  In this work we propose MementoMap, a flexible and adaptive framework to
efficiently summarize holdings of a web archive. We described a simple, yet
extensible, file format suitable for MementoMap. We used the complete index of
the Arquivo.pt comprising 5B mementos (archived web pages/files) to understand
the nature and shape of its holdings. We generated MementoMaps with varying
amount of detail from its HTML pages that have an HTTP status code of 200 OK.
Additionally, we designed a single-pass, memory-efficient, and
parallelization-friendly algorithm to compact a large MementoMap into a small
one and an in-file binary search method for efficient lookup. We analyzed more
than three years of MemGator (a Memento aggregator) logs to understand the
response behavior of 14 public web archives. We evaluated MementoMaps by
measuring their Accuracy using 3.3M unique URIs from MemGator logs. We found
that a MementoMap of less than 1.5% Relative Cost (as compared to the
comprehensive listing of all the unique original URIs) can correctly identify
the presence or absence of 60% of the lookup URIs in the corresponding archive
while maintaining 100% Recall (i.e., zero false negatives).
