Decimal: SetInt: default precision uses whole input magnitude

When z.prec is zero, compute precision based on all decimal digits in
source big.Int, including trailing zeros (i.e. the precision should
match the magnitude of the input, regardless of its actual value).

Before the change, trailing zero digits were ignored.

decimal.go

@@ -1147,12 +1147,16 @@ func (z *Decimal) SetInt(x *big.Int) *Decimal {
 	// TODO(db47h) truncating x could be more efficient if z.prec > 0
 	// but small compared to the size of x.
 	z.mant = z.mant.make(int((prec + _DW - 1) / _DW)).setNat(x.Bits())
-	exp := dnorm(z.mant)
 	if z.prec == 0 {
-		// adjust precision
-		z.prec = uint32(max(len(z.mant)*_DW-int(z.mant.trailingZeroDigits()), DefaultDecimalPrec))
+		// adjust precision. z.mant is not notmalized yet. use nlz() and accept
+		// trailing zeros as part of the required precision
+		digits := int64(len(z.mant))*_DW - int64(nlz10(z.mant[len(z.mant)-1]))
+		if digits > MaxPrec {
+			digits = MaxPrec
+		}
+		z.prec = umax32(uint32(digits), DefaultDecimalPrec)
 	}
-	z.setExpAndRound(int64(len(z.mant))*_DW-exp, 0)
+	z.setExpAndRound(int64(len(z.mant))*_DW-dnorm(z.mant), 0)
 	return z
 }
 
@@ -1584,9 +1588,9 @@ func dnorm(m dec) int64 {
 	if debugDecimal && (len(m) == 0 || m[len(m)-1] == 0) {
 		panic("msw of mantissa is 0")
 	}
-	s := _DW - decDigits(uint(m[len(m)-1]))
-	// partial shift
+	s := nlz10(m[len(m)-1])
 	if s > 0 {
+		// partial shift
 		c := shl10VU(m, m, s)
 		if debugDecimal && c != 0 {
 			panic("nlz or shlVU incorrect")

decimal_test.go

@@ -721,7 +721,7 @@ func TestDecimalSetInt(t *testing.T) {
 		f.SetInt(&x)
 
 		// check precision
-		n := len(strings.TrimRight(want, "0"))
+		n := len(want)
 		if n < DefaultDecimalPrec {
 			n = DefaultDecimalPrec
 		}
@@ -747,7 +747,7 @@ func TestDecimalSetRat(t *testing.T) {
 		{"-1", DefaultDecimalPrec},
 		{"1234567890", DefaultDecimalPrec},
 		{"123456789012345678901234567890", DefaultDecimalPrec},
-		{"123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890", 89},
+		{"123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890", 90},
 		{"1.2", DefaultDecimalPrec},
 		{"3.14159265", DefaultDecimalPrec},
 		// TODO(db47h) expand

decimal_toa.go

@@ -346,7 +346,7 @@ func (x *Decimal) fmtP(buf []byte) []byte {
 	mant, exp := x.toa(10)
 	buf = append(buf, bytes.TrimRight(mant, "0")...)
 	buf = append(buf, 'e')
-	if x.exp >= 0 {
+	if exp >= 0 {
 		buf = append(buf, '+')
 	}
 	return strconv.AppendInt(buf, int64(exp), 10)