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+# Hashing
+
+https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-006-introduction-to-algorithms-fall-2011/lecture-videos/lecture-8-hashing-with-chaining/
+
+movitation
+
+* database
+* compilers and interpreters
+* network router
+* substring search
+* string commonalities
+* file/dir synchronization
+
+Direct Access table:
+
+  ----------
+0 |        |
+1 |        |
+2 |        |
+3 |        |
+4 |        |
+  | ...    |
+  | ...    |
+  | ...    |
+  ----------
+
+Hash requires mapping to an integer.
+
+ideally:
+
+* prehash hash(x) == hash(y) only when x == y
+
+badness:
+
+1. keys may not be non negative integers
+1. gigantic memory hog
+
+hashing -> hatchet
+
+- reduce universe of all possible keys and reduce them down to some reasonable set of integers.
+- idea: m = theta(n)
+  - size of table proportial to size of # of things.
+
+    -----------
+0   | | | | | |
+    ----------
+1   |         |
+    ----------
+2   |         |
+    ----------
+3   |         |
+    ----------
+4   |         |
+    ----------
+    | ...     |
+    ----------
+    | ...     |
+    ----------
+m-1 | ...     |
+    ----------
+
+Collision: Multiple keys map to same slot in hash table.
+* h(ki) == h(kj) but ki <> kj
+* we can use chaining
+
+How to define?
+
+* function (h)
+* all keys map to `h = { 0, ..., m-1 }`
+
+Two ways to deal with collisions:
+
+* Chaining: store collisions as a list. (i.e. linked list)
+* Open addressing
+
+## Chaining
+
+    ----------
+0   |         | --> (item1) --> (item2) --> (nil)
+    ----------
+1   |         |
+    ----------
+2   |         |
+    ----------
+3   |         |
+    ----------
+4   |         |
+    ----------
+    | ...     |
+    ----------
+    | ...     |
+    ----------
+m-1 | ...     |
+    ----------
+
+Worst case:
+
+* theta(n) (i.e. all items have a collision)
+
+Simple uniform hashing:
+
+* Convenient for analysis.
+* each key is equally likely to be hashed to any slot of the table, independent of where other keys hashing.
+
+Analysis:
+
+* expected length of a chain for n keys, m slots.
+  * n/m == alpha == load factor
+  * O(1 + alpha)
+
+## Hash functions:
+
+1. division method:
+  * `h(k) == k mod m`
+  * works okay if `m` is prime and not close to power of 2 or power of 10.
+2. multiplication method:
+  * `h(k) = [(a*k) mod 2^w] >> (w - r)`
+3. Universal hasing:
+  * `h(k) = [(ak+b) mod p] mod m
+    * `a`, `b` are random between 0 and `p-1`. p is a big prime number larger than the universe.
+    * {0, ..,p-1}
+  * worst case keys: k1 != k2
+    * `1/m`