concatMaybeSentinel

pub fn concatMaybeSentinel(allocator: Allocator, comptime T: type, slices: []const []const T, comptime s: ?T) Allocator.Error![]T {
    if (slices.len == 0) return if (s) |sentinel| try allocator.dupe(T, &[1]T{sentinel}) else &[0]T{};

    const total_len = blk: {
        var sum: usize = 0;
        for (slices) |slice| {
            sum += slice.len;
        }

        if (s) |_| {
            sum += 1;
        }

        break :blk sum;
    };

    const buf = try allocator.alloc(T, total_len);
    errdefer allocator.free(buf);

    var buf_index: usize = 0;
    for (slices) |slice| {
        @memcpy(buf[buf_index .. buf_index + slice.len], slice);
        buf_index += slice.len;
    }

    if (s) |sentinel| {
        buf[buf.len - 1] = sentinel;
    }

    // No need for shrink since buf is exactly the correct size.
    return buf;
}