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  • Does it matter the direction of a Huffman's tree child node?

    - by Omega
    So, I'm on my quest about creating a Java implementation of Huffman's algorithm for compressing/decompressing files (as you might know, ever since Why create a Huffman tree per character instead of a Node?) for a school assignment. I now have a better understanding of how is this thing supposed to work. Wikipedia has a great-looking algorithm here that seemed to make my life way easier. Taken from http://en.wikipedia.org/wiki/Huffman_coding: Create a leaf node for each symbol and add it to the priority queue. While there is more than one node in the queue: Remove the two nodes of highest priority (lowest probability) from the queue Create a new internal node with these two nodes as children and with probability equal to the sum of the two nodes' probabilities. Add the new node to the queue. The remaining node is the root node and the tree is complete. It looks simple and great. However, it left me wondering: when I "merge" two nodes (make them children of a new internal node), does it even matter what direction (left or right) will each node be afterwards? I still don't fully understand Huffman coding, and I'm not very sure if there is a criteria used to tell whether a node should go to the right or to the left. I assumed that, perhaps the highest-frequency node would go to the right, but I've seen some Huffman trees in the web that don't seem to follow such criteria. For instance, Wikipedia's example image http://upload.wikimedia.org/wikipedia/commons/thumb/8/82/Huffman_tree_2.svg/625px-Huffman_tree_2.svg.png seems to put the highest ones to the right. But other images like this one http://thalia.spec.gmu.edu/~pparis/classes/notes_101/img25.gif has them all to the left. However, they're never mixed up in the same image (some to the right and others to the left). So, does it matter? Why?

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  • best way to compute vertex normals from a Triangle's list

    - by nkint
    hi i'm a complete newbie in computergraphics so sorry if it's a stupid answer. i'm trying to make a simple 3d engine from scratch, more for educational purpose than for real use. i have a Surface object with inside a Triangle's list. For now i compute normals inside Triangle class, in this way: triangle.computeFaceNormals() { Vec3D u = v1.sub(v3) Vec3D v = v1.sub(v2) Vec3D normal = Vec3D.cross(u,v) normal.normalized() this.n1 = this.n2 = this.n3 = normal } and when building surface: t = new Triangle(v1,v2,v3).computeFaceNormals() surface.addTriangle(t) and i think this is the best way to do that.. isn't it? now.. what about for vertex normals? i've found this simple algorithm: flipcode vertex normal but.. hei this algorithm has.. exponential complexity? (if my memory doesn't fail my computer science background..) (bytheway.. it has 3 nested loops.. i don't think it's the best way to do it..) any suggestion?

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  • Performance issues with visibility detection and object transparency

    - by maul
    I'm working on a 3d game that has a view similar to classic isometric games (diablo, etc.). One of the things I'm trying to implement is the effect of turning walls transparent when the player walks behind them. By itself this is not a huge issue, but I'm having trouble determining which walls should be transparent exactly. I can't use a circle or square mask. There are a lot of cases where the wall piece at the same (relative) position has different visibility depending on the surrounding area. With the help of a friend I came up with this algorithm: Create a grid around the player that contains a lot of "visibility points" (my game is semi tile-based so I create one point for every tile on the grid) - the size of the square's side is close to the radius where I make objects transparent. I found 6x6 to be a good value, so that's 36 visibility points total. For every visibility point on the grid, check if that point is in the player's line of sight. For every visibility point that is in the LOS, cast a ray from the camera to that point and mark all objects the ray hits as transparent. This algorithm works - not perfectly, but only requires some tuning - however this is very slow. As you can see, it requries 36 ray casts minimum, but most of the time 60-70 depending on the position. That's simply too much for the CPU. Is there a better way to do this? I'm using Unity 3D but I'm not looking for an engine-specific solution.

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  • Ruby: implementing alpha-beta pruning for tic-tac-toe

    - by DerNalia
    So, alpha-beta pruning seems to be the most efficient algorithm out there aside from hard coding (for tic tac toe). However, I'm having problems converting the algorithm from the C++ example given in the link: http://www.webkinesia.com/games/gametree.php #based off http://www.webkinesia.com/games/gametree.php # (converted from C++ code from the alpha - beta pruning section) # returns 0 if draw LOSS = -1 DRAW = 0 WIN = 1 @next_move = 0 def calculate_ai_next_move score = self.get_best_move(COMPUTER, WIN, LOSS) return @next_move end def get_best_move(player, alpha, beta) best_score = nil score = nil if not self.has_available_moves? return false elsif self.has_this_player_won?(player) return WIN elsif self.has_this_player_won?(1 - player) return LOSS else best_score = alpha NUM_SQUARES.times do |square| if best_score >= beta break end if self.state[square].nil? self.make_move_with_index(square, player) # set to negative of opponent's best move; we only need the returned score; # the returned move is irrelevant. score = -get_best_move(1-player, -beta, -alpha) if (score > bestScore) @next_move = square best_score = score end undo_move(square) end end end return best_score end the problem is that this is returning nil. some support methods that are used above: WAYS_TO_WIN = [[0, 1, 2], [3, 4, 5], [6, 7, 8], [0, 3, 6], [1, 4, 7], [2, 5, 8],[0, 4, 8], [2, 4, 6]] def has_this_player_won?(player) result = false WAYS_TO_WIN.each {|solution| result = self.state[solution[0]] if contains_win?(solution) } return (result == player) end def contains_win?(ttt_win_state) ttt_win_state.each do |pos| return false if self.state[pos] != self.state[ttt_win_state[0]] or self.state[pos].nil? end return true end def make_move(x, y, player) self.set_square(x,y, player) end

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  • Algorithm to use for shop floor layout?

    - by jkohlhepp
    I ran into a classroom problem yesterday (business oriented class, not computer science) and I found it interesting from an algorithmic perspective. The problem goes something like this: Assume there is a shop floor with N different rooms, and you have N different departments that need to go in those rooms. The departments and the rooms are all the same size, so any department could go in any room. There is a known travel distance from each room to each other room. There is also a known amount of trips necessary from one department to another (trips are counted the same regardless which room they originate from, so a trip from A to B is equivalent to a trip from B to A). Given those inputs, determine a layout of departments into rooms which minimizes travel time. What is the best way to approach this problem algorithmically? Is there already a particular algorithm or class of algorithms designed to solve this type of problem? Does this type of problem have a name in computer science? I am not looking for you to design an algorithm to solve this, although feel free to do so if you would like. I'm wondering if this is a problem space that has already been well defined and studied algorithmically and if so get some links to research further. I can see a lot of different data structures and algorithms that might apply to this and I'm curious which approach would be "best". And don't worry, you are not doing my homework for me. This is not a homework problem per se, as this is a business course and we were simply discussing the concepts and not trying to solve the problem algorithmically.

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  • Grading an algorithm: Readability vs. Compactness

    - by amiregelz
    Consider the following question in a test \ interview: Implement the strcpy() function in C: void strcpy(char *destination, char *source); The strcpy function copies the C string pointed by source into the array pointed by destination, including the terminating null character. Assume that the size of the array pointed by destination is long enough to contain the same C string as source, and does not overlap in memory with source. Say you were the tester, how would you grade the following answers to this question? 1) void strcpy(char *destination, char *source) { while (*source != '\0') { *destination = *source; source++; destionation++; } *destionation = *source; } 2) void strcpy(char *destination, char *source) { while (*(destination++) = *(source++)) ; } The first implementation is straightforward - it is readable and programmer-friendly. The second implementation is shorter (one line of code) but less programmer-friendly; it's not so easy to understand the way this code is working, and if you're not familiar with the priorities in this code then it's a problem. I'm wondering if the first answer would show more complexity and more advanced thinking, in the tester's eyes, even though both algorithms behave the same, and although code readability is considered to be more important than code compactness. It seems to me that since making an algorithm this compact is more difficult to implement, it will show a higher level of thinking as an answer in a test. However, it is also possible that a tester would consider the first answer not good because it's not readable. I would also like to mention that this is not specific to this example, but general for code readability vs. compactness when implementing an algorithm, specifically in tests \ interviews.

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  • Algorithm for optimal control on space ship using accelerometer input data

    - by mm24
    Does someone have a good algorithm for controlling a space ship in a vertical shooter game using acceleration data? I have done a simple algorithm, but works very badly. I save an initial acceleration value (used to calibrate the movement according to the user's initial position) and I do subtract it from the current acceleration so I get a "calibrated" value. The problem is that basing the movement solely on relative acceleration has an effect of loss of sensitivity: certain movements are independent from the initial position. Would anyone be able to share a a better solution? I am wondering if I should use/integrate also inputs from gyroscope hardware. Here is my sample of code for a Cocos2d iOS game: - (void) accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration { if (calibrationLayer.visible){ [self evaluateCalibration:acceleration]; initialAccelleration=acceleration; return; } if([self evaluatePause]){ return; } ShooterScene * shooterScene = (ShooterScene *) [self parent]; ShipEntity *playerSprite = [shooterScene playerShip]; float accellerationtSensitivity = 0.5f; UIAccelerationValue xAccelleration = acceleration.x - initialAccelleration.x; UIAccelerationValue yAccelleration = acceleration.y - initialAccelleration.y; if(xAccelleration > 0.05 || xAccelleration < -0.05) { [playerSprite setPosition:CGPointMake(playerSprite.position.x + xAccelleration * 80, playerSprite.position.y + yAccelleration * 80)]; } else if(yAccelleration > 0.05 || yAccelleration < -0.05) { [playerSprite setPosition:CGPointMake(playerSprite.position.x + xAccelleration * 80, playerSprite.position.y + yAccelleration * 80)]; } }

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  • Algorithm for detecting windows in a room

    - by user2733436
    I am dealing with the following problem and i was looking to write a Pseudo-code for developing an algorithm that can be generic for such a problem. Here is what i have come up with thus far. STEP 1 In this step i try to get the robot where it maybe placed to the top left corner. Turn Left - If no window or Wall detected keep going forward 1 unit.. if window or wall detected -Turn right -- if no window or Wall detected keep going forward.. if window or wall detected then top left corner is reached. STEP 2 (We start counting windows after we get to this stage to avoid miscounting) I would like to declare a variable called turns as it will help me keep track if robot has gone around entire room. Turns = 4; Now we are facing north and placed on top left corner. while(turns0){ If window or wall detected (if window count++) Turn Right Turn--; While(detection!=wall || detection!=window){ move 1 unit forward Turn left (if window count++) Turn right } } I believe in doing so the robot will go around the entire room and count windows and it will stop once it has gone around the entire room as the turns get decremented. I don't feel this is the best solution and would appreciate suggestions on how i can improve my Pseudo-code. I am not looking for any code just a algorithm on solving such a problem and that is why i have not posted this in stack overflow. I apologize if my Pseudo-code is poorly written please make suggestions if i can improve that as i am new to this. Thanks.

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  • Why create a Huffman tree per character instead of a Node?

    - by Omega
    For a school assignment we're supposed to make a Java implementation of a compressor/decompresser using Huffman's algorithm. I've been reading a bit about it, specially this C++ tutorial: http://www.cprogramming.com/tutorial/computersciencetheory/huffman.html In my program, I've been thinking about having Nodes that have the following properties: Total Frequency Character (if a leaf) Right child (if any) Left child (if any) Parent (if any) So when building the Huffman tree, it is just a matter of linking a node to others, etc. However, I'm a bit confused with the following quote (emphasis mine): First, every letter starts off as part of its own tree and the trees are ordered by the frequency of the letters in the original string. Then the two least-frequently used letters are combined into a single tree, and the frequency of that tree is set to be the combined frequency of the two trees that it links together. My question: why should I create a tree per letter, instead of just a node per letter and then do the linking later? I have not begun coding, I'm just studying the algorithm first, so I guess I'm missing an important detail. What is it?

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  • Need a good quality bitmap rotation algorithm for Android

    - by Lumis
    I am creating a kaleidoscopic effect on an android tablet. I am using the code below to rotate a slice of an image, but as you can see in the image when rotating a bitmap 60 degrees it distorts it quite a lot (red rectangles) – it is smudging the image! I have set dither and anti-alias flags but it does not help much. I think it is just not a very sophisticated bitmap rotation algorithm. canvas.save(); canvas.rotate(angle, screenW/2, screenH/2); canvas.drawBitmap(picSlice, screenW/2, screenH/2, pOutput); canvas.restore(); So I wonder if you can help me find a better way to rotate a bitmap. It does not have to be fast, because I intend to use a high quality rotation only when I want to save the screen to the SD card - I would redraw the screen in memory before saving. Do you know any comprehensible or replicable algorithm for bitmap rotation that I could programme or use as a library? Or any other suggestion? EDIT: The answers below made me think if Android OS had bilinear or bicubic interpolation option and after some search I found that it does have its own version of it called FilterBitmap. After applying it to my paint pOutput.setFilterBitmap(true); I get much better result

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  • pie chart (php graph lib) problem !

    - by gin
    i'm using phpgraphlib for creating charts i tried one of the example of the pie charts ex (with lil bit pf changes) which is : include('phpgraphlib.php'); include('phpgraphlib_pie.php'); $graph = new PHPGraphLibPie(400, 200); $data = array("CBS" = 0, "NBC" = 1); $graph-addData($data); $graph-setTitle('8/29/07 Top 5 TV Networks Market Share'); $graph-setLabelTextColor('50,50,50'); $graph-setLegendTextColor('50,50,50'); $graph-createGraph(); ? and whether i put $data = array("CBS" = 0, "NBC" = 1); or $data = array("CBS" = 1, "NBC" = 0); the pie chart have the same color.. shouldn't the whole pie/slice suppose to be colored with some color (i.e. blue) if ("CBS" = 0, "NBC" = 1) , while if ("CBS" = 1, "NBC" = 0) it should be colored with other color (i.e. red)/? is there any way to fix it? i would really appreciate any help ,,

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  • Looking for an algorithm that will show how to fit the most boxes in a container.

    - by Abe Miessler
    I've been interested in writing an application that will show how to fit boxes (of random dimensions) in a container so there is as little space as possible left. A real life example would be something that would tell you how to use the most space in a UPS truck. Does anyone know of a good place to start for something like this? Is there an existing algorithm that does something similar to what I'm talking about?

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  • Efficient algorithm to find a maximum common subset of two sets?

    - by datasunny
    Each set contains bunch of checksums. For example: Set A: { 4445968d0e100ad08323df8c895cea15 a67f8052594d6ba3f75502c0b91b868f 07736dde2f8484a4a3af463e05f039e3 5b1e374ff2ba949ab49870ca24d3163a } Set B: { 6639e1da308fd7b04b7635a17450df7c 4445968d0e100ad08323df8c895cea15 a67f8052594d6ba3f75502c0b91b868f } The maximum common subset of A and B is: { 4445968d0e100ad08323df8c895cea15 a67f8052594d6ba3f75502c0b91b868f } A lot of this operations will be performed, so I'm looking for an efficient algorithm to do so. Thanks for your help.

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  • An algorithm for converting a base-10 number to a base-N number.

    - by roja
    Guys, I am looking for a way to convert a base-10 number into a base-N number where N can be large. Specifically i am looking at converting to base-85 and back again. Does anyone know a simple algorithm to perform the conversion? Ideally it would provide something like: to_radix(83992, 85) - [11, 53, 12] Any ideas are appreciated! Roja

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  • What algorithm .Net use for searching a pattern in a string?

    - by Hun1Ahpu
    I'm studying string searching algorithms now and wondering what algorithm is used for .NET String.Contains function for example. Reflector shows that this function is used but I have no idea what its name means. private static extern int InternalFindNLSStringEx(IntPtr handle, string localeName, int flags, string source, int sourceCount, int startIndex, string target, int targetCount);

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  • Is there a scheduling algorithm that optimizes for "maker's schedules"?

    - by John Feminella
    You may be familiar with Paul Graham's essay, "Maker's Schedule, Manager's Schedule". The crux of the essay is that for creative and technical professionals, meetings are anathema to productivity, because they tend to lead to "schedule fragmentation", breaking up free time into chunks that are too small to acquire the focus needed to solve difficult problems. In my firm we've seen significant benefits by minimizing the amount of disruption caused, but the brute-force algorithm we use to decide schedules is not sophisticated enough to handle scheduling large groups of people well. (*) What I'm looking for is if there's are any well-known algorithms which minimize this productivity disruption, among a group of N makers and managers. In our model, There are N people. Each person pi is either a maker (Mk) or a manager (Mg). Each person has a schedule si. Everyone's schedule is H hours long. A schedule consists of a series of non-overlapping intervals si = [h1, ..., hj]. An interval is either free or busy. Two adjacent free intervals are equivalent to a single free interval that spans both. A maker's productivity is maximized when the number of free intervals is minimized. A manager's productivity is maximized when the total length of free intervals is maximized. Notice that if there are no meetings, both the makers and the managers experience optimum productivity. If meetings must be scheduled, then makers prefer that meetings happen back-to-back, while managers don't care where the meeting goes. Note that because all disruptions are treated as equally harmful to makers, there's no difference between a meeting that lasts 1 second and a meeting that lasts 3 hours if it segments the available free time. The problem is to decide how to schedule M different meetings involving arbitrary numbers of the N people, where each person in a given meeting must place a busy interval into their schedule such that it doesn't overlap with any other busy interval. For each meeting Mt the start time for the busy interval must be the same for all parties. Does an algorithm exist to solve this problem or one similar to it? My first thought was that this looks really similar to defragmentation (minimize number of distinct chunks), and there are a lot of algorithms about that. But defragmentation doesn't have much to do with scheduling. Thoughts? (*) Practically speaking this is not really a problem, because it's rare that we have meetings with more than ~5 people at once, so the space of possibilities is small.

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  • How do I get the Math equation of Python Algorithm?

    - by Gabriel
    ok so I am feeling a little stupid for not knowing this, but a coworker asked so I am asking here: I have written a python algorithm that solves his problem. given x 0 add all numbers together from 1 to x. def fac(x): if x > 0: return x + fac(x - 1) else: return 0 fac(10) 55 first what is this type of equation is this and what is the correct way to get this answer as it is clearly easier using some other method?

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  • How to modify preorder tree traversal algorithm to handle nodes with multiple parents?

    - by poldo
    I've been searching for a while now and can't seem to find an alternative solution. I need the tree traversal algorithm in such a way that a node can have more than 1 parent, if it's possible (found a great article here: Storing Hierarchical Data in a Database). Are there any algorithms so that, starting from a root node, we can determine the sequence and dependencies of nodes (currently reading topological sorting)?

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