* 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, 110 insertions, 46 deletions

diff --git a/generic/tclStrToD.c b/generic/tclStrToD.c
index df2919c..ed2c59c 100755
--- a/generic/tclStrToD.c
+++ b/generic/tclStrToD.c
@@ -14,7 +14,7 @@
  * 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 1.33.2.7 2010/12/01 18:57:11 andreas_kupries Exp $
+ * RCS: @(#) $Id: tclStrToD.c,v 1.33.2.8 2011/01/15 19:07:01 kennykb Exp $
  *
  *----------------------------------------------------------------------
  */
@@ -3046,6 +3046,11 @@ StrictInt64Conversion(Double* dPtr,
 	    if (2*b > S
 		|| (2*b == S && (digit & 1) != 0)) {
 		s = BumpUp(s, retval, &k);
+	    } else {
+		while (*--s == '0') {
+		    /* do nothing */
+		}
+		++s;
 	    }
 	    break;
 	}
@@ -3463,6 +3468,11 @@ StrictBignumConversionPowD(Double* dPtr,
 	if (i == ilim) {
 	    if (ShouldBankerRoundUpPowD(&b, sd, digit&1)) {
 		s = BumpUp(s, retval, &k);
+	    } else {
+		while (*--s == '0') {
+		    /* do nothing */
+		}
+		++s;
 	    }
 	    break;
 	}
@@ -3736,7 +3746,7 @@ ShorteningBignumConversion(Double* dPtr,
 	    mp_div_d(&S, 5, &S, NULL);
 	    --s5;
 	    /* 
-	     * TODO: It might possibly be a win to fall back to
+	     * 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
@@ -3934,8 +3944,13 @@ StrictBignumConversion(Double* dPtr,
 		mp_mul_2d(&b, 1, &b);
 		if (ShouldBankerRoundUp(&b, &S, digit&1)) {
 		    s = BumpUp(s, retval, &k);  
-		}
-		break;
+		} else {
+		    while (*--s == '0') {
+			/* do nothing */
+		    }
+		    ++s;
+		}	
+	    break;
 	    }
 	}
     }
@@ -3996,9 +4011,9 @@ StrictBignumConversion(Double* dPtr,
  *		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.
+ *		is supplied instead, it also returns fewer digits if the 
+ *		shorter string will still reconvert 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.
@@ -4009,6 +4024,7 @@ StrictBignumConversion(Double* dPtr,
  *		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.
+ *		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,
diff --git a/generic/tclUtil.c b/generic/tclUtil.c
index ee17166..ff2fb6f 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 1.97.2.9 2010/12/03 22:27:44 hobbs Exp $
+ * RCS: @(#) $Id: tclUtil.c,v 1.97.2.10 2011/01/15 19:07:01 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) {
@@ -2281,51 +2321,59 @@ Tcl_PrintDouble(
 	}
     p = digits;
     if (exponent < -4 || exponent > 16) {
-	    /*
-	     * E format for numbers < 1e-3 or >= 1e17.
-	     */
-
-	    *dst++ = *p++;
-	    c = *p;
-	    if (c != '\0') {
-		*dst++ = '.';
-		while (c != '\0') {
-		    *dst++ = c;
-		    c = *++p;
-		}
+	/*
+	 * E format for numbers < 1e-3 or >= 1e17.
+	 */
+	
+	*dst++ = *p++;
+	c = *p;
+	if (c != '\0') {
+	    *dst++ = '.';
+	    while (c != '\0') {
+		*dst++ = c;
+		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 {
-	    /*
-	     * F format for others.
-	     */
-
+	    sprintf(dst, "e%+03d", exponent);
+	}
+    } else {
+	/*
+	 * F format for others.
+	 */
+	
 	if (exponent < 0) {
-		*dst++ = '0';
-	    }
-	    c = *p;
+	    *dst++ = '0';
+	}
+	c = *p;
 	while (exponent-- >= 0) {
-		if (c != '\0') {
-		    *dst++ = c;
-		    c = *++p;
-		} else {
-		    *dst++ = '0';
-		}
-	    }
-	    *dst++ = '.';
-	    if (c == '\0') {
-		*dst++ = '0';
+	    if (c != '\0') {
+		*dst++ = c;
+		c = *++p;
 	    } else {
+		*dst++ = '0';
+	    }
+	}
+	*dst++ = '.';
+	if (c == '\0') {
+	    *dst++ = '0';
+	} else {
 	    while (++exponent < -1) {
-		    *dst++ = '0';
-		}
-		while (c != '\0') {
-		    *dst++ = c;
-		    c = *++p;
-		}
+		*dst++ = '0';
+	    }
+	    while (c != '\0') {
+		*dst++ = c;
+		c = *++p;
 	    }
-	    *dst++ = '\0';
 	}
+	*dst++ = '\0';
+    }
     ckfree(digits);
 }