/*程式說明:
*   程式是由順序表實現,功能為查詢兩字串中的最長公共子串;
*   兩條字串分別由兩個順序表儲存;
*   查詢思路是分解出短的那條字串的全部連續子串(例如abc分解為abc,ab,a,bc,b,c),
*   再使用kmp演算法將該全部子串與長的那條字串進行匹配,找到最長的那條子串;

*   完成日期：2017.5.20
*   編譯環境：visual studio 2015
*/

#include<iostream>
using namespace std;
const int MaxNum = 100;
struct SeqString//順序表結構
{
    int
 
 length;//長度
    char c[MaxNum];//儲存資料
};
typedef struct SeqString* PSeqString;

PSeqString creatString();
void find(PSeqString t, PSeqString p);
int KMPmatch(PSeqString t, PSeqString p2, int* next);
void makenext(PSeqString p, int* next);
void display(PSeqString pstr);

int main()
{
    PSeqString p = new
 
 SeqString;
    p = creatString();
    PSeqString t = new SeqString;
    t = creatString();

    find(p, t);
    cin.get(); cin.get();//暫停
}

PSeqString creatString()//建立並初始化一條字串
{
    PSeqString pstr = new SeqString;
    cout << "輸入其中一個字串:\n";
    cin >> pstr->c;
    pstr->length = strlen
 
(pstr->c);
    return pstr;
}

void find(PSeqString t, PSeqString p)//查詢公共子串
{
    PSeqString temp = new SeqString;
    if (t->length < p->length)//交換使得p串為更短的那個字串
    {
        temp = t;
        t = p;
        p = temp;
    }
    int* next = new int[MaxNum];//next陣列
    int len = 0, maxlen = 0, loc = 0;//len為子串長度,maxlen為匹配成功的最長子串長度,loc為開始匹配子串的首下標
    PSeqString p2 = new SeqString;
    int i, k, l; //i,j,k都是是字串的下標

    //利用迴圈取不同的i和len，以達到取p不同的子字串的目的
    for (int i = 0; i < p->length; i++)//改變i,此處i是p字串的下標
    {
        for (len = p->length - i; len > 0; len--)//改變len
        {
            p2->length = len;
            for (k = 0, l = i; k < len; k++, l++)//使得p2為即將進行匹配的子串
            {
                p2->c[k] = p->c[l];
            }
            makenext(p2, next);//獲取p2的next陣列

            if (KMPmatch(t, p2, next) && len > maxlen)//子串匹配成功且len最大則儲存len與i
            {
                maxlen = len;
                loc = i;
            }
        }
    }

    p2->length = maxlen;
    for (k = 0, l = loc; k < maxlen; k++, l++)//使得p2為最長公共子串
    {
        p2->c[k] = p->c[l];
    }

    cout << "最長的公共子串為: ";
    display(p2);
    delete(p2);
    delete(temp);
}

int KMPmatch(PSeqString t, PSeqString p2, int* next)//kmp演算法字串匹配
{
    int i = 0, j = 0;
    while (i < p2->length&&j < t->length)
    {
        if (i == -1 || p2->c[i] == t->c[j])
        {
            i++, j++;
        }
        else
            i = next[i];
    }
    if (i >= p2->length)
        return 1;
    else
        return 0;
}

void makenext(PSeqString p, int* next)//kmp演算法求next陣列
{
    int i = 0, k = -1;
    next[0] = -1;
    while (i < p->length - 1)
    {
        while (k >= 0 && p->c[i] != p->c[k])
            k = next[k];
        i++, k++;
        if (p->c[i] == p->c[k])
            next[i] = next[k];
        else
            next[i] = k;
    }
}

void display(PSeqString pstr)//顯示pstr順序表中的資料
{
    for (int i = 0; i < pstr->length; i++)
    {
        cout << pstr->c[i];
    }
}