start with real sections

README.md

@@ -370,12 +370,15 @@ STATUS  SECTION               SIZE             DESCRIPTION
 
 Section creation order:
 
-1. Groupmembers, UserGids. No dependencies.
+1. `bdz_*`. No depdendencies.
-2. ShellIndex, ShellBlob. No dependencies.
+1. ShellIndex, ShellBlob. No dependencies.
-3. `bdz_*`. No depdendencies.
+1. UserGids. No dependencies.
-4. Groups. Requires Groupmembers.
+1. Users, but without `additional_gids_offset`. No dependencies.
-5. Users. Requires Groupmembers and ShellIndex.
+1. Groupmembers. Depends on Users, ex. `additional_gids_offset`.
-6. `idx_*`. Requires offsets to Groups and Users.
+1. Groups. Requires Groupmembers.
+1. Mutate `Users.additional_gids_offset`. Requires Groupmembers and ShellIndex.
+1. `idx_*`. Requires offsets to Groups and Users.
+1. Header.
 
 [git-subtrac]: https://apenwarr.ca/log/20191109
 [cmph]: http://cmph.sourceforge.net/

src/sections.zig

@@ -3,11 +3,13 @@ const unicode = std.unicode;
 const sort = std.sort;
 const Allocator = std.mem.Allocator;
 const ArrayListUnmanaged = std.ArrayListUnmanaged;
+const ArrayList = std.ArrayList;
 const StringHashMap = std.StringHashMap;
 const AutoHashMap = std.AutoHashMap;
 const BufSet = std.BufSet;
 
 const pad = @import("padding.zig");
+const compress = @import("compress.zig");
 const userImport = @import("user.zig");
 const groupImport = @import("group.zig");
 const User = userImport.User;
@@ -139,6 +141,46 @@ const Corpus = struct {
     }
 };
 
+pub const Sections = struct {
+    allocator: Allocator,
+    corpus: *const Corpus,
+
+    pub fn init(allocator: Allocator, corpus: *const Corpus) Sections {
+        return Sections{
+            .allocator = allocator,
+            .corpus = corpus,
+        };
+    }
+
+    pub const GroupMembers = struct {
+        offsets: []const usize,
+        bytes: []const u8,
+    };
+
+    const groupMembersErr = error{Overflow} || Allocator.Error;
+
+    pub fn groupMembers(self: *const Sections) groupMembersErr!GroupMembers {
+        var buf: [compress.maxVarintLen64]u8 = undefined;
+        var offsets = ArrayListUnmanaged(usize).initCapacity(
+            self.allocator,
+            self.corpus.groups.len,
+        );
+        var bytes = ArrayList(u8).init(self.allocator);
+        var offset: usize = 0;
+        for (self.corpus.groups) |group, i| {
+            offsets[i] = offset;
+            const users = self.corpus.groupname2users.get(group.name).?;
+            const len = try compress.putVarint(&buf, users.len);
+            offset += len;
+            try bytes.appendSlice(buf[0..len]);
+            for (users) |user| {
+                // TODO: offset into the User's record
+                _ = user;
+            }
+        }
+    }
+};
+
 // cmpUser compares two users for sorting. By username's utf8 codepoints, ascending.
 fn cmpUser(_: void, a: User, b: User) bool {
     var utf8_a = (unicode.Utf8View.init(a.name) catch unreachable).iterator();