Previously we have looked at the methods of implementing sparse mappings which conduct searches according to some preplanned scanning strategy. In this chapter we discuss a family of methods where the position of an entry in a directory is related to the value of its key; these methods are known as hashing techniques. Because hashing involves explicitly computing the position of an entry from its key, the technique is only applicable where there is fixed upper bound on the number of directory entries. Hashed directories will therefore be considered as array structures
array [0.. n] of entry
though in essence it is immaterial whether the array is held in main store or on backing store as a randomly accessible file.
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