/*  
	September 6, 1999.
  Submission to MacTech Programmer's Challenge for September 99.
  Copyright © 1999, Ernst Munter, Kanata, ON, Canada.
  
  				"Playfair Decipher"
  				
  Version 2
  
Problem Statement
-----------------
Given a dictionary, a cipher text, and the encoding method, break the code
and return the deciphered plaintext.

Solution
--------
My strategy is to decode the ciphertext with each possible keyword
until a plain text results which is accepted by the spell checker.

In InitPlayfair() the dictionary is scanned twice.

First, the set of decoders, one for each keyword, is constructed.  Because
similar keywords can result in identical coding matrices, redundant matrices
are discarded where the keywords are within a certain distance of each other
in the dictionary.

Secondly, a spell check tree is built which indexes all words in the 
dictionary that are distinct in the first 5 characters (stem).  
Longer words are checked by indexing to the stem, and scanning the group of
words which share the same stem directly in the dictionary.  

In addition, all trigraphs (sequences of 3 chars) in the dictionary words
and digraphs marking start and end of each word are collected.  

In DecodePlayfair() we use each of the keywords in turn to decode the cipher
text into a tentative plain text.  If during the plain text construction
any illegal trigraphs are encountered, the decoding breaks off, and the next
keyword decoder is selected.

If decoding passes the trigraph check (about 1 in 20 to 1 in 50), the text
is submitted to the spell checker which recursively tries to match the text
to dictionary words until the whole plain text has passed the check.  
The spell checker builds a stack of dictionary indices matching the found
words.  The first successfully checked plain text is returned to the caller
by copying the indexed dictionary words into the final plain text.

Complications
-------------
There are two sources of complications for the spell check.

The letters 'J' and 'I' are encoded as if they are the same.  The decoding
step always yields 'I'.  The spell checker allows this aliasing with 
additional tree branches (alias nodes).  Nevertheless, there can be true 
ambiguity which can not be resolved.  For example the dictionary words 
"JON" and "ION" cannot be distinguished.

The encoder inserts 'X' (or 'Z' in some cases) if the two characters of a 
letterpair are the same ("BLEEP" becomes "BLEXEP".  But it does not insert 
'X' between pairs, thus "BEEP" remains "BEEP".  In addition, there are words
which might be ambiguous, e.g. SEES and SEXES, both of which are in the
dictionary.  The spell checker also provides alias nodes for such cases.

My spell checker is built on the simplifying assumption that no double 
letters occur in the raw decoded text.  'X' is always expected between 
double letters.  This also improves the efficiency of trigraph checking.
To make this assumption true, the decoder inserts 'X' between letter pairs 
if necessary, and marks any 'X' which occurs as the first character in a 
letter pair as a "hard" X, because this 'X' is never a filler 'X'.  

Limits and Assumptions
----------------------
CipherText and the dictionary contain only words using 'A' - 'Z'.

The dictionary is sorted.

The program uses the standard (64K) program stack and allocates a word 
stack on the heap for up to 400 words in the resulting plain text. 

The program uses a recursive function which takes up to 144 bytes on the 
call stack and recurses 1 level per word found.  The standard stack size 
of 64K is only sufficient for plain text messages of about 300 words.

NOTE: 	If it is desired to decode longer texts than about 300 words,
		Please change the constant "kMaxWords" accordingly and increase 
		the call stack allocation by 144K for every 1000 words.

The array parameter "char* plainText" in DecodePlayfair() is assumed to
be of sufficient size to hold the resulting plain text PLUS an allowance
for a slightly larger intermediate raw plain text (50% extra will cover
even pathological cases). 

Version 2 changes
-----------------
Fine tuning for speed (15% gain), but no logical changes.  Techniques:
- use of unsigned char instead of char to avoid extend-signbit instruction
- reordering of tests in Legal() to favour most frequent case
- review of need of masking of characters.  1 << c, where c is an
  uppercase character.  This works without mask because shift is modulo 64.
*/
#include "Playfair.h"
#include <string.h>

typedef unsigned char uchar;
typedef unsigned long ulong;
typedef unsigned short ushort;
typedef const char* Cptr;

static const char** dict;
static int 	numDecoders;

enum {
	kMaxWords 	= 400,	// Increase for longer texts, also increase stack
	kMaxNodes	= 27*32*32,
	kDepth		= 16
};

enum {
	kXFLAG		= 1,
	kJFLAG		= 1<<27,
	kZFLAG		= 1<<28,
	kAll		= 0x07FFFFFF,
	
	kStart		= 27,
 	kStartOfWord= 1<<kStart,
	kEndOfWord	= 1,
	
	kJbit		= (1 << (31 & 'J')),
	kCharSet	= 0x07FFFFFE & ~kJbit
};

// Rule implements the encoding rule in a static look up table
static struct Rule {
	short	LUT[25][32];
	Rule()
	{
		int spot1=0;
		for (int r1=0;r1<5;r1++)
		for (int c1=0;c1<5;c1++,spot1++)
		{
			int spot2=0;
			for (int r2=0;r2<5;r2++)
			for (int c2=0;c2<5;c2++,spot2++)
			{
				int vr1,vr2,vc1,vc2;
				if (r1==r2)
				{
					vr1=r1;
					if (c1) vc1=c1-1;else vc1=4;
					vr2=r2;
					if (c2) vc2=c2-1;else vc2=4;
				}
				else if (c1==c2)
				{
					vc1=c1;
					if (r1) vr1=r1-1;else vr1=4;
					vc2=c2;
					if (r2) vr2=r2-1;else vr2=4;
				}
				else
				{
					vr1=r1;vc1=c2;
					vr2=r2;vc2=c1;
				}
				LUT[spot1][spot2]=
					((vr1*5+vc1) << 8) 
					+(vr2*5+vc2);
			}
		}
	}
} R;
	
/******* Trigraphs and set of related functions **********/

static ulong trigraph[27][32];

inline void Register(ulong c)
// Single character words 	
 	{
 		for (int i=1;i<=26;i++)
 		{
 			trigraph[i][c] |= kEndOfWord;
 			trigraph[c][i] |= kStartOfWord;
 		}
 	}
inline void Register(ulong c1,ulong c2)
// Two character words 
 	{
 		trigraph[31 & c1][31 & c2] |= kEndOfWord | kStartOfWord;
 	}
inline void Register(ulong c1,ulong c2,ulong c3)
// Three character words 
 	{
 		trigraph[31 & c1][31 & c2] |= kStartOfWord | (1 << c3);
 		trigraph[31 & c2][31 & c3] |= kEndOfWord;
 	}	
inline void Register(ulong c1,ulong c2,ulong c3,ulong c4)
// Four character words 
 	{
 		trigraph[31 & c1][31 & c2] |= kStartOfWord | (1 << c3);
 		trigraph[31 & c2][31 & c3] |= 1 << c4;
 		trigraph[31 & c3][31 & c4] |= kEndOfWord;
 	}	
inline void RegisterHead(ulong c1,ulong c2,ulong c3,ulong c4,ulong c5)
 	{
 		trigraph[31 & c1][31 & c2] 		|= kStartOfWord | (1 << c3);
 		trigraph[31 & c2][31 & c3] 		|= 1 << c4;
 		trigraph[31 & c3][31 & c4] 		|= 1 << c5;
 	}	
inline void RegisterTail(ulong c1,ulong c2,const uchar* w)
 	{
 		ulong c3=31 & *w;
 		while (c3)
 		{
 			trigraph[31 & c1][31 & c2] |= 1 << c3;
 			c1=c2;c2=c3;c3=*++w;
 		} 
 		trigraph[31 & c1][31 & c2] |= kEndOfWord;	
 	}	
 	
static void RemoveDoubleLetters()
// Removes all double letter cases from the trigraphs and replaces
// them with the escape sequences using 'X' or 'Z' as appropriate	
	{
		for (int c1=1;c1<=26;c1++)
		{
			ulong set=trigraph[c1][c1];
			if (0==set) continue;
			int sub;
			if (c1==(31 & 'X')) sub=31 & 'Z'; else sub=31 & 'X';			
			trigraph[c1][sub] |= 
				(set & kStartOfWord) | (1 << c1);
			if (set & kEndOfWord)
				trigraph[sub][c1] |= kEndOfWord;
				
			trigraph[c1][c1] = 0;
			for (int c2=1;c2<=26;c2++)
			{
				set=trigraph[c1][c2];
				if (set & (1 << c2))
				{
					if (c2==(31 & 'X')) sub=31 & 'Z'; else sub=31 & 'X';	
					trigraph[c1][c2]=set & (~(1 << c2)) | (1 << sub);
				}
			}
		}
	}
	
	
inline bool LegalStart(ulong cpair)
{
	return (trigraph[31 & (cpair>>8)][31 & cpair] & kStartOfWord);
}
inline bool LegalEnd(ulong cpair)
{
	return (trigraph[31 & (cpair>>8)][31 & cpair] & kEndOfWord);
}
inline bool Legal(ulong cp1,ulong cp2)
{
	return (
		(trigraph[31 & (cp1>>8)][31 & cp1] & 		// ABC.?
			(kEndOfWord | (1 << (31 & (cp2 >> 8))))) ||
// note: mask needed here because cp2.hi might be lower case 'x'
//		 however, it costs nothing here since >>8 and &31 is one instr.
			
		(trigraph[31 & cp1][31 & (cp2 >> 8)] & 		// ?.BCD
			(kStartOfWord | (1 << (cp2)))) ||
			
		(LegalEnd(cp1) && LegalStart(cp2)) 			// AB.CD
	);
}

/************** Decoder Array, one per keyword ***************/

static struct Decoder{
	uchar	matrix[25];
	uchar	spot[27];	
#if KWD	
	const char*	kwd;	// useful in debugging
#endif	

	void Init(const char* keyword)
	{		
// Sets up the unique matrix and its inverse (=spot) for one keyword 	
#if KWD	
		kwd=keyword;
#endif						
		const	char* kp=keyword;
		uchar	line[25];
		uchar*	lp=line;
		
		uchar 	nextC=*kp;
		ulong	mask=kCharSet;// make sure there is no 'J'
			
		// write unique letters in a line	
		for (;;)
		{
			uchar c=nextC;
			nextC=*++kp;
			if (c==0) break;
			
			if (c=='J') c='I';
			int bit=1 << c;
						
			if (mask & bit) {
				mask &= ~bit;
				*lp++ = c;
			}
		}
		int numUnique=lp-line;
	
		// write remaining letters
		for (int c='A';c<='Z';c++)
		{
			if ((mask & 2)) *lp++=c;
			mask >>= 1;
		}
		
	
		// transpose line into matrix
		uchar* mp=matrix;
		int k=0;
		for (int i=0;i<numUnique;i++) {
			for (int j=i;j<25;j+=numUnique) {
				int c=line[j];					
				*mp++ = c;
				spot[31 & c]=k++;
			}	
		}
	}
	
	bool SameMatrix(Decoder* dp)
	{
// Compares matrizes to help in eliminating unneeded duplicates	
		ulong* a=(ulong*)matrix;
		ulong* b=(ulong*)(dp->matrix);
		if (a[0] != b[0]) return false;
		if (a[1] != b[1]) return false;
		if (a[2] != b[2]) return false;
		if (a[3] != b[3]) return false;
		if (a[4] != b[4]) return false;
		if (a[5] != b[5]) return false;
		return true;
	}
	
	ulong Decode(ulong cipherPair)
	{
// Decodes one character pair
// Hard 'X' is identified by lower case 'x'	
		int spot0=spot[31 & (cipherPair>>8)];
		int spot1=spot[31 & cipherPair];
		int spotPair=R.LUT[spot0][spot1];
		uchar c0=matrix[spotPair >> 8];
		if (c0=='X') c0='x';			// "hard" X
		uchar c1=matrix[31 & spotPair];
		return (c0 << 8) | c1; 
	}
	
	int DecodeCipher(const char* cipherText,char* plainText)
	{
// Decodes the entire cipherText into a tentative plaintext
// Inserts 'X' or 'Z' between double letters at pair boundaries
// 		to accomodate spell checker method	
		const char* ct=cipherText;
		char* pt=plainText;										
		
		ulong cipherPair=*((ushort*)ct);ct+=2;	
		ulong plainPair1=Decode(cipherPair);
		
		if (0==LegalStart(plainPair1)) {						
			return 0;
		}	
		
		*((ushort*)pt)=plainPair1;pt+=2;
		
			
		for (;;ct+=4)
		{
			ulong cipherWord=*((ulong*)ct);					
			if ((cipherWord & 0xFF000000) == 0)
				break;
			
			ulong plainPair0=Decode(cipherWord >> 16);
			if (0==(0x1F & ((plainPair0 >> 8) ^ plainPair1)))
			{
				if ('X'==(0x5F & plainPair1)) *pt++ = 'Z';
				else *pt++ = 'X';
			} else if (!Legal(plainPair1,plainPair0)) 		
				return 0;
				
			*((ushort*)pt)=plainPair0;pt+=2;
			if ((cipherWord & 0x0000FF00) == 0) 
				break;
			
			plainPair1=Decode(cipherWord);
			if (0==(0x1F & ((plainPair1 >> 8) ^ plainPair0)))
			{
				if ('X'==(0x5F & plainPair0)) *pt++ = 'Z';
				else *pt++ = 'X';
			} else if (!Legal(plainPair0,plainPair1)) 	
				return 0;
			
			*((ushort*)pt)=plainPair1;pt+=2;
		}
		pt[0]=0;										
		return pt-plainText;	
	}
	
} *D;

// The spell checker tree is a three level hierarchy:
//		Node -> List -> Stem -> dictionaryWord

// Nodes are in a 3-dimensional table accessed by the first 3 characters 
// A node contains pointers to up to 26 lists ,
// A list contains up to 26 stems ,
// A stem contains a pointer to a dictionary word,
//		and the number of words in the group with the same stem.

// Shorter words are referenced by using the 0-index at each level.

struct Stem {
	const 	char** word;// a dictionary word
	int 	numWords;	// number of words with common stem
	
	void Add(ulong w)
	{
		if (word==0) {
			word=dict+w;
			numWords=1;
		}	
		else numWords++;				
	}
	
	bool InsertedX(const uchar c,const uchar* pt,int len) const 
	{
		uchar c0=pt[len-1];
		if ( (c0==pt[len+1]) &&
			 ( (c=='X') || ((c=='Z') && (c0=='X')) ) )
		{
			return true;
		} 
		return false;	 
	}
	
	bool JtoI(const uchar c1,const uchar c2) const
	{
		return ((c1=='J') && (c2=='I'));
	}
	
	const char* MatchTail(const uchar* pt,int & len,int numX,
		int curWord) const
// Matches string starting at the 5th letter - numX
// numX is the number of inserted X (or Z) in the first 5 letters	
	{
		if (curWord < numWords)
		{
			const char* wp=word[curWord];
			const char* temp=wp+5-numX;
			len=5;
			uchar c1;	
			while (0 != (c1 = *temp)) {
				uchar c2=pt[len];
				
				if (31 & (c1 ^ c2))
				{
					 if (InsertedX(c2,pt,len))
					 {
					 	temp--;// skipped X, repeat comparison
					 } else if (!JtoI(c1,c2))
					 	break;
				}
				len++;	
				temp++;	
			}
			if (0==*temp)
				return wp;
		}
		return 0;	
	}
};

// Lists and Nodes are dynamically allocated.
// They are descended from LinkedNode and maintained in a linked list.
// This is to allow them to be deleted when TermPlayfair() is called.
struct LinkedNode {
	LinkedNode*		link;
	LinkedNode(LinkedNode* lk):link(lk){}
}; 

struct List:LinkedNode {
	Stem	group[27];
	List*	alias;	
	ulong	xjFlag;
	
	List(LinkedNode* lk,ulong flag):
		LinkedNode(lk),alias(0),xjFlag(flag)
		{memset(group,0,sizeof(group));}
		
	void Add(ulong c,ulong w){group[c].Add(w);}
		
	const char* GetListLeader() const
	{
		ulong n=group[0].numWords;
		if (n)
			return group[0].word[0];
		return (const char*)n;//== 0; cast saves 2 instructions!
	}
};

struct Node:LinkedNode {
	List*	list[27];
	Node*	alias;
	ulong	xjFlag;
	Node(LinkedNode* lk,ulong flag):
	LinkedNode(lk),alias(0),xjFlag(flag)
	{memset(list,0,sizeof(list));}
		
	void Add(ulong c,List* lp){list[c]=lp;}
	const char* GetNodeLeader() const
	{
		List* lp=list[0];
		if (lp)
		{
			if (lp->group[0].numWords){
				return lp->group[0].word[0];
			}	
		}
		return (const char*)lp;// == 0; cast saves 6 instructions!!
	}
};

/*************** CodeBreaker ********************/

static struct CodeBreaker {
	LinkedNode*	nodeList;
	Node*		base[kMaxNodes];
	Cptr		wordStack[kMaxWords];
	Cptr*		stackPtr;
	int			cache;
	
	CodeBreaker(const char *words[],long numDictionaryWords)
		:nodeList(0),cache(0),stackPtr(wordStack)
	{
// The constructor creates the index tree from the dictionary words			
		dict=words;
		SetupDecoders(numDictionaryWords);
		memset(base,0,sizeof(base));
		for (ulong w=0;w < numDictionaryWords;w++)
		{
			const 	char* word=dict[w];
			ulong 	first=*((const ulong*)word);
			ulong	c0=31 & (first >> 24),
					c1=31 & (first >> 16),	
					c2=31 & (first >>  8),	
					c3=31 & first,
					c4=31 & word[4];
							
// Change all 'J' to 'I' as we analyse the letters	
			ulong nodeJflag=0;		
			if (c0==(31 & 'J'))
			{
				 c0=(31 & 'I');
				 nodeJflag=kJFLAG;
			}	 
			if (c1==0)		
			{
				List* lp=NewList(0,0,w);		// only a 1-letter word
				Node* np=NewNode(0);				
				base[32*32*c0]=np;
				np->Add(0,lp);
				Register(c0);
				continue;
			} 
			
// At least 2 letters:
			ulong nodeXflag=0;	
			if (c1==(31 & 'J')) 
			{
				 c1=(31 & 'I');
				 nodeJflag |= kJFLAG<<1;
			}	 
			
			if (c0 == c1)
			{
				c4=c3;c3=c2;c2=c1;
				if (c0 == (31 & 'X')) {
					c1 = 31 & 'Z';
					nodeXflag |= kZFLAG;
				} else c1 = 31 & 'X';
				nodeXflag |= kXFLAG;
			}
			
			if (c2==0)
			{
				List* lp=NewList(0,0,w);	// only a 2-letter word
				Node* np=NewNode(0);
				base[32*(32*c0+c1)+c2]=np;
				np->Add(0,lp);
				Register(c0,c1);
				continue;
			}
			
// At least 3 letters:
			if (c2==(31 & 'J')) 
			{
				 c2=(31 & 'I');
				 nodeJflag |= kJFLAG<<2;
			}	  
			if (c1 == c2)
			{
				c4=c3;c3=c2;
				if (c1 == (31 & 'X')) {
					c2 = 31 & 'Z';
					nodeXflag |= kZFLAG;
				} else c2 = 31 & 'X';
				nodeXflag |= kXFLAG;
			}
			
			
			Node* np=base[32*(32*c0+c1)+c2];
			if (np==0)
				base[32*(32*c0+c1)+c2]=
				np=NewNode(nodeXflag | nodeJflag);
			else np=GetAlias(np,nodeXflag | nodeJflag);
			
			if (c3==0)
			{
				List* lp=NewList(0);
				np->Add(0,lp);	// only a 3-letter word
				lp->Add(0,w);				
				Register(c0,c1,c2);
			} else
// more than 3 letters			
			{		
				ulong listXflag=0,listJflag=0;
				ulong numX=1 & nodeXflag;
				if (c3==(31 & 'J')) 
				{
					 c3=(31 & 'I');
					 listJflag = kJFLAG;
				} 
				if (c2 == c3)
				{
					c4=c3;
					if (c2 == (31 & 'X')) {
						c3 = 31 & 'Z';
						listXflag |= kZFLAG;
					} else c3 = 31 & 'X';
					listXflag |= kAll;// all stems in list affected
					numX++;
				} 
			
				List* lp=np->list[c3];
				if (lp==0) 
				{
					lp=NewList(listXflag);
					np->Add(c3,lp);	
				}		
				
				if (c4==(31 & 'J')) 
					c4=(31 & 'I');// no need for flag; J follows I
				if (c3 == c4)
				{
					if (c3 == (31 & 'X')) {
						c4 = 31 & 'Z';
						listXflag |= kZFLAG;
					} else c4 = 31 & 'X';
					listXflag |= 1 << c4;// only [c4] stem affected 
					numX++;
				} 
				
				lp=GetAlias(lp,listXflag | listJflag);
				
				lp->Add(c4,w);		
				
// Register all trigraphs from this word and stem				
				if (c4==0) Register(c0,c1,c2,c3);
				else
				{	
					RegisterHead(c0,c1,c2,c3,c4);
					RegisterTail(c3,c4,(uchar*)word+5-numX);
				}	
			}		
					
		} // end of loop through the dictionary	
		
// Fixup trigraphs		
		RemoveDoubleLetters();
	}
	
	~CodeBreaker()
	{
// Destructor releases all dynamically allocated nodes and lists	
		LinkedNode* np=nodeList;
		while(np)
		{
			LinkedNode* next=np->link;
			delete np;						
			np=next;
		}
		delete [] D;
	}
	
	void SetupDecoders(long numDictionaryWords)
	{
// Scans the dictionary and allocates 1 decoder per potential keyword	
 		D=new Decoder[numDictionaryWords];		
 		Decoder* dp=D;
 		const char** wp=dict;
 		for (int i=0;i<numDictionaryWords;i++,wp++)
 		{
 			dp->Init(*wp);
 			Decoder* dx=dp-1;	
 			int k=kDepth;
// Scans backwards to discover perhaps identical matrizes, and assigns
// this matrix only if it is unique.
// (Example GUESS and GUESSES yield the same matrix, no need to keep both) 			
 			for (;dx>=D;dx--,k--){			
 				if (k<=0) break;
 				if (dp->SameMatrix(dx))
 					goto cont;	
			}
 			dp++;	
 			cont:;		
 		}
		numDecoders=dp-D;			
 	}
 	
// Allocation of lists and nodes, and linkage in chain for memory mgmnt 	
	Node*	NewNode(ulong xFlag){
		Node* np=new Node(nodeList,xFlag);
		nodeList=np;								
		return np;
	}
	List*	NewList(ulong flag){
		List* lp=new List(nodeList,flag);
		nodeList=lp;								
		return lp;
	}
	List*	NewList(int index,ulong flag,int w){
		List* lp=NewList(flag);			
		lp->Add(index,w);				
		return lp;
	}
	
// Alias nodes and lists are chained off the first node or list
// with the same 3- or 4-letter key.	
	Node* GetAlias(Node* np,const ulong flag) 
	{
		if (np->xjFlag == flag) return np;
		if (np->alias==0) 
		{
			np->alias=NewNode(flag);
			return np->alias;
		} else return GetAlias(np->alias,flag);	
	}
	List* GetAlias(List* lp,const ulong flag)
	{
		if (lp->xjFlag == flag) return lp;
		if (lp->alias==0) 
		{
			lp->alias=NewList(flag);
			return lp->alias;
		} else return GetAlias(lp->alias,flag);
	}
	
	bool Spell(const uchar* pt,int tailLen,const uchar lastChar,int numWords);
// Recursive, not inlined, defined below. 
	
	void Push(const Cptr wp) {*stackPtr++ = wp;}
	
	void CopyStack(char* text)
// Copies the word stack of solution words back into plaintext
// This overwrites the decoded plaintext with dictionary words
// and automatically takes care of I/J equivalents and X/Z fillers.	
	{
		while (stackPtr > wordStack) {
			Cptr wp=*--stackPtr;
			uchar c=*wp;
			while (c) {
				*text++=c;
				c=*++wp;	
			}
		} 
		*text=0;
	}
	
	void Cache(Decoder* lastUsed)
	{
// Builds a cache of recently used decoders by moving lastUsed forward	
		Decoder* nextCache=D+cache;
		if (nextCache < lastUsed) 
		{
			Decoder temp=*nextCache;
			*nextCache=*lastUsed;
			*lastUsed=temp;
			cache++;
		}
	}
} *C;


// 			CodeBreaker::Spell
//  Returns true if the string in pt[] of length tailLen can be parsed
//	into dictionary words, allowing for J/I substitutions and X/Z fillers.	

//	On each recursion it attempts to match the head of pt[] with a
//	dictionary word, and if successful, stacks a pointer to the word
//	and calls itself with the shortened string until the end of pt[] is
//	reached.

//	Note: tailLen and wordlengths refer to decoded plain text and
//	words before filler 'X' and 'Z' characters are removed.	
	bool CodeBreaker::
	Spell(const uchar* pt,int tailLen,const uchar lastChar,int numWords)
	{
	
		if (--numWords <=0)// runtime check against stack overflow
		{
			numWords++;
			return false;
		}	
		
		const char* wp;			
		
//	Get Node matching the first 3 characters
		ulong pt0123=*((ulong*)pt);	
		Node* np=base[32*(32*
			(31 & (pt0123 >> 24)) +
			(31 & (pt0123 >> 16))) +
			(31 & (pt0123 >> 8))];
			
		if (np)
		{	
			ulong hardX=pt0123 & 0x20202000;
		
			if (hardX && 
				(1 & np->xjFlag) &&
				(0==(kZFLAG & np->xjFlag))
				) np=np->alias;
		
			while (np) {
//	Get list matching the first 4 characters	
				List* lp=np->list[31 & pt0123];
				hardX=0x20 & (31 & pt0123 | pt[4]);
			
				if (hardX && lp 
					&& (1 & lp->xjFlag) 
					&& (0==(kZFLAG & lp->xjFlag))
					) lp=lp->alias;
			
				while (lp) {
//	Get stem matching the first 5 characters
					Stem* gp=&lp->group[31 & pt[4]];
			
					if (gp->numWords) {
//	Explore all words matching the first 5 characters starting with 
//	the last word in the group (hoping to catch	longer words first)		
						int curWord=gp->numWords;
						int numX = 		// make this a function
							(1 & np->xjFlag) + 
							(1 & (lp->xjFlag >> pt[4]));
						
						do {
							int len;
							wp=gp->MatchTail(pt,len,numX,--curWord);
							if (wp)
							{
								if ((len==tailLen) 
								|| (Spell(pt+len,tailLen-len,pt[len-1],numWords)))
								{
									Push(wp);
									numWords++;
									return true;	
								}	
							}
						} while (curWord > 0);
					}
				
// Try the 4-letter word
					wp=lp->GetListLeader();
					if (wp)
					{ 
						if ((4==tailLen) 
						|| (Spell(pt+4,tailLen-4,pt[3],numWords)))
						{
							Push(wp);
							numWords++;
							return true;	
						}	
					}
					lp=lp->alias;
				}
				
// No luck so far, try the 3-letter word
			
				wp=np->GetNodeLeader();
				if (wp)
				{ 
					if ((3==tailLen) 
					|| (Spell(pt+3,tailLen-3,pt[2],numWords))){
						Push(wp);
						numWords++;
						return true;	
					}	
				}
				np=np->alias;
			}	// end of tests where first 3 letters match	
		}
		
// Try the 2-letter word
		np=base[32*(32*(31 & pt[0])+(31 & pt[1]))+0];
		if (np)
		{
			wp=np->GetNodeLeader();
			if (wp)
			{ 
				if ((2==tailLen) 
				|| (Spell(pt+2,tailLen-2,pt[1],numWords))){
					Push(wp);
					numWords++;
					return true;	
				}	
			}
		}
		
// Try the 1-letter word
		np=base[32*(32*(31 & pt[0])+0)+0];
		if (np)
		{
			wp=np->GetNodeLeader();
			if (wp)
			{ 
				if ((1==tailLen) 
				|| (Spell(pt+1,tailLen-1,pt[0],numWords))){
					Push(wp);
					numWords++;
					return true;	
				}	
			}		
		}
		
// Finally try if this is a filler 'X' or 'Z' between words
		if ((pt[0]=='X') || 
			((0==(31 & (lastChar ^ 'X'))) && (pt[0]=='Z'))) 
		{
			if (tailLen==1){
				numWords++;
				return true;
			}	
			if (0==(31 & (lastChar ^ pt[1])) 
			&& Spell(pt+1,tailLen-1,0,numWords))
			{
				// nothing to stack, just carry on unwinding	
				numWords++;
				return true;
			}	
		}
		
// Spellcheck failed: no word matches at this offset in plaintext	
		numWords++;	
		return false;
	}
	


void InitPlayfair(
  const char *words[],		/* dictionary words */
  long numDictionaryWords	/* number of null-terminated words in dictionary */
) 
{
 	C = new CodeBreaker(words,numDictionaryWords);// all that's needed				
}

void DecodePlayfair(
  const char *cipherText,	/* null-terminated text to decode */
  char *plainText			/* null-terminated decoded text */
)
{
// Decodes cipherText into plainText. 
// If no solution is found, plainText will be a gibberish string
// of the same length as cipherText, or possibly a bit longer.

	if ((0==*cipherText) || (strlen(cipherText) & 1))
// Cannot handle empty or odd-length cipher text strings
		return;
		
 	Decoder* dp=D;
 	Decoder* dEnd=D+numDecoders;									
 	for (;dp<dEnd;dp++)
 	{	
 		int len=dp->DecodeCipher(cipherText,plainText);
 		
		if (len && C->Spell((uchar*)plainText,len,0,kMaxWords)) 
		{
			C->CopyStack(plainText);
			C->Cache(dp);
 			break;
 		}	
 													
 	}
}

void TermPlayfair(void)
{
	delete C;		
}