Use ideas from T. J. Dekker in "A Floating-Point Technique for Extending the

Available Precision" (1971-07-26) to enhance the accuracy of the SUM()
aggregate function in cases where input magnitudes vary wildly.

FossilOrigin-Name: 439fc00fee62b4db3751860485e21a99cae4fd1f5d911b2c08651a1466245ecc

src/sqliteInt.h

@@ -1234,6 +1234,7 @@ typedef struct Cte Cte;
 typedef struct CteUse CteUse;
 typedef struct Db Db;
 typedef struct DbFixer DbFixer;
+typedef struct DblDbl DblDbl;
 typedef struct Schema Schema;
 typedef struct Expr Expr;
 typedef struct ExprList ExprList;
@@ -5035,6 +5036,15 @@ int sqlite3FixSrcList(DbFixer*, SrcList*);
 int sqlite3FixSelect(DbFixer*, Select*);
 int sqlite3FixExpr(DbFixer*, Expr*);
 int sqlite3FixTriggerStep(DbFixer*, TriggerStep*);
+
+
+/* Representation of an extended precision floating point value.
+** The actual value is the sum r and rr.  See the dbldbl.c file.
+*/
+void sqlite3DDFromInt(i64,double*);
+void sqlite3DDAdd(double,double,double,double,double*);
+void sqlite3DDSub(double,double,double,double,double*);
+
 int sqlite3RealSameAsInt(double,sqlite3_int64);
 i64 sqlite3RealToI64(double);
 int sqlite3Int64ToText(i64,char*);