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🔗 Computing

Constraint programming (CP) is a paradigm for solving combinatorial problems that draws on a wide range of techniques from artificial intelligence, computer science, and operations research. In constraint programming, users declaratively state the constraints on the feasible solutions for a set of decision variables. Constraints differ from the common primitives of imperative programming languages in that they do not specify a step or sequence of steps to execute, but rather the properties of a solution to be found. In addition to constraints, users also need to specify a method to solve these constraints. This typically draws upon standard methods like chronological backtracking and constraint propagation, but may use customized code like a problem specific branching heuristic.

Constraint programming takes its root from and can be expressed in the form of constraint logic programming, which embeds constraints into a logic program. This variant of logic programming is due to Jaffar and Lassez, who extended in 1987 a specific class of constraints that were introduced in Prolog II. The first implementations of constraint logic programming were Prolog III, CLP(R), and CHIP.

Instead of logic programming, constraints can be mixed with functional programming, term rewriting, and imperative languages. Programming languages with built-in support for constraints include Oz (functional programming) and Kaleidoscope (imperative programming). Mostly, constraints are implemented in imperative languages via constraint solving toolkits, which are separate libraries for an existing imperative language.

🔗 Apple Inc. 🔗 Computing 🔗 Computing/Software

Contiki is an operating system for networked, memory-constrained systems with a focus on low-power wireless Internet of Things (IoT) devices. Extant uses for Contiki include systems for street lighting, sound monitoring for smart cities, radiation monitoring, and alarms. It is open-source software released under the BSD-3-Clause license.

Contiki was created by Adam Dunkels in 2002 and has been further developed by a worldwide team of developers from Texas Instruments, Atmel, Cisco, ENEA, ETH Zurich, Redwire, RWTH Aachen University, Oxford University, SAP, Sensinode, Swedish Institute of Computer Science, ST Microelectronics, Zolertia, and many others. Contiki gained popularity because of its built in TCP/IP stack and lightweight preemptive scheduling over event-driven kernel which is a very motivating feature for IoT. The name Contiki comes from Thor Heyerdahl's famous Kon-Tiki raft.

Contiki provides multitasking and a built-in Internet Protocol Suite (TCP/IP stack), yet needs only about 10 kilobytes of random-access memory (RAM) and 30 kilobytes of read-only memory (ROM). A full system, including a graphical user interface, needs about 30 kilobytes of RAM.

A new branch has recently been created, known as Contiki-NG: The OS for Next Generation IoT Devices

🔗 Computing 🔗 Classical Greece and Rome 🔗 Greece 🔗 Astronomy 🔗 History of Science 🔗 Alternative Views 🔗 Time

The Antikythera mechanism (, ) is an ancient hand powered Greek analogue computer which has also been described as the first example of such device used to predict astronomical positions and eclipses for calendar and astrological purposes decades in advance. It could also be used to track the four-year cycle of athletic games which was similar to an Olympiad, the cycle of the ancient Olympic Games.

This artefact was retrieved from the sea in 1901, and identified on 17 May 1902 as containing a gear by archaeologist Valerios Stais, among wreckage retrieved from a shipwreck off the coast of the Greek island Antikythera. The instrument is believed to have been designed and constructed by Greek scientists and has been variously dated to about 87 BC, or between 150 and 100 BC, or to 205 BC, or to within a generation before the shipwreck, which has been dated to approximately 70–60 BC.

The device, housed in the remains of a 34 cm × 18 cm × 9 cm (13.4 in × 7.1 in × 3.5 in) wooden box, was found as one lump, later separated into three main fragments which are now divided into 82 separate fragments after conservation efforts. Four of these fragments contain gears, while inscriptions are found on many others. The largest gear is approximately 14 centimetres (5.5 in) in diameter and originally had 223 teeth.

It is a complex clockwork mechanism composed of at least 30 meshing bronze gears. A team led by Mike Edmunds and Tony Freeth at Cardiff University used modern computer x-ray tomography and high resolution surface scanning to image inside fragments of the crust-encased mechanism and read the faintest inscriptions that once covered the outer casing of the machine.

Detailed imaging of the mechanism suggests that it had 37 gear wheels enabling it to follow the movements of the Moon and the Sun through the zodiac, to predict eclipses and even to model the irregular orbit of the Moon, where the Moon's velocity is higher in its perigee than in its apogee. This motion was studied in the 2nd century BC by astronomer Hipparchus of Rhodes, and it is speculated that he may have been consulted in the machine's construction.

The knowledge of this technology was lost at some point in antiquity. Similar technological works later appeared in the medieval Byzantine and Islamic worlds, but works with similar complexity did not appear again until the development of mechanical astronomical clocks in Europe in the fourteenth century. All known fragments of the Antikythera mechanism are now kept at the National Archaeological Museum in Athens, along with a number of artistic reconstructions and replicas of the mechanism to demonstrate how it may have looked and worked.

🔗 Computing 🔗 Computer science

Eiffel is an object-oriented programming language designed by Bertrand Meyer (an object-orientation proponent and author of Object-Oriented Software Construction) and Eiffel Software. Meyer conceived the language in 1985 with the goal of increasing the reliability of commercial software development; the first version becoming available in 1986. In 2005, Eiffel became an ISO-standardized language.

The design of the language is closely connected with the Eiffel programming method. Both are based on a set of principles, including design by contract, command–query separation, the uniform-access principle, the single-choice principle, the open–closed principle, and option–operand separation.

Many concepts initially introduced by Eiffel later found their way into Java, C#, and other languages. New language design ideas, particularly through the Ecma/ISO standardization process, continue to be incorporated into the Eiffel language.

🔗 Computing 🔗 Computing/Software 🔗 Computing/Free and open-source software

Maxima () is a computer algebra system (CAS) based on a 1982 version of Macsyma. It is written in Common Lisp and runs on all POSIX platforms such as macOS, Unix, BSD, and Linux, as well as under Microsoft Windows and Android. It is free software released under the terms of the GNU General Public License (GPL).

🔗 Computing

A stack-oriented programming language is one that relies on a stack machine model for passing parameters. Several programming languages fit this description, notably Forth, RPL, PostScript, BibTeX style design language and many assembly languages (on a much lower level).

Stack-oriented languages operate on one or more stacks, each of which may serve a different purpose. Thus, programming constructs in other programming languages may need to be modified for use in a stack-oriented system. Further, some stack-oriented languages operate in postfix or Reverse Polish notation, that is, any arguments or parameters for a command are stated before that command. For example, postfix notation would be written 2, 3, multiply instead of multiply, 2, 3 (prefix or Polish notation), or 2 multiply 3 (infix notation).

🔗 Computing

In computing, the Two Generals' Problem is a thought experiment meant to illustrate the pitfalls and design challenges of attempting to coordinate an action by communicating over an unreliable link. In the experiment, two generals are only able to communicate with one another by sending a messenger through enemy territory. The experiment asks how they might reach an agreement on the time to launch an attack, while knowing that any messenger they send could be captured.

It is related to the more general Byzantine Generals Problem and appears often in introductory classes about computer networking (particularly with regard to the Transmission Control Protocol, where it shows that TCP can't guarantee state consistency between endpoints and why this is the case), though it applies to any type of two-party communication where failures of communication are possible. A key concept in epistemic logic, this problem highlights the importance of common knowledge. Some authors also refer to this as the Two Generals' Paradox, the Two Armies Problem, or the Coordinated Attack Problem. The Two Generals' Problem was the first computer communication problem to be proved to be unsolvable. An important consequence of this proof is that generalizations like the Byzantine Generals problem are also unsolvable in the face of arbitrary communication failures, thus providing a base of realistic expectations for any distributed consistency protocols.

🔗 Computing 🔗 Africa 🔗 Ancient Egypt 🔗 Novels 🔗 Novels/Science fiction 🔗 Science Fiction 🔗 Women writers 🔗 Egypt

The Mummy! A Tale of the Twenty-Second Century is an 1827 three-volume novel written by Jane Webb (later Jane C. Loudon). It concerns the Egyptian mummy of Cheops, who is brought back to life in the year 2126. The novel describes a future filled with advanced technology, and was the first English-language story to feature a reanimated mummy.

After her father's death, making her an orphan at the age of 17, Webb found that:

on the winding up of his affairs that it would be necessary to do something for my support. I had written a strange, wild novel, called the Mummy, in which I had laid the scene in the twenty-second century, and attempted to predict the state of improvement to which this country might possibly arrive.

She may have drawn inspiration from the general fashion for anything pharaonic, inspired by the French researches during the Napoleonic invasion of Egypt; the 1821 public unwrappings of Egyptian mummies in a theatre near Piccadilly, which she may have attended as a girl; and, very likely, the 1818 novel by Mary Shelley, Frankenstein; or, The Modern Prometheus. As Shelley had written of Frankenstein's creation, "A mummy again endued with animation could not be so hideous as that wretch," which may have triggered her later concept. In any case, at many points she deals in greater clarity with elements from the earlier book such as the loathing for the much-desired object and the immediate arrest for crime and attempt to lie one's way out of it. However, unlike the Frankenstein monster, the hideous revived Cheops is not shuffling around dealing out horror and death, but giving canny advice on politics and life to those who befriend him. In some ways The Mummy! may be seen as her reaction to themes in Frankenstein: her mummy specifically says he is allowed life only by divine favour, rather than being indisputably vivified only by mortal science, and so on, as Hopkins' 2003 essay covers in detail.

Unlike many early science fiction works (Shelley's The Last Man, and The Reign of King George VI, 1900–1925, written anonymously in 1763), Loudon did not portray the future as her own day with only political changes. She filled her world with foreseeable changes in technology, society, and even fashion. The hero, Edric Montague, lived in a peaceful and Catholic England under the rule of Queen Claudia. Her court ladies wear trousers and hair ornaments of controlled flame. Surgeons and lawyers may be steam-powered automatons. Air travel, by balloon, is commonplace. A kind of Internet is predicted in it. Besides trying to account for the revivification of the mummy in scientific terms—galvanic shock rather than incantations—"she embodied ideas of scientific progress and discovery, that now read like prophecies" to those later in the 19th century. Many of the incidents in the book can be seen as satirical or humorous. Her social attitudes have resulted in this book being ranked among feminist novels.

The Mummy!: Or a Tale of the Twenty-Second Century was published anonymously in 1827 by Henry Colburn in three volumes, as was usual in that day so that each small volume could be easily carried around. It drew many favourable reviews, including one in 1829 in The Gardener's Magazine on the inventions proposed in it. In 1830, the 46-year-old reviewer, John Claudius Loudon, sought out the 22-year-old Webb, and they married the next year.

🔗 Computing 🔗 Computing/Software

This article compares variety of different X window managers. For an introduction to the topic, see X Window System.

🔗 Computing 🔗 Computing/Computer hardware

Rekursiv was a computer processor designed by David M. Harland in the mid-1980s for Linn Smart Computing in Glasgow, Scotland. It was one of the few computer architectures intended to implement object-oriented concepts directly in hardware, a form of high-level language computer architecture. The Rekursiv operated directly on objects rather than bits, nibbles, bytes and words. Virtual memory was used as a persistent object store and unusually, the processor instruction set supported recursion (hence the name).

The project originated in an initiative within the hi-fi manufacturer Linn Products to improve its manufacturing automation systems, which at the time ran on a DEC VAX minicomputer. This resulted in the design of Lingo, an object-oriented programming language derived from Smalltalk and ALGOL. Due to the poor performance of Lingo on the VAX, a subsidiary company, Linn Smart Computing Ltd., was formed to develop a new processor to efficiently run Lingo.

The Rekursiv processor consisted of four gate-array chips named Numerik (32-bit ALU), Logik (instruction sequencer), Objekt (object-oriented memory management unit) and Klock (processor clock and support logic). A small number of prototype VMEbus boards, called Hades, comprising these four chips plus 80 MB of RAM were produced. These were intended for installation in a host system such as a Sun-3 workstation.

Although the Rekursiv was never fully developed and was not a commercial success, several Hades boards were used in academic research projects in the UK. The last known copy of a Rekursiv computer ended up at the bottom of the Forth and Clyde canal in Glasgow.