use ArrayList instead of slices

The API is more ergonimic.

src/padding.zig

@@ -1,5 +1,7 @@
 const std = @import("std");
 const assert = std.debug.assert;
+const Allocator = std.mem.Allocator;
+const ArrayList = std.ArrayList;
 
 // rounds up an int to the nearest factor of nbits.
 pub fn roundUp(comptime T: type, comptime nbits: u8, n: T) T {
@@ -14,6 +16,12 @@ pub fn roundUpPadding(comptime T: type, comptime nbits: u8, n: T) T {
     return roundUp(T, nbits, n) - n;
 }
 
+// arrayList adds padding to an ArrayList(u8) for a given number of nbits
+pub fn arrayList(arr: *ArrayList(u8), comptime nbits: u8) Allocator.Error!void {
+    const padding = roundUpPadding(u64, nbits, arr.items.len);
+    try arr.*.appendNTimes(0, padding);
+}
+
 const testing = std.testing;
 
 test "padding" {
@@ -30,3 +38,16 @@ test "padding" {
     try testing.expectEqual(roundUpPadding(u12, 2, 4091), 1);
     try testing.expectEqual(roundUpPadding(u12, 2, 4092), 0);
 }
+
+test "arrayList" {
+    var buf = try ArrayList(u8).initCapacity(testing.allocator, 16);
+    defer buf.deinit();
+
+    buf.appendAssumeCapacity(1);
+    try arrayList(&buf, 3);
+    try testing.expectEqual(buf.items.len, 8);
+
+    buf.appendAssumeCapacity(2);
+    try arrayList(&buf, 10);
+    try testing.expectEqual(buf.items.len, 1024);
+}

src/user.zig

@@ -141,7 +141,12 @@ pub const PackedUser = struct {
     // 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.
-    pub fn packTo(buf: *[]u8, user: User, idxFn: shellIndexFn) errInvalid!void {
+    const packErr = errInvalid || Allocator.Error;
+    pub fn packTo(
+        arr: *ArrayList(u8),
+        user: User,
+        idxFn: shellIndexFn,
+    ) packErr!void {
         // function arguments are consts. We need to mutate the underlying
         // slice, so passing it via pointer instead.
         const home_len = try downCast(u6, user.home.len - 1);
@@ -167,35 +172,21 @@ pub const PackedUser = struct {
         };
         const innerBytes = mem.asBytes(&inner);
 
-        var pos: usize = buf.*.len;
-        buf.*.len += InnerSize +
-            user.home.len +
-            user.gecos.len;
-
         // innerBytes.len is longer than InnerSize. We want to copy
         // only the InnerSize-number of bytes.
-        mem.copy(u8, buf.*[pos .. pos + InnerSize], innerBytes[0..InnerSize]);
-        pos += InnerSize;
-        mem.copy(u8, buf.*[pos .. pos + user.home.len], user.home);
-        pos += user.home.len;
+        try arr.*.appendSlice(innerBytes[0..InnerSize]);
+        try arr.*.appendSlice(user.home);
 
         if (!inner.name_is_a_suffix) {
-            buf.*.len += user.name.len;
-            mem.copy(u8, buf.*[pos .. pos + user.name.len], user.name);
-            pos += user.name.len;
+            try arr.*.appendSlice(user.name);
         }
-        mem.copy(u8, buf.*[pos .. pos + user.gecos.len], user.gecos);
-        pos += user.gecos.len;
+        try arr.*.appendSlice(user.gecos);
 
         if (inner.shell_here) {
-            buf.*.len += user.shell.len;
-            mem.copy(u8, buf.*[pos .. pos + user.shell.len], user.shell);
-            pos += user.shell.len;
+            try arr.*.appendSlice(user.shell);
         }
 
-        const padding = pad.roundUpPadding(u64, AlignmentBits, pos);
-        buf.*.len += padding;
-        mem.set(u8, buf.*[pos .. pos + padding], 0);
+        try pad.arrayList(arr, AlignmentBits);
     }
 
     // maxSize is the maximum number of records a PackedUser can take
@@ -331,7 +322,7 @@ test "construct PackedUser section" {
     } };
     for (users) |user| {
         try buf.ensureUnusedCapacity(PackedUser.maxSize());
-        try PackedUser.packTo(&buf.items, user, testShellIndex);
+        try PackedUser.packTo(&buf, user, testShellIndex);
     }
 
     var it = userIterator(buf.items, testShell);
@@ -361,6 +352,6 @@ test "PackedUser.maxSize()" {
         .home = "Home" ** 16, // 64
         .shell = "She.LllL" ** 8, // 64
     };
-    try PackedUser.packTo(&buf.items, largeUser, testShellIndex);
+    try PackedUser.packTo(&buf, largeUser, testShellIndex);
     try testing.expectEqual(PackedUser.maxSize(), buf.items.len);
 }