diff --git a/lib/PackedUser.zig b/lib/PackedUser.zig
index 805038f..27603b7 100644
--- a/lib/PackedUser.zig
+++ b/lib/PackedUser.zig
@@ -5,6 +5,7 @@ const math = std.math;
 const Allocator = mem.Allocator;
 const ArrayList = std.ArrayList;
 const StringHashMap = std.StringHashMap;
+const fieldInfo = std.meta.fieldInfo;
 
 const pad = @import("padding.zig");
 const validate = @import("validate.zig");
@@ -129,9 +130,7 @@ pub fn iterator(section: []const u8, shell_reader: shellImport.ShellReader) Iter
     return Iterator{ .section = section, .shell_reader = shell_reader };
 }
 
-// packTo packs the User record and copies it to the given byte slice.
-// The slice must have at least maxRecordSize() bytes available. The
-// slice is passed as a pointer, so it can be mutated.
+// packTo packs the User record and copies it to the given arraylist.
 pub fn packTo(
     arr: *ArrayList(u8),
     user: User,
@@ -141,10 +140,11 @@ pub fn packTo(
     std.debug.assert(arr.items.len & 7 == 0);
     // function arguments are consts. We need to mutate the underlying
     // slice, so passing it via pointer instead.
-    const home_len = try validate.downCast(u6, user.home.len - 1);
-    const name_len = try validate.downCast(u5, user.name.len - 1);
-    const shell_len = try validate.downCast(u8, user.shell.len - 1);
-    const gecos_len = try validate.downCast(u8, user.gecos.len);
+    //const home_len = try validate.downCast(u6, user.home.len - 1);
+    const home_len = try validate.downCast(fieldInfo(Inner, .home_len).field_type, user.home.len - 1);
+    const name_len = try validate.downCast(fieldInfo(Inner, .name_len).field_type, user.name.len - 1);
+    const shell_len = try validate.downCast(fieldInfo(Inner, .shell_len_or_idx).field_type, user.shell.len - 1);
+    const gecos_len = try validate.downCast(fieldInfo(Inner, .gecos_len).field_type, user.gecos.len);
 
     try validate.utf8(user.home);
     try validate.utf8(user.name);
@@ -211,6 +211,11 @@ pub fn shell(self: PackedUser, shell_reader: shellImport.ShellReader) []const u8
     return shell_reader.get(self.inner.shell_len_or_idx);
 }
 
+pub const max_str_len = math.maxInt(fieldInfo(Inner, .shell_len_or_idx).field_type) + 1 +
+    math.maxInt(fieldInfo(Inner, .home_len).field_type) + 1 +
+    math.maxInt(fieldInfo(Inner, .name_len).field_type) + 1 +
+    math.maxInt(fieldInfo(Inner, .gecos_len).field_type);
+
 const testing = std.testing;
 
 test "PackedUser internal and external alignment" {
@@ -232,6 +237,33 @@ const test_shell_reader = shellImport.ShellReader{
     .index = &[_]u16{ 0, 9, 17 },
 };
 
+const max_user = User{
+    .uid = 0,
+    .gid = math.maxInt(u32),
+    .name = "Name" ** 8,
+    .gecos = "realname" ** 255 ++ "realnam",
+    .home = "Home" ** 16,
+    .shell = "She.LllL" ** 32,
+};
+
+test "max_user and max_str_len are consistent" {
+    const total_len = max_user.name.len +
+        max_user.gecos.len +
+        max_user.home.len +
+        max_user.shell.len;
+    try testing.expectEqual(total_len, max_str_len);
+}
+
+test "pack max_user" {
+    var arr = ArrayList(u8).init(testing.allocator);
+    defer arr.deinit();
+
+    var idx_noop = StringHashMap(u8).init(testing.allocator);
+    defer idx_noop.deinit();
+
+    try packTo(&arr, max_user, 0, idx_noop);
+}
+
 test "construct PackedUser section" {
     var buf = ArrayList(u8).init(testing.allocator);
     defer buf.deinit();
@@ -250,14 +282,7 @@ test "construct PackedUser section" {
         .gecos = "Service Account",
         .home = "/home/service1",
         .shell = "/usr/bin/nologin",
-    }, User{
-        .uid = 0,
-        .gid = math.maxInt(u32),
-        .name = "Name" ** 8,
-        .gecos = "Gecos" ** 51,
-        .home = "Home" ** 16,
-        .shell = "She.LllL" ** 32,
-    }, User{
+    }, max_user, User{
         .uid = 1002,
         .gid = 1002,
         .name = "svc-bar",
diff --git a/lib/User.zig b/lib/User.zig
index a250447..4152ab6 100644
--- a/lib/User.zig
+++ b/lib/User.zig
@@ -1,6 +1,8 @@
 const std = @import("std");
 const mem = std.mem;
 const Allocator = mem.Allocator;
+const ArrayList = std.ArrayList;
+const maxInt = std.math.maxInt;
 
 const User = @This();
 
@@ -48,6 +50,17 @@ pub fn deinit(self: *User, allocator: Allocator) void {
     self.* = undefined;
 }
 
+pub fn fromReader(allocator: Allocator, reader: anytype) ![]User {
+    var users = ArrayList(User).init(allocator);
+    var scratch = ArrayList(u8).init(allocator);
+    defer scratch.deinit();
+    while (true) {
+        const line = reader.readUntilDelimiterArrayList(&scratch, '\n', maxInt(usize));
+        _ = line;
+    }
+    _ = users;
+}
+
 const testing = std.testing;
 
 test "User.clone" {