programming

# Postgre OO database example

CREATE TABLE capitals (
name text,
population real,
altitude int, — (in ft)
state char(2)
);
CREATE TABLE non_capitals (
name text,
population real,
altitude int — (in ft)
);
CREATE VIEW cities AS
SELECT name, population, altitude FROM capitals
UNION
SELECT name, population, altitude FROM non_capitals;

This works OK as far as querying goes, but it gets ugly when you need to update several rows, to name
one thing.

A better solution is this:
CREATE TABLE cities (
name text,
population real,
altitude int — (in ft)
);
CREATE TABLE capitals (
state char(2)
) INHERITS (cities);

Standard
programming

# Merge 2 BST’s

• Flattening a BST into a sorted list is O(N)
• It’s just “in-order” iteration on the whole tree.
• Doing it for both is O(n1+n2)
• Merging two sorted lists is into one sorted list is O(n1+n2).
• Keep pointers to the heads of both lists
• This is how the merge of merge-sort works
• Creating a perfectly balanced BST from a sorted list is O(N)
• The value at the middle would be the root, and recurse.
• In our case the sorted list is of size n1+n2. so O(n1+n2)
• The resulting tree would be the conceptual BST of binary searching the list

`BinaryTree* sortedArrayToBST(``int` `arr[], ``int` `start, ``int` `end) {`

`  ``if` `(start > end) ``return` `NULL;`
`  ``// same as (start+end)/2, avoids overflow.`
`  ``int` `mid = start + (end - start) / 2;`
`  ``BinaryTree *node = ``new` `BinaryTree(arr[mid]);`
`  ``node->left = sortedArrayToBST(arr, start, mid-1);`
`  ``node->right = sortedArrayToBST(arr, mid+1, end);`
`  ``return` `node;`
`}`
`BinaryTree* sortedArrayToBST(``int` `arr[], ``int` `n) {`
`  ``return` `sortedArrayToBST(arr, 0, n-1);`
`}`
Standard
programming

# Dijkstra’s Shortest Path Algorithm in Java

http://www.vogella.com/articles/JavaAlgorithmsDijkstra/article.html

Dijkstra’s Shortest Path Algorithm in Java

Dijkstra’s Algorithms describes how to find the shortest path from one node to another node in a directed weighted graph. This article presents a Java implementation of this algorithms.

## 1. Shortest Path through a network – Dijkstra Algorithm

### 1.1. Overview

Dijkstra finds the shortest path from a source to all destination in a directed graph (single source shortest path problem). During this process it will also determine a spanning tree for the graph.

### 1.2. Graph

A graph is made of out nodes and directed edges which defines a connection from one node to another node. A node (or vertex) is a discrete position in a graph. Edges can be directed an undirected. Edges have an associated distance (also called costs or weight). The distance between two nodes a and b is labeled as [a,b].

The mathematical description for graphs is G= {V,E}, meaning that a graph is defined by a set of vertexes (V) and a collection of edges.

The “order” of a graph is the number of nodes.

The “size” of a graph is the number of edges.

Typical graph problems are:

• finding the shortest path from a specific node to another node
• finding the maximum possible flow through a network where each edges has a pre-defined maximum capacity (maximum-flow minimum-cut problem)

The following will focus on finding the shortest path from one node to another node in a graph.

### 1.3. Algorithms Description

The idea of Dijkstra is simple.

Dijkstra partitions all nodes into two distinct sets. Unsettled and settled. Initially all nodes are in the unsettled sets, e.g. they must be still evaluated. A node is moved to the settled set if a shortest path from the source to this node has been found.

Initially the distance of each node to the source is set to a very high value.

First only the source is in the set of unsettledNodes.

The algorithms runs until the unsettledNodes are empty. In earch iteration it selects the node with the lowest distance from the source out the unsettled nodes. If reads all edges which are outgoing from the source and evaluates for each destination node in these edges which is not yet settled if the known distance from the source to this node can be reduced if the selected edge is used. If this can be done then the distance is updated and the node is added to the nodes which need evaluation.

In Pseudocode the algorihm can be described as follows. Please note that Dijkstra also determines the presuccessor of each node on its way to the source. I’ll leave that out of the pseudo code to simplify it.

```

Foreach node set distance[node] = HIGH
SettledNodes = empty
UnSettledNodes = empty

distance[sourceNode]= 0

while (UnSettledNodes is not empty) {
evaluationNode = getNodeWithLowestDistance(UnSettledNodes)
remove evaluationNode from UnSettledNodes
evaluatedNeighbors(evaluationNode)
}

getNodeWithLowestDistance(UnSettledNodes){
find the node with the lowest distance in UnSettledNodes and return it
}

evaluatedNeighbors(evaluationNode){
Foreach destinationNode which can be reached via an edge from evaluationNode AND which is not in SettledNodes {
edgeDistance = getDistance(edge(evaluationNode, destinationNode))
newDistance = distance[evaluationNode] + edgeDistance
if (distance[destinationNode]  > newDistance ) {
distance[destinationNode]  = newDistance
}
}
}```

## 2. Model

A graph consists out of vertices and edges. These are represented by the following model.

```
package de.vogella.algorithms.dijkstra.model;

public class Vertex {
final private String id;
final private String name;

public Vertex(String id, String name) {
this.id = id;
this.name = name;
}
public String getId() {
return id;
}

public String getName() {
return name;
}

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((id == null) ? 0 : id.hashCode());
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Vertex other = (Vertex) obj;
if (id == null) {
if (other.id != null)
return false;
} else if (!id.equals(other.id))
return false;
return true;
}

@Override
public String toString() {
return name;
}

}```

A edge has a source and a destination.

```
package de.vogella.algorithms.dijkstra.model;

public class Edge  {
private final String id;
private final Vertex source;
private final Vertex destination;
private final int weight;

public Edge(String id, Vertex source, Vertex destination, int weight) {
this.id = id;
this.source = source;
this.destination = destination;
this.weight = weight;
}

public String getId() {
return id;
}
public Vertex getDestination() {
return destination;
}

public Vertex getSource() {
return source;
}
public int getWeight() {
return weight;
}

@Override
public String toString() {
return source + " " + destination;
}

}```

Both represent a graph.

```
package de.vogella.algorithms.dijkstra.model;

import java.util.List;

public class Graph {
private final List<Vertex> vertexes;
private final List<Edge> edges;

public Graph(List<Vertex> vertexes, List<Edge> edges) {
this.vertexes = vertexes;
this.edges = edges;
}

public List<Vertex> getVertexes() {
return vertexes;
}

public List<Edge> getEdges() {
return edges;
}

}```

## 3. Algorithmus

### 3.1. Implementation

The following is a simple implementation of Dijkstra’s algorithm. It does not use any performance optimization (e.g. by using a PriorityQueue for the UnSettledNodes of does not cache the result of the target evaluation of the edges) to make the algorihms as simple as possible.

```
package de.vogella.algorithms.dijkstra.engine;

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import de.vogella.algorithms.dijkstra.model.Edge;
import de.vogella.algorithms.dijkstra.model.Graph;
import de.vogella.algorithms.dijkstra.model.Vertex;

public class DijkstraAlgorithm {

private final List<Vertex> nodes;
private final List<Edge> edges;
private Set<Vertex> settledNodes;
private Set<Vertex> unSettledNodes;
private Map<Vertex, Vertex> predecessors;
private Map<Vertex, Integer> distance;

public DijkstraAlgorithm(Graph graph) {
// Create a copy of the array so that we can operate on this array
this.nodes = new ArrayList<Vertex>(graph.getVertexes());
this.edges = new ArrayList<Edge>(graph.getEdges());
}

public void execute(Vertex source) {
settledNodes = new HashSet<Vertex>();
unSettledNodes = new HashSet<Vertex>();
distance = new HashMap<Vertex, Integer>();
predecessors = new HashMap<Vertex, Vertex>();
distance.put(source, 0);
while (unSettledNodes.size() > 0) {
Vertex node = getMinimum(unSettledNodes);
unSettledNodes.remove(node);
findMinimalDistances(node);
}
}

private void findMinimalDistances(Vertex node) {
for (Vertex target : adjacentNodes) {
if (getShortestDistance(target) > getShortestDistance(node)
+ getDistance(node, target)) {
distance.put(target, getShortestDistance(node)
+ getDistance(node, target));
predecessors.put(target, node);
}
}

}

private int getDistance(Vertex node, Vertex target) {
for (Edge edge : edges) {
if (edge.getSource().equals(node)
&& edge.getDestination().equals(target)) {
return edge.getWeight();
}
}
throw new RuntimeException("Should not happen");
}

private List<Vertex> getNeighbors(Vertex node) {
List<Vertex> neighbors = new ArrayList<Vertex>();
for (Edge edge : edges) {
if (edge.getSource().equals(node)
&& !isSettled(edge.getDestination())) {
}
}
return neighbors;
}

private Vertex getMinimum(Set<Vertex> vertexes) {
Vertex minimum = null;
for (Vertex vertex : vertexes) {
if (minimum == null) {
minimum = vertex;
} else {
if (getShortestDistance(vertex) < getShortestDistance(minimum)) {
minimum = vertex;
}
}
}
return minimum;
}

private boolean isSettled(Vertex vertex) {
return settledNodes.contains(vertex);
}

private int getShortestDistance(Vertex destination) {
Integer d = distance.get(destination);
if (d == null) {
return Integer.MAX_VALUE;
} else {
return d;
}
}

/* * This method returns the path from the source to the selected target and * NULL if no path exists */
Vertex step = target;
// Check if a path exists
if (predecessors.get(step) == null) {
return null;
}
while (predecessors.get(step) != null) {
step = predecessors.get(step);
}
// Put it into the correct order
Collections.reverse(path);
return path;
}

}```

### 3.2. Test

The following is a small JUnit Test to validate the correctness of the algorithm.

```
package de.vogella.algorithms.dijkstra.test;

import java.util.ArrayList;
import java.util.List;

import org.junit.Test;

import de.vogella.algorithms.dijkstra.engine.DijkstraAlgorithm;
import de.vogella.algorithms.dijkstra.model.Edge;
import de.vogella.algorithms.dijkstra.model.Graph;
import de.vogella.algorithms.dijkstra.model.Vertex;

import static org.junit.Assert.assertNotNull;
import static org.junit.Assert.assertTrue;

public class TestDijkstraAlgorithm {

private List<Vertex> nodes;
private List<Edge> edges;

@Test
public void testExcute() {
nodes = new ArrayList<Vertex>();
edges = new ArrayList<Edge>();
for (int i = 0; i < 11; i++) {
Vertex location = new Vertex("Node_" + i, "Node_" + i);
}

// Lets check from location Loc_1 to Loc_10
Graph graph = new Graph(nodes, edges);
DijkstraAlgorithm dijkstra = new DijkstraAlgorithm(graph);
dijkstra.execute(nodes.get(0));

assertNotNull(path);
assertTrue(path.size() > 0);

for (Vertex vertex : path) {
System.out.println(vertex);
}

}

private void addLane(String laneId, int sourceLocNo, int destLocNo,
int duration) {
Edge lane = new Edge(laneId,nodes.get(sourceLocNo), nodes.get(destLocNo), duration );
}
}```

Standard

# memcached love story chinese

\$MEMCACHE_SERVERS = array(

“10.1.1.1”, //web1
“10.1.1.2”, //web2
“10.1.1.3”, //web3 );

\$memcache = new Memcache();
foreach(\$MEMCACHE_SERVERS as \$server){

Select语句所得的结果在我的memcache里面吗? 如果不在, 就执行查询并把结果放到memcache里:

\$huge_data_for_frong_page = \$memcache->get(“huge_data_for_frong_page”); if(\$huge_data_for_frong_page === false){

\$huge_data_for_frong_page = array();
\$sql = “SELECT * FROM hugetable WHERE timestamp > lastweek
ORDER BY timestamp ASC LIMIT 50000″;
\$res = mysql_query(\$sql, \$mysql_connection);
while(\$rec = mysql_fetch_assoc(\$res)){
\$huge_data_for_frong_page[] = \$rec;
}
// cache for 10 minutes
\$memcache->set(“huge_data_for_frong_page”, \$huge_data_for_frong_page, 600); }
// use \$huge_data_for_frong_page how you please

“好吧, 我手头上还有一个很慢的查询语句, 而且每秒还需要执行100次, 或许我们可以把它也缓存起来” 李雷二话不说就去编程了, 就像之前那样, 数据库的负载下降到8了!

“很好”韩梅梅说, “让我们继续” 她继续观察系统的图表, 三台memcached都运行起来了! 他们都在处理请求!

“mmm…,不过好像还是有问题, 其中一台memcached的机器已经过时了, 需要更新升级, 这样的话我就必须把这台机器给关了,这样一来我心爱的memcached会咋样啊~?” “那…让我试试看!” 韩梅梅二话不说,关掉了其中一台机器,然后她小心翼翼地观察系统曲线变化… “糟了,数据库的负载又直线上升了! 1…现在是2了,不过还算可以接受吧.

“缓存又自动恢复了! 我懂了, 如果其中有部分机器下线了,只是说会有部分应该缓存的数据不存在而已, 没什么大不了的, 偶稀饭!”

Standard
programming

# The Java serialization algorithm revealed

http://www.javaworld.com/community/node/2915

Serialization is the process of saving an object’s state to a sequence of bytes; deserialization is the process of rebuilding those bytes into a live object. The Java Serialization API provides a standard mechanism for developers to handle object serialization. In this tip, you will see how to serialize an object, and why serialization is sometimes necessary. You’ll learn about the serialization algorithm used in Java, and see an example that illustrates the serialized format of an object. By the time you’re done, you should have a solid knowledge of how the serialization algorithm works and what entities are serialized as part of the object at a low level.

## Why is serialization required?

In today’s world, a typical enterprise application will have multiple components and will be distributed across various systems and networks. In Java, everything is represented as objects; if two Java components want to communicate with each other, there needs be a mechanism to exchange data. One way to achieve this is to define your own protocol and transfer an object. This means that the receiving end must know the protocol used by the sender to re-create the object, which would make it very difficult to talk to third-party components. Hence, there needs to be a generic and efficient protocol to transfer the object between components. Serialization is defined for this purpose, and Java components use this protocol to transfer objects.

Figure 1 shows a high-level view of client/server communication, where an object is transferred from the client to the server through serialization. ## How to serialize an object

In order to serialize an object, you need to ensure that the class of the object implements the `java.io.Serializable`interface, as shown in Listing 1.

#### Listing 1. Implementing Serializable

```import java.io.Serializable; ```

```class TestSerial implements Serializable { public byte version = 100; public byte count = 0; }```

In Listing 1, the only thing you had to do differently from creating a normal class is implement the `java.io.Serializable`interface. The `Serializable` interface is a marker interface; it declares no methods at all. It tells the serialization mechanism that the class can be serialized.

Now that you have made the class eligible for serialization, the next step is to actually serialize the object. That is done by calling the `writeObject()` method of the `java.io.ObjectOutputStream` class, as shown in Listing 2.

#### Listing 2. Calling writeObject()

```public static void main(String args[]) throws IOException { FileOutputStream fos = new FileOutputStream("temp.out"); ObjectOutputStream oos = new ObjectOutputStream(fos); TestSerial ts = new TestSerial(); oos.writeObject(ts); oos.flush(); oos.close(); }```

Listing 2 stores the state of the `TestSerial` object in a file called `temp.out``oos.writeObject(ts);` actually kicks off the serialization algorithm, which in turn writes the object to `temp.out`.

To re-create the object from the persistent file, you would employ the code in Listing 3.

#### Listing 3. Recreating a serialized object

```public static void main(String args[]) throws IOException { FileInputStream fis = new FileInputStream("temp.out"); ObjectInputStream oin = new ObjectInputStream(fis); TestSerial ts = (TestSerial) oin.readObject(); System.out.println("version="+ts.version); }```

In Listing 3, the object’s restoration occurs with the `oin.readObject()` method call. This method call reads in the raw bytes that we previously persisted and creates a live object that is an exact replica of the original object graph. Because`readObject()` can read any serializable object, a cast to the correct type is required.

Executing this code will print `version=100` on the standard output.

## The serialized format of an object

What does the serialized version of the object look like? Remember, the sample code in the previous section saved the serialized version of the `TestSerial` object into the file `temp.out`. Listing 4 shows the contents of `temp.out`, displayed in hexadecimal. (You need a hexadecimal editor to see the output in hexadecimal format.)

#### Listing 4. Hexadecimal form of TestSerial

```AC ED 00 05 73 72 00 0A 53 65 72 69 61 6C 54 65 73 74 A0 0C 34 00 FE B1 DD F9 02 00 02 42 00 05 63 6F 75 6E 74 42 00 07 76 65 72 73 69 6F 6E 78 70 00 64```

If you look again at the actual `TestSerial` object, you’ll see that it has only two byte members, as shown in Listing 5.

#### Listing 5. TestSerial’s byte members

``` public byte version = 100; public byte count = 0;```

The size of a byte variable is one byte, and hence the total size of the object (without the header) is two bytes. But if you look at the size of the serialized object in Listing 4, you’ll see 51 bytes. Surprise! Where did the extra bytes come from, and what is their significance? They are introduced by the serialization algorithm, and are required in order to to re-create the object. In the next section, you’ll explore this algorithm in detail.

## Java’s serialization algorithm

By now, you should have a pretty good knowledge of how to serialize an object. But how does the process work under the hood? In general the serialization algorithm does the following:

• It writes out the metadata of the class associated with an instance.
• It recursively writes out the description of the superclass until it finds `java.lang.object`.
• Once it finishes writing the metadata information, it then starts with the actual data associated with the instance. But this time, it starts from the topmost superclass.
• It recursively writes the data associated with the instance, starting from the least superclass to the most-derived class.

I’ve written a different example object for this section that will cover all possible cases. The new sample object to be serialized is shown in Listing 6.

#### Listing 6. Sample serialized object

```class parent implements Serializable { int parentVersion = 10; } class contain implements Serializable{ int containVersion = 11; } public class SerialTest extends parent implements Serializable { int version = 66; contain con = new contain(); ```

```public int getVersion() { return version; } public static void main(String args[]) throws IOException { FileOutputStream fos = new FileOutputStream("temp.out"); ObjectOutputStream oos = new ObjectOutputStream(fos); SerialTest st = new SerialTest(); oos.writeObject(st); oos.flush(); oos.close(); } }```

This example is a straightforward one. It serializes an object of type `SerialTest`, which is derived from `parent` and has a container object, `contain`. The serialized format of this object is shown in Listing 7.

#### Listing 7. Serialized form of sample object

```AC ED 00 05 73 72 00 0A 53 65 72 69 61 6C 54 65 73 74 05 52 81 5A AC 66 02 F6 02 00 02 49 00 07 76 65 72 73 69 6F 6E 4C 00 03 63 6F 6E 74 00 09 4C 63 6F 6E 74 61 69 6E 3B 78 72 00 06 70 61 72 65 6E 74 0E DB D2 BD 85 EE 63 7A 02 00 01 49 00 0D 70 61 72 65 6E 74 56 65 72 73 69 6F 6E 78 70 00 00 00 0A 00 00 00 42 73 72 00 07 63 6F 6E 74 61 69 6E FC BB E6 0E FB CB 60 C7 02 00 01 49 00 0E 63 6F 6E 74 61 69 6E 56 65 72 73 69 6F 6E 78 70 00 00 00 0B```

Figure 2 offers a high-level look at the serialization algorithm for this scenario. #### Figure 2. An outline of the serialization algorithm

Let’s go through the serialized format of the object in detail and see what each byte represents. Begin with the serialization protocol information:

• `AC ED``STREAM_MAGIC`. Specifies that this is a serialization protocol.
• `00 05``STREAM_VERSION`. The serialization version.
• `0x73``TC_OBJECT`. Specifies that this is a new `Object`.

The first step of the serialization algorithm is to write the description of the class associated with an instance. The example serializes an object of type `SerialTest`, so the algorithm starts by writing the description of the `SerialTest` class.

• `0x72``TC_CLASSDESC`. Specifies that this is a new class.
• `00 0A`: Length of the class name.
• `53 65 72 69 61 6c 54 65 73 74``SerialTest`, the name of the class.
• `05 52 81 5A AC 66 02 F6``SerialVersionUID`, the serial version identifier of this class.
• `0x02`: Various flags. This particular flag says that the object supports serialization.
• `00 02`: Number of fields in this class.

Next, the algorithm writes the field `int version = 66;`.

• `0x49`: Field type code. 49 represents “I”, which stands for `Int`.
• `00 07`: Length of the field name.
• `76 65 72 73 69 6F 6E``version`, the name of the field.

And then the algorithm writes the next field, `contain con = new contain();`. This is an object, so it will write the canonical JVM signature of this field.

• `0x74``TC_STRING`. Represents a new string.
• `00 09`: Length of the string.
• `4C 63 6F 6E 74 61 69 6E 3B``Lcontain;`, the canonical JVM signature.
• `0x78``TC_ENDBLOCKDATA`, the end of the optional block data for an object.

The next step of the algorithm is to write the description of the `parent` class, which is the immediate superclass of `SerialTest`.

• `0x72``TC_CLASSDESC`. Specifies that this is a new class.
• `00 06`: Length of the class name.
• `70 61 72 65 6E 74``SerialTest`, the name of the class
• `0E DB D2 BD 85 EE 63 7A``SerialVersionUID`, the serial version identifier of this class.
• `0x02`: Various flags. This flag notes that the object supports serialization.
• `00 01`: Number of fields in this class.

Now the algorithm will write the field description for the `parent` class. `parent` has one field, `int parentVersion = 100;`.

• `0x49`: Field type code. 49 represents “I”, which stands for `Int`.
• `00 0D`: Length of the field name.
• `70 61 72 65 6E 74 56 65 72 73 69 6F 6E``parentVersion`, the name of the field.
• `0x78``TC_ENDBLOCKDATA`, the end of block data for this object.
• `0x70``TC_NULL`, which represents the fact that there are no more superclasses because we have reached the top of the class hierarchy.

So far, the serialization algorithm has written the description of the class associated with the instance and all its superclasses. Next, it will write the actual data associated with the instance. It writes the parent class members first:

• `00 00 00 0A`: 10, the value of `parentVersion`.

Then it moves on to `SerialTest`.

• `00 00 00 42`: 66, the value of `version`.

The next few bytes are interesting. The algorithm needs to write the information about the `contain` object, shown in Listing 8.

#### Listing 8. The contain object

`contain con = new contain();`

Remember, the serialization algorithm hasn’t written the class description for the `contain` class yet. This is the opportunity to write this description.

• `0x73``TC_OBJECT`, designating a new object.
• `0x72``TC_CLASSDESC`.
• `00 07`: Length of the class name.
• `63 6F 6E 74 61 69 6E``contain`, the name of the class.
• `FC BB E6 0E FB CB 60 C7``SerialVersionUID`, the serial version identifier of this class.
• `0x02`: Various flags. This flag indicates that this class supports serialization.
• `00 01`: Number of fields in this class.

Next, the algorithm must write the description for `contain`‘s only field, `int containVersion = 11;`.

• `0x49`: Field type code. 49 represents “I”, which stands for `Int`.
• `00 0E`: Length of the field name.
• `63 6F 6E 74 61 69 6E 56 65 72 73 69 6F 6E``containVersion`, the name of the field.
• `0x78``TC_ENDBLOCKDATA`.

Next, the serialization algorithm checks to see if `contain` has any parent classes. If it did, the algorithm would start writing that class; but in this case there is no superclass for `contain`, so the algorithm writes `TC_NULL`.

• `0x70``TC_NULL`.

Finally, the algorithm writes the actual data associated with `contain`.

• `00 00 00 0B`: 11, the value of `containVersion`.

## Conclusion

In this tip, you have seen how to serialize an object, and learned how the serialization algorithm works in detail. I hope this article gives you more detail on what happens when you actually serialize an object.