A finite-state machine or finite-state automaton, finite automaton, or a state machine, is a mathematical model of computation. It is an abstract machine that can be in one of a finite number of states at any given time; the FSM can change from one state to another in response to some external inputs. An FSM is defined by a list of its states, its initial state, the conditions for each transition. Finite state machines are of two types – deterministic finite state machines and non-deterministic finite state machines. A deterministic finite-state machine can be constructed equivalent to any non-deterministic one; the behavior of state machines can be observed in many devices in modern society that perform a predetermined sequence of actions depending on a sequence of events with which they are presented. Simple examples are vending machines, which dispense products when the proper combination of coins is deposited, whose sequence of stops is determined by the floors requested by riders, traffic lights, which change sequence when cars are waiting, combination locks, which require the input of combination numbers in the proper order.
The finite state machine has less computational power than some other models of computation such as the Turing machine. The computational power distinction means there are computational tasks that a Turing machine can do but a FSM cannot; this is because a FSM's memory is limited by the number of states it has. FSMs are studied in the more general field of automata theory. An example of a simple mechanism that can be modeled by a state machine is a turnstile. A turnstile, used to control access to subways and amusement park rides, is a gate with three rotating arms at waist height, one across the entryway; the arms are locked, blocking the entry, preventing patrons from passing through. Depositing a coin or token in a slot on the turnstile unlocks the arms, allowing a single customer to push through. After the customer passes through, the arms are locked again. Considered as a state machine, the turnstile has two possible states: Unlocked. There are two possible inputs that affect its state: pushing the arm.
In the locked state, pushing on the arm has no effect. Putting a coin in – that is, giving the machine a coin input – shifts the state from Locked to Unlocked. In the unlocked state, putting additional coins in has no effect. However, a customer pushing through the arms, giving a push input, shifts the state back to Locked; the turnstile state machine can be represented by a state transition table, showing for each possible state, the transitions between them and the outputs resulting from each input: The turnstile state machine can be represented by a directed graph called a state diagram. Each state is represented by a node. Edges show the transitions from one state to another; each arrow is labeled with the input. An input that doesn't cause a change of state is represented by a circular arrow returning to the original state; the arrow into the Locked node from the black dot indicates. A state is a description of the status of a system, waiting to execute a transition. A transition is a set of actions to be executed when a condition is fulfilled or when an event is received.
For example, when using an audio system to listen to the radio, receiving a "next" stimulus results in moving to the next station. When the system is in the "CD" state, the "next" stimulus results in moving to the next track. Identical stimuli trigger different actions depending on the current state. In some finite-state machine representations, it is possible to associate actions with a state: an entry action: performed when entering the state, an exit action: performed when exiting the state. Several state transition table types are used; the most common representation is shown below: the combination of current state and input shows the next state. The complete action's information is not directly described in the table and can only be added using footnotes. A FSM definition including the full actions information is possible using state tables; the Unified Modeling Language has a notation for describing state machines. UML state machines overcome the limitations of traditional finite state machines while retaining their main benefits.
UML state machines introduce the new concepts of hierarchically nested states and orthogonal regions, while extending the notion of actions. UML state machines have the characteristics of Moore machines, they support actions that depend on both the state of the system and the triggering event, as in Mealy machines, as well as entry and exit actions, which are associated with states rather than transitions, as in Moore machines. The Specification and Description Language is a standard from ITU that includes graphical symbols to describe actions in the transition: send an event receive an event start a timer cancel a timer start another concurrent state machine decisionSDL embeds basic data types called "Abstract Data Types", an action language, an execution semantic in order to make the finite state machine executable. There are a large number of variants to represent an FSM such as the one in figure 3. In addition to their use in modeling reactive systems
SCXML stands for State Chart XML: State Machine Notation for Control Abstraction. It is an XML-based markup language that provides a generic state-machine-based execution environment based on Harel statecharts. SCXML is able to describe complex finite state machines. For example, it is possible to describe notations such as sub-states, parallel states, synchronization, or concurrency, in SCXML; the objective of this standard is to genericize state diagram notations that are used in other XML contexts. For example, it is expected that SCXML notations will replace the State machines notations used in the next CCXML 2.0 version. It could be used as a multimodal control language in the Multimodal Interaction Activity. One of the goals of this language is to make sure that the language is compatible with CCXML and that there is an easy path for existing CCXML scripts to be converted to SCXML without major changes to the programming model or document structure; the current version of the specification was released by the W3C in September 2015.
According to the W3C SCXML specification, SCXML is a general-purpose event-based state machine language that can be used in many ways, including: As a high-level dialog language controlling VoiceXML 3.0's encapsulated speech modules As a voice application metalanguage, where in addition to VoiceXML 3.0 functionality, it may control database access and business logic modules. As a multimodal control language in the MultiModal Interaction framework, combining VoiceXML 3.0 dialogs with dialogs in other modalities including keyboard and mouse, vision, etc. It may control combined modalities such as lipreading speech input with keyboard as fallback, multiple keyboards for multi-user editing; as the state machine framework for a future version of CCXML. As an extended call center management language, combining CCXML call control functionality with computer-telephony integration for call centers that integrate telephone calls with computer screen pops, as well as other types of message exchange such as chats, instant messaging, etc.
As a general process control language in other contexts not involving speech processing. The draft W3C VoiceXML 3.0 specification includes State Chart and SCXML Representation to define functionality. Multimodal application designs can use different modalities for different parts of a communication best suited to it. For example, voice input can be used to avoid having to type on the small screen of a mobile phone, but the screen may be a faster way of communicating a list or map, compared to listening to long descriptions of available options. SCXML makes it easy to do several things in parallel, the Interaction Manager SCXML application will maintain the synchronization between Voice and Visual dialogues; the W3C document Authoring Applications for the Multimodal Architecture describes a multimodal system that implements the W3C Multimodal Architecture and gives an example of a simple multimodal application authored using various W3C markup languages, including SCXML, CCXML, VoiceXML 2.1 and HTML. scxmlcc An efficient scxml to C++ compiler.
It supports some additional features such as custom tag libraries and includes. It is not W3C compliant. PySCXML a Python-implementation. Supports a wide range of technologies, including websockets and SOAP. Standards-compliant. Supports the ECMAScript datamodel; the PySCXML Console a web-based interactive SCXML console for running and interacting with SCXML documents. Supports the ECMAScript datamodel. SCXML4Flex ActionScript/Flex partial port of PySCXML. SCXMLgui Java Visual Editor for SCXML. VoiceXML CCXML W3C SCXML specification 1.0 SCXML Commons Usecases - Stopwatch example
Cascading Style Sheets
This cascading priority scheme is predictable. The CSS specifications are maintained by the World Wide Web Consortium. Internet media type text/css is registered for use with CSS by RFC 2318; the W3C operates a free CSS validation service for CSS documents. In addition to HTML, other markup languages support the use of CSS including XHTML, plain XML, SVG, XUL. CSS has a simple syntax and uses a number of English keywords to specify the names of various style properties. A style sheet consists of a list of rules; each rule or rule-set consists of one or more selectors, a declaration block. In CSS, selectors declare which part of the markup a style applies to by matching tags and attributes in the markup itself. Selectors may apply to the following: all elements of a specific type, e.g. the second-level headers h2 elements specified by attribute, in particular: id: an identifier unique within the document class: an identifier that can annotate multiple elements in a document elements depending on how they are placed relative to others in the document tree.
Classes and IDs are case-sensitive, start with letters, can include alphanumeric characters and underscores. A class may apply to any number of instances of any elements. An ID may only be applied to a single element. Pseudo-classes are used in CSS selectors to permit formatting based on information, not contained in the document tree. One example of a used pseudo-class is:hover, which identifies content only when the user “points to” the visible element by holding the mouse cursor over it, it is #elementid: hover. A pseudo-class classifies document elements, such as:link or:visited, whereas a pseudo-element makes a selection that may consist of partial elements, such as::first-line or::first-letter. Selectors may be combined in many ways to achieve great flexibility. Multiple selectors may be joined in a spaced list to specify elements by location, element type, id, class, or any combination thereof; the order of the selectors is important. For example, div.myClass applies to all elements of class myClass that are inside div elements, whereas.myClass div applies to all div elements that are in elements of class myClass.
The following table provides a summary of selector syntax indicating usage and the version of CSS that introduced it. A declaration block consists of a list of declarations in braces; each declaration itself consists of a property, a colon, a value. If there are multiple declarations in a block, a semi-colon must be inserted to separate each declaration. Properties are specified in the CSS standard; each property has a set of possible values. Some properties can affect any type of element, others apply only to particular groups of elements. Values may be keywords, such as "center" or "inherit", or numerical values, such as 200px, 50vw or 80%. Color values can be specified with keywords, hexadecimal values, RGB values on a 0 to 255 scale, RGBA values that specify both color and alpha transparency, or HSL or HSLA values. Before CSS, nearly all presentational attributes of HTML documents were contained within the HTML markup. All font colors, background styles, element alignments and sizes had to be explicitly described repeatedly, within the HTML.
CSS lets authors move much of that information to another file, the style sheet, resulting in simpler HTML. For example, sub-headings, sub-sub-headings, etc. are defined structurally using HTML. In print and on the screen, choice of font, size and emphasis for these elements is presentational. Before CSS, document authors who wanted to assign such typographic characteristics to, all h2 headings had to repeat HTML presentational markup for each occurrence of that heading type; this made documents more complex and more error-prone and difficult to maintain. CSS allows the separation of presentation from structure. CSS can define color, text alignment, borders, spacing and many other typographic characteristics, can do so independently for on-screen and printed views. CSS defines non-visual styles, such as reading speed and emphasis for aural text readers; the W3C has now deprecated the use of all presentational HTML markup. For example, under pre-CSS HTML, a heading element defined with red text would be written as: Using CSS, the sam
It describes 18 elements comprising the initial simple design of HTML. Except for the hyperlink tag, these were influenced by SGMLguid, an in-house Standard Generalized Markup Language -based documentation format at CERN. Eleven of these elements still exist in HTML 4. HTML is a markup language that web browsers use to interpret and compose text and other material into visual or audible web pages. Default characteristics for every item of HTML markup are defined in the browser, these characteristics can be altered or enhanced by the web page designer's additional use of CSS. Many of the text elements are found in the 1988 ISO technical report TR 9537 Techniques for using SGML, which in turn covers the features of early text formatting languages such as that used by the RUNOFF command developed in the early 1960s for the CTSS operating system: these formatting commands were derived from the commands used by typesetters to manually format documents. However, the SGML concept of generalized markup is based on elements rather than print effects, with the separation of structure and markup.
Berners-Lee considered HTML to be an application of SGML. It was formally defined as such by the Internet Engineering Task Force with the mid-1993 publication of the first proposal for an HTML specification, the "Hypertext Markup Language" Internet Draft by Berners-Lee and Dan Connolly, which included an SGML Document type definition to define the grammar; the draft expired after six months, but was notable for its acknowledgment of the NCSA Mosaic browser's custom tag for embedding in-line images, reflecting the IETF's philosophy of basing standards on successful prototypes. Dave Raggett's competing Internet-Draft, "HTML+", from late 1993, suggested standardizing already-implemented features like tables and fill-out forms. After the HTML and HTML+ drafts expired in early 1994, the IETF created an HTML Working Group, which in 1995 completed "HTML 2.0", the first HTML specification intended to be treated as a standard against which future implementations should be based. Further development under the auspices of the IETF was stalled by competing interests.
Since 1996, the HTML specifications have been maintained, with input from commercial software vendors, by the World Wide Web Consortium. However, in 2000, HTML became an international standard. HTML 4.01 was published in late 1999, with further errata published through 2001. In 2004, development began on HTML5 in the Web Hypertext Application Technology Working Group, which became a joint deliverable with the W3C in 2008, completed and standardized on 28 October 2014. November 24, 1995 HTML 2.0 was published as RFC 1866. Supplemental RFCs added capabilities: November 25, 1995: RFC 1867 May 1996: RFC 1942 August 1996: RFC 1980 January 1997: RFC 2070 January 14, 1997 HTML 3.2 was published as a W3C Recommendation. It was the first version developed and standardized by the W3C, as the IETF had closed its HTML Working Group on September 12, 1996. Code-named "Wilbur", HTML 3.2 dropped math formulas reconciled overlap among various proprietary extensions and adopted most of Netscape's visual markup tags.
Netscape's blink element and Microsoft's marquee element were omitted due to a mutual agreement between the two companies. A markup for mathematical formu
The Platform for Privacy Preferences Project is an obsolete protocol allowing websites to declare their intended use of information they collect about web browser users. Designed to give users more control of their personal information when browsing, P3P was developed by the World Wide Web Consortium and recommended on April 16, 2002. Development ceased shortly thereafter and there have been few implementations of P3P. Microsoft Internet Explorer and Edge were the only major browsers to support P3P. Microsoft has ended support from Windows 10 onwards. Microsoft Internet Explorer and Edge on Windows 10 will no longer support P3P; the president of TRUSTe has stated that P3P has not been implemented due to the difficulty and lack of value. As the World Wide Web became a genuine medium in which to sell products and services, electronic commerce websites tried to collect more information about the people who purchased their merchandise; some companies used controversial practices such as tracker cookies to ascertain the users' demographic information and buying habits, using this information to provide targeted advertisements.
Users who saw this as an invasion of privacy would sometimes turn off HTTP cookies or use proxy servers to keep their personal information secure. P3P is designed to give users a more precise control of the kind of information that they allow to release. According to the W3C, the main goal of P3P “is to increase user trust and confidence in the Web through technical empowerment.”P3P is a machine-readable language that helps to express a website’s data management practices. P3P manages information through privacy policies; when a website uses P3P, they set up a set of policies that allows them to state their intended uses of personal information that may be gathered from their site visitors. When a user decides to use P3P, they set their own set of policies and state what personal information they will allow to be seen by the sites that they visit; when a user visits a site, P3P will compare what personal information the user is willing to release, what information the server wants to get – if the two do not match, P3P will inform the user and ask if he/she is willing to proceed to the site, risk giving up more personal information.
IE provides the ability to display P3P privacy policies, compare the P3P policy with the browser's settings to decide whether or not to allow cookies from a particular site. However, the P3P functionality in Internet Explorer extends only to cookie blocking, will not alert the user to an entire web site that violates active privacy preferences. Microsoft considers the feature deprecated in its browsers and removed P3P support on Windows 10. Mozilla supported some P3P features for a few years, but all P3P related source code was removed by 2007; the Privacy Finder service was created by Carnegie Mellon's Usable Privacy and Security Laboratory. It is a publicly available "P3P-enabled search engine." A user can enter a search term along with their stated privacy preferences, is presented with a list of search results which are ordered based on whether the sites comply with their preferences. This works by crawling the web and maintaining a P3P cache for every site that appears in a search query.
ActivityPub is an open, decentralized social networking protocol based on Pump.io's ActivityPump protocol. It provides a client/server API for creating and deleting content, as well as a federated server-to-server API for delivering notifications and content. ActivityPub is a standard for the Internet in the Social Web Networking Group of the World Wide Web Consortium. At an earlier stage, the name of the protocol was "ActivityPump", but it was felt that ActivityPub better indicated the cross-publishing purpose of the protocol, it learned from the experiences with the older standard called OStatus. In January 2018, the World Wide Web Consortium published the ActivityPub standard as a Recommendation. Former Diaspora community manager Sean Tilley wrote an article that suggests ActivityPub protocols may provide a way to federate Internet platforms. Mastodon, a social networking software, implemented ActivityPub in version 1.6, released on 10 September 2017. It is intended that ActivityPub offer more security for private messages than the previous OStatus protocol does.
Pleroma, a social networking software implementing ActivityPub. Misskey, a social networking software implementing ActivityPub. Hubzilla a community CMS software platform that uses Zot, added ActivityPub support from version 2.8 with a plugin. Nextcloud, a federated service for file hosting. PeerTube, a federated service for video streaming. Pixelfed, a federated service for image sharing. Friendica, a social networking software, implemented ActivityPub in version 2019.01. Osada, a social networking software implementing Zot & ActivityPub The following solutions are clear client based implementations of ActivityPub: dokielie a client side editor using WebAnnotation and ActivityPub. Go-fed a library that implements ActivityPub in Go; the following solutions are clear server based implementations of ActivityPub: microblog.pub is under development and a self-hosted, single-user microblog implementation for a basic ActivityPub server. Distbin is a distributed pastebin service implemented ActivityPub.
Micropub Comparison of software and protocols for distributed social networking Comparison of microblogging services Fediverse "Socialwg - W3C Wiki". Www.w3.org. Retrieved 2017-11-05. "ActivityPub". Www.w3.org. Retrieved 2017-11-05. "ActivityPub Rocks!". Activitypub.rocks. Retrieved 2017-11-05. Contribute to activitypub development by creating an account on GitHub, World Wide Web Consortium, 2017-11-02, retrieved 2017-11-05
A regular expression, regex or regexp is a sequence of characters that define a search pattern. This pattern is used by string searching algorithms for "find" or "find and replace" operations on strings, or for input validation, it is a technique developed in formal language theory. The concept arose in the 1950s when the American mathematician Stephen Cole Kleene formalized the description of a regular language; the concept came into common use with Unix text-processing utilities. Since the 1980s, different syntaxes for writing regular expressions exist, one being the POSIX standard and another used, being the Perl syntax. Regular expressions are used in search engines and replace dialogs of word processors and text editors, in text processing utilities such as sed and AWK and in lexical analysis. Many programming languages provide regex capabilities, built-in or via libraries; the phrase regular expressions, regexes, is used to mean the specific, standard textual syntax for representing patterns for matching text.
Each character in a regular expression is either a metacharacter, having a special meaning, or a regular character that has a literal meaning. For example, in the regex a. A is a literal character which matches just'a', while'.' is a meta character that matches every character except a newline. Therefore, this regex matches, for example,'a', or'ax', or'a0'. Together and literal characters can be used to identify text of a given pattern, or process a number of instances of it. Pattern matches may vary from a precise equality to a general similarity, as controlled by the metacharacters. For example. Is a general pattern, is less general and a is a precise pattern; the metacharacter syntax is designed to represent prescribed targets in a concise and flexible way to direct the automation of text processing of a variety of input data, in a form easy to type using a standard ASCII keyboard. A simple case of a regular expression in this syntax is to locate a word spelled two different ways in a text editor, the regular expression serialie matches both "serialise" and "serialize".
Wildcards achieve this, but are more limited in what they can pattern, as they have fewer metacharacters and a simple language-base. The usual context of wildcard characters is in globbing similar names in a list of files, whereas regexes are employed in applications that pattern-match text strings in general. For example, the regex ^+|+$ matches excess whitespace at the beginning or end of a line. An advanced regular expression that matches any numeral is??. A regex processor translates a regular expression in the above syntax into an internal representation which can be executed and matched against a string representing the text being searched in. One possible approach is the Thompson's construction algorithm to construct a nondeterministic finite automaton, made deterministic and the resulting deterministic finite automaton is run on the target text string to recognize substrings that match the regular expression; the picture shows the NFA scheme N obtained from the regular expression s*, where s denotes a simpler regular expression in turn, recursively translated to the NFA N.
Regular expressions originated in 1951, when mathematician Stephen Cole Kleene described regular languages using his mathematical notation called regular sets. These arose in theoretical computer science, in the subfields of automata theory and the description and classification of formal languages. Other early implementations of pattern matching include the SNOBOL language, which did not use regular expressions, but instead its own pattern matching constructs. Regular expressions entered popular use from 1968 in two uses: pattern matching in a text editor and lexical analysis in a compiler. Among the first appearances of regular expressions in program form was when Ken Thompson built Kleene's notation into the editor QED as a means to match patterns in text files. For speed, Thompson implemented regular expression matching by just-in-time compilation to IBM 7094 code on the Compatible Time-Sharing System, an important early example of JIT compilation, he added this capability to the Unix editor ed, which led to the popular search tool grep's use of regular expressions.
Around the same time when Thompson developed QED, a group of researchers including Douglas T. Ross implemented a tool based on regular expressions, used for lexical analysis in compiler design. Many variations of these original forms of regular expressions were used in Unix programs at Bell Labs in the 1970s, including vi, sed, AWK, expr, in other programs such as Emacs. Regexes were subsequently adopted by a wide range of programs, with these early forms standardized in the POSIX.2 standard in 1992. In the 1980s the more complicated regexes arose in Perl, which derived from a regex library written by Henry Spencer, who wrote an implementation of Advanced Regular Expressions for Tcl; the Tcl library is a hybrid NFA/DFA implementation with improved performance characteristics. Software projects that have adopted Spencer's Tcl regular expression implementation include PostgreSQL. Perl expanded on Spencer's original library