Function analysis and source code of hash guessing game system development

　　 Hash (Hash) Algorithm , Hash function . It's a one-way cryptosystem , That is, it is an irreversible mapping from plaintext to ciphertext , Only the encryption process , No decryption process . meanwhile , Hash function can change the input of any length to get fixed length output . This one-way feature of the hash function and the fixed length of the output data make it possible to generate messages or data .

　　 The hash algorithm ( Hash algorithm or message digest algorithm ) It is a basic technology used in information storage and query , It is based on Hash File construction method of function , Map a given key value of any length to a hash value of fixed length , Generally used for authentication 、 authentication 、 encryption 、 Index, etc. . Its main advantage is that the operation is simple , Short pretreatment time , Low memory consumption , Matching search speed is relatively fast , Easy to maintain and refresh , Support more matching rules .

　　Hash Methods of constructors

　　1. direct addressing ：

　　 Direct addressing is based on data element keywords k Itself or its linear function as its hash address , namely ：H（k）=k or H（k）=a×k+b;( among a,b Constant )

　　2. Digital analysis ：

　　 Suppose that every keyword in the keyword set is created by s Digit composition (u1,u2,…,us), Analyze the whole set of keywords , And a number of evenly distributed bits or their combination are extracted as addresses .

　　 Numerical analysis is a method of taking some digits with uniform values in the keywords of data elements as hash addresses . That is, when there are many digits of keywords , You can analyze the keywords , Lose the unevenly distributed bits , As Hashi is worth . It is only suitable for all cases where the keyword value is known . By analyzing the distribution, the keyword value range is transformed into a smaller keyword value range .

　　3. Folding method ：

　　 Split keywords into parts , Then take the sum of them as the hash address . Two methods of superposition processing ： Shift stacking : Align and add the low order of the divided parts ; Boundary overlay : Fold back and forth along the division boundary from one end , And then align and add .

　　 The so-called folding method is to divide keywords into parts with the same number of digits （ The number of digits in the last part can be different ）, And then you take the sum of these parts （ Give up the carry ）, This method is called folding method . This method is applicable to keywords with many digits , And the number distribution on each digit in the keyword is roughly uniform .

　　 In the folding method, digital folding is divided into two methods: shift superposition and boundary superposition , Shift superposition is to align the lowest bit of each part after segmentation , Then I add ; Boundary superposition is folding back and forth along the division boundary from one end to the other , And then align and add .

　　 Hash property ：

　　(1) One way .That is,given an input number,it is easy to calculate its hash value,but given a hash value,the original input number cannot be obtained according to the same algorithm.

　　(2) Weak anti-collision .That is,given an input number,it is computationally infeasible to find another hash value to get a given number when using the same method.

　　(3) Strong collision resistance .That is,for any two different input numbers,it is not feasible to calculate the same hash value according to the same algorithm.