add some clarifying comments

src/user.zig

@@ -7,10 +7,12 @@ const ArrayList = std.ArrayList;
 const math = std.math;
 const mem = std.mem;
 
-// TODO(motiejus) move to the struct where it's used.
-const shellIndexProto = fn (u6) []const u8;
-const PackedUserAlignmentBits = 3;
+// ShellIndexProto is a function prototype that, given a shell's index (in
+// global shell section), will return a shell string. Matches ShelReader.get.
+const ShellIndexProto = fn (u6) []const u8;
 
+// User is a convenient public struct for record construction and
+// serialization. Iterator can help retrieve these records.
 pub const User = struct {
     uid: u32,
     gid: u32,
@@ -21,6 +23,7 @@ pub const User = struct {
 };
 
 pub const PackedUser = struct {
+    const AlignmentBits = 3;
     const shellIndexFnType = fn ([]const u8) ?u6;
 
     const InnerSize = @divExact(@bitSizeOf(Inner), 8);
@@ -87,6 +90,8 @@ pub const PackedUser = struct {
         }
     };
 
+    // PackedUser does not allocate; it re-interprets the "bytes" blob field.
+    // Both of those fields are pointers to "our representation" of that field.
     inner: *const Inner,
     userdata: []const u8,
 
@@ -98,6 +103,7 @@ pub const PackedUser = struct {
     pub fn fromBytes(bytes: []const u8) Entry {
         const inner = std.mem.bytesAsValue(
             Inner,
+            // Should use InnerSize instead of sizeOf, see
             // https://github.com/ziglang/zig/issues/10958
             bytes[0..@sizeOf(Inner)],
         );
@@ -105,7 +111,7 @@ pub const PackedUser = struct {
         const startBlob = InnerSize;
         const endBlob = startBlob + inner.blobLength();
 
-        const nextStart = pad.roundUp(usize, PackedUserAlignmentBits, endBlob);
+        const nextStart = pad.roundUp(usize, AlignmentBits, endBlob);
         var next: ?[]const u8 = null;
         if (nextStart < bytes.len) {
             next = bytes[nextStart..];
@@ -188,7 +194,7 @@ pub const PackedUser = struct {
             pos += user.shell.len;
         }
 
-        const padding = pad.roundUpPadding(u64, PackedUserAlignmentBits, pos);
+        const padding = pad.roundUpPadding(u64, AlignmentBits, pos);
         buf.*.len += padding;
         mem.set(u8, buf.*[pos .. pos + padding], 0);
     }
@@ -202,7 +208,7 @@ pub const PackedUser = struct {
                 std.math.maxInt(u5) + // name
                 std.math.maxInt(u6) + // shell
                 std.math.maxInt(u8); // gecos
-            return pad.roundUp(u64, PackedUserAlignmentBits, unpadded);
+            return pad.roundUp(u64, AlignmentBits, unpadded);
         }
     }
 
@@ -230,7 +236,7 @@ pub const PackedUser = struct {
         return self.userdata[gecos_pos .. gecos_pos + gecos_len];
     }
 
-    pub fn shell(self: *const PackedUser, shellIndex: shellIndexProto) []const u8 {
+    pub fn shell(self: *const PackedUser, shellIndex: ShellIndexProto) []const u8 {
         if (self.inner.shell_here) {
             const shell_pos = self.inner.maybeShellStart();
             const shell_len = self.inner.shellLen();
@@ -240,7 +246,7 @@ pub const PackedUser = struct {
     }
 };
 
-pub fn userIterator(section: []const u8, shellIndex: shellIndexProto) Iterator {
+pub fn userIterator(section: []const u8, shellIndex: ShellIndexProto) Iterator {
     return Iterator{
         .section = section,
         .shellIndex = shellIndex,
@@ -249,7 +255,7 @@ pub fn userIterator(section: []const u8, shellIndex: shellIndexProto) Iterator {
 
 pub const Iterator = struct {
     section: ?[]const u8,
-    shellIndex: shellIndexProto,
+    shellIndex: ShellIndexProto,
 
     pub fn next(it: *Iterator) ?PackedUser {
         if (it.section) |section| {
@@ -264,7 +270,7 @@ pub const Iterator = struct {
 const testing = std.testing;
 
 test "PackedUser internal and external alignment" {
-    // External padding (PackedUserAlignmentBits) must be higher or equal to
+    // External padding (AlignmentBits) must be higher or equal to
     // the "internal" PackedUser alignment. By aligning PackedUser we are also
     // working around https://github.com/ziglang/zig/issues/10958 ; PackedUser
     // cannot be converted from/to [@bitSizeOf(PackedUser)/8]u8;