Saturday, December 27, 2014

Branching out in Martial Arts

I've been training in Shotokan karate on-and-off for the past 15 years or so.  The original Okinawan curriculum, dating back to Itosu in the late 19th century, consisted exclusively of katas. Hence, the katase must be taken as the core representation of the art form and combat system.

The modern curriculum tends to focus on sharp strikes, punches, and kicks. This is great, but the modern curriculum tends to under-emphasize the joint-locks, grappling, throws, and takedowns found throughout the katas. In fact, the kihon [basics] and kumite [sparring] curriculum tends to look quite different from the katas. One interpretation of this discrepancy, is that the katas are antiquated, mysterious, or impractical. My preferred interpretation is that we're missing something in our basics and sparring training. The grappling/throwing aspect is often under-emphasized.

I'm not the originator of this interpretation. Established practitioners and writers like Bruce Clayton, Iain Abernathy, and Kousaku Yokota have been popularizing this under-emphasized portion of the curriculum for many years. The under-emphasis comes from an over-emphasis in training in a way that complies with tournament rules. Throwing an opponent will not gain points under standard karate tournament rules, but punching or kicking him will.

Anyway, from time-to-time, I've started leading classes at my local shotokan dojo. I make sure to emphasize the grappling and throwing, and make sure the students practice these in pairs, as well as practicing how to fall correctly. I'm also taking a beginner's course in Judo, to get a more systematic treatment of throwing.

At the end of the day, there is nothing mystical about martial arts technique. It's just physics and anatomy. This is why there is great commonality in punching technique across art-forms, from boxing to karate to muay thai.

Friday, November 14, 2014

Pallindromes in C++

Sometimes you come across puzzles or interview questions along the lines of "Find the longest palindrome in a string." To be honest, I've never looked for palindromes in my work, but I think it's a good exercise nonetheless. For one thing, the "best practices" strategies can be applied to other problems. For another, the search for symmetric subsets in datasets of all types definitely does come up.

So, we might start this exercise by first writing a function to classify a string as a palindrome or not. A nice way to do this is to start with a pointer at either end of the string, and iterate them inwards, checking the lower and upper characters against each other at each stage.  For example, if the first and last characters are equal, we iterate inwards. If not, we can return "false" immmediately. The following snippet shows this function.


 bool is_pallindrome(string input){  
      int lower=0;  
      int upper=input.length()-1;  
      while (upper>lower){  
           if (input[upper]!=input[lower]) //If we find differing characters, not a pallindrome  
                return false;  
           upper--;  
           lower++;  
      }  
      return true;  
 }  

So now, to find the longest palindrome in a string, we could just iterate over the string, looking at all substrings, and calling is_pallindrome() [notice my typo in the function name]. That sounds pretty straight-forward, but searching all substrings becomes O(N^2), kind of cumbersome.

A nice way to use the is_pallindrome() function is to think about strings that are centered at index i. So, we have a pointer that searches left from i, and a pointer that searches right from i, and we stop iterating when input[lower]!=input[upper]. This is very similar to the is_pallindrome() function.

There's another approach that's more efficient than the above. Sometimes a good way to think about such a problem is to find the desired subset that ends at index i. If I'm searching for a palindrome that ends at index i, I can store the longest palindrome that ended at index i-1. Then I simply have to look at the first index of that palindrome, and the character that precedes it in the input string.

The storage/caching can be implemented with a hashmap, so we store the longest palindrome that ends at each index, and then we can look it up at the next step. This means, each additional step only features two more comparisons. The following snippet implements this approach:

 string longest_pallindrome_map(string input){  
      map<int, string> pal; //hashmap from index to longest pallindrome that *ends* at index  
      int max_length=0; int max_index=0;  
      for (int i=0; i<input.size(); i++){  
           int prev_index=i-pal[i-1].length()-1;            
           if (pal[i-1][0]==input[i]) //Looking for consecutive duplicate letters  
                pal[i]=pal[i-1]+input[i];            
           else if (input[i]==input[prev_index])  
                pal[i]=input[prev_index]+pal[i-1]+input[i]; //Looking at the character prior to previous pallindrome  
           else  
                pal[i]=string(input.begin()+i, input.begin()+i+1); //Longest pallindrome is current character  
   
           if (pal[i].length()>max_length){  
                max_index=i;  
                max_length=pal[i].length();            
           }            
      }  
      return pal[max_index];  
 }  

This is actually quite a bit more efficient in the searching, more like pseudo-linear instead of quadratic search. [Again notice my mispelling of palindrome.]  The only thing is, we are storing a bunch of strings in this hashmap. We don't actually need to do this.

Instead, we can just store the indices or pointers that begin the palindromes that ends at each index in the input string. The following function does this.

 string longest_pallindrome(string input){       
      map<int, int> pal; //hashmap from current index, to the begin-index of the longest pallindrome that *ends* here  
      int max_length=0;  
      int max_ind=0;  
      for (int i=0; i<input.size(); i++){                           
           int prev_index=pal[i-1];                                
           if (input[i]==input[prev_index]){ //Looking for consecutive duplicate characters  
                pal[i]=prev_index;  
           }  
           else if (input[i]==input[prev_index-1]) //Looking at the index prior to the last pallindrome  
                pal[i]=prev_index-1;  
           else  
                pal[i]=i; //Longest pallindrome is simply the current character  
           if ((i-pal[i])>max_length){ //Finding the longest one along the way  
                max_ind=i;  
                max_length=i-pal[i];  
           }                           
      }            
      return string(input.begin()+pal[max_ind], input.begin()+max_ind+1);  
 }  

So now we have, via dynamic programming, a pseudo-linear search, in that we basically have a linear search over the input string, but we have to store O(N) indices.

Did I really need the hashmap? Not necessarily; I could have used a regular old array, and had some code to check_bounds. On the other hand, the hashmap approach is generalizable to other problems, where the key values might not be simple ordinal numbers.

Thursday, November 6, 2014

Housing Affordability in San Francisco [and the Bay Area]

If you live in the Bay Area, you know that the cost of living is high. By "cost of living," I don't mean food prices, fuel prices, or many other goods and services. I mainly mean the high prices for renting or purchasing property.

While many areas offer an array of low-end to high-end housing solutions, where the renter/buyer can adapt his budget to the options, San Francisco offers almost no availability at the low-end, and almost everything else at high-end prices, with premium-end a growing market share.

There is an array of historical reasons behind this strange, skewed distribution of housing options. The underlying reason is a geographically constrained area, with ample economic opportunities, which attracts a lot of people from all over the world. This underlying problem is exacerbated by Not-In-My-Backyard policies that make it difficult to develop higher-rise, higher-density housing.

Prop 13 severely reduces liquidity in the property market, because if you already own property and decide to move, you stand to increase your tax burden; all property owners stay put. Finally, rent control ensures that renters also stay put.

In 1978, people got tired of property tax increases, so they passed Prop 13 to limit/eliminate property tax increases, and consequences be damned! In 1979, San Franciscans got tired of rent increases, so they passed rent-control, and consequences be damned! Each of these policies are best-described as a "We were here first!" tax on newcomers.

Down where I live, on the Peninsula/Silicon Valley, there is no rent control, but rents are still very high. We have similarly high demand for housing here, and many high-paying employers nearby. One additional reason appears to be monopolistic ownership of rental properties. The same 2-3 companies own all the apartment complexes in town, so they alone control the going rents.

So what's wrong with high rents/home prices? Well, for the lower-income, they're a massive burden. For the middle and higher-income, they're a huge waste of resources. The excess money someone spends on rent or mortgage could go to purchasing other goods and services, creating more economic activity. 

In any case, there's a great article on techcrunch that details the housing situation, and historical factors in San Francisco. It's well-researched, and a long but informative read.

Tuesday, October 14, 2014

Matplotlib Animations, Using Stored Data, Writing Video

There are some good matplotlib animations examples floating around the internet, including here and here. These were useful and I gained a great deal from them, but my particular problem was not directly addressed.

The scenario we're looking at today is the following. We have some stored data on disk, that we would like to plot using a Matplotlib animation, and save this animation as a video file. This is a reasonably common occurrence for researchers, but I had trouble finding tutorials online for this problem. Many of the tutorials generated their data via simulations on the fly, which didn't directly map to the case of recorded data.

There were two stumbling blocks I ran into, that had non-obvious solutions. First, Matplotlib's animation takes a function pointer to a user-defined function, for example def animate(i), but the animate function is then controlled by the iterator i. The second major stumbling block I hit, is that your animation function def animate(i): should return the data structures that need to be updated.

In this example, I have saved some data to a file in JSON format; each entry has a position and a velocity parameter. I read this file in line-by-line, and create a scrolling plot of the time series. I save this animated scrolling plot as a video file to disk, using ffmpeg and x264 for video encoding.


 import sys  
 import os  
 import json  
 import numpy as np  
 import matplotlib  
 import matplotlib.pyplot as plt  
 import matplotlib.animation as animation  
 from time import time  
 import itertools  
   
 from matplotlib.path import Path  
 import matplotlib.patches as patches  
 import threading  
   
   
   
 def init():  
   plotline.set_data([], [])  
   dplotline.set_data([],[])  
   return plotline,dplotline,  
   
 def animate(i):        
   #ax.clear()  
     
   global frame_num, buffersize  
     
   if (frame_num%100==0):  
             print frame_num    
   line=f.readline()  
   jsondata=json.loads(line)  
   frame_num+=1  
     
   if "Position" in jsondata:  
     pos= jsondata["Position"]  
       
   if "Velocity" in jsondata:  
             vel=jsondata["Velocity"]  
   
   pos_buf.append(pos)  
   vel_buf.append(vel)  
     
   while len(pos_buf)>buffersize:  
     pos_buf.pop(0)  
   while len(vel_buf)>buffersize:  
     vel_buf.pop(0)  
     
   x=np.linspace(-len(pos_buf)/framerate,0,len(pos_buf))  
   dx=np.linspace(-len(vel_buf)/framerate,0,len(vel_buf))        
     
   plotline.set_data(x,headpose_buf)  
   dplotline.set_data(dx,headvel_buf)    
   return plotline, dplotline,  
   
 def folder_from_path(file):  
   split_path=file.split("/")  
   folder=split_path[0]  
   for i in range(1,len(split_path)-1):  
     folder+="/"+split_path[i]  
   return folder  
   
 #Setting up globals  
 file=""    
 frame_num=0    
   
 framerate=20.0  
 buffersize=5*20  
 vel_buf=[]  
 pos_buf=[]  
   
 fig, ax = plt.subplots()  
 ax.set_xlim(-5.0,2.0)  
 ax.set_ylim(-180,180)  
 my_dpi=96  
 fig.set_size_inches(640/my_dpi, 360/my_dpi)    
 plotline, =ax.plot([],[],lw=2,color='b')  
 dplotline, =ax.plot([],[], lw=2, color='r')  
   
 if __name__=='__main__':  
   if len(sys.argv)<2:  
     print "Usage:\nplot_data.py file"  
     exit()  
   #Parsing command-line arguments, checking the input file exists.  
   file= sys.argv[1]  
   print file  
   if (os.path.isfile(file)==False):  
       print "Problem with file"  
       exit()   
   folder=folder_from_path(file)  
   #Writing video to same folder  
   outfile=folder+"/data.avi"  
   print folder  
   #Getting number of data points from file. A little hackey, but I couldn't control animate() with the file open()            
   line_count=0  
   with open(file, 'r') as f:  
     for read_data in f:      
       line_count=line_count+1  
   print line_count  
   f=open(file,'r')  
   Writer = animation.writers['ffmpeg']  
   writer = Writer(fps=25, metadata=dict(artist='Sayanan'), extra_args=['-vcodec','libx264'])      
   anim = animation.FuncAnimation(fig, animate,init_func=init, frames=line_count-1,blit=True, interval=1, repeat=False)      
   anim.save(outfile,writer=writer)  
   f.close()  
   #plt.show()  
     

So there you go. A basic program that plots animation, and saves the resulting video to disk, using recorded data, stored in JSON on disk.

Edit: Just a quick addendum. I also had some trouble plotting animations of a grid of custom polygons, which could be useful for mapping, occupancy grids, or similar data. Here's a quick demo for how to generate animations of this sort:


 "Demo of a grid of PathPatch objects."  
 import numpy as np  
 import matplotlib  
 import matplotlib.pyplot as plt  
 import matplotlib.animation as animation  
 from time import time  
 import itertools  
   
 from matplotlib.path import Path  
 import matplotlib.patches as patches  
   
 def animate(i):       
   prob=np.random.rand(np.size(yr)*np.size(xl))  
   ax.clear()  
   cells=[]  
   for i in range(0,len(P1)):    
     verts=[P1[i], P2[i], P3[i], P4[i], P1[i] ]  
     path = Path(verts, codes)  
     patch = patches.PathPatch(path, color=[1-prob[i],prob[i],0], lw=2)  
     cells.append(ax.add_patch(patch))  
   return cells  
     
   
 fig, ax = plt.subplots()  
 fig.set_size_inches(5,20)  
   
 width=3.5  
 height=5.0  
   
 xl=np.arange(-8.75,8.75,width)  
 yr=np.arange(-50.0,50.0,height)  
 ax.set_xlim(xl[0],xl[np.size(xl)-1]+width)  
 ax.set_ylim(yr[0],yr[np.size(yr)-1]+height)  
 cells=[]  
 print yr[10]  
 print np.size(yr)  
 print np.size(xl)  
 P1=list(itertools.product(xl,yr+height))  
 P2=list(itertools.product(xl+width, yr+height))  
 P3=list(itertools.product(xl+width, yr))  
 P4=list(itertools.product(xl,yr))  
 prob=np.random.rand(np.size(yr)*np.size(xl))  
 codes = [Path.MOVETO,  
      Path.LINETO,  
      Path.LINETO,  
      Path.LINETO,  
      Path.CLOSEPOLY,  
      ]  
 for i in range(0,len(P1)):  
   verts=[P1[i], P2[i], P3[i], P4[i], P1[i] ]  
   path = Path(verts, codes)  
   patch = patches.PathPatch(path, color=[1-prob[i],prob[i],0], lw=2)  
   cells.append(ax.add_patch(patch))  
 anim = animation.FuncAnimation(fig, animate, blit=True, save_count=0,interval=10)  
 plt.show()  
   

Friday, September 26, 2014

Matplotlib Animation Save with FFmpeg Freezes After N Frames

Matplotlib is a go-to plotting tool in Python, which has a lot of useful features, like compatibility with the Scipy/Numpy stack. Sometimes it's useful to visualize time-series data temporally.

One very useful feature that Matplotlib has is the animations module, which allows you to animate plots with changing data [over time, for example], and save the data as a video file using FFmpeg. There's a great tutorial for Matplotlib animations here.

My only complaint about the Matplotlib animations, is that the documentation and examples are not super-comprehensive. I might create a follow-up post with some sample programs, to add to the documented examples floating around the interwebs.

In any case, once I got the animations working, I wanted to save them as a video for later on-demand viewing. I set up an animation writer with ffmpeg, x264, and let it run. Each time I ran the script, and the video saving would freeze after 820 frames.

It took a fair amount of online searching to find the answer here. It turns out that the stock Ubuntu/Debian FFmpeg and LibAV have branched apart at some point. You can obtain the "real" FFmpeg here. Once you install this, the Matplotlib animations save videos just fine.


Sunday, September 14, 2014

USB Serial COM Ports and Python

I was working with a simple device that provides data via USB Serial COM. For whatever reason, the C++ library I was using to read from the device was not working consistently on different machines.

USB Serial COM is akin to socket programming. The routines are somewhat platform-dependent, and today's developer doesn't usually care about the details. Just give me an abstraction that lets me communicate!

Then I realized that Python has PySerial. Using this library, I was easily able to communicate with the device, and send it via localhost socket to my C++ program. Done and done.

Check out the super simple source code:

 import serial  
 import socket  
   
 IP="127.0.0.1"  
 PORT=3001  
 sock=socket.socket(socket.AF_INET,socket.SOCK_DGRAM) #Sending data to localhost via UDP  
   
 ser= serial.Serial(2,timeout=2) #COM3 is port number 2.  
 print ser.name  
 line=''  
 while (True):  
      line=ser.readline()  
      print line  
      sock.sendto(line,(IP,PORT))  

Python for the win.

Sunday, August 3, 2014

Karate Interpretations: Sport, Fighting, Self-defense

Over the past year, I've gotten back into Shotokan karate. I grew up doing karate, and completed my blackbelt exam when I was 15. There was a great sense of community with my dojo growing up, in Randallstown, Maryland.  The training there was pretty intense, and not geared towards tournaments.

 Over the years, I've been on and off with it. Practical considerations, like time, or distance to the neareset dojo, obviated my formal training when I was in San Diego. I used to train by myself.

It's great for fitness, teaches you about bio-mechanics and anatomy, and it's solid for self-defense. The full system consists of various punches, strikes, kicks, blocks, joint locks, grappling, throws, and take-downs. The traditional style are tailor made for close-range and medium-range hand-to-hand combat.

A problematic aspect arises when a dojo trains with too much emphasis on tournament competition. Tournaments have an artificial set of rules, that really favor high kicks, and fast punches to the body. These techniques alone are fairly impractical for self-defense.

If a dojo trains with too much emphasis on winning tournaments, it tends to ignore the more practical, self-defense oriented styles and techniques. Common moves like arm-bars, wrist-locks, and throws tend to be completely ignored in such a dojo. This is especially problematic for students, who think they are learning how to defend themselves, but are actually learning how to score points in tournament-style sparring.

There are some voices of reason out there, who train traditional karate, and include the grappling and throwing. A couple are Bruce Clayton, and Iain Abernathy. These two have produced books and dvds on the subject, the historical evolution of modern karate, and even lead practical self-defense karate clinics.

No single self-defense system is comprehensive, and so it's good to keep your eyes open. When I watch elite boxers, I see a lot of the same technique and form. When I watch brazilian jiu-jutsu guys, I see a lot of familiar grappling, but enhanced and taken to the ground-fighting scenario. Martial arts are enjoyable for sure.


Saturday, July 26, 2014

Winsock TCP Client in C++

Recently I've been looking at a server-client software architecture for machine learning.  A lot of really great machine learning and mathematical modeling toolboxes are now written in Python. However, if you're working directly with a sensor-based system, your sensor capture code is often written in C/C++.

Yes, you could write a wrapper for that code, but it can be a slow and redundant process.

Instead, you can set up a Python TCP server like so , even on the localhost on the same machine, send a JSON structure with your feature vector, do your heavy number-crunching in Python, and send the result back to the C++ client.

There's a lot of documentation out there for TCP servers and clients on *nix in C and C++, but the documentation for Windows is less available, written in pure C, or cryptic to understand. I wanted a class that would abstract away most of the nuts and bolts.

Using the Windows Winsock documentation example, I've written a simple Winsock TCP client class in C++. This code will build with any semi-recent version of Visual Studio Express or Professional. It's really simple stuff, just easier to read.

Here's the header file, which I called TCPClient.h

#ifndef TCP_CLIENT_H
#define TCP_CLIENT_H
#define WIN32_LEAN_AND_MEAN

#include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdlib.h>
#include <stdio.h>
#include <string>
#include <iostream>
#include <sstream>

#pragma comment (lib, "Ws2_32.lib")
#pragma comment (lib, "Mswsock.lib")
#pragma comment (lib, "AdvApi32.lib")


class TCPClient{
public:
    TCPClient(); //Default constructor
    TCPClient(const char* host, const char* port); //Constructor with host-ipaddress, port number
    TCPClient(std::string host, std::string port); //Constructor with host-ipaddress, port number
    TCPClient(const char* host, const char* port, const int buffer_size); //Constructor with host-ipaddress, port number, buffersize [rarely used]
    TCPClient(std::string host, std::string port, const int buffer_size); //Constructor with host-ipaddress, port number, buffersize [rarely used]
    int init(); //initialization, called by constructors
    ~TCPClient(); //clean up winsock environment
    bool open(); //open connection
    bool open(const char* host, const char* port); //open connection with host-ipaddress, port number
    bool open(std::string host, std::string port); //open connection with host-ipaddress, port number
    int send_buf(const char* sendbuf);    //send a message. Returns the number of bytes successfully sent. Returns -1 if failed.
    int send_buf(std::string sendbuffer); //send a message. Returns the number of bytes successfully sent. Returns -1 if failed.
    int receive(std::string& recvbuffer); //receive a message. Returns the number of bytes received.
    bool shutdown_send(); //shutdown the outgoing connection
    bool close(); //close the socket    
private:
    int resolve(); //used internally, resolving host. Returns -1 if failed.
    int connect_to(); //used internally, actually connect. Returns -1 if failed.

    int _buffer_size;    //buffer size in bytes
    int iResult;        //errorcodes from Winsock

    WSADATA wsaData;   //Winsock stuff
    SOCKET ConnectSocket; //Socket
    struct addrinfo *result, *ptr, hints; //Winsock stuff

    std::string host; //host ipaddress
    std::string port; //port number
    bool _connection_open; // Is the connection open?
    bool _ipset; //Did we initialize with host and port number?
    bool _shutdown_sent; //Have we sent a shutdown message to the host?
};

#endif

Here's the class implementation. Again, really simple stuff.

#include "TCPClient.h"

int TCPClient::init(){
    ConnectSocket = INVALID_SOCKET;
    result=NULL;
    ptr=NULL;

    iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
    if (iResult != 0) {
        std::cerr<<"WSAStartup failed with error: "<<iResult<<"\n";
        return -1;
    }
    ZeroMemory( &hints, sizeof(hints) );
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;
    this->_buffer_size=4096;
    this->_shutdown_sent=false;
    return 0;
}
TCPClient::TCPClient(){
    this->_ipset=false;
    if (init()<0){
        std::cerr<<"Error initializing WinSock\n";
        exit(1);
    }
}
TCPClient::TCPClient(const char* host, const char* port){
    this->host=host;
    this->port=port;
    this->_ipset=true;
    if (init()<0){
        std::cerr<<"Error initializing WinSock\n";
        exit(1);
    }
}
TCPClient::TCPClient(std::string host, std::string port){
    this->host=host;
    this->port=port;
    this->_ipset=true;
    if (init()<0){
        std::cerr<<"Error initializing WinSock\n";
        exit(1);
    }
}
TCPClient::TCPClient(const char* host, const char* port, const int buffer_size){
    this->host=host;
    this->port=port;
    this->_ipset=true;
    this->_buffer_size=buffer_size;
    if (init()<0){
        std::cerr<<"Error initializing WinSock\n";
        exit(1);
    }
}
TCPClient::TCPClient(std::string host, std::string port, const int buffer_size){
    this->host=host;
    this->port=port;
    this->_ipset=true;
    this->_buffer_size=buffer_size;
    if (init()<0){
        std::cerr<<"Error initializing WinSock\n";
        exit(1);
    }
}
TCPClient::~TCPClient(){
    WSACleanup();
}

int TCPClient::resolve(){
    // Resolve the server address and port
    iResult = getaddrinfo(host.c_str(), port.c_str(), &hints, &result);
    if ( iResult != 0 ) {
        std::cerr<<"getaddrinfo failed with error: "<<iResult<<"\n";
        WSACleanup();
        return -1;
    }
    return 0;
}
int TCPClient::connect_to(){
    // Attempt to connect to an address until one succeeds
    for(ptr=result; ptr != NULL ;ptr=ptr->ai_next) {
        // Create a SOCKET for connecting to server
        ConnectSocket = socket(ptr->ai_family, ptr->ai_socktype, 
            ptr->ai_protocol);
        if (ConnectSocket == INVALID_SOCKET) {           
            std::cerr<<"socket failed with error: "<<WSAGetLastError()<<"\n";
            WSACleanup();
            return -1;
        }
        // Connect to server.
        iResult = connect( ConnectSocket, ptr->ai_addr, (int)ptr->ai_addrlen);
        if (iResult == SOCKET_ERROR) {
            closesocket(ConnectSocket);
            ConnectSocket = INVALID_SOCKET;
            continue;
        }
        break;
    }
    freeaddrinfo(result);
    if (ConnectSocket == INVALID_SOCKET) {
        std::cerr<<"Unable to connect to server!\n";
        WSACleanup();
        return -1;
    }
    return 0;
}

bool TCPClient::open(){
    if (this->_ipset==false)
        return false;
    if (resolve()<0)
        return false;
    if (connect_to()<0)
        return false;
    return true;
}
bool TCPClient::open(const char* host, const char* port){
    this->host=host;
    this->port=port;
    this->_ipset=true;
    this->_connection_open=this->open();
    return _connection_open;
}
bool TCPClient::open(std::string host, std::string port){
    this->host=host;
    this->port=port;
    this->_ipset=true;
    this->_connection_open=this->open();
    return _connection_open;
}
int TCPClient::send_buf(const char* sendbuf){
    if (_connection_open==false)
        return -1;
    // Send an initial buffer

    iResult = send( ConnectSocket, sendbuf, (int)strlen(sendbuf), 0 );
    if (iResult == SOCKET_ERROR) {
        std::cerr<<"socket failed with error: "<<WSAGetLastError()<<"\n";    
        WSACleanup();
        return -1;
    }   
    return iResult;
}
int TCPClient::send_buf(std::string sendbuffer){
    
    int length=sendbuffer.length();
    if (_connection_open==false)
        return -1;
    // Send an initial buffer
    iResult = send( ConnectSocket, sendbuffer.c_str(), length, 0 );

    if (iResult == SOCKET_ERROR) {
        std::cerr<<"socket failed with error: "<<WSAGetLastError()<<"\n";
        this->close();
        WSACleanup();
        return -1;
    }   
    return iResult;
}
int TCPClient::receive(std::string& recvbuffer){
    // Receive up to _buffer_size bytes
    recvbuffer.clear();
    int total=0;
    char* recvbuf=new char[this->_buffer_size];    
    iResult = recv(ConnectSocket, recvbuf, _buffer_size, 0);
        if ( iResult > 0 ){            
            std::string received_str=recvbuf;
            received_str.resize(iResult);
            recvbuffer+=received_str;
            total+=iResult;            
        }
        else if ( iResult == 0 ){
            std::cout<<"\nConnection closed\n";            
        }
        else            
            std::cerr<<"recv failed with error: "<<WSAGetLastError()<<"\n";
   
    delete [] recvbuf;
    return total;
}
bool TCPClient::shutdown_send(){
     // shutdown the connection since no more data will be sent
    iResult = shutdown(ConnectSocket, SD_SEND);    
    if (iResult == SOCKET_ERROR) {
       std::cerr<<"shutdown failed with error: "<<WSAGetLastError()<<"\n";       
        this->close();
        WSACleanup();
        return false;
    }
    this->_shutdown_sent=true;
    return true;
}
bool TCPClient::close(){
    if (!_shutdown_sent)
        shutdown_send();        
     // cleanup
    closesocket(ConnectSocket);
    return true;
}

And finally, this is the main function, which shows how to invoke the class. I had a very simple TCP server written in Python, running on a different machine on the same local network.

The main program simple gets your keyboard input and sends it to the server. If you input the string "exit", the program exits cleanly.


int main (int argc, char* argv[]){
    TCPClient myClient("192.168.1.2","5000");    
    if (!myClient.open()){
        char a;
        std::cout<<"Enter any key to exit\n";
        std::cin>>a;
        return 0;
    }

    while (true){        
        std::string input; 
        std::cout<<"Enter something to send to the server\n";
        getline(std::cin,input);
        if (!input.compare("exit")){
            myClient.close();
            break;
        }
        if (myClient.send_buf(input)<0)    
            break;    
        std::string received;
        if    (myClient.receive(received)<0)
            break;    
        std::cout<<"Received:\n"<<received<<"\n\n";            
    }
    return 0;
}

Much easier to read and understand, than the original code on the Microsoft documentation site. Cheers!

Friday, July 11, 2014

El Nino, California Drought

NOAA performs a vital a national and international service in monitoring ocean and atmospheric conditions, recognizing patterns, and classifying and predicting significant climate events.

One of the somewhat-well-understood phenomena is the El Nino Southern Oscillation cycle. Characterized by warmer-than-average ocean temperatures across the Pacific Ocean, and a weakening or reversal of the prevailing Pacific Trade winds, El Nino years tend to bring unusual weather and some extreme weather events. In past El Nino years, we've seen unusual shifts in precipitation, including drought in some places, flooding and even snow in others.

All this is of interest to the drought-stricken American West and Southwest, because strong El Nino years tend to correlate with above-average rainfall in these regions. NOAA maintains a public communication, including monthly updates here. Currently, we are in an El Nino advisory state. A more detailed document, detailing various observations and methodologies is here.

Overall, NOAA predicts the probability of an El Nino by fall or winter at about 80%, but the El Nino is likely going to be "weak to moderate'.




I really enjoyed viewing the satellite imagery, and temperature time series in the .pdf document. I also enjoyed the numerous temperature prediction models, split into "Statistical" and "Dynamical" models. They also included an ensemble prediction. Good stuff.

Thursday, June 26, 2014

Conferences

Last week, I attended the IEEE Intelligent Vehicles Symposium, hosted this year in Dearborn, Michigan. It was nice to run into old colleagues from UCSD, as well as former collaborators from places like Toyota, and researchers from around the world.

I've always enjoyed attending conferences, as they provide a forum for exchanging new ideas and learning about new developments. In our own labs and workplaces, we can get stuck, recycling the same ideas and approaches, trapped in our echo chambers. At a conference, we can learn from each other.

There was a lot of work presented on autonomous driving. Honestly, the topic is not very exciting anymore. Autonomous driving has been done reliably by many groups, and has really transitioned from a research problem into an engineering problem. Now car companies are figuring out how to bring autonomous driving to the market and make profits from it.

In particular, a former collaborator at UCSD, Eshed Ohn-Bar, presented some of his work on hand-tracking in the vehicle, which he's had quite a bit of experience with. He also clued me in to some new work from Pietro Perona's group on object detection. It turns out that Serge Belongie, who sat on my PhD committee, is also a co-author on the paper. Good stuff.

There was a time when I was sick of doing computer vision. I took a break from it for a little while, working with some other sensors, and learning about some other areas. My plan is to get back into some computer vision on a work project later this year.


Saturday, May 10, 2014

Springtime is here!

Springtime, baby. Sunshine, warmth, hiking, camping, grilling, gardening, and more outdoor activities are on the schedule. To be fair, the Bay area doesn't get much of a winter, just a couple of months of rainy weather, which frankly, are integral to the eco-system.

That said, the transition to more a real winter, moving from San Diego to the Bay, was not insignificant. Glad to be in the sunshine for the next few months.

Saturday, May 3, 2014

Reversing C-Style Strings With Pointers

It's a common interview question to ask an applicant to reverse a c-style string, without using string utility functions. In practice, this is something you almost never do, as modern programming languages have nice, safe string classes and interfaces, most likely complete with their own string::reverse() methods.

However, the test has some value. Depending on how the applicant approaches the task, you can gauge his comfort level with pointers. You can get a feel for the applicant's efficiency with memory and operations. Do they reverse the string in place, or allocate new memory?

In particular, swapping the values of two variables, is an interesting sub-test. Can the applicant do this correctly? How do they choose to do it? They can do it in a separate function, with pointers, with references, etc.

Finally, testing. Does the applicant simply write an answer and say "tada"? Or do they try to walk through the code and test what it does, maybe catching mistakes or corner-cases?

As long as the applicant can correctly complete the task, there are no wrong answers. This test really elucidates their style and thought process.

Even though a person might scoff at such a "trivial" test that "doesn't prove anything," it turns out that the test is not so trivial for most programmers. Notably, most applicants cannot correctly complete this task the first time.

Now to the code:

1:  void reverse_str(char* str){  
2:       //Receive a c-style string, meaning a pointer to an array of type char,  
3:       //Reverse in place  
4:       char* lower=str;  
5:       char* upper=lower;            
6:       while (*upper!='\0')     //Searching for the end of a null-terminated string       
7:            upper++;       
8:       upper--;          //Decrement it; we want the last non-null element  
9:       while (upper>lower){       
10:            char temp=*lower;  
11:            *lower=*upper;  
12:            *upper=temp;                                          
13:            upper--;  
14:            lower++;  
15:       }  
16:  }  
In this example, I've used pointers. To find the length of the string, I iterate the pointer [upper] until I find the null-character. I then decrement this pointer, to the last non-null character.

Then, I swap the values of the lower and upper pointers, incrementing the lower and decrementing the upper, until they meet in the middle. And that's it.

If I wanted to be a little more elegant with my swapping, I might write a separate function to swap the values of the pointers. How might I do that?

1:  template<class T>  
2:  void swap(T* a, T* b){  
3:       //Swap the values of two pointers  
4:       T temp=*a;  
5:       *a=*b;  
6:       *b=temp;  
7:  }  

In this case, I've chosen to swap using pointers. So, lines 10-12 in reverse_str() would change to:
1:  swap(lower,upper);  

One other option could be to swap using references. It's actually pretty important that an applicant understands references in C++, as this is a common and integral feature of the language. [Maybe I'll do a follow-up entry on this at some point]. Here's a swap with references:

1:  template<class T>  
2:  void swap(T& a, T& b){  
3:       //Swap the values of two variables  
4:       T temp=a;  
5:       a=b;  
6:       b=temp;  
7:  }  

In this case, lines 10-12 of reverse_str() would need the pointers to be dereferenced:

1:  swap(*lower,*upper);  

So yeah, there you go. There are myriads of other ways to approach this problem. For such a seemingly simple task, you get a lot of mileage and information about a software engineer.

The Only Democrat in CA Who Won't Blow the Budget

This year, amidst a tax increase and an economic recovery, after years of budget cuts and massive deficits, the state of California is turning a nice surplus. Most Democrats in Sacramento want to immediately establish new permanent programs.

 Perhaps in some cases, they argue for reinstating some of the social welfare programs that were cut during the Great Recession. In other cases, they'd like to establish brand new programs, because if there's some extra money this year, why no establish new spending obligations with no guarantee of budget surpluses next year?

Need we remind them that the voter-approved tax increase was temporary, and that economic booms [Silicon Valley, Wall Street] are also temporary?

 Jerry Brown is proposing an initiative that would set aside a rainy day fund, if the fiscal year's capital gains tax comprises beyond a certain percentage of overall tax revenue. He's the most responsible politician in Sacramento. 

Thursday, May 1, 2014

Binary Search With Pointers, C++

Today I'm posting my version of binary search, using pointers in C++. Binary search is a fundamental algorithm, which searches for an element in a sorted array. It is considered the mother of all divide-and-conquer algorithms, and executes in log(N) time, where N is the size of the array. The log is taken with respect to base 2 [binary].

Implementing Binary Search with pointers is a nice exercise that brings together a few concepts.

1. While binary search is an important algorithm from a conceptual standpoint, it's also highly practical and has come up in my research and work. Consider captured data that is indexed by its timestamp, e.g.

                timestamp :  value 1, value 2,....value N
             
The most efficient way to search for the values at a particular timestamp is binary search. In reality, we may want the nearest timestamp to a given reference, but that's a minor adjustment to the algorithm.

2. Implementing binary search with pointers helps to reinforce understanding of pointers, a concept which tends to trip up many C++ programmers.

3. Despite the fact that binary search is a fundamental algorithm, very few job applicants are able to code it correctly on a whiteboard. In short, it's good practice.

In our example, we initialize an array with random elements, then sort it. First we demonstrate how we find an element in the array. Second, we demonstrate how the function behaves when we search for an unknown element. Now, to the code:

 #include <iostream> //For std::cout  
 #include <algorithm> //For std::sort  
 #include <stdlib.h> //For srand, rand   
 #include <time.h>  //For time   
   
 template<class T> //template class means we can accept arguments of different classes: int, float, double, etc.  
 int binarysearch(T* arr /*pointer to beginning of array*/, T* low, T* high, T searchkey /*seeking this value in array*/){  
      int index=-1; //Default value, if we don't find searchkey  
      if (*low>*high) //Double-checking that the array's actually sorted  
           return index;  
      if (low>high) //At this point, we've searched the whole array  
           return index;  
   
      T* mid=(high-low)/2+low; //The array index midway between high and low  
      if (*mid==searchkey)   //We've found the searchkey  
           return mid-arr;  
      if (searchkey<*mid)  
           return binarysearch(arr,low,mid-1,searchkey); //Recursive call to search the lower half of the array  
      else if (searchkey>*mid)  
           return binarysearch(arr,mid+1,high,searchkey); //Recursive call to search the upper half of the array  
   
      return index;  
 }  
   
 int main(int argc, char* argv[]){  
      //Using srand to initialize rand() with time()  
      srand (time(NULL));  
   
      //Size of our array will be 15  
      int size=15;   
      int* arr=new int [size]; //Note that arr is the array, and is also a pointer. It points to a memory block of 15 integers.  
      for (int i=0; i<size; i++){            
           arr[i]=rand()%100; //Randomly initialize the elements of the array, integers between 0 and 100  
           std::cout<<arr[i]<<",";  
      }  
      std::cout<<"\n";                 
      std::sort(arr,arr+size); //Sort the array  
      for (int i=0; i<size; i++)  
           std::cout<<arr[i]<<",";  
      std::cout<<"\n";  
   
      //Finding element in array.   
      int to_find=arr[rand()%size]; //Choosing an element at random from the array to search using binary search  
      int found_index=binarysearch(arr,arr,arr+size-1,to_find);  
      if (found_index>-1)  
           std::cout<<"Found "<<to_find<<" at location "<<found_index<<"\n\n";  
      else  
           std::cout<<"Couldn't find "<<to_find<<" in array\n\n";  
   
      //Finding unknown element  
      to_find=100; //We initialized the array with elements strictly smaller than 100  
      found_index=binarysearch(arr,arr,arr+size-1,to_find);  
      if (found_index>-1)  
           std::cout<<"Found "<<to_find<<" at location "<<found_index<<"\n";  
      else  
           std::cout<<"Couldn't find "<<to_find<<" in array\n";  
   
      delete [] arr; //Clean up dynamically allocated memory  
      return 0;  
 }  

Tuesday, April 8, 2014

Money and free speech

Corporations became people, and money became free speech. So progressed the Second Gilded Age.

While the Supreme Court has deliberated on the limits that the government can set on free speech, with respect to election financing, they have missed a major point: Corporations are not people, and spending money is not the same as speaking freely.

If I support a candidate, I can tell all my friends about him. I can write about him on my website, social media, or newspaper editorial, in unlimited quantities. None of these actions cost a penny. Campaign finance laws were not designed to stifle free speech. In fact, they enhance it, by rendering money alone slightly less potent. Giving somebody money is not free speech.

Corporations are not people. Corporations are legal entities, set up to limit the liability of individuals who directly participate in the market system. If you own stocks in a company that tanks, you do not lose your home, just the value of your stocks. This is the intended purpose of corporations: limited liability.

It's clear that Justice John Roberts considers any limits on free speech abhorrent, which is commendable. However, in recent years he has conflated "people" with "limited liability organizations designed to protect the personal assets of people." He has confused "speech" with "money that can be spent on, among other things, disseminating opinions."

Tuesday, April 1, 2014

Initializing the site...

Initializing this blog

I'm just creating a first post, to initialize the blog, and figure out how some other things work. 

It's just rained for the past week, which is great news, given California's ongoing drought situation. The rainfall will do little to make up for 3 years of drought, but at least the hills will be green for a couple of months.