* doc/tclvars.n:

* generic/tclStrToD.c:
* generic/tclUtil.c (Tcl_PrintDouble):
* tests/util.test (util-16.*): Restored full Tcl 8.4 compatibility
for the formatting of floating point numbers when $::tcl_precision
is not zero. Added compatibility tests to make sure that excess
trailing zeroes are suppressed for all eight major code paths.
[Bug 3157475]

2 files changed, 77 insertions, 9 deletions

diff --git a/generic/tclStrToD.c b/generic/tclStrToD.c
index 51a1a58..53ffedd 100755
--- a/generic/tclStrToD.c
+++ b/generic/tclStrToD.c
@@ -12,7 +12,7 @@
  * See the file "license.terms" for information on usage and redistribution of
  * this file, and for a DISCLAIMER OF ALL WARRANTIES.
  *
- * RCS: @(#) $Id: tclStrToD.c,v 1.52 2010/12/07 16:27:39 dkf Exp $
+ * RCS: @(#) $Id: tclStrToD.c,v 1.53 2011/01/15 18:10:19 kennykb Exp $
  */
 
 #include "tclInt.h"
@@ -3086,6 +3086,11 @@ StrictInt64Conversion(
 	if (i == ilim) {
 	    if (2*b > S || (2*b == S && (digit & 1) != 0)) {
 		s = BumpUp(s, retval, &k);
+	    } else {
+		while (*--s == '0') {
+		    /* do nothing */
+		}
+		++s;
 	    }
 	    break;
 	}
@@ -3330,6 +3335,11 @@ ShorteningBignumConversionPowD(
 		    *s++ = '9';
 		    s = BumpUp(s, retval, &k);
 		    break;
+		} else if (mp_iszero(&b)) {
+		    while (*--s == '0') {
+			/* do nothing */
+		    }
+		    ++s;
 		}
 	    }
 
@@ -3494,6 +3504,10 @@ StrictBignumConversionPowD(
 	    if (ShouldBankerRoundUpPowD(&b, sd, digit&1)) {
 		s = BumpUp(s, retval, &k);
 	    }
+	    while (*--s == '0') {
+		/* do nothing */
+	    }
+	    ++s;
 	    break;
 	}
 
@@ -3766,7 +3780,7 @@ ShorteningBignumConversion(
 	    --s5;
 
 	    /*
-	     * TODO: It might possibly be a win to fall back to int64
+	     * IDEA: It might possibly be a win to fall back to int64
 	     *       arithmetic here if S < 2**64/10. But it's a win only for
 	     *       a fairly narrow range of magnitudes so perhaps not worth
 	     *       bothering.  We already know that we shorten the
@@ -3966,6 +3980,10 @@ StrictBignumConversion(
 	    }
 	}
     }
+    while (*--s == '0') {
+	/* do nothing */
+    }
+    ++s;
 
     /*
      * Endgame - store the location of the decimal point and the end of the
@@ -4023,9 +4041,10 @@ StrictBignumConversion(
  *		choosing the one that is closest to the given number (and
  *		resolving ties with 'round to even').  It is allowed to return
  *		fewer than 'ndigits' if the number converts exactly; if the
- *		TCL_DD_E_FORMAT|TCL_DD_SHORTEN_FLAG is supplied instead, it is
- *		also allowed to return fewer digits if the shorter string will
- *		still reconvert to the given input number.
+ *		TCL_DD_E_FORMAT|TCL_DD_SHORTEN_FLAG is supplied instead, it 
+ *		also returns fewer digits if the shorter string will still
+ *		reconvert without loss to the given input number. In any case,
+ *		strings of trailing zeroes are suppressed.
  *	TCL_DD_F_FORMAT - This value is used to prepare numbers for %f format
  *		conversion. It requests that conversion proceed until
  *		'ndigits' digits after the decimal point have been converted.
@@ -4035,7 +4054,8 @@ StrictBignumConversion(
  *		number that converts exactly, and changing the argument to
  *		TCL_DD_F_FORMAT|TCL_DD_SHORTEN_FLAG will allow the routine
  *		also to return fewer digits if the shorter string will still
- *		reconvert without loss to the given input number.
+ *		reconvert without loss to the given input number. Strings of
+ *		trailing zeroes are suppressed.
  *
  *	To any of these flags may be OR'ed TCL_DD_NO_QUICK; this flag requires
  *	all calculations to be done in exact arithmetic. Normally, E and F
diff --git a/generic/tclUtil.c b/generic/tclUtil.c
index 984ba0a..f746aed 100644
--- a/generic/tclUtil.c
+++ b/generic/tclUtil.c
@@ -11,7 +11,7 @@
  * See the file "license.terms" for information on usage and redistribution of
  * this file, and for a DISCLAIMER OF ALL WARRANTIES.
  *
- * RCS: @(#) $Id: tclUtil.c,v 1.123 2010/12/03 22:30:56 hobbs Exp $
+ * RCS: @(#) $Id: tclUtil.c,v 1.124 2011/01/15 18:10:19 kennykb Exp $
  */
 
 #include "tclInt.h"
@@ -2273,7 +2273,47 @@ Tcl_PrintDouble(
 	digits = TclDoubleDigits(value, -1, TCL_DD_SHORTEST,
 				 &exponent, &signum, &end);
     } else {
-	digits = TclDoubleDigits(value, *precisionPtr, TCL_DD_E_FORMAT, 
+	/*
+	 * There are at least two possible interpretations for tcl_precision.
+	 *
+	 * The first is, "choose the decimal representation having
+	 * $tcl_precision digits of significance that is nearest to the
+	 * given number, breaking ties by rounding to even, and then
+	 * trimming trailing zeros." This gives the greatest possible
+	 * precision in the decimal string, but offers the anomaly that
+	 * [expr 0.1] will be "0.10000000000000001".
+	 *
+	 * The second is "choose the decimal representation having at
+	 * most $tcl_precision digits of significance that is nearest
+	 * to the given number. If no such representation converts
+	 * exactly to the given number, choose the one that is closest,
+	 * breaking ties by rounding to even. If more than one such
+	 * representation converts exactly to the given number, choose
+	 * the shortest, breaking ties in favour of the nearest, breaking
+	 * remaining ties in favour of the one ending in an even digit."
+	 *
+	 * Tcl 8.4 implements the first of these, which gives rise to
+	 * anomalies in formatting:
+	 *
+	 * % expr 0.1
+	 * 0.10000000000000001
+	 * % expr 0.01
+	 * 0.01
+	 * % expr 1e-7
+	 * 9.9999999999999995e-08
+	 *
+	 * For human readability, it appears better to choose the second rule,
+	 * and let [expr 0.1] return 0.1. But for 8.4 compatibility, we
+	 * prefer the first (the recommended zero value for tcl_precision
+	 * avoids the problem entirely).
+	 *
+	 * Uncomment TCL_DD_SHORTEN_FLAG in the next call to prefer the
+	 * method that allows floating point values to be shortened if
+	 * it can be done without loss of precision.
+	 */
+
+	digits = TclDoubleDigits(value, *precisionPtr,
+				 TCL_DD_E_FORMAT /* | TCL_DD_SHORTEN_FLAG */, 
 				 &exponent, &signum, &end);
     }
     if (signum) {
@@ -2294,7 +2334,15 @@ Tcl_PrintDouble(
 		c = *++p;
 	    }
 	}
-	sprintf(dst, "e%+d", exponent);
+	/*
+	 * Tcl 8.4 appears to format with at least a two-digit exponent;
+	 * preserve that behaviour when tcl_precision != 0
+	 */
+	if (*precisionPtr == 0) {
+	    sprintf(dst, "e%+d", exponent);
+	} else {
+	    sprintf(dst, "e%+03d", exponent);
+	}
     } else {
 	/*
 	 * F format for others.