add unit tests for padding

src/header.zig

@@ -1,7 +1,7 @@
 const std = @import("std");
 const shell = @import("shell.zig");
 
-const HeaderSize = @sizeOf(Header);
+const HeaderSize = @divExact(@bitSizeOf(Header), 8);
 const Magic = [4]u8{ 0xf0, 0x9f, 0xa4, 0xb7 };
 const Version = 0;
 const Bom = 0x1234;
@@ -73,10 +73,6 @@ const Header = packed struct {
 
 const testing = std.testing;
 
-test "header is byte-aligned" {
-    try testing.expectEqual(HeaderSize * 8, @bitSizeOf(Header));
-}
-
 test "Section length is a power of two" {
     try testing.expect(std.math.isPowerOfTwo(SectionLength));
 }

src/shell.zig

@@ -170,7 +170,12 @@ pub const ShellWriter = struct {
 // rounds up a u12 to the nearest factor of 4 and returns the difference
 // (padding)
 inline fn roundUp4Padding(n: u12) u12 {
-    return ((n + 3) & ~@intCast(u12, 3)) - n;
+    return roundUp4(n) - n;
+}
+
+// rounds up a u12 to the nearest factor of 4.
+inline fn roundUp4(n: u12) u12 {
+    return ((n + 3) & ~@intCast(u12, 3));
 }
 
 // ShellIndex is an index to the shell strings. As shell can be up to 64 bytes
@@ -192,12 +197,12 @@ test "basic shellpopcon" {
 
     const bash = "/bin/bash"; // 9 chars
     const zsh = "/bin/zsh"; // 8 chars
+    const long = "/bin/very-long-shell-name-ought-to-be-first"; // 43 chars
     const nobody = "/bin/nobody"; // only 1 instance, ought to ignore
-    const long = "/bin/very-long-shell-name-ought-to-be-first";
     const input = [_][]const u8{
         zsh, zsh, zsh, zsh, // zsh score 8*4=32
         bash, bash, bash, nobody, // bash score 3*9=27
-        long, long, // long score 2*42=84
+        long, long, // long score 2*43=86
     };
 
     for (input) |shell| {
@@ -212,6 +217,10 @@ test "basic shellpopcon" {
     try testing.expectEqual(sections.getIndex(zsh).?, 1);
     try testing.expectEqual(sections.getIndex(bash).?, 2);
     try testing.expectEqual(sections.getIndex(nobody), null);
+    try testing.expectEqual(
+        sections.sectionBlob().len,
+        roundUp4(bash.len) + roundUp4(zsh.len) + roundUp4(long.len),
+    );
 
     const shellReader = ShellReader.init(
         sections.sectionIndex(),
@@ -220,6 +229,8 @@ test "basic shellpopcon" {
     try testing.expectEqualStrings(shellReader.get(0), long);
     try testing.expectEqualStrings(shellReader.get(1), zsh);
     try testing.expectEqualStrings(shellReader.get(2), bash);
+
+    try testing.expectEqual(shellReader.sectionIndex.len, 3);
 }
 
 test "padding" {

src/user.zig

@@ -3,7 +3,7 @@ const std = @import("std");
 const Allocator = std.mem.Allocator;
 const cast = std.math.cast;
 
-pub const PackedUserSize = @sizeOf(PackedUser);
+pub const PackedUserSize = @divExact(@bitSizeOf(PackedUser), 8);
 pub const PackedUser = packed struct {
     uid: u32,
     gid: u32,