OAuth2 Notes

very hard to understand if you just check it from different web sources: people simply had wrong understanding or assumption on it.

OAuth:
it is not authentication nor authorization, it is about delegation protocol that is scalable.
I delegate access to someone to do something for me.

Client id: client should be registered with oauth server the first. along its id, usually there is a redirect_uri.
redirect_uri: is uased to verify the request is valid, and used to send authentication code back if the app is a web application.

access token: like session. used for secured api calls
refresh token: like a password to get new access token.

by reference token: reference is stored somewhere, it will convert reference to by value token for accessing apis
by value token: token will full information.

bearer token: like cash, when you spend it, no body ask for identification
holder of key token: like credit card. asking for identification. not shared by other users.

id token is for client to build a meaningful session between its client app and server app.
access token is meant for apis.

four roles:

user/me, client/application, authorization server/oauth server, resource server/where api or data is

steps:typical scenario

--client asks authorization erver for accessing resources on resource server,
--authorization server says sure if user agreed. it redirect client to user login page
--user signs in authorizatoin to complete authorization. once the authenticcation is successful, authorization server issues an authentication code to client app via redirect_uri.
--client app uses authentication code, its clilent id, and secrect to ask for an access token.
--client app now uses access token to access resources owned by user/on behalf of user.
--resource server can call authorization server to check if the token is valid. usually it does not need to and it simply check if the signature is trustable.
--resource server then provides resources to client app if token is valid.

it usually works with openid. after user logging in, the authorization server also returns id token that contains information about the user.
client app(server end) uses this id token to build an user session between client app's client and server.

in microservice, let each service understand JWT. and pass around JWT when it needs to call out for other services.

ID token is JWT token, JWT can also be access token.

token can expire, usually a refresh token is given at same time for client app to renew access token.

about exchanging token.
https://www.youtube.com/watch?v=1ZX7554l8hY

token has access scope
access token can be in bearer header, query string or payload, depending on oauth provider.

Client:
confidential client: web server etc
public client: model app, javascript in useragent etc

grant type:
two legged:
client credentials: accessing own resources.
    you provie client id and client secret/password to get access token.
    usually used on server side since it is OK when you can hide the client secret in server side configuration or code.
 
resource owner credentials:
    this is user strongly trust client app, and give out its own user and password to client app.

implicit:usually in javascript code
    https://tools.ietf.org/html/rfc6749#section-4.2
    It is designed for applications that access APIs only while the user is present at the application
 
    client app directs user to auth server to express authentication
    oauth server redirect res owner back to client app along with access token
    client app uses access token to access res user's resources on behalf of user
    it does not have refresh token since the client app is not authenticated.it was driven directly by user himself.
    (authentication code is for server that is proxy of user.)
    since access token is viewiable to user on same computer, it's required to be passed only within secured transport.
 
redirect_uri is defined as part of client login in oauth server, it includes redirect_uri as optional configuration. redirect_uri is
a mean of verification in implicit grant, not a mean of communication. but it is mean of communication in authentication code grant.
 
 
 
three-legged:
    authentication code: accessing other's resources

About Hash

Hash table

This is a key-value look up data structure.

You can think it is an Array coupled with hash function. Hash function takes in key and output an integer as index in array, then it stores the key and value under the index.

Key is required to be stored for the reason of collision hanlding. Key's equals() function is used to determine a key that is in hash collision.

Hashtable is roughly same as HashMap in Java, except it's multi-thread safe and does not allow null key and null value.

Hash Set

In java it is a hash table that stores key itself as its look up value.

Hash Map

This is a hash table, but not thread safe and allow null key and null value.


Collision

solution is collision is linear probing and (separate) chaining, as well as doubling hashing. linear probing can lead to a problem of clustering (major drawback of linear probing) when a lot of collisions happen. chaining is a solution Java is using.

double hashing use a fomular with second hash function involved when first hash function has a collision.

Tried to google Java or XSLT solution to convert XML to tabular data format, butcould not find an easy to understand or to use one. So that I made one myself.

https://github.com/shijiema/JavaXmlTabulator

Java Xml Tabulator

Converting XML to tabular form of data in pure Java implementation. No third party library required.
For am Xml such as

<Relations>
 <Relationship p1="v1">some text
     <id>1</id>
  <Type>OneToMany</Type>other text
  <Weight>1.0</Weight>
  <Score>100.0</Score>
 </Relationship>
 <Relationship>
  <id>2</id>   noise 3
  <Type>ManytoOne</Type>
  <Weight>1.0</Weight>
  <Score>90.0</Score>
 </Relationship>
</Relations>

It will convert it to a flatten version of data that if iterating, looks like this:

[Relations_Relationship, Relations_Relationship_Score, Relations_Relationship_Type, Relations_Relationship_Weight, Relations_Relationship_id, Relations_Relationship_p1]
[some textother text, 100.0, OneToMany, 1.0, 1, v1]
[noise 3, 90.0, ManytoOne, 1.0, 2, null]

It has two ways to access transformed data.
First method is through its iterator() method, which will return each row as List. First row is header and subsequent rows are body content. Headers and body content have been aligned.
Second method is through its getHeaders() and getBody() if partial data access is what is wanted.

Algorithm

Observations:

1. non-repeat elements in XML could be treated as parent node's attributes
2. repeat element in XML usually means multiple rows after being flattened
3. path from root to node makes the columns in tabular format

tablify(){
    with XML tree,
    1. merge non-repeat elements to their parents
        1.1 from leaf to root, merge non-repeat children element to its parent as its parent's attributes. this includes both text and its attributes
            attribute name for child node is child node element name; attribute value for child node element is child node's text value
            attribute name for child node attributes are child node element name + child node attribute name; value is child element attribute value
        1.2 remove these children from their parents
        1.3 repeat 1.1 and 1.2 until no more such children exists
    2. make node production from leaf to root
        2.1 for a node, make its equivalent node production
                2.1.1 leaf node's children node is null
                2.1.2 rows of repeat children element 1 * rows repeat children element 2 * rows repeat children element n
                2.1.3 insert this parent node to head of each produced row from 2.1.1

        2.2 do 2.1 for all parent nodes(null node's parent is the leaf node), but stop at root element
        2.3 do 2.1 for root element(this is because different path has different depth, they have to wait to do final production)
    3. in each row in final node production, convert node to columns
        3.1 node column name = path to node; node value = text content in node
        3.2 node attribute column name = path to node + attribute name; node attribute column value = attribute value
        (this works well for non-repeat node wrapped as parent node's attribute)
    4.return key-value paired node production
}

• Shi Jie Ma - Initial work

Note: Statistics, Data mining, Maching learning and Artifical integgligence

Reading from http://stats.stackexchange.com/questions/5026/what-is-the-difference-between-data-mining-statistics-machine-learning-and-ai

 'unsupervised' (we don't know the answer--discovery) or 'supervised' (we know the answer--prediction

• Statistics quantifies numbers
• Data Mining explains patterns
• Machine Learning predicts with models
• Artificial Intelligence behaves and reasons

Recursion: Davis' Staircase

Recursion, if computation is repeated, memorization can be used to make it linear. Some of them, if asking summary of something, DP can usually be used to solve it.

In this post, I am providing DP solution instead of recursion, which should be easily be written with memorization as an optimization.

Davis has staircases in his house and he likes to climb each staircase , , or steps at a time. Being a very precocious child, he wonders how many ways there are to reach the top of the staircase.
Given the respective heights for each of the staircases in his house, find and print the number of ways he can climb each staircase on a new line.

Input Format

The first line contains a single integer, , denoting the number of staircases in his house.
Each line of the subsequent lines contains a single integer, , denoting the height of staircase .

Constraints

• 
  
• 
  

Subtasks

• for of the maximum score.

Output Format

For each staircase, print the number of ways Davis can climb it in a new line.

Sample Input

3
1
3
7

Sample Output

1
4
44

Explanation

Let's calculate the number of ways of climbing the first two of the Davis' staircases:

1. The first staircase only has step, so there is only one way for him to climb it (i.e., by jumping step). Thus, we print on a new line.
2. The second staircase has steps and he can climb it in any of the four following ways:
   
   1. 
      
   2. 
      
   3. 
      
   4. 
      

Thus, we print on a new line.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

public class Solution { static void numWays(int n){ //let dp[i] is num ways to reach stair i. //each dp[i] is an accumulation of ways to reach to there int dp[] = new int[n+1]; //assuming you are at nth stair, to reach there //you can step 1 from n-1th stair //or step 2 from n-2th stair or //step 3 from n-3th stair //so you have that many ways to reach to nth stair. //adding them up is the ways to reach to nth stair //dp[3]=dp[2]+dp[1]+dp[0]; /* to reach to stair 1, you have one way:dp[1]=1=dp[0] to reach to stair 2, you have: 1 from dp[0], that is 1 way,2. from dp[1], that's 1 way. to reach to stair 3, you can : 1. dp[0]->dp[3],2.dp[2]->dp[3],3. dp[1]->dp[3] whatever you use to reach dp[2] now contributes to dp[3] */ if(n==1){ System.out.println(1); }else if(n==2){ System.out.println(2); }else{ dp[0]=1;//0 is base dp[1]=1;dp[2]=dp[1]+dp[0]; for(int i=3;i<=n;i++){ dp[i]=dp[i-1]+dp[i-2]+dp[i-3]; } System.out.println(dp[n]); } } public static void main(String[] args) { Scanner in = new Scanner(System.in); int s = in.nextInt(); for(int a0 = 0; a0 < s; a0++){ int n = in.nextInt(); numWays(n); } } }

Coin Change-- number of ways

Given a number of dollars, , and a list of dollar values for  distinct coins, , find and print the number of different ways you can make change for  dollars if each coin is available in an infinite quantity.

Hints:

• You can solve this problem recursively, but you must optimize your solution to eliminate overlapping subproblems using Dynamic Programming if you wish to pass all test cases. More specifically, think of ways to store the checked solutions and use the stored values to avoid repeatedly calculating the same values.
• Think about the degenerate cases: 
  
  • How many ways can you make change for  dollars?
  • How many ways can you make change for less than  dollars if you have no coins?
• If you are having trouble defining the storage for your precomputed values, then think about it in terms of the base case .

Input Format

The first line contain two space-separated integers describing the respective values of  and .
The second line contains  space-separated integers describing the respective values of , where each integer denotes the dollar value of a distinct coin available in an infinite quantity.

Constraints

• The list of coins contains  distinct integers where each integer denotes the dollar value of a coin available in an infinite quantity.

Output Format

Print a single integer denoting the number of ways we can make change for  dollars using an infinite supply of our  types of coins.

Sample Input 0

4 3
1 2 3 

Sample Output 0

4

Explanation 0
For  and  there are four solutions:

Thus, we print  on a new line.

Sample Input 1

10 4
2 5 3 6

Sample Output 1

5

Explanation 1
For  and  there are five solutions:

Thus, we print  on a new line.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

package dp; import java.util.Scanner; /** * first successful story by analyzing it and coming up an algorithm by myself * --even though I saw this question before, I basically forgot. * Them came up a acceptable solution within 15 minutes. * * Cheers to myself. * @author Andrew Ma * */ public class CoinChangeNumOfWays { public static long makeChange(int[] coins, int money) { //I assumes 0 denom having 0 solutions to any amount of money //also assumes any denom having 0 solution to 0 amount of money //it turns out to be right! to be brave to make assumptions! //making room for 0 denom and 0 amount of money long dp[][] = new long[coins.length+1][money+1]; //initialization the known solutions //0 denom for(int i=0;i<money+1;i++){ dp[0][i]=0;//no solution for each money value } //0 money value for(int i=0;i<coins.length+1;i++){ dp[i][0]=0;//no coin can do 0 change } //then build up values towards to final solution //c and m here are for dp array's dimensions /* * define dp[c][m] as accumulated solutions at c and m * dp[c][m]= * 1. dp[c-1][m]. when money value is less than denom, then get solution from last denom * --here it needs to have a 0 denom * 2. dp[c-1][m]+1. when money value equals denom value, we get one more solution * 3. dp[c-1][m] + dp[c][m-coins[c-1]]. when money valus is greater than denom value, * it adds up accumulation from previous denom and same denom for meney value * not including this denom. * damn, when did I become so able to analyze? I guess drawing it out and practice on paper or white board * really helped to see one's thoughts and then you just need to write the code to reflect the thoughts. * and you are confident that you are able to write code to reflect your thoughts. */ for(int c=1;c<coins.length+1;c++){ for(int m=1;m<money+1;m++){ //when it comes to refer values in coins, c need to be converted back to 0 based if(m<coins[c-1]){ //no change at this, copy the last denom's accumulated solutions dp[c][m] = dp[c-1][m]; } else if(m==coins[c-1]){ //we get one more solution dp[c][m] = dp[c-1][m] +1; } else //if (m>coins[c-1]) { //then it is last denom's accumulated solution + lesser value's accumulated solution dp[c][m] = dp[c-1][m] + dp[c][m-coins[c-1]]; } } } return dp[coins.length][money]; } public static void main(String[] args) { Scanner in = new Scanner(System.in); int n = in.nextInt(); int m = in.nextInt(); int coins[] = new int[m]; for(int coins_i=0; coins_i < m; coins_i++){ coins[coins_i] = in.nextInt(); } System.out.println(makeChange(coins, n)); } }