An idea for improving accuracy of fp-to-decimal conversion for system…

…s that

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

FossilOrigin-Name: 702243333843219f8904ee0fd12283080d6c6b3fc1ffb36c534cdefb3563c00d

manifest

@@ -1,5 +1,5 @@
-C This\scheck-in\sadds\sthe\suse\sof\s"long\sdouble"\sto\ssqlite3FpDecode()\swhich\swhen\nrun\son\sx86\susing\sa\scompiler\sthat\stranslates\s"long\sdouble"\sinto\sbinary80,\ncauses\sall\slegacy\stests\sto\spass.\s\sBut\son\sa\ssystem\swhere\s"long\sdouble"\sis\sjust\nan\salias\sfor\s"double",\ssome\sof\sthe\stests\sare\sstill\sfailing.
-D 2023-07-01T20:21:27.087
+C An\sidea\sfor\simproving\saccuracy\sof\sfp-to-decimal\sconversion\sfor\ssystems\sthat\ndo\snot\shave\s"long\sdouble".\s\sDoes\snot\squite\swork.\s\sRetained\sonly\sfor\ndocumentation\spurposes.
+D 2023-07-03T00:40:37.359
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -705,7 +705,7 @@ F src/trigger.c ad6ab9452715fa9a8075442e15196022275b414b9141b566af8cdb7a1605f2b0
 F src/update.c 0aa36561167a7c40d01163238c297297962f31a15a8d742216b3c37cdf25f731
 F src/upsert.c 5303dc6c518fa7d4b280ec65170f465c7a70b7ac2b22491598f6d0b4875b3145
 F src/utf.c ee39565f0843775cc2c81135751ddd93eceb91a673ea2c57f61c76f288b041a0
-F src/util.c d68692390be083744bd0dfafc797472b0d304743040220e18b93bc9bad8c8d6e
+F src/util.c 12741de9969af5134488b7c89a97ecc22144f93f62c6255f797abb9b3121608a
 F src/vacuum.c 604fcdaebe76f3497c855afcbf91b8fa5046b32de3045bab89cc008d68e40104
 F src/vdbe.c 74282a947234513872a83b0bab1b8c644ece64b3e27b053ef17677c8ff9c81e0
 F src/vdbe.h 41485521f68e9437fdb7ec4a90f9d86ab294e9bb8281e33b235915e29122cfc0
@@ -2041,8 +2041,11 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 07eab52e0801bb0e4743b304a06ad16e18cdf8eaa18c0faf8d47a1f5d8576ea0
-R c4051b06b266c29a490b4cec354913fc
+P ce06982f880339cf46704e95c907249827c3e44af2b9420005200ca8abd3f371
+R ae7783ab51788007ac81a8600cb6b0a8
+T *branch * fp-to-decimal-branch1
+T *sym-fp-to-decimal-branch1 *
+T -sym-fp-to-decimal-refactor *
 U drh
-Z 95eef79511e3e10371b4b76031a5a06d
+Z 0f216e6dd96bccb8187eb2a64c983082
 # Remove this line to create a well-formed Fossil manifest.

manifest.uuid

@@ -1 +1 @@
-ce06982f880339cf46704e95c907249827c3e44af2b9420005200ca8abd3f371
+702243333843219f8904ee0fd12283080d6c6b3fc1ffb36c534cdefb3563c00d

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;
@@ -955,31 +957,55 @@ 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);
     p->n = 0;
     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;