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• Pohlig–Hellman algorithm for computing discrete logarithms

  - by drelihan

  Hi Folks, I'm working on coding the Pohlig-Hellman Algorithm but I am having problem understand the steps in the algorithm based on the definition of the algorithm. Going by the Wiki of the algorithm: http://en.wikipedia.org/wiki/Pohlig%E2%80%93Hellman_algorithm I know the first part 1) is to calculate the prime factor of p-1 - which is fine. Howeever, I am not sure what I need to do in steps 2) and 3). Can someone help with explaining this in plain english (i) - or pseudocode. I want to code the solution myself obviously but I cannot make any more progress unless i understand the algorithm. Note: I have done a lot of searching for this and I read S. Pohlig and M. Hellman (1978). "An Improved Algorithm for Computing Logarithms over GF(p) and its Cryptographic Significance but its still not really making sense to me. Thanks in advance

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• Is there an algorithm to securely split a message into x parts requiring at least y parts to reassem

  - by Aaron

  Is there an algorithm to securely split a message into x parts requiring at least y parts to reassemble? Obviously, y <= x. An example: Say that I have a secret message that I only want to be read in the event of my death. As a way to ensure this, I give a fraction of the message to ten friends. Now, I can't guaranty that all my friends will be able to put their messages together to recover the original. So, I construct each message fraction in such a way so as to only require any 5 friends to put their parts together to reconstruct the whole. However, owning less than 5 parts will not give anything away about the message, except possibly the length. My question is, is this possible? What algorithms might I look at to accomplish this? Clarification edit: The important part of this is the cryptographic strength. An attacker should not be able to recover the message, either in whole or in part with less than y parts.

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• Proving that the distance values extracted in Dijkstra's algorithm is non-decreasing?

  - by Gail

  I'm reviewing my old algorithms notes and have come across this proof. It was from an assignment I had and I got it correct, but I feel that the proof certainly lacks. The question is to prove that the distance values taken from the priority queue in Dijkstra's algorithm is a non-decreasing sequence. My proof goes as follows: Proof by contradiction. Fist, assume that we pull a vertex from Q with d-value 'i'. Next time, we pull a vertex with d-value 'j'. When we pulled i, we have finalised our d-value and computed the shortest-path from the start vertex, s, to i. Since we have positive edge weights, it is impossible for our d-values to shrink as we add vertices to our path. If after pulling i from Q, we pull j with a smaller d-value, we may not have a shortest path to i, since we may be able to reach i through j. However, we have already computed the shortest path to i. We did not check a possible path. We no longer have a guaranteed path. Contradiction.

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• What algorithm can I use to determine points within a semi-circle?

  - by khayman218

  I have a list of two-dimensional points and I want to obtain which of them fall within a semi-circle. Originally, the target shape was a rectangle aligned with the x and y axis. So the current algorithm sorts the pairs by their X coord and binary searches to the first one that could fall within the rectangle. Then it iterates over each point sequentially. It stops when it hits one that is beyond both the X and Y upper-bound of the target rectangle. This does not work for a semi-circle as you cannot determine an effective upper/lower x and y bounds for it. The semi-circle can have any orientation. Worst case, I will find the least value of a dimension (say x) in the semi-circle, binary search to the first point which is beyond it and then sequentially test the points until I get beyond the upper bound of that dimension. Basically testing an entire band's worth of points on the grid. The problem being this will end up checking many points which are not within the bounds.

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• Algorithm to determine indices i..j of array A containing all the elements of another array B

  - by Skylark

  I came across this question on an interview questions thread. Here is the question: Given two integer arrays A [1..n] and B[1..m], find the smallest window in A that contains all elements of B. In other words, find a pair < i , j such that A[i..j] contains B[1..m]. If A doesn't contain all the elements of B, then i,j can be returned as -1. The integers in A need not be in the same order as they are in B. If there are more than one smallest window (different, but have the same size), then its enough to return one of them. Example: A[1,2,5,11,2,6,8,24,101,17,8] and B[5,2,11,8,17]. The algorithm should return i = 2 (index of 5 in A) and j = 9 (index of 17 in A). Now I can think of two variations. Let's suppose that B has duplicates. This variation doesn't consider the number of times each element occurs in B. It just checks for all the unique elements that occur in B and finds the smallest corresponding window in A that satisfies the above problem. For example, if A[1,2,4,5,7] and B[2,2,5], this variation doesn't bother about there being two 2's in B and just checks A for the unique integers in B namely 2 and 5 and hence returns i=1, j=3. This variation accounts for duplicates in B. If there are two 2's in B, then it expects to see at least two 2's in A as well. If not, it returns -1,-1. When you answer, please do let me know which variation you are answering. Pseudocode should do. Please mention space and time complexity if it is tricky to calculate it. Mention if your solution assumes array indices to start at 1 or 0 too. Thanks in advance.

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• Algorithm to determine if array contains n...n+m?

  - by Kyle Cronin

  I saw this question on Reddit, and there were no positive solutions presented, and I thought it would be a perfect question to ask here. This was in a thread about interview questions: Write a method that takes an int array of size m, and returns (True/False) if the array consists of the numbers n...n+m-1, all numbers in that range and only numbers in that range. The array is not guaranteed to be sorted. (For instance, {2,3,4} would return true. {1,3,1} would return false, {1,2,4} would return false. The problem I had with this one is that my interviewer kept asking me to optimize (faster O(n), less memory, etc), to the point where he claimed you could do it in one pass of the array using a constant amount of memory. Never figured that one out. Along with your solutions please indicate if they assume that the array contains unique items. Also indicate if your solution assumes the sequence starts at 1. (I've modified the question slightly to allow cases where it goes 2, 3, 4...) edit: I am now of the opinion that there does not exist a linear in time and constant in space algorithm that handles duplicates. Can anyone verify this? The duplicate problem boils down to testing to see if the array contains duplicates in O(n) time, O(1) space. If this can be done you can simply test first and if there are no duplicates run the algorithms posted. So can you test for dupes in O(n) time O(1) space?

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• Spring security or BCrypt algorithm which one is good for accounts like project?

  - by Ranjith Kumar Nethaji

  I am using spring security for hashing my password.And is it safe ,because am using spring security for first time. my code here <security:http auto-config="true"> <security:intercept-url pattern="/welcome*" access="ROLE_USER" /> <security:form-login login-page="/login" default-target-url="/welcome" authentication-failure-url="/loginfailed" /> <security:logout logout-success-url="/logout" /> </security:http> authentication-failure-url="/loginfailed" /> <security:logout logout-success-url="/logout" /> </security:http> <authentication-manager> <authentication-provider> <password-encoder hash="sha" /> <user-service> <user name="k" password="7c4a8d09ca3762af61e59520943dc26494f8941b" authorities="ROLE_USER" /> </user-service> </authentication-provider> </authentication-manager> .And I havnt used bcrypt algorithm.what is your feedback for both?any recommendation?

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• Why is Reinforcement Learning so rarely used in pathfinding?

  - by doug

  The venerable shortest-path graph theoretic algorithm A* and subsequent improvements (e.g., Hierarchical Annotated A*) is clearly the technique of choice for pathfinding in game development. Instead, it just seems to me that RL is a more natural paradigm to move a character around a game space. And yet I'm not aware of a single game developer who has implemented a Reinforcement Learning-based pathfinding engine. (I don't infer from this that the application of RL in pathfinding is 0, just that it's very small relative to A* and friends.) Whatever the reason, it's not because these developers are unaware of RL, as evidenced by the fact that RL is frequently used elsewhere in the game engine. This question is not a pretext for offering an opinion on RL in pathfinding; in fact, i am assuming that the tacit preference for A* et al. over RL is correct--but that preference is not obviously to me and i'm very curious about the reason for it, particularly from anyone who has tried to use RL for pathfinding.

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• Help with algorithmic complexity in custom merge sort implementation

  - by bitcycle

  I've got an implementation of the merge sort in C++ using a custom doubly linked list. I'm coming up with a big O complexity of n^2, based on the merge_sort() slice operation. But, from what I've read, this algorithm should be n*log(n), where the log has a base of two. Can someone help me determine if I'm just determining the complexity incorrectly, or if the implementation can/should be improved to achieve n*log(n) complexity? If you would like some background on my goals for this project, see my blog. I've added comments in the code outlining what I understand the complexity of each method to be. Clarification - I'm focusing on the C++ implementation with this question. I've got another implementation written in Python, but that was something that was added in addition to my original goal(s).

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• Matrix Pattern Recognition Algorithm

  - by Andres

  I am designing a logic analyzer and I would like to implement some Matrix Algorithm. I have several channels each one represented by a row in the matrix and every element in the column would be the state, for example: Channel 1 1 0 0 1 0 1 1 0 1 Channel 2 1 1 0 1 1 0 0 1 1 Channel 3 0 1 0 1 1 0 1 0 0 Channel 4 0 0 1 0 0 1 0 0 1 I would like to detect a pattern inside my matrix for example, detect if exist and where the sub-matrix or pattern: 1 0 1 1 I think it can be accomplished testing element by element but I think there should be a better way of doing it. Is there any Java API or any way to do it ? If there is a API ARM optimized for NEON instructions would be great also but not mandatory. Thank you very much in advance.

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• "Marching cubes" voxel terrain - triplanar texturing with depth?

  - by Dan the Man

  I am currently working on a voxel terrain that uses the marching cubes algorithm for polygonizing the scalar field of voxels. I am using a triplanar texturing shader for texturing. say I have a grass texture set to the Y axis and a dirt texture for both the X and Z axes. Now, when my player digs downwards, it still appears as grass. How would I make it to appear as dirt? I have been thinking about this for a while, and the only thing I can think of to make this effect, would be to mark vertices that have been dug with a certain vertex color. When it has that vertex color, the shader would apply that dirt texture to the vertices marked. Is there a better method?

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• Efficient visualization of a large voxelized volume

  - by Alejandro Piad

  Lets consider a large voxelized volume stored in an oct-tree or any other convenient structure. This volume represents, for instance, a landscape, where each block is either empty (air), or it has an specific material that will be later used to apply a texture. Voxels that are next to each other represent connected sections of the surface. What I need is an algorithm to generate a mesh from this voxels that represents the volume, with the following caracteristics: All the "holes" in the voxelized volume are correct. All the connections are correct, i.e. seamless. The surface appears smooth. In a broad sense, I want to somehow preserve the surface topology, meaning that connected sections remain connected in the resulting mesh and that the surface has a curvature that responds to the voxels topology. Imagine trying to render the Minecraft world but getting the mountain ladders to be smooth instead of blocky.

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• Designing Algorithm Flowchart Application

  - by l46kok

  I need to develop an GUI application in C# where users can freely add conditional/statement blocks on the algorithm flowchart like the one shown below. By freely, I mean users can add a block on wherever the arrows are. I'm having some problems brainstorming how to approach this problem, especially what to choose for my datastructure to store the blocks. I was thinking LinkedList since everything follows a linear fashion and every node always has a head and tail, but the If/Else block (ba) has two branches (heads) to store, so this complicates things a little bit. How would a smart one approach problems like this? My apologies if this question isn't suited for Programmers stackexchange, but this is more of a conceptual problem rather than implementation problem so I figured this place was appropriate for the question.

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• Difference between a pseudo code and algorithm?

  - by Vamsi Emani

  Technically, Is there a difference between these two words or can we use them interchangeably? Both of them more or less describe the logical sequence of steps that follow in solving a problem. ain't it? SO why do we actually use two such words if they are meant to talk of the same? Or, In case if they aren't synonymous words, What is it that differentiates them? In what contexts are we supposed to use the word pseudo code vs the word algorithm? Thanks.

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• Is Morton Code algorithm patented?

  - by Statement

  Synonyms: Morton Code, Morton Curve, Z-Order Curve, invented 1966 by G. M. Morton according to wiki. Not to be confused with Hilbert Curves which is closely related and have similar name. I was wondering if mentioned algorithm is patented. More generally, is there any place one can browse patented algorithms? I am quite new to all this legal stuff and I am keen to know more. The real question I have, I guess, is: Can I make money off a library that provides Morton Coding, legally?

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• How do I determine the draw order in an isometric view flash game?

  - by Gajet

  This is for a flash game, with isometric view. I need to know how to sort object so that there is no need for z-buffer checking when drawing. This might seem easy but there is another restriction, a scene can have 10,000+ objects so the algorithm needs to be run in less than O(n^2). All objects are rectangular boxes, and there are 3-4 objects moving in the scene. What's the best way to do this? UPDATE in each tile there is only object (I mean objects can stack on top of each other). and we access to both map of Objects and Objects have their own position.

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• Breakout clone, how to handle/design for collision detection/physics between objects?

  - by Zolomon

  I'm working on a breakout clone, and I wish to create some realistic physics effects for collision - angles on the paddle should allow the ball to bounce, as well as doing curve balls etc. I could use per-pixel based collision detection, but then I thought it might be easier with line/circle intersection testing. So, then I naturally consider making a polygon class for the line-based objects and use the built-in circle class for the circular objects. That sounds like an OK approach, right? And then just check for collision using the specified algorithm based on the objects that might be within each other's range?

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• Rubik's cube array rotation

  - by Ace

  I'm about to make a 3D Rubik's cube based game in Flash AS3 and Away3d. I don't really know how to manage the 2D arrays of the Rubik's cube. For example, how do I rotate the corresponding arrays if I rotate a side, or just rotate a middle part? In this stage I also don't know how to rotate those smaller cube parts all together if a side is rotating. First I was thinking of "groups" ( like in sketchup or 3ds max, blender), but that would be tricky, because the group components would change every time. So I was thinking of just rotating each individual piece along a global axis. However, I just know the Away3d functions to rotate the cube of his local X , Y or Z axis, but how to rotate in global axis? Does anyone know of a algorithm for doing these types of rotations?

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• Circular motion on low powered hardware

  - by Akroy

  I was thinking about platforms and enemies moving in circles in old 2D games, and I was wondering how that was done. I understand parametric equations, and it's trivial to use sin and cos to do it, but could an NES or SNES make real time trig calls? I admit heavy ignorance, but I thought those were expensive operations. Is there some clever way to calculate that motion more cheaply? I've been working on deriving an algorithm from trig sum identities that would only use precalculated trig, but that seems convoluted.

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• Monkey Hunter algorithm - Interview question [closed]

  - by Estefany Velez

  Question asked in an Interview: You are a hunter in the forest. A monkey is in the trees, but you don't know where and you can't see it. You can shoot at the trees, you have unlimited ammunition. Immediately after you shoot at a tree, if the monkey was in the tree, he falls and you win. If the monkey was not in the tree, he jumps (randomly) to an adjacent tree (he has to). Find an algorithm to get the monkey in the fewest shots possible. SOLUTION: The correct answer according to me was in the comments, credit to @rtperson: You could eliminate this possibility by shooting each tree twice as you sweep left, giving you a worst case of O(2n). EDIT: ...that is, a worst case of O(2n-1). You don't need to shoot the last tree twice.

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• Date calculation algorithm

  - by Julian Cuevas

  I'm working on a project to schedule a machine shop, basically I've got everything covered BUT date calculations, I've got a method called schedule (working on PHP here): public function schedule($start_date, $duration_in_minutes) Now my problem is, currently I'm calculating end time manually because time calculations have the following rules: During weekdays, work with business hours (7:00 AM to 5:00 PM) Work on Saturdays from 7:00 AM to 2:00 PM Ignore holidays (in Colombia we have A LOT of holidays) I already have a lookup table for holidays, I also have a Java version of this algorithm that I wrote for a previous version of the project, but that one's also manual. Is there any way to calculate an end time from a start time given duration?, my problem is that I have to consider the above rules, I'm looking for a (maybe?) math based solution, however I currently don't have the mind to devise such a solution myself. I'll be happy to provide code samples if necessary.

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• Algorithm for creating spheres?

  - by Dan the Man

  Does anyone have an algorithm for creating a sphere proceduraly with la amount of latitude lines, lo amount of longitude lines, and a radius of r? I need it to work with Unity, so the vertex positions need to be defined and then, the triangles defined via indexes (more info). EDIT I managed to get the code working in unity. But I think I might have done something wrong. When I turn up the detailLevel, All it does is add more vertices and polygons without moving them around. Did I forget something?

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• Chambers In A Castle Algorithm

  - by 7Aces

  Problem Statement - Given a NxM grid of 1s & 0s (1s mark walls, while 0s indicate empty chambers), the task is to identify the number of chambers & the size of the largest. And just to whet my curiosity, to find in which chamber, a cell belongs. It seems like an ad hoc problem, since the regular algorithms just don't fit in. I just can't get the logic for writing an algorithm for the problem. If you get it, pseudo-code would be of great help! Note - I have tried the regular grid search algorithms, but they don't suffice the problem requirements. Source - INOI Q Paper 2003

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• Permission Management Algorithm

  - by Emerald214

  I have 3 levels of permission to see the product: Brand - Allow/Deny Category - Allow/Deny Product - Allow/Deny For example, product A has: Category: Allow Product: Deny = product A cannot be seen because product A isn't allowed in Product level. if(allowForCategory == true) { if(allowForProduct == false) return false; if(allowForProduct == true) return true; } else { ... } This is not a good choice because it will become more complex if we add brand level. if() { if() { if() {} } } So is there any general algorithm to deal with the permission problem just like 777 solution in Linux?

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• Drawing an outline around an arbitrary group of hexagons

  - by Perky

  Is there an algorithm for drawing an outline around around an arbitrary group of hexagons? The polygon outline drawn may be concave. See the images below, the green line is what I am trying to achieve. The hexagons are stored as vertices and drawn as polygons. Edit: I've uploaded images that should explain more. I want to favour convex hulls because it's conveys an area of control more quickly. Each hexagon is stored in a multidimensional array so they all have x and y coordinates, I can easily find adjacent hexagons and the opposite vertex, i.e. adjacentHexagon = getAdjacentHexagon( someHexagon, NORTHWEST ) if there isn't a hexagon immediately adjacent it will continue to search in that direction until it finds one or hits the map edges.

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