sfio: correct floating decimal point scaling of fractions (#131)

_sfcvt(), "convert a floating point value to ASCII", did not adjust
for negative decimal place movement as what happens with leading
zeroes. This caused ksh's 'printf %f' formatter to fail to round
floating point values correctly.

src/lib/libast/sfio/sfcvt.c:
- Removed constraint of <1e-8 for doubles by matching what was done
  for long doubles having <.1.
- Corrected a condition when the next power of 10 occurred and that
  new 1 digit was being overwritten by a 0.

 src/cmd/ksh93/tests/math.sh:
- Validate that typeset -E/F formatting matches that of their
  equivalent printf formatting options as well as checking for
  correct float scaling of the fractional parts.

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-09-14:
+
+- Corrected rounding of floating point values by ksh's printf %f formatting
+  operator. Fix contributed by @hyenias.
+
 2020-09-11:
 
 - The 'command' regular builtin utility (which runs a simple command, removing

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-09-11"
+#define SH_RELEASE	"93u+m 2020-09-14"

src/cmd/ksh93/tests/math.sh

@@ -193,4 +193,48 @@ function test_has_iszero
 test_arithmetric_expression_accesss_array_element_through_nameref
 test_has_iszero
 
+# ======
+# Validate that typeset -E/F formatting matches that of their equivalent
+# printf formatting options as well as checking for correct float scaling
+# of the fractional parts.
+
+unset i tf pf; typeset -F 3 tf
+for i in {'','-'}{0..1}.{0,9}{0,9}{0,1,9}{0,1,9}
+do	tf=$i
+	pf=${ printf '%.3f' tf ;}
+	if	[[ $tf != "$pf" ]]
+	then	err_exit "typeset -F formatted data does not match its printf. typeset -F 3: $tf != $pf"
+		break
+	fi
+done
+unset i tf pf; typeset -lF 3 tf
+for i in {'','-'}{0..1}.{0,9}{0,9}{0,1,9}{0,1,9}
+do	tf=$i
+	pf=${ printf '%.3Lf' tf ;}
+	if	[[ $tf != "$pf" ]]
+	then	err_exit "typeset -lF formatted data does not match its printf. typeset -lF 3: $tf != $pf"
+		break
+	fi
+done
+unset i tf pf; typeset -E 3 tf
+for i in {'','-'}{0..1}.{0,9}{0,9}{0,1,9}{0,1,9}
+do	tf=$i
+	pf=${ printf '%.3g' tf ;}
+	if	[[ $tf != "$pf" ]]
+	then	err_exit "typeset -E formatted data does not match its printf. typeset -E 3: $tf != $pf"
+		break
+	fi
+done
+unset i tf pf; typeset -lE 3 tf
+for i in {'','-'}{0..1}.{0,9}{0,9}{0,1,9}{0,1,9}
+do	tf=$i
+	pf=${ printf '%.3Lg' tf ;}
+	if	[[ $tf != "$pf" ]]
+	then	err_exit "typeset -lE formatted data does not match its printf. typeset -lE 3: $tf != $pf"
+		break
+	fi
+done
+unset i tf pf
+
+# ======
 exit $((Errors<125?Errors:125))

src/lib/libast/sfio/sfcvt.c

@@ -287,7 +287,7 @@ int		format;		/* conversion format		*/
 
 			while(sp < ep)
 			{	/* generate fractional digits */
-				if(f <= 0.)
+				if(f <= 0. && *decpt >= 0)
 				{	/* fill with 0's */
 					do { *sp++ = '0'; } while(sp < ep);
 					goto done;
@@ -404,7 +404,7 @@ int		format;		/* conversion format		*/
 				}
 			} while(f >= (double)CVT_DBL_MAXINT);
 		}
-		else if(f > 0.0 && f < 1e-8)
+		else if(f > 0.0 && f < 0.1)
 		{	/* scale to avoid excessive multiply by 10 below */
 			v = SF_MAXEXP10-1;
 			do
@@ -459,7 +459,7 @@ int		format;		/* conversion format		*/
 
 			while(sp < ep)
 			{	/* generate fractional digits */
-				if(f <= 0.)
+				if(f <= 0. && *decpt >= 0)
 				{	/* fill with 0's */
 					do { *sp++ = '0'; } while(sp < ep);
 					goto done;
@@ -480,20 +480,29 @@ int		format;		/* conversion format		*/
 		ep = b+1;
 	else if(ep < endsp)
 	{	/* round the last digit */
-		*--sp += 5;
+		if (!(format&SFFMT_EFORMAT) && *decpt < 0)
+			sp += *decpt-1;
+		else
+			--sp;
+		*sp += 5;
 		while(*sp > '9')
-		{	*sp = '0';
-			if(sp > b)
+		{	if(sp > b)
+			{	*sp = '0';
 				*--sp += 1;
+			}
 			else
-			{	/* next power of 10 */
+			{	/* next power of 10 and at beginning */
 				*sp = '1';
 				*decpt += 1;
 				if(!(format&SFFMT_EFORMAT))
 				{	/* add one more 0 for %f precision */
-					ep[-1] = '0';
+					if (sp != &ep[-1])
+					{	/* prevents overwriting the previous 1 with 0 */
+						ep[-1] = '0';
+					}
 					ep += 1;
 				}
+				break;
 			}
 		}
 	}