while (map.contains(cur_idx)) {
+                    const blk = map.fetchSwapRemove(cur_idx).?.value;
+                    defer self.alloc.free(blk);
+                    if (blk.len > 0) {
+                        _ = wrt.write(blk) catch |err| {
+                            std.debug.print("ERROR: {}\n", .{err});
+                            //TODO: handle properly.
+                        };
+                    }
+                    cur_idx += 1;
+                }
+            }
+            if (comptime @TypeOf(finish) != @TypeOf(null) and @TypeOf(finish_args) != @TypeOf(null)) @call(.auto, finish, finish_args);
+        }
+        fn decompThread(
+            self: Self,
+            idx: usize,
+            map: *CompletionMap,
+            mut: *std.Thread.Mutex,
+            cond: *std.Thread.Condition,
+        ) void {}
     };
 }