CS/COE 1501 Project 1 Solved

   Goal:
To gain a better understanding of search algorithms and symbol tables by
implementing an autocompletion engine.

   Background:
In order to make typing on a mobile device quick and easy, an autocomplete
feature is commonly implemented to try to guess what word a user wishes to type
before they are finished. Such a feature requires extensive use of search
algorithms.

   Specifications:
* First, download a copy of the English dictionary that you will use for this 
  project from [here](http://people.cs.pitt.edu/~nlf4/cs1501/handouts/dictionary.txt)
  and place it in your repository folder
    * **Do not** add this file to your git repository, however! (i.e., do not run
      `git add dictionary.txt`).
* You must implement a De La Briandais (DLB) trie data structure (as described
  in lecture) to use in your project.
* When your program is run, it should first create a new DLB trie and add all
  of the words in `dictionary.txt` to that trie (this will be the dictionary
  trie).
* Once the dictionary trie is established, you should prompt the user
  to start typing a word.  For this project, you will be writing only a simple
  program to test autocomplete functionality. Due to complexities with
  gathering single character input in Java, you should accept a single character at a time, each followed by `Enter`. After each character, present the user with the list of predictions of the word that they are trying to type.
  If the user types a number (1-5), you should consider the user to have selected
  the corresponding prediction, and restart the process for a new word.  Consider `$` to be a terminator character; if the user enters a
  `$`, consider the characters input so far to be the full word as intended (regardless of suggestions).  Finally, if 
  the user enters a `!` at any point, your program should exit.
* To generate the list of predictions, your program should not only consult
  the dictionary trie, but also keep track of what words the user has entered
  in the past. If the user has previously entered the same sequence of
  characters as a prefix to a word, you should prioritize the words that most frequently resulted from this sequence
  previously. If the user has never entered the current sequence before, or
  has entered fewer than 5 words with the current seequence as a prefix
  (i.e., not enough words to complete the list of 5 predictions), your program
  should suggest words from dictionary.txt that have the current sequence as a
  prefix.
* You program should propose at most 5 suggestions at a time. If there are
  fewer than 5 suggestions available from the user's history and the dictionary
  trie, then only print out the available suggestions.
* If the current sequence of characters has not been entered by the user
  before and does not appear in `dictionary.txt`, you should display a 
  message to the user stating that no predicions were found, and allow
  the user to continue entering characters one at a time.  Once the user
  enters a "$", you should consider the word to be finished and add it
  to the user's history so that you can predict it in the future.
* Your program should run in a case sensitive manner in order to allow for
  easier prediction of proper nouns and acronyms.
* The design of the data structure that keeps track of a user's previously 
  entered words is entirely up to you. You must create a file named
  `approach.txt` that both describes your approach to implementing this symbol
  table and justifies your decision to take this approach.  Note that this file
  does not need to be extensive, just a few lines so the TA is aware of what to
  look for in your code and why you chose this approach.
* The history of the user's entered words should persist across runs of your
  program.  To enable this, your program should save a representation of
  this data structure to the file `user_history.txt` before exiting.
    * Do not add this file to your git repository! (i.e., do not run `git 
      add user_history.txt`)  It should be generated by your program
      if it does not exist.
* Each time the user enters a character, you should use Java's
  [`System.nanoTime()`]
  (https://docs.oracle.com/javase/8/docs/api/java/lang/System.html nanoTime--)
  to calculate how long your program takes to find the predictions. You should 
  display this time along with the list of predictions.
* After the user enters `!`, your program should output the average time
  that was required to produce a list of predictions.

An example run of the program would proceed as follows:

```
Enter your first character: t

(0.000251 s)
Predictions:
(1) t    (2) ta    (3) tab    (4) tab's    (5) tabbed    

Enter the next character:  h               

(0.000159 s)
Predictions:
(1) thalami    (2) thalamus    (3) thalamus's    (4) thalidomide    (5) thalidomide's    

Enter the next character:  e

(0.000052 s)
Predictions:
(1) the    (2) theater    (3) theater's    (4) theatergoer    (5) theatergoer's    

Enter the next character:  r

(0.000225 s)
Predictions:
(1) therapeutic    (2) therapeutically    (3) therapeutics    (4) therapeutics's    (5) therapies    

Enter the next character:  e

(0.000182 s)
Predictions:
(1) there    (2) there's    (3) thereabout    (4) thereabouts    (5) thereafter    

Enter the next character:  3


 WORD COMPLETED:  thereabout

Enter first character of the next word: t

(0.000128 s)
Predictions:
(1) thereabout    (2) t    (3) ta    (4) tab    (5) tab's    

Enter the next character:  h

(0.000094 s)
Predictions:
(1) thereabout    (2) thalami    (3) thalamus    (4) thalamus's    (5) thalidomide    

Enter the next character:  e

(0.000085 s)
Predictions:
(1) thereabout    (2) the    (3) theater    (4) theater's    (5) theatergoer    

Enter the next character:  r

(0.000145 s)
Predictions:
(1) thereabout    (2) therapeutic    (3) therapeutically    (4) therapeutics    (5) therapeutics's    

Enter the next character:  e

(0.000130 s)
Predictions:
(1) thereabout    (2) there    (3) there's    (4) thereabouts    (5) thereafter    

Enter the next character:  !


Average time:  0.000145 s
Bye!
```

   Submission Guidelines:
* **DO NOT** add `dictionary.txt` to your git repository.
* **DO NOT** add `user_history.txt` to your git repository, it must be
  generated by your program.
* **DO NOT SUBMIT** any IDE package files.
* You must name your main program file `ac_test.java`.
* You must be able to compile your program by running
  `javac ac_test.java`.
* You must be able to run your program by running `java ac_test`.
* You must fill out `info_sheet.txt`.
* Be sure to remember to push the latest copy of your code back to your GitHub
  repository before the the assignment is due.  At the deadline, the
  repositories will automatically be copied for grading.  Whatever is present
  in your GitHub repository at that time will be considered your submission for
  this assignment.

   Additional Notes:
* You are free to use any data structures written by you, the textbook authors,
  or in the Java standard library to implement the user history symbol table.
  However, if you use code that you do not write yourself, you must research
  the runtime of that particular implementation and discuss that in your
  `approach.txt`.
* Note that if your user history predictions contain a word that is also
  contained in the dictionary predictions, this word should not be presented
  as a suggestion to the user twice in the same prompt (e.g., the final list
  of predictions in the example above).
* You do not need to implement any sort of autocorrect.  You can assume
  that each character entered by the user is intentional.
* You can assume that the user will not try to enter any numerical characters
  aside from selecting a prediction.

   Grading Rubric:
* DLB trie implemented as described in class:  20
* Dictionary predictions are correctly populated:  25
* User history predictions are correctly populated:  30
* Sufficient justification of user history approach:  10
* User interaction works as specified:  5
* Timing data presented: 5
* Assignment info sheet/submission:  5