diff --git a/src/user.zig b/src/user.zig
index e54aa47..44b24fc 100644
--- a/src/user.zig
+++ b/src/user.zig
@@ -1,6 +1,7 @@
 const std = @import("std");
 
 const Allocator = std.mem.Allocator;
+const ArrayList = std.ArrayList;
 const cast = std.math.cast;
 
 pub const PackedUserSize = @divExact(@bitSizeOf(PackedUser), 8);
@@ -27,81 +28,50 @@ pub const User = struct {
 
 // UserWriter accepts a naive User struct and returns a PackedUser
 pub const UserWriter = struct {
-
     // shellIndexFnType is a signature for a function that accepts a shell
     // string and returns it's index in the global shell section. Passing a
     // function makes tests easier, and removes the Shell dependency of this
     // module.
     const shellIndexFnType = fn ([]const u8) ?u6;
-    allocator: Allocator,
+    appendTo: ArrayList(u8),
     shellIndexFn: shellIndexFnType,
 
-    pub fn init(allocator: Allocator, shellIndexFn: shellIndexFnType) UserWriter {
+    pub fn init(appendTo: ArrayList(u8), shellIndexFn: shellIndexFnType) UserWriter {
         return UserWriter{
-            .allocator = allocator,
+            .appendTo = appendTo,
             .shellIndexFn = shellIndexFn,
         };
     }
 
-    const fromUserErr = std.mem.Allocator.Error || error{InvalidRecord};
+    const fromUserErr = error{InvalidRecord};
 
-    pub fn fromUser(self: *UserWriter, user: User) fromUserErr![]const u8 {
-        const home_len = std.math.cast(u6, user.home.len - 1) catch return error.InvalidRecord;
+    pub fn appendUser(self: *UserWriter, user: User) !void {
+        const home_len = cast(u6, user.home.len - 1) catch return error.InvalidRecord;
         const name_len = cast(u5, user.name.len - 1) catch return error.InvalidRecord;
         const shell_len = cast(u6, user.shell.len - 1) catch return error.InvalidRecord;
         const gecos_len = cast(u8, user.gecos.len) catch return error.InvalidRecord;
-        var bindata_len: u32 = home_len;
 
         var puser = PackedUser{
-            .uid = @as(u32, user.uid),
-            .gid = @as(u32, user.gid),
+            .uid = user.uid,
+            .gid = user.gid,
             .additional_gids_offset = std.math.maxInt(u29), // needs second pass
-            .shell_here = undefined,
-            .shell_len_or_idx = undefined,
+            .shell_here = self.shellIndexFn(user.shell) == null,
+            .shell_len_or_idx = self.shellIndexFn(user.shell) orelse shell_len,
             .home_len = home_len,
-            .name_is_a_suffix = undefined,
+            .name_is_a_suffix = std.mem.endsWith(u8, user.home, user.name),
             .name_len = name_len,
             .gecos_len = gecos_len,
         };
 
-        if (std.mem.endsWith(u8, user.home, user.name)) {
-            puser.name_is_a_suffix = true;
-        } else {
-            puser.name_is_a_suffix = false;
-            bindata_len += name_len;
-        }
-
-        bindata_len += gecos_len;
-
-        if (self.shellIndexFn(user.shell)) |idx| {
-            puser.shell_here = false;
-            puser.shell_len_or_idx = idx;
-        } else {
-            puser.shell_here = true;
-            puser.shell_len_or_idx = shell_len;
-            bindata_len += shell_len;
-        }
-
-        var result = try self.allocator.alloc(u8, PackedUserSize + bindata_len);
-
-        const userPointer = @ptrCast([*]const u8, &puser);
-        {
-            var i: u32 = 0;
-            while (i < PackedUserSize) {
-                result[i] = userPointer[i];
-                i += 1;
-            }
-        }
-        std.mem.copy(u8, result, user.home);
+        try self.appendTo.appendSlice(std.mem.asBytes(&puser));
+        try self.appendTo.appendSlice(user.home);
         if (!puser.name_is_a_suffix) {
-            std.mem.copy(u8, result, user.name);
+            try self.appendTo.appendSlice(user.name);
         }
-        std.mem.copy(u8, result, user.gecos);
+        try self.appendTo.appendSlice(user.gecos);
         if (puser.shell_here) {
-            std.mem.copy(u8, result, user.shell);
+            try self.appendTo.appendSlice(user.shell);
         }
-
-        return result;
     }
 };
 
@@ -121,7 +91,10 @@ fn testShellIndex(shell: []const u8) ?u6 {
 }
 
 test "construct PackedUser blob" {
-    var writer = UserWriter.init(testing.allocator, testShellIndex);
+    var buf = ArrayList(u8).init(testing.allocator);
+    defer buf.deinit();
+
+    var writer = UserWriter.init(buf, testShellIndex);
     const user1 = User{
         .uid = 1000,
         .gid = 1000,
@@ -138,8 +111,6 @@ test "construct PackedUser blob" {
         .home = "/home/service1",
         .shell = "/usr/bin/nologin",
     };
-    const puser1 = try writer.fromUser(user1);
-    const puser2 = try writer.fromUser(user2);
-    defer testing.allocator.free(puser1);
-    defer testing.allocator.free(puser2);
+    try writer.appendUser(user1);
+    try writer.appendUser(user2);
 }