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Showing posts from February, 2017

Kotlin Language Features Related to Null Handling

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Any software engineer with a Java background would find the null handling features in the Kotlin language interesting. Let's summarize this topic with some examples. Nullable types: In Kotlin, types are non-nullable by default. If you want a variable to be able to hold a null value, you need to explicitly declare its type as nullable using the Type? syntax. For example, String? denotes a nullable string, while String represents a non-nullable string. Safe calls (?.): Kotlin introduces the safe call operator (?.) for handling nullable types. It allows you to safely invoke a method or access a property on a nullable object. If the object is null, the expression returns null instead of throwing a NullPointerException. Example: data class Person(val name: String, val age: Int, val address: String?) fun main() {     // Create a person with a nullable address     val person1 = Person("John Doe", 25, "123 Main Street")     val person2 = Person("Jane Doe", 30,
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Understanding SimpleScalar codebase

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Among others, sim-safe is the most user-friendly simulator and the implementation is crafted for clarity. Generally there are two types of simulators: Functional simulators implement architecture (what programmers see) perform the actual execution sim-fast, sim-safe , sim-profile, sim-cache Performance simulators implement micro-architecture model system internals measure time sim-outorder Question: What is Dlite debugger? How can we debug a simulator?? DLite is a symbolic debugger. To use it, start the simulator with -i option Then user the debugger commands, for example: "step" Question: To work with TLB, do I have to fully understand sim-outorder ?? http://www.simplescalar.com/docs/simple_tutorial_v2.pdf page:14 http://www.simplescalar.com/docs/hack_guide_v2.pdf Question: How does sim-safe count number of instructions? Where is iTLB related code? In main.c file: /* options database*/ struct opt_odb_t *sim_odb; /* simulated re
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More on Virtual Memory and TLB

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Below is a series of videos on virtual memory: https://www.youtube.com/watch?v=KNUJhZCQZ9c Virtual memory is a layer of indirection. Without virtual memory the program address would be equal to the RAM address.  Discs are 1000x slower than RAM. What happens if a page is not on RAM?  Page table tells that the page is on disc. Hardware (CPU) generates a page fault exception . This exception will be picked up by the operating system. ( page fault handler ) Operating system kicks out (evicts) another page from RAM and writes it to disc If the page is dirty, it needs to be written back to disc first. Then OS reads the page from disk and puts it in RAM Then OS updates the page table to map the new page Finally OS jumps back to the instruction that caused the page fault A page fault is the slowest possible thing that can happen to a computer. (except for human interaction) Some operating sytems do not use paging: iOS kills the program if it uses too much memory. TLB sh
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Trie Data Structure and Finding Patterns in a Collection of Words

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 I faced a very hard problem recently about finding substrings based on a collection of patterns.  Example For  words = ["Apple", "Melon", "Orange", "Watermelon"]  and  parts = ["a", "mel", "lon", "el", "An"] , the output should be  findSubstrings(words, parts) = ["Apple", "Me[lon]", "Or[a]nge", "Water[mel]on"] . While  "Watermelon"  contains three substrings from the  parts  array,  "a" ,  "mel" , and  "lon" ,  "mel"  is the longest substring that appears first in the string. Note: Number of words and number of parts can be huge! A brute force method or careless usage of Java String methods would make it impossible to handle a large number of words and parts (patterns). Solution Approach Whenever we want to search patterns inside a string, we should think about the Trie data structure:  https://en.wikipedia.
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swapLexOrder: Finding lexicographically largest string

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The following coding question is really amazing: Given a string  str  and array of  pairs  that indicates which indices in the string can be swapped, return the  lexicographically largest  string that results from doing the allowed swaps. You can swap indices any number of times. Example For  str = "abdc"  and  pairs = [[1, 4], [3, 4]] , the output should be swapLexOrder(str, pairs) = "dbca" . By swapping the given indices, you get the strings:  "cbda" ,  "cbad" ,  "dbac" ,  "dbca" . The lexicographically largest string in this list is  "dbca" . Input/Output [execution time limit] 3 seconds (java) [input] string str A string consisting only of lowercase English letters. Guaranteed constraints: 1 ≤ str.length ≤ 10 4 . [input] array.array.integer pairs An array containing pairs of indices that can be swapped in  str  (1-based). This means that for each  pairs[i] , you can swap elements in  str  that have the indices  p
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A Graph Application in Java: Using WordNet to Find Outcast Words

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I'll explain our intent with an example: Let's say we have the following words: horse, zebra, cat, bear, table Which one is the outcast? The table.. But how can a computer system find this? We are talking about the whole English language and any word can be in this list. The number of words in the list can be huge, so the solution should use appropriate data structures and algorithms.  We'll use WordNet which is a graph of English words. It contains the semantic relationships between words and our concern is the "is a" relationship. For our example, we know that horse and cat are both animals. "Table" is not an animal and it is the outcast in this situation. But we even have the ability to tell that horse and zebra is very close. Below is a unit test that shows a high level view of our application in Java: public class OutcastTest {     private WordNet wordNet;     private Outcast outcast;     @Before  
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