Online Algorithms for the Santa Claus Problem

Max Springer, MohammadTaghi Hajiaghayi, Debmalya Panigrahi, MohammadReza Khani

Keywords: fair allocation, online algorithm, maxmin, random order model, optimal competitive ratio

TL;DR: We provide an online algorithm for the classic Santa Claus problem that achieves an optimal $(1-\epsilon)$ competitive ratio, supplementing the result with a nearly tight lower bound.

Abstract: The Santa Claus problem is a fundamental problem in {\em fair division}: the goal is to partition a set of {\em heterogeneous} items among {\em heterogeneous} agents so as to maximize the minimum value of items received by any agent. In this paper, we study the online version of this problem where the items are not known in advance and have to be assigned to agents as they arrive over time. If the arrival order of items is arbitrary, then no good assignment rule exists in the worst case. However, we show that, if the arrival order is random, then for $n$ agents and any $\varepsilon > 0$, we can obtain a competitive ratio of $1-\varepsilon$ when the optimal assignment gives value at least $\Omega(\log n / \varepsilon^2)$ to every agent (assuming each item has at most unit value). We also show that this result is almost tight: namely, if the optimal solution has value at most $C \ln n / \varepsilon$ for some constant $C$, then there is no $(1-\varepsilon)$-competitive algorithm even for random arrival order.