turbonss/lib/PackedGroup.zig

const std = @import("std");

const pad = @import("padding.zig");
const validate = @import("validate.zig");
const compress = @import("compress.zig");
const InvalidRecord = validate.InvalidRecord;

const mem = std.mem;
const Allocator = mem.Allocator;
const ArrayList = std.ArrayList;
const BufSet = std.BufSet;

const PackedGroup = @This();

pub const GroupStored = struct {
    gid: u32,
    name: []const u8,
    members_offset: u64,
};

pub const alignment_bits = 3;

const Inner = packed struct {
    gid: u32,
    padding: u3 = 0,
    groupname_len: u5,

    pub fn groupnameLen(self: *const Inner) usize {
        return @as(usize, self.groupname_len) + 1;
    }
};

inner: *const Inner,
groupdata: []const u8,
members_offset: u64,

pub const Entry = struct {
    group: PackedGroup,
    next: ?[]const u8,
};

pub fn fromBytes(bytes: []const u8) Entry {
    const inner = mem.bytesAsValue(Inner, bytes[0..@sizeOf(Inner)]);
    const start_blob = @sizeOf(Inner);
    const end_strings = @sizeOf(Inner) + inner.groupnameLen();
    const members_offset = compress.uvarint(bytes[end_strings..]) catch |err| switch (err) {
        error.Overflow => unreachable,
    };
    const end_blob = end_strings + members_offset.bytes_read;
    const next_start = pad.roundUp(usize, alignment_bits, end_blob);

    var next: ?[]const u8 = null;
    if (next_start < bytes.len)
        next = bytes[next_start..];

    return Entry{
        .group = PackedGroup{
            .inner = inner,
            .groupdata = bytes[start_blob..end_strings],
            .members_offset = members_offset.value,
        },
        .next = next,
    };
}

fn validateUtf8(s: []const u8) InvalidRecord!void {
    if (!std.unicode.utf8ValidateSlice(s))
        return error.InvalidRecord;
}

pub const Iterator = struct {
    section: ?[]const u8,

    pub fn next(it: *Iterator) ?PackedGroup {
        if (it.section) |section| {
            const entry = fromBytes(section);
            it.section = entry.next;
            return entry.group;
        }
        return null;
    }
};

pub fn iterator(section: []const u8) Iterator {
    return Iterator{ .section = section };
}

pub fn gid(self: *const PackedGroup) u32 {
    return self.inner.gid;
}

pub fn membersOffset(self: *const PackedGroup) u64 {
    return self.members_offset;
}

pub fn name(self: *const PackedGroup) []const u8 {
    return self.groupdata;
}

pub fn packTo(
    arr: *ArrayList(u8),
    group: GroupStored,
) error{ InvalidRecord, OutOfMemory }!void {
    std.debug.assert(arr.items.len & 7 == 0);
    try validate.utf8(group.name);
    const len = try validate.downCast(u5, group.name.len - 1);
    const inner = Inner{ .gid = group.gid, .groupname_len = len };
    try arr.*.appendSlice(mem.asBytes(&inner));
    try arr.*.appendSlice(group.name);
    try compress.appendUvarint(arr, group.members_offset);
}

const testing = std.testing;

test "PackedGroup alignment" {
    try testing.expectEqual(@sizeOf(PackedGroup) * 8, @bitSizeOf(PackedGroup));
}

test "construct PackedGroups" {
    var buf = ArrayList(u8).init(testing.allocator);
    defer buf.deinit();

    const groups = [_]GroupStored{
        GroupStored{
            .gid = 1000,
            .name = "sudo",
            .members_offset = 1,
        },
        GroupStored{
            .gid = std.math.maxInt(u32),
            .name = "Name" ** 8, // 32
            .members_offset = std.math.maxInt(u64),
        },
    };

    for (groups) |group| {
        try PackedGroup.packTo(&buf, group);
        try pad.arrayList(&buf, PackedGroup.alignment_bits);
    }

    var i: u29 = 0;
    var it = PackedGroup.iterator(buf.items);
    while (it.next()) |group| : (i += 1) {
        try testing.expectEqual(groups[i].gid, group.gid());
        try testing.expectEqualStrings(groups[i].name, group.name());
        try testing.expectEqual(groups[i].members_offset, group.membersOffset());
    }
    try testing.expectEqual(groups.len, i);
}