update padded user alignment

src/shell.zig

@@ -10,6 +10,7 @@ const StringContext = std.hash_map.StringContext;
 // MaxShells is the maximum number of "popular" shells.
 pub const MaxShells = 63;
 pub const MaxShellLen = 64;
+const ShellAlignment = 2; // bits
 
 // ShellReader interprets "Shell Index" and "Shell Blob" sections.
 pub const ShellReader = struct {
@@ -71,7 +72,7 @@ pub const ShellWriter = struct {
                 });
 
                 fullOffset += len;
-                const padding = pad.roundUpPadding(u12, 2, fullOffset);
+                const padding = pad.roundUpPadding(u12, ShellAlignment, fullOffset);
                 fullOffset += padding;
                 //const stderr = std.io.getStdErr().writer();
                 //stderr.print("\n", .{}) catch unreachable;

src/user.zig

@@ -17,6 +17,7 @@ pub const PackedUser = packed struct {
     name_len: u5,
     gecos_len: u8,
 };
+const PackedUserAlignment = 3; // bits
 
 pub const User = struct {
     uid: u32,
@@ -46,11 +47,21 @@ pub const UserWriter = struct {
 
     const fromUserErr = error{InvalidRecord};
 
+    pub fn downCast(comptime T: type, n: u64) fromUserErr!T {
+        if (std.math.cast(T, n)) |result| {
+            return result;
+        } else |err| switch (err) {
+            error.Overflow => {
+                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;
+        const home_len = try downCast(u6, user.home.len - 1);
+        const name_len = try downCast(u5, user.name.len - 1);
+        const shell_len = try downCast(u6, user.shell.len - 1);
+        const gecos_len = try downCast(u8, user.gecos.len);
 
         var puser = PackedUser{
             .uid = user.uid,
@@ -74,8 +85,11 @@ pub const UserWriter = struct {
             try self.appendTo.appendSlice(user.shell);
         }
 
-        const padding = pad.roundUpPadding(u64, 2, self.appendTo.items.len);
-        try self.appendTo.appendNTimes(0, padding);
+        try self.appendTo.appendNTimes(0, pad.roundUpPadding(
+            u64,
+            PackedUserAlignment,
+            self.appendTo.items.len,
+        ));
     }
 };