dec: fix setUint64 for _BD <= x <= 1<<_W

dec.go

@@ -107,12 +107,12 @@ func (z dec) setWord(x Word) dec {
 
 func (z dec) setUint64(x uint64) (dec, int32) {
 	dig := int32(decDigits64(x))
-	if w := Word(x); uint64(w) == x {
+	if w := Word(x); uint64(w) == x && w < _BD {
 		return z.setWord(w), dig
 	}
 	// x could be a 2 to 3 words value
 	z = z.make(int(dig+_WD-1) / _WD)
-	for i := 0; i < len(z) && x != 0; i++ {
+	for i := 0; i < len(z); i++ {
 		hi, lo := bits.Div64(0, x, _BD)
 		z[i] = Word(lo)
 		x = hi
@@ -237,7 +237,6 @@ func (z dec) shl(x dec, s uint) dec {
 
 	n := m + int(s/_WD)
 	z = z.make(n + 1)
-	// TODO(db47h): optimize and bench shifts when s%_WD == 0
 	z[n] = shl10VU(z[n-m:n], x, s%_WD)
 	z[0 : n-m].clear()
 

dec_test.go

@@ -49,3 +49,31 @@ func TestDec_digit(t *testing.T) {
 		})
 	}
 }
+
+func TestDecSetUint64(t *testing.T) {
+	data := []struct {
+		in  uint64
+		exp int32
+	}{
+		{_BD + 1, int32(_WD + 1)},
+		{_BD - 1, int32(_WD)},
+		{9999, 4},
+	}
+	var z dec
+	for _, d := range data {
+		out := fmt.Sprintf("%d", d.in)
+		t.Run(out, func(t *testing.T) {
+			var exp int32
+			z, exp = z.setUint64(d.in)
+			if a := string(z.utoa(10)); a != out {
+				t.Fatalf("expected mantissa %v, got %v", out, a)
+			}
+			if exp != d.exp {
+				t.Fatalf("expected exponent %v, got %v", d.exp, exp)
+			}
+			if l := (exp + _WD - 1) / _WD; l != int32(len(z)) {
+				t.Fatalf("expected length %v, got %v", l, len(z))
+			}
+		})
+	}
+}