turbonss/src/PackedGroup.zig

const std = @import("std");

const mem = std.mem;
const assert = std.debug.assert;
const Allocator = mem.Allocator;
const ArrayListAligned = std.ArrayListAligned;
const BufSet = std.BufSet;

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

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,
    end: usize,
};

pub fn fromBytes(bytes: []align(8) 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;

    return Entry{
        .group = PackedGroup{
            .inner = inner,
            .groupdata = bytes[start_blob..end_strings],
            .members_offset = members_offset.value,
        },
        .end = mem.alignForward(end_blob, 8),
    };
}

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

pub const Iterator = struct {
    section: []align(8) const u8,
    next_start: usize = 0,
    idx: u32 = 0,
    total: u32,
    advanced_by: usize = 0,

    pub fn next(it: *Iterator) ?PackedGroup {
        if (it.idx == it.total) return null;
        const entry = fromBytes(@alignCast(8, it.section[it.next_start..]));
        it.idx += 1;
        it.next_start += entry.end;
        it.advanced_by = entry.end;
        return entry.group;
    }

    pub fn rollback(it: *Iterator) void {
        assert(it.advanced_by > 0);
        it.idx -= 1;
        it.next_start -= it.advanced_by;
        it.advanced_by = 0;
    }
};

pub fn iterator(section: []align(8) const u8, total: u32) Iterator {
    return Iterator{
        .section = section,
        .total = total,
    };
}

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

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

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

pub fn packTo(
    arr: *ArrayListAligned(u8, 8),
    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 "PackedGroup construct" {
    var buf = ArrayListAligned(u8, 8).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 buf.appendNTimes(0, mem.alignForward(buf.items.len, 8) - buf.items.len);
    }

    var i: u29 = 0;
    var it = PackedGroup.iterator(buf.items, groups.len);
    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);
}