Practical Computer Science Concepts Simplified

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Practical Computer Science Concepts Simpliﬁed Joshua Silver VP, Product Development Patientco

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Housekeeping This is YOUR time, so: –  Try another session if this one isn’t for you. Please REVIEW + COMMENT https://joind.in/13719

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Agenda •  About me •  Approach •  Deep dives: – Regex – Finite State Machines – Graph Theory •  Overview of other topics

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About Me •  CS degree from GA Tech •  Helped internationalize CareerBuilder.com platform •  Technical co-founder of Patientco •  Fun Fact: –  Travel junkie: 25 countries by the time I was 25

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About Patientco •  Simpliﬁes patient billing for healthcare providers •  Bills that patients can actually understand! •  Founded in 2009 in Atlanta •  Inc. 500 Fastest Growing Company •  We’re hiring: http://www.patientcolife.com

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Let’s Go!

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“Don’t reinvent the wheel”

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“Don’t reinvent the wheel” Only works if you know the wheel exists!

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Let’s explore some wheels…

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Regular Expressions ‘Regex’

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Typical Uses of Regex •  Pattern matching within strings •  Find & replace •  Input Validation •  Text ﬁltering

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Regex Example Validate a US phone number. Acceptable format: 000-000-0000

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Regex Example Validate a US phone number. Acceptable formats include: 000-000-0000 (000) 000-0000

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! $phoneNumbers = array(! "202-555-1234", "1-789-555-0281", "(770) 555-7422", ! "789 555-7898", "555-8790", "890-000-233", ! "(789 029-8293", "903.555.1093" );! ! $regex = '/^(\d{3}\-|$\d{3}$ )[0-9]{3}-\d{4}$/';! ! foreach ($phoneNumbers as $phoneNumber)! if (preg_match($regex, $phoneNumber) === 1)! echo $phoneNumber . ” is valid.";! else! echo $phoneNumber . ” is NOT valid.”;!

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Output:! ! 202-555-1234 is valid.! 1-789-555-0281 is NOT valid.! (770) 555-7422 is valid.! 789 555-7898 is NOT valid.! 555-8790 ! is NOT valid.! 890-000-233 is NOT valid.! (789 029-8293 is NOT valid.! 903.555.1093 is NOT valid.!

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$regex = '/^(\d{3}\-|$\d{3}$ )[0-9]{3}-\d{4}$/';! ! ^ start of string! (! \d{3}\- exactly 3 digits, followed by a -! | OR! $\d{3}$ open paren, 3 digits, close paren! )! [0-9]{3} alternative notation for 3 digits! -! \d{4} exactly 4 digits! $ end of string! !

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(?:(?:\r\n)?[ \t])*(?:(?:(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t]! )+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:! \r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(! ?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ ! \t]))*"(?:(?:\r\n)?[ \t])*))*@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\0! 31]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\! ](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+! (?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:! (?:\r\n)?[ \t])*))*|(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z! |(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)! ?[ \t])*)*\<(?:(?:\r\n)?[ \t])*(?:@(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\! r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[! \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)! ?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t]! )*))*(?:,@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[! \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*! )(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t]! )+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*)! *:(?:(?:\r\n)?[ \t])*)?(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+! Regexes can get CRAZY. Don’t do this. ** FROM: h;p://www.ex-‐parrot.com/~pdw/Mail-‐RFC822-‐Address.html

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Resources General info: http://en.wikipedia.org/wiki/Regular_expression http://php.net/manual/en/function.preg-match.php http://www.regular-expressions.info/ Online Regex Tester: https://regex101.com/

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Finite State Machines

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State Machine Example From: h;p://en.wikipedia.org/wiki/Finite-‐state_machine

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Typical Uses of State Machines •  Business rules requiring ﬂows –  Order management –  Payment Processing •  Complex parsing applications •  Embedded systems •  Background-type jobs (“servers”) •  Many game engines

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How to implement? 1.  Draw a state machine diagram 2.  Convert to variable-based state table 3.  Implement getStatus() in code 4.  Implement transition functions (with error checking) in code 5.  PROFIT$$!!

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Order Management System Conﬁrmed Cancelled On Hold Completed Ready to ship Order Created Order Cancelled Card Declined Order Cancelled Card Successful Card Successful Order Shipped Card Declined

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Order Management System Status timeCancelled timeShipped lastBillingOutcome Conﬁrmed NULL NULL NULL Ready to ship NULL NULL SUCCESS Completed NULL NOT NULL SUCCESS Cancelled NOT NULL NULL -- On Hold NULL NULL DECLINED Conﬁrmed Cancelled On Hold Completed Ready to ship

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public function getStatus() {! if ($this->timeCancelled === NULL &&! $this->timeShipped === NULL &&! $this->lastBillingOutcome === NULL){! return self::ORDER_STATUS_CONFIRMED;! } ! ! if ($this->timeCancelled === NULL &&! $this->timeShipped === NULL &&! $this->lastBillingOutcome === "SUCCESS"){! return self::ORDER_STATUS_READYTOSHIP;! }! !

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public function getStatus() {! . . .! if ($this->timeCancelled !== NULL &&! $this->timeShipped === NULL){! return self::ORDER_STATUS_CANCELLED;}! ! if ($this->timeCancelled === NULL &&! $this->timeShipped === NULL &&! $this->lastBillingOutcome === "DECLINED"){! return self::ORDER_STATUS_ONHOLD;}! ! die("ERROR: INVALID STATUS\n");! }! !

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public function chargeCard() {! if ($this->getStatus() !== self::ORDER_STATUS_CONFIRMED ! && $this->getStatus() !== ORDER_STATUS_ONHOLD) {! die("ERROR: Can't charge card - INVALID STATUS\n");! }! ! if (wasCardChargeSuccessful() === true)! $this->lastBillingOutcome = "SUCCESS";! else! $this->lastBillingOutcome = "DECLINED”;! }! ! !

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! public function ship() {! if ($this->getStatus() !== self::ORDER_STATUS_READYTOSHIP) {! die("ERROR: Can't ship order - INVALID STATUS\n");! }! $this->timeShipped = time();! }!

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$o1 = new Order();! echo $o1->getStatus();! $o1->chargeCard();! echo $o1->getStatus();! $o1->ship();! echo $o1->getStatus();! ! --------! Confirmed! Ready To Ship! Completed!

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$o2 = new Order();! echo $o2->getStatus();! $o2->ship();! echo $o2->getStatus();! ! --------! Confirmed! ERROR: Can't ship order- INVALID STATUS!

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Why use a state machine? •  Business logic self-contained and can be centralized –  e.g. If rules for when an order is ready to ship ever change, only need to change a single function. •  Complex rules can be implemented simply and compactly •  Easier to test

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Related to regex? •  Regular Expressions are a speciﬁc case of state machine •  EVERY regex can be converted to a state machine •  State machines can also represent much more complex grammars

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Resources General info: http://en.wikipedia.org/wiki/Automata_theory http://www.i-programmer.info/babbages-bag/223- ﬁnite-state-machines.html http://en.wikipedia.org/wiki/Finite-state_machine

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Graph Theory

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What’s a graph? •  Set of vertices (the nodes) •  Set of edges connecting nodes –  Can be directed or undirected –  Can optionally be weighted

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Typical Uses of Graphs •  More than just “Traveling salesman problem” •  Job/resource scheduling •  Navigation, maps •  Ordering links (eg. Google ranking) •  Social graph analysis (eg. Facebook, LinkedIn) •  Matching (eg. Dating app) •  Indexing, searching

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Graphs are hard easy 1.  Determine if the problem is really a graph problem 2.  Determine appropriate algorithm 3.  Implement algorithm properly

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Trees •  Directed, connected, acyclic graph •  Example: –  Binary Search Tree

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Dĳkstra's algorithm •  Finds the shortest path between nodes in a graph –  Directions between cities –  Finding cheapest ﬂight –  Network packing routing

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Dĳkstra's algorithm SFO DFW ORD NYC ATL 70 180 100 30 40 80 80

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Dĳkstra's algorithm SFO DFW ORD NYC ATL 70 180 100 30 40 80 80

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Matching •  Bipartite graphs used for matching –  Match boys / girls on a dating site –  Match medical students & residency programs •  Implementation –  Max-ﬂow min-cut theorem –  Hall's marriage theorem –  König's theorem

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Social Graph Searching

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Social Graph Searching

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Resources General info: http://en.wikipedia.org/wiki/Graph_(mathematics) http://en.wikipedia.org/wiki/Graph_traversal http://en.wikipedia.org/wiki/Category:Graph_algorithms

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Other common “reinventions” •  Mutexes, locks, semaphores –  Manage exclusive access of a resource •  Queues, Priority Queues –  Don’t put them in a DB!!! –  Any tasks that can be done asynchronously •  Caching –  Don’t roll your own –  Cache invalidation

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Questions? [email protected] http://www.PatientcoLife.com/ https://joind.in/13719