Further constraints:
<UL>
  <LI>the array cannot possibly be large enough to have a position for
      every possible key.<P>
  <LI>certain types of keys (e.g. strings) cannot be used directly as
      indices: they must be converted to e.g. integers first.
</UL>

<P><B>Hashing Function:</B> converts a key into a valid array index.

<P>a SIN can be mapped to an index in the range 0-99 by taking it mod
100.

<P>For the operations to be constant time, the hashing function must
be constant time.

<P>Original premise: different keys correspond to different positions
in the array, i.e. if K1 =/= K2, then H(K1) =/= H(K2). Such a function
is called a <EM>perfect hashing function</EM>.

<P>In general H is not perfect, and <EM>collisions</EM> will happen.
<UL>
  <LI>minimize number of collisions.
  <LI>resolve collisions with no degradation in performance.
</UL>