Begin adding the data-structure explaining subsystem. All is contained within

FossilOrigin-Name: 79ae51c5b1b20ed0a425a87e65a32a096a80b7e1

src/vdbeInt.h

@@ -33,6 +33,9 @@ typedef unsigned char Bool;
 /* Opaque type used by code in vdbesort.c */
 typedef struct VdbeSorter VdbeSorter;
 
+/* Opaque type used by the explainer */
+typedef struct Explain Explain;
+
 /*
 ** A cursor is a pointer into a single BTree within a database file.
 ** The cursor can seek to a BTree entry with a particular key, or
@@ -255,6 +258,18 @@ struct sqlite3_context {
   CollSeq *pColl;       /* Collating sequence */
 };
 
+/*
+** An Explain object accumulates indented output which is helpful
+** in describing recursive data structures.
+*/
+struct Explain {
+  Vdbe *pVdbe;       /* Attach the explanation to this Vdbe */
+  StrAccum str;      /* The string being accumulated */
+  int nIndent;       /* Number of elements in aIndent */
+  u16 aIndent[100];  /* Levels of indentation */
+  char zBase[100];   /* Initial space */
+};
+
 /*
 ** An instance of the virtual machine.  This structure contains the complete
 ** state of the virtual machine.
@@ -320,6 +335,8 @@ struct Vdbe {
   void *pFree;            /* Free this when deleting the vdbe */
 #ifdef SQLITE_DEBUG
   FILE *trace;            /* Write an execution trace here, if not NULL */
+  Explain *pExplain;      /* The explainer */
+  char *zExplain;         /* Explanation of data structures */
 #endif
   VdbeFrame *pFrame;      /* Parent frame */
   VdbeFrame *pDelFrame;   /* List of frame objects to free on VM reset */