fix the rounding bug in roundShortest method (#161)

* fix: fix rounding in FormatFloat fallback path

decimal_test.go

@@ -57,6 +57,9 @@ var testTable = []*testEnt{
 	{2e250, "20000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000", "", ""},
 	{math.MaxInt64, strconv.FormatFloat(float64(math.MaxInt64), 'f', -1, 64), "", strconv.FormatInt(math.MaxInt64, 10)},
 	{1.29067116156722e-309, "0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000129067116156722", "", "0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001290671161567218558822290567835270536800098852722416870074139002112543896676308448335063375297788379444685193974290737962187240854947838776604607190387984577130572928111657710645015086812756013489109884753559084166516937690932698276436869274093950997935137476803610007959500457935217950764794724766740819156974617155861568214427828145972181876775307023388139991104942469299524961281641158436752347582767153796914843896176260096039358494077706152272661453132497761307744086665088096215425146090058519888494342944692629602847826300550628670375451325582843627504604013541465361435761965354140678551369499812124085312128659002910905639984075064968459581691226705666561364681985266583563078466180095375402399087817404368974165082030458595596655868575908243656158447265625000000000000000000000000000000000000004440000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"},
+	// go Issue 29491.
+	{498484681984085570, "498484681984085570", "", ""},
+	{5.8339553793802237e+23, "583395537938022370000000", "", ""},
 }
 
 var testTableScientificNotation = map[string]string{

rounding.go

@@ -80,39 +80,80 @@ func roundShortest(d *decimal, mant uint64, exp int, flt *floatInfo) {
 	// would round to the original mantissa and not the neighbors.
 	inclusive := mant%2 == 0
 
+	// As we walk the digits we want to know whether rounding up would fall
+	// within the upper bound. This is tracked by upperdelta:
+	//
+	// If upperdelta == 0, the digits of d and upper are the same so far.
+	//
+	// If upperdelta == 1, we saw a difference of 1 between d and upper on a
+	// previous digit and subsequently only 9s for d and 0s for upper.
+	// (Thus rounding up may fall outside the bound, if it is exclusive.)
+	//
+	// If upperdelta == 2, then the difference is greater than 1
+	// and we know that rounding up falls within the bound.
+	var upperdelta uint8
+
 	// Now we can figure out the minimum number of digits required.
 	// Walk along until d has distinguished itself from upper and lower.
-	for i := 0; i < d.nd; i++ {
-		l := byte('0') // lower digit
-		if i < lower.nd {
-			l = lower.d[i]
+	for ui := 0; ; ui++ {
+		// lower, d, and upper may have the decimal points at different
+		// places. In this case upper is the longest, so we iterate from
+		// ui==0 and start li and mi at (possibly) -1.
+		mi := ui - upper.dp + d.dp
+		if mi >= d.nd {
+			break
+		}
+		li := ui - upper.dp + lower.dp
+		l := byte('0') // lower digit
+		if li >= 0 && li < lower.nd {
+			l = lower.d[li]
+		}
+		m := byte('0') // middle digit
+		if mi >= 0 {
+			m = d.d[mi]
 		}
-		m := d.d[i]    // middle digit
 		u := byte('0') // upper digit
-		if i < upper.nd {
-			u = upper.d[i]
+		if ui < upper.nd {
+			u = upper.d[ui]
 		}
 
 		// Okay to round down (truncate) if lower has a different digit
 		// or if lower is inclusive and is exactly the result of rounding
 		// down (i.e., and we have reached the final digit of lower).
-		okdown := l != m || inclusive && i+1 == lower.nd
+		okdown := l != m || inclusive && li+1 == lower.nd
 
+		switch {
+		case upperdelta == 0 && m+1 < u:
+			// Example:
+			// m = 12345xxx
+			// u = 12347xxx
+			upperdelta = 2
+		case upperdelta == 0 && m != u:
+			// Example:
+			// m = 12345xxx
+			// u = 12346xxx
+			upperdelta = 1
+		case upperdelta == 1 && (m != '9' || u != '0'):
+			// Example:
+			// m = 1234598x
+			// u = 1234600x
+			upperdelta = 2
+		}
 		// Okay to round up if upper has a different digit and either upper
 		// is inclusive or upper is bigger than the result of rounding up.
-		okup := m != u && (inclusive || m+1 < u || i+1 < upper.nd)
+		okup := upperdelta > 0 && (inclusive || upperdelta > 1 || ui+1 < upper.nd)
 
 		// If it's okay to do either, then round to the nearest one.
 		// If it's okay to do only one, do it.
 		switch {
 		case okdown && okup:
-			d.Round(i + 1)
+			d.Round(mi + 1)
 			return
 		case okdown:
-			d.RoundDown(i + 1)
+			d.RoundDown(mi + 1)
 			return
 		case okup:
-			d.RoundUp(i + 1)
+			d.RoundUp(mi + 1)
 			return
 		}
 	}