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• Recursive N-way merge/diff algorithm for directory trees?

  - by BobMcGee

  What algorithms or Java libraries are available to do N-way, recursive diff/merge of directories? I need to be able to generate a list of folder trees that have many identical files, and have subdirectories with many similar files. I want to be able to use 2-way merge operations to quickly remove as much redundancy as possible. Goals: Find pairs of directories that have many similar files between them. Generate short list of directory pairs that can be synchronized with 2-way merge to eliminate duplicates Should operate recursively (there may be nested duplicates of higher-level directories) Run time and storage should be O(n log n) in numbers of directories and files Should be able to use an embedded DB or page to disk for processing more files than fit in memory (100,000+). Optional: generate an ancestry and change-set between folders Optional: sort the merge operations by how many duplicates they can elliminate I know how to use hashes to find duplicate files in roughly O(n) space, but I'm at a loss for how to go from this to finding partially overlapping sets between folders and their children. EDIT: some clarification The tricky part is the difference between "exact same" contents (otherwise hashing file hashes would work) and "similar" (which will not). Basically, I want to feed this algorithm at a set of directories and have it return a set of 2-way merge operations I can perform in order to reduce duplicates as much as possible with as few conflicts possible. It's effectively constructing an ancestry tree showing which folders are derived from each other. The end goal is to let me incorporate a bunch of different folders into one common tree. For example, I may have a folder holding programming projects, and then copy some of its contents to another computer to work on it. Then I might back up and intermediate version to flash drive. Except I may have 8 or 10 different versions, with slightly different organizational structures or folder names. I need to be able to merge them one step at a time, so I can chose how to incorporate changes at each step of the way. This is actually more or less what I intend to do with my utility (bring together a bunch of scattered backups from different points in time). I figure if I can do it right I may as well release it as a small open source util. I think the same tricks might be useful for comparing XML trees though.

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• What's the standard algorithm for syncing two lists of objects?

  - by Oliver Giesen

  I'm pretty sure this must be in some kind of text book (or more likely in all of them) but I seem to be using the wrong keywords to search for it... :( A common task I'm facing while programming is that I am dealing with lists of objects from different sources which I need to keep in sync somehow. Typically there's some sort of "master list" e.g. returned by some external API and then a list of objects I create myself each of which corresponds to an object in the master list. Sometimes the nature of the external API will not allow me to do a live sync: For instance the external list might not implement notifications about items being added or removed or it might notify me but not give me a reference to the actual item that was added or removed. Furthermore, refreshing the external list might return a completely new set of instances even though they still represent the same information so simply storing references to the external objects might also not always be feasible. Another characteristic of the problem is that both lists cannot be sorted in any meaningful way. You should also assume that initializing new objects in the "slave list" is expensive, i.e. simply clearing and rebuilding it from scratch is not an option. So how would I typically tackle this? What's the name of the algorithm I should google for? In the past I have implemented this in various ways (see below for an example) but it always felt like there should be a cleaner and more efficient way. Here's an example approach: Iterate over the master list Look up each item in the "slave list" Add items that do not yet exist Somehow keep track of items that already exist in both lists (e.g. by tagging them or keeping yet another list) When done iterate once more over the slave list Remove all objects that have not been tagged (see 4.) Update Thanks for all your responses so far! I will need some time to look at the links. Maybe one more thing worthy of note: In many of the situations where I needed this the implementation of the "master list" is completely hidden from me. In the most extreme cases the only access I might have to the master list might be a COM-interface that exposes nothing but GetFirst-, GetNext-style methods. I'm mentioning this because of the suggestions to either sort the list or to subclass it both of which is unfortunately not practical in these cases unless I copy the elements into a list of my own and I don't think that would be very efficient. I also might not have made it clear enough that the elements in the two lists are of different types, i.e. not assignment-compatible: Especially, the elements in the master list might be available as interface references only.

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• How can I implement the Gale-Shapley stable marriage algorithm in Perl?

  - by srk

  Problem : We have equal number of men and women.each men has a preference score toward each woman. So do the woman for each man. each of the men and women have certain interests. Based on the interest we calculate the preference scores. So initially we have an input in a file having x columns. First column is the person(men/woman) id. id are nothing but 0.. n numbers.(first half are men and next half woman) the remaining x-1 columns will have the interests. these are integers too. now using this n by x-1 matrix... we have come up with a n by n/2 matrix. the new matrix has all men and woman as their rows and scores for opposite sex in columns. We have to sort the scores in descending order, also we need to know the id of person related to the scores after sorting. So here i wanted to use hash table. once we get the scores we need to make up pairs.. for which we need to follow some rules. My trouble is with the second matrix of n by n/2 that needs to give information of which man/woman has how much preference on a woman/man. I need these scores sorted so that i know who is the first preferred woman/man, 2nd preferred and so on for a man/woman. I hope to get good suggestions on the data structures i use.. I prefer php or perl. Thank you in advance Hey guys this is not an home work. This a little modified version of stable marriage algorithm. I have working solution. I am only working on optimizing my code. more info: It is very similar to stable marriage problem but here we need to calculate the scores based on the interests they share. So i have implemented it as the way you see in the wiki page http://en.wikipedia.org/wiki/Stable_marriage_problem. my problem is not solving the problem. i solved it and can run it. I am just trying to have a better solution. so i am asking suggestions on the type of data structure to use. Conceptually I tried using an array of hashes. where the array index give the person id and the hash in it gives the id's <= score's in sorted manner. I initially start with an array of hashes. now i sort the hashes on values, but i could not store the sorted hashes back in an array.So just stored the keys after sorting and used these to get the values from my initial unsorted hashes. Can we store the hashes after sorting ? Can you suggest a better structure ?

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• More localized, efficient Lowest Common Ancestor algorithm given multiple binary trees?

  - by mstksg

  I have multiple binary trees stored as an array. In each slot is either nil (or null; pick your language) or a fixed tuple storing two numbers: the indices of the two "children". No node will have only one child -- it's either none or two. Think of each slot as a binary node that only stores pointers to its children, and no inherent value. Take this system of binary trees: 0 1 / \ / \ 2 3 4 5 / \ / \ 6 7 8 9 / \ 10 11 The associated array would be: 0 1 2 3 4 5 6 7 8 9 10 11 [ [2,3] , [4,5] , [6,7] , nil , nil , [8,9] , nil , [10,11] , nil , nil , nil , nil ] I've already written simple functions to find direct parents of nodes (simply by searching from the front until there is a node that contains the child) Furthermore, let us say that at relevant times, both all trees are anywhere between a few to a few thousand levels deep. I'd like to find a function P(m,n) to find the lowest common ancestor of m and n -- to put more formally, the LCA is defined as the "lowest", or deepest node in which have m and n as descendants (children, or children of children, etc.). If there is none, a nil would be a valid return. Some examples, given our given tree: P( 6,11) # => 2 P( 3,10) # => 0 P( 8, 6) # => nil P( 2,11) # => 2 The main method I've been able to find is one that uses an Euler trace, which turns the given tree, with a node A to be the invisible parent of 0 and 1 with a depth of -1, into: A-0-2-6-2-7-10-7-11-7-2-0-3-0-A-1-4-1-5-8-5-9-5-1-A And from that, simply find the node between your given m and n that has the lowest number; For example, to find P(6,11), look for a 6 and an 11 on the trace. The number between them that is the lowest is 2, and that's your answer. If A is in between them, return nil. -- Calculating P(6,11) -- A-0-2-6-2-7-10-7-11-7-2-0-3-0-A-1-4-1-5-8-5-9-5-1-A ^ ^ ^ | | | m lowest n Unfortunately, I do believe that finding the Euler trace of a tree that can be several thousands of levels deep is a bit machine-taxing...and because my tree is constantly being changed throughout the course of the programming, every time I wanted to find the LCA, I'd have to re-calculate the Euler trace and hold it in memory every time. Is there a more memory efficient way, given the framework I'm using? One that maybe iterates upwards? One way I could think of would be the "count" the generation/depth of both nodes, and climb the lowest node until it matched the depth of the highest, and increment both until they find someone similar. But that'd involve climbing up from level, say, 3025, back to 0, twice, to count the generation, and using a terribly inefficient climbing-up algorithm in the first place, and then re-climbing back up. Are there any other better ways?

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• How can I modify my Shunting-Yard Algorithm so it accepts unary operators?

  - by KingNestor

  I've been working on implementing the Shunting-Yard Algorithm in JavaScript for class. Here is my work so far: var userInput = prompt("Enter in a mathematical expression:"); var postFix = InfixToPostfix(userInput); var result = EvaluateExpression(postFix); document.write("Infix: " + userInput + "<br/>"); document.write("Postfix (RPN): " + postFix + "<br/>"); document.write("Result: " + result + "<br/>"); function EvaluateExpression(expression) { var tokens = expression.split(/([0-9]+|[*+-\/()])/); var evalStack = []; while (tokens.length != 0) { var currentToken = tokens.shift(); if (isNumber(currentToken)) { evalStack.push(currentToken); } else if (isOperator(currentToken)) { var operand1 = evalStack.pop(); var operand2 = evalStack.pop(); var result = PerformOperation(parseInt(operand1), parseInt(operand2), currentToken); evalStack.push(result); } } return evalStack.pop(); } function PerformOperation(operand1, operand2, operator) { switch(operator) { case '+': return operand1 + operand2; case '-': return operand1 - operand2; case '*': return operand1 * operand2; case '/': return operand1 / operand2; default: return; } } function InfixToPostfix(expression) { var tokens = expression.split(/([0-9]+|[*+-\/()])/); var outputQueue = []; var operatorStack = []; while (tokens.length != 0) { var currentToken = tokens.shift(); if (isNumber(currentToken)) { outputQueue.push(currentToken); } else if (isOperator(currentToken)) { while ((getAssociativity(currentToken) == 'left' && getPrecedence(currentToken) <= getPrecedence(operatorStack[operatorStack.length-1])) || (getAssociativity(currentToken) == 'right' && getPrecedence(currentToken) < getPrecedence(operatorStack[operatorStack.length-1]))) { outputQueue.push(operatorStack.pop()) } operatorStack.push(currentToken); } else if (currentToken == '(') { operatorStack.push(currentToken); } else if (currentToken == ')') { while (operatorStack[operatorStack.length-1] != '(') { if (operatorStack.length == 0) throw("Parenthesis balancing error! Shame on you!"); outputQueue.push(operatorStack.pop()); } operatorStack.pop(); } } while (operatorStack.length != 0) { if (!operatorStack[operatorStack.length-1].match(/([()])/)) outputQueue.push(operatorStack.pop()); else throw("Parenthesis balancing error! Shame on you!"); } return outputQueue.join(" "); } function isOperator(token) { if (!token.match(/([*+-\/])/)) return false; else return true; } function isNumber(token) { if (!token.match(/([0-9]+)/)) return false; else return true; } function getPrecedence(token) { switch (token) { case '^': return 9; case '*': case '/': case '%': return 8; case '+': case '-': return 6; default: return -1; } } function getAssociativity(token) { switch(token) { case '+': case '-': case '*': case '/': return 'left'; case '^': return 'right'; } } It works fine so far. If I give it: ((5+3) * 8) It will output: Infix: ((5+3) * 8) Postfix (RPN): 5 3 + 8 * Result: 64 However, I'm struggling with implementing the unary operators so I could do something like: ((-5+3) * 8) What would be the best way to implement unary operators (negation, etc)? Also, does anyone have any suggestions for handling floating point numbers as well? One last thing, if anyone sees me doing anything weird in JavaScript let me know. This is my first JavaScript program and I'm not used to it yet.

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• A star algorithm implementation problems

  - by bryan226

  I’m having some trouble implementing the A* algorithm in a 2D tile based game. The problem is basically that the algorithm gets stuck when something gets in its direct way (e.g. walls) Note that it only allows Horizontal and Vertical movement. Here's a picture as it works fine across the map without something in its direct way: (Green tile = destination, Blue = In closed list, Green = in open list) This is what happens if I try to walk 'around' a wall: I calculate costs with the F = G + H formula: G = 1 Cost per Step H = 10 Cost per Step //Count how many tiles are between current-tile & destination-tile The functions: short c_astar::GuessH(short Startx,short Starty,short Destinationx,short Destinationy) { hgeVector Start, Destination; Start.x = Startx; Start.y = Starty; Destination.x = Destinationx; Destination.y = Destinationy; short a = 0; short b = 0; if(Start.x > Destination.x) a = Start.x - Destination.x; else a = Destination.x - Start.x; if(Start.y > Destination.y) b = Start.y - Destination.y; else b = Destination.y - Start.y; return (a+b)*10; } short c_astar::GuessG(short Startx,short Starty,short Currentx,short Currenty) { hgeVector Start, Destination; Start.x = Startx; Start.y = Starty; Destination.x = Currentx; Destination.y = Currenty; short a = 0; short b = 0; if(Start.x > Destination.x) a = Start.x - Destination.x; else a = Destination.x - Start.x; if(Start.y > Destination.y) b = Start.y - Destination.y; else b = Destination.y - Start.y; return (a+b); } At the end of the loop I check which tile is the cheapest to go according to its F value: Then some quick checks are done for each tile (UP,DOWN,LEFT,RIGHT): //...CX are holding the F value of the TILE specified // Info: C0 = Center (Current) // C1 = UP // C2 = DOWN // C3 = LEFT // C4 = RIGHT //Quick checks if(((C1 < C2) && (C1 < C3) && (C1 < C4))) { Current.y -= 1; bSimilar = false; if(DEBUG) hge->System_Log("C1 < ALL"); } //.. same for C2,C3 & C4 If there are multiple tiles with the same F value: It’s actually a switch for DOWNLEFT,UPRIGHT.. etc. Here’s one of it: case UPRIGHT: { //UP Temporary = Current; Temporary.y -= 1; bTileStatus[0] = IsTileWalkable(Temporary.x,Temporary.y); if(bTileStatus[0]) { //Proceed normal we are OK & walkable Tilex.Tile = map.at(Temporary.y).at(Temporary.x); //Search in lists if(SearchInClosedList(Tilex.Tile.ID,C0)) bFoundInClosedList[0] = true; if(SearchInOpenList(Tilex.Tile.ID,C0)) bFoundInOpenList[0] = true; //RIGHT Temporary = Current; Temporary.x += 1; bTileStatus[1] = IsTileWalkable(Temporary.x,Temporary.y); if(bTileStatus[1]) { //Proceed normal we are OK & walkable Tilex.Tile = map.at(Temporary.y).at(Temporary.x); //Search in lists if(SearchInClosedList(Tilex.Tile.ID,C0)) bFoundInClosedList[1] = true; if(SearchInOpenList(Tilex.Tile.ID,C0)) bFoundInOpenList[1] = true; //************************************************* // Purpose: ClosedList behavior //************************************************* if(bFoundInClosedList[0] && !bFoundInClosedList[1]) { //UP found in ClosedList. Go RIGHT return RIGHT; } if(!bFoundInClosedList[0] && bFoundInClosedList[1]) { //RIGHT found in ClosedList. Go UP return UP; } if(bFoundInClosedList[0] && bFoundInClosedList[1]) { //Both found in ClosedList. Random value switch(hge->Random_Int(8,9)) { case 8: return UP; break; case 9: return RIGHT; break; } } //************************************************* // Purpose: OpenList behavior //************************************************* if(bFoundInOpenList[0] && !bFoundInOpenList[1]) { //UP found in OpenList. Go RIGHT return RIGHT; } if(!bFoundInOpenList[0] && bFoundInOpenList[1]) { //RIGHT found in OpenList. Go UP return UP; } if(bFoundInOpenList[0] && bFoundInOpenList[1]) { //Both found in OpenList. Random value switch(hge->Random_Int(8,9)) { case 8: return UP; break; case 9: return RIGHT; break; } } } else if(!bTileStatus[1]) { //RIGHT is not walkable OR out of range //Choose UP return UP; } } else if(!bTileStatus[0]) { //UP is not walkable OR out of range //Fast check RIGHT Temporary = Current; Temporary.x += 1; bTileStatus[1] = IsTileWalkable(Temporary.x,Temporary.y); if(bTileStatus[1]) { return RIGHT; } else return FAILED; //Failed, no valid path found! } } break; A log for the second picture: (Cut down to ten passes, because it’s just repeating itself) ----------------------------------------------------- PASS: 1 | C1: 211 | C2: 191 | C3: 211 | C4: 191 DOWN + RIGHT SIMILAR Going DOWN ----------------------------------------------------- PASS: 2 | C1: 200 | C2: 182 | C3: 202 | C4: 182 DOWN + RIGHT SIMILAR Going DOWN ----------------------------------------------------- PASS: 3 | C1: 191 | C2: 193 | C3: 193 | C4: 173 C4 < ALL Tile(12.000000,6.000000) not walkable. MAX_F_VALUE set. ----------------------------------------------------- PASS: 4 | C1: 182 | C2: 184 | C3: 182 | C4: 999 UP + LEFT SIMILAR Going UP Tile(12.000000,5.000000) not walkable. MAX_F_VALUE set. ----------------------------------------------------- PASS: 5 | C1: 191 | C2: 173 | C3: 191 | C4: 999 C2 < ALL Tile(12.000000,6.000000) not walkable. MAX_F_VALUE set. ----------------------------------------------------- PASS: 6 | C1: 182 | C2: 184 | C3: 182 | C4: 999 UP + LEFT SIMILAR Going UP Tile(12.000000,5.000000) not walkable. MAX_F_VALUE set. ----------------------------------------------------- PASS: 7 | C1: 191 | C2: 173 | C3: 191 | C4: 999 C2 < ALL Tile(12.000000,6.000000) not walkable. MAX_F_VALUE set. ----------------------------------------------------- PASS: 8 | C1: 182 | C2: 184 | C3: 182 | C4: 999 UP + LEFT SIMILAR Going LEFT ----------------------------------------------------- PASS: 9 | C1: 191 | C2: 193 | C3: 193 | C4: 173 C4 < ALL Tile(12.000000,6.000000) not walkable. MAX_F_VALUE set. ----------------------------------------------------- PASS: 10 | C1: 182 | C2: 184 | C3: 182 | C4: 999 UP + LEFT SIMILAR Going LEFT ----------------------------------------------------- Its always going after the cheapest F value, which seems to be wrong. If someone could point me to the right direction I'd be thankful. Regards, bryan226

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• What are some C++ Standard Library usage best practices?

  - by apphacker

  I'm learning C++ and the book I'm reading (The C++ Programming Language) says to not reinvent the wheel, to rely on the standard libraries. In C, I often end up creating a linked list, and link list iteration over and over again (maybe I'm doing that wrong not sure), so the ideas of containers available in C++, and strings, and algorithms really appeal to me. However I have read a little online, and heard some criticisms from my friends and coworkers about STL, so I thought I maybe I'd pick some brains here. What are some best practices for using STL, and what lessons have you learned about STL?

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• How lucene indexing ?

  - by user312140

  Hello I read some document about lucene ; also i read the document in this link ( http://lucene.sourceforge.net/talks/pisa ) . I don't really understand how lucene index documents and don't understand lucene work with which algorithm for indexing ? On above link , said lucene use this algorithm for indexing : * incremental algorithm: o maintain a stack of segment indices o create index for each incoming document o push new indexes onto the stack o let b=10 be the merge factor; M=8 for (size = 1; size < M; size *= b) { if (there are b indexes with size docs on top of the stack) { pop them off the stack; merge them into a single index; push the merged index onto the stack; } else { break; } } How this algorithm help us to have an optimize indexing ? Does lucene use B-tree algorithm or any other algorithm like that for indexing or have a paticular algorithm ? Thank you for reading my post .

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• Composite pattern in C++ problem

  - by annouk

  Hello! I have to work with an application in C++ similar to a phone book: the class Agenda with an STL list of Contacts.Regarding the contacts hierarchy,there is a base-class named Contact(an abstract one),and the derived classes Friend and Acquaintance(the types of contact). These classes have,for instance, a virtual method called getName,which returns the name of the contact. Now I must implement the Composite pattern by adding another type of contact,Company(being derived from Contact),which also contains a collection of Contacts(an STL list as well),that can be either of the "leaf" type(Friends or Acquaintances),or they can be Companies as well. Therefore,Company is the Compound type. The question is: how and where can I implement an STL find_if to search the contact with a given name(via getName function or suggest me smth else) both among the "leaf"-type Contact and inside the Company collection? In other words,how do I traverse the tree in order to find possible matches there too,using an uniform function definition? I hope I was pretty clear...

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• Sublinear Extra Space MergeSort

  - by hulkmeister

  I am reviewing basic algorithms from a book called Algorithms by Robert Sedgewick, and I came across a problem in MergeSort that I am, sad to say, having difficulty solving. The problem is below: Sublinear Extra Space. Develop a merge implementation that reduces that extra space requirement to max(M, N/M), based on the following idea: Divide the array into N/M blocks of size M (for simplicity in this description, assume that N is a multiple of M). Then, (i) considering the blocks as items with their first key as the sort key, sort them using selection sort; and (ii) run through the array merging the first block with the second, then the second block with the third, and so forth. The problem I have with the problem is that based on the idea Sedgewick recommends, the following set of arrays will not be sorted: {0, 10, 12}, {3, 9, 11}, {5, 8, 13}. The algorithm I use is the following: Divide the full array into subarrays of size M. Run Selection Sort on each of the subarrays. Merge each of the subarrays using the method Sedgwick recommends in (ii). (This is where I encounter the problem of where to store the results after the merge.) This leads to wanting to increase the size of the auxiliary space needed to handle at least two subarrays at a time (for merging), but based on the specifications of the problem, that is not allowed. I have also considered using the original array as space for one subarray and using the auxiliary space for the second subarray. However, I can't envision a solution that does not end up overwriting the entries of the first subarray. Any ideas on other ways this can be done? NOTE: If this is suppose to be on StackOverflow.com, please let me know how I can move it. I posted here because the question was academic.

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• Procedural... house with rooms generator

  - by pek

  I've been looking at some algorithms and articles about procedurally generating a dungeon. The problem is, I'm trying to generate a house with rooms, and they don't seem to fit my requirements. For one, dungeons have corridors, where houses have halls. And while initially they might seem the same, a hall is nothing more than the area that isn't a room, whereas a corridor is specifically designed to connect one area to another. Another important difference with a house is that you have a specific width and height, and you have to fill the entire thing with rooms and halls, whereas with a dungeon, there is empty space. I think halls in a house is something in between a dungeon corridor (gets you to other rooms) and an empty space in the dungeon (it's not explicitly defined in code). More specifically, the requirements are: There is a set of predefined rooms I cannot create walls and doors on the fly. Rooms can be rotated but not resized Again, because I have a predefined set of rooms, I can only rotate them, not resize them. The house dimensions are set and has to be entirely filled with rooms (or halls) I.e. I want to fill a 14x20 house with the available rooms making sure there is no empty space. Here are some images to make this a little more clear: As you can see, in the house, the "empty space" is still walkable and it gets you from one room to another. So, having said all this, maybe a house is just a really really tightly packed dungeon with corridors. Or it's something easier than a dungeon. Maybe there is something out there and I haven't found it because I don't really know what to search for. This is where I'd like your help: could you give me pointers on how to design this algorithm? Any thoughts on what steps it will take? If you have created a dungeon generator, how would you modify it to fit my requirements? You can be as specific or as generic as you like. I'm looking to pick your brains, really.

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• Diamond-square terrain generation problem

  - by kafka

  I've implemented a diamond-square algorithm according to this article: http://www.lighthouse3d.com/opengl/terrain/index.php?mpd2 The problem is that I get these steep cliffs all over the map. It happens on the edges, when the terrain is recursively subdivided: Here is the source: void DiamondSquare(unsigned x1,unsigned y1,unsigned x2,unsigned y2,float range) { int c1 = (int)x2 - (int)x1; int c2 = (int)y2 - (int)y1; unsigned hx = (x2 - x1)/2; unsigned hy = (y2 - y1)/2; if((c1 <= 1) || (c2 <= 1)) return; // Diamond stage float a = m_heightmap[x1][y1]; float b = m_heightmap[x2][y1]; float c = m_heightmap[x1][y2]; float d = m_heightmap[x2][y2]; float e = (a+b+c+d) / 4 + GetRnd() * range; m_heightmap[x1 + hx][y1 + hy] = e; // Square stage float f = (a + c + e + e) / 4 + GetRnd() * range; m_heightmap[x1][y1+hy] = f; float g = (a + b + e + e) / 4 + GetRnd() * range; m_heightmap[x1+hx][y1] = g; float h = (b + d + e + e) / 4 + GetRnd() * range; m_heightmap[x2][y1+hy] = h; float i = (c + d + e + e) / 4 + GetRnd() * range; m_heightmap[x1+hx][y2] = i; DiamondSquare(x1, y1, x1+hx, y1+hy, range / 2.0); // Upper left DiamondSquare(x1+hx, y1, x2, y1+hy, range / 2.0); // Upper right DiamondSquare(x1, y1+hy, x1+hx, y2, range / 2.0); // Lower left DiamondSquare(x1+hx, y1+hy, x2, y2, range / 2.0); // Lower right } Parameters: (x1,y1),(x2,y2) - coordinates that define a region on a heightmap (default (0,0)(128,128)). range - basically max. height. (default 32) Help would be greatly appreciated.

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• Fastest way to group units that can see each other?

  - by mac

  In the 2D game I'm working with, the game engine is able to give me, for each unit, the list of other units that are in its view range. I would like to know if there is an established algorithm to sort the units in groups, where each group would be defined by all those units which are "connected" to each other (even through others). An example might help understand the question better (E=enemy, O=own unit). First the data that I would get from the game engine: E1 can see E2, E3, O5 E2 can see E1 E3 can see E1 E4 can see O5 E5 can see O2 E6 can see E7, O9, O1 E7 can see E6 O1 can see E6 O2 can see O5, E5 O5 can see E1, E4, O2 O9 can see E6 Then I should compute the groups as follow: G1 = E1, E2, E3, E4, E5, O2, O5 G2 = O1, O9, E6, E7 It can be safely assumed that there is a transitive property for the field of view: [if A sees B, then B sees A]. Just to clarify: I already wrote a naïve implementation that loops on each row of the game engine info, but from the look of it, it seems a problem general enough for it to have been studied in depth and have various established algorithms (maybe passing through some tree-like structure?). My problem is that I couldn't find a way to describe my problem that returned useful google hits. Thank you in advance for your help!

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• Algorithm to figure out appointment times?

  - by Rachel

  I have a weird situation where a client would like a script that automatically sets up thousands of appointments over several days. The tricky part is the appointments are for a variety of US time zones, and I need to take the consumer's local time zone into account when generating appointment dates and times for each record. Appointment Rules: Appointments should be set from 8AM to 8PM Eastern Standard Time, with breaks from 12P-2P and 4P-6P. This leaves a total of 8 hours per day available for setting appointments. Appointments should be scheduled 5 minutes apart. 8 hours of 5-minute intervals means 96 appointments per day. There will be 5 users at a time handling appointments. 96 appointments per day multiplied by 5 users equals 480, so the maximum number of appointments that can be set per day is 480. Now the tricky requirement: Appointments are restricted to 8am to 8pm in the consumer's local time zone. This means that the earliest time allowed for each appointment is different depending on the consumer's time zone: Eastern: 8A Central: 9A Mountain: 10A Pacific: 11A Alaska: 12P Hawaii or Undefined: 2P Arizona: 10A or 11A based on current Daylight Savings Time Assuming a data set can be several thousand records, and each record will contain a timezone value, is there an algorithm I could use to determine a Date and Time for every record that matches the rules above?

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• Partial recalculation of visibility on a 2D uniform grid

  - by Martin Källman

  Problem Imagine that we have a 2D uniform grid of dimensions N x N. For this grid we have also pre-computed a visibility look-up table, e.g. with DDA, which answers the boolean query is cell X visible from cell Y? The look-up table is a complete graph KN of the cells V in the grid, with each edge E being a binary value denoting the visibility between its vertices. Question If any given cell has its visibility modified, is it possible to extract the subset Edelta of edges which must have their visibility recomputed due to the change, so as to avoid a full-on recomputation for the entire grid? (Which is N(N-1) / 2 or N2 depending on the implementation) Update If is not possible to solve thi in closed form, then maintaining a separate mapping of each cell and every cell pair who's line intersects said cell might also be an option. This obviously consumes more memory, but the data is static. The increased memory requirement could be reduced by introducing a hierarchy, subdividing the grid into smaller parts, and by doing so the above mapping can be reused for each sub-grid. This would come at a cost in terms of increased computation relative to the number of subdivisions; also requiring a resumable ray-casting algorithm.

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• Algorithm allowing a good waypoint path following?

  - by Thierry Savard Saucier

  I'm more looking into how should I implement this, either a tutorial or even the name of the concept I'm missing. I'm pretty sure some basic pathfinding algorithm could help me here, but I dont know which one ... I have a worldmap, with different cities on it. The player can choose a city from a menu, or click on an available cities on the world map, and the toon should walk over there. But I want him to follow a predefine path. Lets say our hero is on the city 1. He clicks on city 4. I want him to follow the path to city 2 and from there to city 4. I was handling this easily with arrow movement (left right top bottom) since its a single check. Now I'm not sure how I should do this. Should I loop threw each possible path and check which one leads me to D the fastest ... and if I do how do I avoid running in circle forever with cities 1-5-2 ?

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• Negamax implementation doesn't appear to work with tic-tac-toe

  - by George Jiglau

  I've implemented Negamax as it can be found on wikipedia, which includes alpha/beta pruning. However, it seems to favor a losing move, which should be an invalid result. The game is Tic-Tac-Toe, I've abstracted most of the game play so it should be rather easy to spot an error within the algorithm. Here is the code, nextMove, negamax or evaluate are probably the functions that contain the fault: #include <list> #include <climits> #include <iostream> //#define DEBUG 1 using namespace std; struct Move { int row, col; Move(int row, int col) : row(row), col(col) { } Move(const Move& m) { row = m.row; col = m.col; } }; struct Board { char player; char opponent; char board[3][3]; Board() { } void read(istream& stream) { stream >> player; opponent = player == 'X' ? 'O' : 'X'; for(int row = 0; row < 3; row++) { for(int col = 0; col < 3; col++) { char playa; stream >> playa; board[row][col] = playa == '_' ? 0 : playa == player ? 1 : -1; } } } void print(ostream& stream) { for(int row = 0; row < 3; row++) { for(int col = 0; col < 3; col++) { switch(board[row][col]) { case -1: stream << opponent; break; case 0: stream << '_'; break; case 1: stream << player; break; } } stream << endl; } } void do_move(const Move& move, int player) { board[move.row][move.col] = player; } void undo_move(const Move& move) { board[move.row][move.col] = 0; } bool isWon() { if (board[0][0] != 0) { if (board[0][0] == board[0][1] && board[0][1] == board[0][2]) return true; if (board[0][0] == board[1][0] && board[1][0] == board[2][0]) return true; } if (board[2][2] != 0) { if (board[2][0] == board[2][1] && board[2][1] == board[2][2]) return true; if (board[0][2] == board[1][2] && board[1][2] == board[2][2]) return true; } if (board[1][1] != 0) { if (board[0][1] == board[1][1] && board[1][1] == board[2][1]) return true; if (board[1][0] == board[1][1] && board[1][1] == board[1][2]) return true; if (board[0][0] == board[1][1] && board[1][1] == board[2][2]) return true; if (board[0][2] == board [1][1] && board[1][1] == board[2][0]) return true; } return false; } list<Move> getMoves() { list<Move> moveList; for(int row = 0; row < 3; row++) for(int col = 0; col < 3; col++) if (board[row][col] == 0) moveList.push_back(Move(row, col)); return moveList; } }; ostream& operator<< (ostream& stream, Board& board) { board.print(stream); return stream; } istream& operator>> (istream& stream, Board& board) { board.read(stream); return stream; } int evaluate(Board& board) { int score = board.isWon() ? 100 : 0; for(int row = 0; row < 3; row++) for(int col = 0; col < 3; col++) if (board.board[row][col] == 0) score += 1; return score; } int negamax(Board& board, int depth, int player, int alpha, int beta) { if (board.isWon() || depth <= 0) { #if DEBUG > 1 cout << "Found winner board at depth " << depth << endl; cout << board << endl; #endif return player * evaluate(board); } list<Move> allMoves = board.getMoves(); if (allMoves.size() == 0) return player * evaluate(board); for(list<Move>::iterator it = allMoves.begin(); it != allMoves.end(); it++) { board.do_move(*it, -player); int val = -negamax(board, depth - 1, -player, -beta, -alpha); board.undo_move(*it); if (val >= beta) return val; if (val > alpha) alpha = val; } return alpha; } void nextMove(Board& board) { list<Move> allMoves = board.getMoves(); Move* bestMove = NULL; int bestScore = INT_MIN; for(list<Move>::iterator it = allMoves.begin(); it != allMoves.end(); it++) { board.do_move(*it, 1); int score = -negamax(board, 100, 1, INT_MIN + 1, INT_MAX); board.undo_move(*it); #if DEBUG cout << it->row << ' ' << it->col << " = " << score << endl; #endif if (score > bestScore) { bestMove = &*it; bestScore = score; } } if (!bestMove) return; cout << bestMove->row << ' ' << bestMove->col << endl; #if DEBUG board.do_move(*bestMove, 1); cout << board; #endif } int main() { Board board; cin >> board; #if DEBUG cout << "Starting board:" << endl; cout << board; #endif nextMove(board); return 0; } Giving this input: O X__ ___ ___ The algorithm chooses to place a piece at 0, 1, causing a guaranteed loss, do to this trap(nothing can be done to win or end in a draw): XO_ X__ ___ Perhaps it has something to do with the evaluation function? If so, how could I fix it?

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• Floodfill algorithm for GO

  - by user1048606

  The floodfill algorithm is used in the bucket tool in MS paint and photoshop, but it can also be used for GO and minesweeper. http://en.wikipedia.org/wiki/Flood_fill In go you can capture groups of stones, this website portrays it with two stones. http://www.connectedglobe.com/mindy/cap6.html This is my floodfill method in Java, it is not capturing a group of stones and I have no idea why because to me it makes sense. public void floodfill(int turn, int col, int row){ for(int a = col; a<19; a++){ for(int b = row; b<19; b++){ if(turn == black){ if(stones[col][row] == white){ stones[col][row] = 0; floodfill(black, col-1, row); floodfill(black, col+1, row); floodfill(black, col, row-1); floodfill(black, col, row+1); } } } } } It searches up, down, left, right for all the stones on the board. If the stones are white it captures them by making them 0, which represents empty.

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• Regulating how much to draw based on how much was drawn last frame.

  - by Mike Howard

  I have a 3D game world on an iPhone (limited graphics speed), and I'm already regulating whether I draw each shape on the screen based on it's size and distance from the camera. Something like... if (how_big_it_looks_from_the_camera > constant) then draw What I want to do now is also take into account how many shapes are being drawn, so that in busier areas of the game world I can draw less than I otherwise would. I tried to do this by dividing how_big_it_looks by the number of shapes that were drawn last frame (well, the square root of this but I'm simplifying - the problem is the same). if (how_big_it_looks / shapes_drawn > constant2) then draw But the check happens at the level of objects which represent many drawn shapes, and if an object containing many shapes is switched on, it increases shapes_drawn lots and switches itself back off the next frame. It flickers on and off. I tried keeping a kind of weighted average of previous values, by each frame doing something like shapes_drawn_recently = 0.9 * shapes_drawn_recently + 0.1 * shapes_just_drawn, but of course it only slows the flickering down because of the nature of the feedback loop. Is there a good way of solving this? My project is in Objective-C, but a general algorithm or pseudo-code is good too. Thanks.

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• Algorithm to reduce calls to mapping API

  - by aidan

  A random distribution of points lies on a map. This data lies behind an API, and I want to grab the complete set of points within a given bounding box. I can query the API with the bounding box and the API will return the set of points that fall within that box. The problem is that the API will limit the result set to 10 items, with no pagination and no indication if there are more points that have been omitted. So I made a recursive algorithm that takes a bounding box and requests the points that lie within it. If the result set is exactly 10 items, then I split the bounding box into four quadrants and recurse. It works fine but my question is this: if want to minimize the number of API calls, what is the optimal way to split the bounding box? Splitting it into quadrants was just an arbitrary decision. When there are a lot of points on the map, I have to drill down many levels before I start getting meaningful results. So I imagine it might be faster to split the box into, say, 9, 16, or more sections. But if I do that, then I eventually get to a point where a lot of requests are returning 0 results which isn't so efficient. Also, does the size of the limit on the results set affect the answer? (This is all assuming that I have no prior knowledge of nominal point density in the bounding box)

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• XNA: How to make the Vaus Spacecraft move left and right on directional keys pressed?

  - by Will Marcouiller

  I'm currently learning XNA per suggestion from this question's accepted answer: Where to start writing games, any tutorials or the like? I have then installed everything to get ready to work with XNA Game Studio 4.0. General Objective Writing an Arkanoid-like game. I want to make my ship move when I press either left or right keys. Code Sample protected override void Update(GameTime gameTime) { // Allows the game to exit if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed) this.Exit(); // TODO: Add your update logic here #if WINDOWS if (Keyboard.GetState().IsKeyDown(Keys.Escape)) this.Exit(); else { if (Keyboard.GetState().IsKeyDown(Keys.Left)) MoveLeft(gameTime); } #endif // Move the sprite around. BounceEnergyBall(gameTime); base.Update(gameTime); } void MoveLeft(GameTime gameTime) { // I'm not sure how to play with the Vector2 object and its position here!... _vausSpacecraftPos /= _vausSpacecraftSpeed.X; // This line makes the spacecraft move diagnol-top-left. } Question What formula shall I use, or what algorithm shall I consider to make my spaceship move as expected left and right properly? Thanks for your thoughts! Any clue will be appreciated. I am on my learning curve though I have years of development behind me (already)!

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• Optimized algorithm for line-sphere intersection in GLSL

  - by fernacolo

  Well, hello then! I need to find intersection between line and sphere in GLSL. Right now my solution is based on Paul Bourke's page and was ported to GLSL this way: // The line passes through p1 and p2: vec3 p1 = (...); vec3 p2 = (...); // Sphere center is p3, radius is r: vec3 p3 = (...); float r = ...; float x1 = p1.x; float y1 = p1.y; float z1 = p1.z; float x2 = p2.x; float y2 = p2.y; float z2 = p2.z; float x3 = p3.x; float y3 = p3.y; float z3 = p3.z; float dx = x2 - x1; float dy = y2 - y1; float dz = z2 - z1; float a = dx*dx + dy*dy + dz*dz; float b = 2.0 * (dx * (x1 - x3) + dy * (y1 - y3) + dz * (z1 - z3)); float c = x3*x3 + y3*y3 + z3*z3 + x1*x1 + y1*y1 + z1*z1 - 2.0 * (x3*x1 + y3*y1 + z3*z1) - r*r; float test = b*b - 4.0*a*c; if (test >= 0.0) { // Hit (according to Treebeard, "a fine hit"). float u = (-b - sqrt(test)) / (2.0 * a); vec3 hitp = p1 + u * (p2 - p1); // Now use hitp. } It works perfectly! But it seems slow... I'm new at GLSL. You can answer this questions in two ways: Tell me there is no solution, showing some proof or strong evidence. Tell me about GLSL features (vector APIs, primitive operations) that makes the above algorithm faster, showing some example. Thanks a lot!

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• questions on a particular algorithm

  - by paul smith

  Upon searching for a fast primr algorithm, I stumbled upon this: public static boolean isP(long n) { if (n==2 || n==3) return true; if ((n&0x1)==0 || n%3==0 || n<2) return false; long root=(long)Math.sqrt(n)+1L; // we check just numbers of the form 6*k+1 and 6*k-1 for (long k=6;k<=root;k+=6) { if (n%(k-1)==0) return false; if (n%(k+1)==0) return false; } return true; } My questions are: Why is long being used everywhere instead of int? Because with a long type the argument could be much larger than Integer.MAX thus making the method more flexible? In the second 'if', is n&0x1 the same as n%2? If so why didn't the author just use n%2? To me it's more readable. The line that sets the 'root' variable, why add the 1L? What is the run-time complexity? Is it O(sqrt(n/6)) or O(sqrt(n)/6)? Or would we just say O(n)?

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• Algorithm to shoot at a target in a 3d game

  - by Sebastian Bugiu

  For those of you remembering Descent Freespace it had a nice feature to help you aim at the enemy when shooting non-homing missiles or lasers: it showed a crosshair in front of the ship you chased telling you where to shoot in order to hit the moving target. I tried using the answer from http://stackoverflow.com/questions/4107403/ai-algorithm-to-shoot-at-a-target-in-a-2d-game?lq=1 but it's for 2D so I tried adapting it. I first decomposed the calculation to solve the intersection point for XoZ plane and saved the x and z coordinates and then solving the intersection point for XoY plane and adding the y coordinate to a final xyz that I then transformed to clipspace and put a texture at those coordinates. But of course it doesn't work as it should or else I wouldn't have posted the question. From what I notice the after finding x in XoZ plane and the in XoY the x is not the same so something must be wrong. float a = ENG_Math.sqr(targetVelocity.x) + ENG_Math.sqr(targetVelocity.y) - ENG_Math.sqr(projectileSpeed); float b = 2.0f * (targetVelocity.x * targetPos.x + targetVelocity.y * targetPos.y); float c = ENG_Math.sqr(targetPos.x) + ENG_Math.sqr(targetPos.y); ENG_Math.solveQuadraticEquation(a, b, c, collisionTime); First time targetVelocity.y is actually targetVelocity.z (the same for targetPos) and the second time it's actually targetVelocity.y. The final position after XoZ is crossPosition.set(minTime * finalEntityVelocity.x + finalTargetPos4D.x, 0.0f, minTime * finalEntityVelocity.z + finalTargetPos4D.z); and after XoY crossPosition.y = minTime * finalEntityVelocity.y + finalTargetPos4D.y; Is my approach of separating into 2 planes and calculating any good? Or for 3D there is a whole different approach? sqr() is square not sqrt - avoiding a confusion.

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• algorithm for Virtual Machine(VM) Consolidation in Cloud

  - by devansh dalal

  PROBLEM: We have N physical machines(PMs) each with ram Ri, cpu Ci and a set of currently scheduled VMs each with ram requirement ri and ci respectively Moving(Migrating) any VM from one PM to other has a cost associated which depends on its ram ri. A PM with no VMs is shut down to save power. Our target is to minimize the weighted sum of (N,migration cost) by migrating some VMs i.e. minimize the number of working PMs as well as not to degrade the service level due to excessive migrations. My Approach: Brute Force approach is choosing the minimum loaded PM and try to fit its VMs to other PMs by First Fit Decreasing algorithm or we can select the victim PMs and target PMs based on their loading level and shut down victims if possible by moving their VMs to targets. I tried this Greedy approach on the Data of Baadal(IIT-D cloud) but It isn't giving promising results. I have also tried to study the Ant colony optimization for dynamic VM consolidating but was unable to understand very much. I used the links. http://dumas.ccsd.cnrs.fr/docs/00/72/52/15/PDF/Esnault.pdf http://hal.archives-ouvertes.fr/docs/00/72/38/56/PDF/RR-8032.pdf Would anyone please explain the solution or suggest any new approach for better performance soon. Thanks in advance.

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