An idea for improving accuracy of fp-to-decimal conversion for systems that

do not have "long double".  Does not quite work.  Retained only for
documentation purposes.

FossilOrigin-Name: 702243333843219f8904ee0fd12283080d6c6b3fc1ffb36c534cdefb3563c00d

1 files changed, 45 insertions, 19 deletions

diff --git a/src/util.c b/src/util.c
index a560f2613..9bc37c4f9 100644
--- a/src/util.c
+++ b/src/util.c
@@ -937,16 +937,18 @@ int sqlite3Atoi(const char *z){
 ** decimal point if n is negative.  No rounding is performed if
 ** n is zero.
 */
-void sqlite3FpDecode(FpDecode *p, double rr, int iRound, int mxRound){
+void sqlite3FpDecode(FpDecode *p, double r, int iRound, int mxRound){
   int i;
   u64 v;
   int e, exp = 0;
-  long double r;
   p->isSpecial = 0;
-  if( rr<0.0 ){
+
+  /* Convert negative numbers to positive.  Deal with Infinity, 0.0, and
+  ** NaN. */
+  if( r<0.0 ){
     p->sign = '-';
-    rr = -rr;
-  }else if( rr==0.0 ){
+    r = -r;
+  }else if( r==0.0 ){
     p->sign = '+';
     p->n = 1;
     p->iDP = 1;
@@ -955,7 +957,7 @@ void sqlite3FpDecode(FpDecode *p, double rr, int iRound, int mxRound){
   }else{
     p->sign = '+';
   }
-  memcpy(&v,&rr,8);
+  memcpy(&v,&r,8);
   e = v>>52;
   if( (e&0x7ff)==0x7ff ){
     p->isSpecial = 1 + (v!=0x7ff0000000000000L);
@@ -963,23 +965,47 @@ void sqlite3FpDecode(FpDecode *p, double rr, int iRound, int mxRound){
     p->iDP = 0;
     return;
   }
-  r = rr;
 
-  /* At this point, r is positive (non-zero) and is not Inf or NaN.
-  ** The strategy is to multiple or divide r by powers of 10 until
-  ** it is in between 1.0e+17 and 1.0e+19.  Then convert r into
-  ** an unsigned 64-bit integer v, and extract digits from v.
+  /* Multiply r by powers of ten until it lands somewhere in between
+  ** 1.0e+19 and 1.0e+17.
   */
-  if( r>=1.0e+19 ){
-    while( r>=1.0e+119L ){ exp+=100; r *= 1.0e-100L; }
-    while( r>=1.0e+29L  ){ exp+=10;  r *= 1.0e-10L;  }
-    while( r>=1.0e+19L  ){ exp++;    r *= 1.0e-1L;   }
+  if( sizeof(long double)>8
+   || (v & 0x0000000000ffffffLL)==0
+   || (v & 0xffffffffff000000LL)==0
+  ){
+    long double rr = r;
+    if( rr>=1.0e+19 ){
+      while( rr>=1.0e+119L ){ exp+=100; rr *= 1.0e-100L; }
+      while( rr>=1.0e+29L  ){ exp+=10;  rr *= 1.0e-10L;  }
+      while( rr>=1.0e+19L  ){ exp++;    rr *= 1.0e-1L;   }
+    }else{
+      while( rr<1.0e-97L   ){ exp-=100; rr *= 1.0e+100L; }
+      while( rr<1.0e+07L   ){ exp-=10;  rr *= 1.0e+10L;  }
+      while( rr<1.0e+17L   ){ exp--;    rr *= 1.0e+1L;   }
+    }
+    v = (u64)rr;
   }else{
-    while( r<1.0e-97L   ){ exp-=100; r *= 1.0e+100L; }
-    while( r<1.0e+07L   ){ exp-=10;  r *= 1.0e+10L;  }
-    while( r<1.0e+17L   ){ exp--;    r *= 1.0e+1L;   }
+    double r2, r3;
+    v &= 0xffffffffff000000LL;
+    memcpy(&r2, &v, 8);
+    assert( r2<r );
+    assert( r2>0.0 );
+    r3 = r - r2;
+    assert( r3>0.0 );
+    if( r2>=1.0e+19 ){
+      while( r2>=1.0e+119L ){ exp+=100; r2 *= 1.0e-100; r3 *= 1.0e-100; }
+      while( r2>=1.0e+29L  ){ exp+=10;  r2 *= 1.0e-10;  r3 *= 1.0e-10;  }
+      while( r2>=1.0e+19L  ){ exp++;    r2 *= 1.0e-1;   r3 *= 1.0e-1;   }
+    }else{
+      while( r2<1.0e-97L   ){ exp-=100; r2 *= 1.0e+100; r3 *= 1.0e+100; }
+      while( r2<1.0e+07L   ){ exp-=10;  r2 *= 1.0e+10;  r3 *= 1.0e+10;  }
+      while( r2<1.0e+17L   ){ exp--;    r2 *= 1.0e+1;   r3 *= 1.0e+1;   }
+    }
+    v = (u64)(r2+r3);
   }
-  v = (u64)r;
+
+
+  /* Extract significant digits. */
   i = sizeof(p->z)-1;
   while( v ){  p->z[i--] = (v%10) + '0'; v /= 10; }
   p->n = sizeof(p->z) - 1 - i;