math/big: implemented Frexp, Ldexp, IsInt, Copy, bug fixes, more tests
- Frexp, Ldexp are equivalents to the corresponding math functions.
- Set now has the same prec behavior as the other functions
- Copy is a true assignment (replaces old version of Set)
- Cmp now handles infinities
- more tests
Change-Id: I0d33980c08be3095b25d7b3d16bcad1aa7abbd0f
Reviewed-on: https://go-review.googlesource.com/4292
Reviewed-by: Alan Donovan <adonovan@google.com>
diff --git a/src/math/big/float.go b/src/math/big/float.go
index d280916..d911143 100644
--- a/src/math/big/float.go
+++ b/src/math/big/float.go
@@ -172,6 +172,86 @@
return x.mode
}
+// Sign returns:
+//
+// -1 if x < 0
+// 0 if x == 0 or x == -0
+// +1 if x > 0
+//
+func (x *Float) Sign() int {
+ s := 0
+ if len(x.mant) != 0 || x.exp == infExp {
+ s = 1 // non-zero x
+ }
+ if x.neg {
+ s = -s
+ }
+ return s
+}
+
+// MantExp breaks x into its mantissa and exponent components.
+// It returns mant and exp satisfying x == mant × 2**exp, with
+// the absolute value of mant satisfying 0.5 <= |mant| < 1.0.
+// mant has the same precision and rounding mode as x.
+//
+// Special cases are:
+//
+// ( ±0).MantExp() = ±0, 0
+// (±Inf).MantExp() = ±Inf, 0
+//
+// MantExp does not modify x; the result mant is a new Float.
+func (x *Float) MantExp() (mant *Float, exp int) {
+ mant = new(Float).Copy(x)
+ if x.exp != infExp {
+ mant.exp = 0
+ exp = int(x.exp)
+ }
+ return
+}
+
+// SetMantExp is the inverse of MantExp. It sets z to mant × 2**exp and
+// and returns z. The result z has the same precision and rounding mode
+// as mant.
+//
+// Special cases are:
+//
+// z.SetMantExp( ±0, exp) = ±0
+// z.SetMantExp(±Inf, exp) = ±Inf
+//
+// The result is ±Inf if the magnitude of exp is > MaxExp.
+func (z *Float) SetMantExp(mant *Float, exp int) *Float {
+ z.Copy(mant)
+ if len(z.mant) == 0 || z.exp == infExp {
+ return z
+ }
+ z.setExp(int64(exp))
+ return z
+}
+
+// IsInt reports whether x is an integer.
+// ±Inf are not considered integers.
+func (x *Float) IsInt() bool {
+ // pick off easy cases
+ if len(x.mant) == 0 {
+ return x.exp != infExp // x == 0
+ }
+ // x != 0
+ if x.exp <= 0 {
+ return false // 0 < |x| <= 0.5
+ }
+ // x.exp > 0
+ if uint(x.exp) >= x.prec {
+ return true // not enough precision for fractional mantissa
+ }
+ if debugFloat {
+ x.validate()
+ }
+ // x.mant[len(x.mant)-1] != 0
+ // determine minimum required precision for x
+ minPrec := uint(len(x.mant))*_W - x.mant.trailingZeroBits()
+ return uint(x.exp) >= minPrec
+}
+
// IsInf reports whether x is an infinity, according to sign.
// If sign > 0, IsInf reports whether x is positive infinity.
// If sign < 0, IsInf reports whether x is negative infinity.
@@ -181,7 +261,7 @@
}
// setExp sets the exponent for z.
-// If the exponent's magnitude is too large, z becomes +/-Inf.
+// If the exponent's magnitude is too large, z becomes ±Inf.
func (z *Float) setExp(e int64) {
if -MaxExp <= e && e <= MaxExp {
z.exp = int32(e)
@@ -374,9 +454,8 @@
// Round sets z to the value of x rounded according to mode to prec bits and returns z.
// TODO(gri) rethink this signature.
-// TODO(gri) adjust this to match precision semantics.
func (z *Float) Round(x *Float, prec uint, mode RoundingMode) *Float {
- z.Set(x)
+ z.Copy(x)
z.prec = prec
z.mode = mode
z.round(0)
@@ -530,14 +609,38 @@
return z.Quo(&a, &b)
}
-// Set sets z to x, with the same precision as x, and returns z.
-// TODO(gri) adjust this to match precision semantics.
+// Set sets z to the (possibly rounded) value of x and returns z.
+// If z's precision is 0, it is changed to the precision of x
+// before setting z (and rounding will have no effect).
+// Rounding is performed according to z's precision and rounding
+// mode; and z's accuracy reports the result error relative to the
+// exact (not rounded) result.
func (z *Float) Set(x *Float) *Float {
if z != x {
+ if z.prec == 0 {
+ z.prec = x.prec
+ }
+ z.acc = Exact
+ z.neg = x.neg
+ z.exp = x.exp
+ z.mant = z.mant.set(x.mant)
+ if z.prec < x.prec {
+ z.round(0)
+ }
+ }
+ return z
+}
+
+// Copy sets z to x, with the same precision and rounding mode as x,
+// and returns z.
+func (z *Float) Copy(x *Float) *Float {
+ if z != x {
+ z.acc = Exact
z.neg = x.neg
z.exp = x.exp
z.mant = z.mant.set(x.mant)
z.prec = x.prec
+ z.mode = x.mode
}
return z
}
@@ -581,7 +684,7 @@
// by rounding to nearest with 53 bits precision.
// TODO(gri) implement/document error scenarios.
func (x *Float) Float64() (float64, Accuracy) {
- // x == +/-Inf
+ // x == ±Inf
if x.exp == infExp {
var sign int
if x.neg {
@@ -604,40 +707,26 @@
return math.Float64frombits(s | e<<52 | m), r.acc
}
-func (x *Float) Int() *Int {
- if len(x.mant) == 0 {
- return new(Int)
- }
+// BUG(gri) Int is not yet implemented
+func (x *Float) Int() (*Int, Accuracy) {
panic("unimplemented")
}
+// BUG(gri) Rat is not yet implemented
func (x *Float) Rat() *Rat {
panic("unimplemented")
}
-func (x *Float) IsInt() bool {
- if len(x.mant) == 0 {
- return true
- }
- if x.exp <= 0 {
- return false
- }
- if uint(x.exp) >= x.prec {
- return true
- }
- panic("unimplemented")
-}
-
-// Abs sets z to |x| (the absolute value of x) and returns z.
-// TODO(gri) adjust this to match precision semantics.
+// Abs sets z to the (possibly rounded) value |x| (the absolute value of x)
+// and returns z.
func (z *Float) Abs(x *Float) *Float {
z.Set(x)
z.neg = false
return z
}
-// Neg sets z to x with its sign negated, and returns z.
-// TODO(gri) adjust this to match precision semantics.
+// Neg sets z to the (possibly rounded) value of x with its sign negated,
+// and returns z.
func (z *Float) Neg(x *Float) *Float {
z.Set(x)
z.neg = !z.neg
@@ -1022,57 +1111,59 @@
// +1 if x > y
//
func (x *Float) Cmp(y *Float) int {
- // TODO(gri) handle Inf
-
- // special cases
- switch {
- case len(x.mant) == 0:
- // 0 cmp y == -sign(y)
- return -y.Sign()
- case len(y.mant) == 0:
- // x cmp 0 == sign(x)
- return x.Sign()
+ if debugFloat {
+ x.validate()
+ y.validate()
}
- // x != 0 && y != 0
- // x cmp y == x cmp y
- // x cmp (-y) == 1
- // (-x) cmp y == -1
- // (-x) cmp (-y) == -(x cmp y)
+ mx := x.mag()
+ my := y.mag()
+
switch {
- case x.neg == y.neg:
- r := x.ucmp(y)
- if x.neg {
- r = -r
- }
- return r
- case x.neg:
+ case mx < my:
return -1
- default:
- return 1
+ case mx > my:
+ return +1
}
+ // mx == my
+
+ // only if |mx| == 1 we have to compare the mantissae
+ switch mx {
+ case -1:
+ return -x.ucmp(y)
+ case +1:
+ return +x.ucmp(y)
+ }
+
return 0
}
-// Sign returns:
-//
-// -1 if x < 0
-// 0 if x == 0 (incl. x == -0) // TODO(gri) is this correct?
-// +1 if x > 0
-//
-func (x *Float) Sign() int {
- if len(x.mant) == 0 {
- return 0
- }
- if x.neg {
- return -1
- }
- return 1
-}
-
func umax(x, y uint) uint {
if x > y {
return x
}
return y
}
+
+// mag returns:
+//
+// -2 if x == -Inf
+// -1 if x < 0
+// 0 if x == -0 or x == +0
+// +1 if x > 0
+// +2 if x == +Inf
+//
+// mag is a helper function for Cmp.
+func (x *Float) mag() int {
+ m := 1
+ if len(x.mant) == 0 {
+ m = 0
+ if x.exp == infExp {
+ m = 2
+ }
+ }
+ if x.neg {
+ m = -m
+ }
+ return m
+}
diff --git a/src/math/big/float_test.go b/src/math/big/float_test.go
index 8921209..e4c2e1a 100644
--- a/src/math/big/float_test.go
+++ b/src/math/big/float_test.go
@@ -9,6 +9,7 @@
"math"
"sort"
"strconv"
+ "strings"
"testing"
)
@@ -66,7 +67,126 @@
// TODO(gri) test how precision is set for zero value results
}
-func TestFloatInf(t *testing.T) {
+func makeFloat(s string) *Float {
+ if s == "Inf" || s == "+Inf" {
+ return NewInf(+1)
+ }
+ if s == "-Inf" {
+ return NewInf(-1)
+ }
+ var x Float
+ x.prec = 100 // TODO(gri) find a better way to do this
+ if _, ok := x.SetString(s); !ok {
+ panic(fmt.Sprintf("%q is not a valid float", s))
+ }
+ return &x
+}
+
+func TestFloatSign(t *testing.T) {
+ for _, test := range []struct {
+ x string
+ s int
+ }{
+ {"-Inf", -1},
+ {"-1", -1},
+ {"-0", 0},
+ {"+0", 0},
+ {"+1", +1},
+ {"+Inf", +1},
+ } {
+ x := makeFloat(test.x)
+ s := x.Sign()
+ if s != test.s {
+ t.Errorf("%s.Sign() = %d; want %d", test.x, s, test.s)
+ }
+ }
+}
+
+// feq(x, y) is like x.Cmp(y) == 0 but it also considers the sign of 0 (0 != -0).
+func feq(x, y *Float) bool {
+ return x.Cmp(y) == 0 && x.neg == y.neg
+}
+
+func TestFloatMantExp(t *testing.T) {
+ for _, test := range []struct {
+ x string
+ frac string
+ exp int
+ }{
+ {"0", "0", 0},
+ {"+0", "0", 0},
+ {"-0", "-0", 0},
+ {"Inf", "+Inf", 0},
+ {"+Inf", "+Inf", 0},
+ {"-Inf", "-Inf", 0},
+ {"1.5", "0.75", 1},
+ {"1.024e3", "0.5", 11},
+ {"-0.125", "-0.5", -2},
+ } {
+ x := makeFloat(test.x)
+ frac := makeFloat(test.frac)
+ f, e := x.MantExp()
+ if !feq(f, frac) || e != test.exp {
+ t.Errorf("%s.MantExp() = %s, %d; want %s, %d", test.x, f.Format('g', 10), e, test.frac, test.exp)
+ }
+ }
+}
+
+func TestFloatSetMantExp(t *testing.T) {
+ for _, test := range []struct {
+ frac string
+ exp int
+ z string
+ }{
+ {"0", 0, "0"},
+ {"+0", 0, "0"},
+ {"-0", 0, "-0"},
+ {"Inf", 1234, "+Inf"},
+ {"+Inf", -1234, "+Inf"},
+ {"-Inf", -1234, "-Inf"},
+ {"0", -MaxExp - 1, "0"},
+ {"1", -MaxExp - 1, "+Inf"}, // exponent magnitude too large
+ {"-1", -MaxExp - 1, "-Inf"}, // exponent magnitude too large
+ {"0.75", 1, "1.5"},
+ {"0.5", 11, "1024"},
+ {"-0.5", -2, "-0.125"},
+ } {
+ frac := makeFloat(test.frac)
+ want := makeFloat(test.z)
+ var z Float
+ z.SetMantExp(frac, test.exp)
+ if !feq(&z, want) {
+ t.Errorf("SetMantExp(%s, %d) = %s; want %s", test.frac, test.exp, z.Format('g', 10), test.z)
+ }
+ }
+}
+
+func TestFloatIsInt(t *testing.T) {
+ for _, test := range []string{
+ "0 int",
+ "-0 int",
+ "1 int",
+ "-1 int",
+ "0.5",
+ "1.23",
+ "1.23e1",
+ "1.23e2 int",
+ "0.000000001e+8",
+ "0.000000001e+9 int",
+ "1.2345e200 int",
+ "Inf",
+ "+Inf",
+ "-Inf",
+ } {
+ s := strings.TrimSuffix(test, " int")
+ want := s != test
+ if got := makeFloat(s).IsInt(); got != want {
+ t.Errorf("%s.IsInt() == %t", s, got)
+ }
+ }
+}
+
+func TestFloatIsInf(t *testing.T) {
// TODO(gri) implement this
}
@@ -709,6 +829,10 @@
}
}
+func TestFloatCmp(t *testing.T) {
+ // TODO(gri) implement this
+}
+
// normBits returns the normalized bits for x: It
// removes multiple equal entries by treating them
// as an addition (e.g., []int{5, 5} => []int{6}),
diff --git a/src/math/big/floatconv.go b/src/math/big/floatconv.go
index e3611b2..71920cd 100644
--- a/src/math/big/floatconv.go
+++ b/src/math/big/floatconv.go
@@ -57,6 +57,7 @@
// with base 0 or 10 corresponds to the value 1.2 * 2**3.
//
// BUG(gri) This signature conflicts with Scan(s fmt.ScanState, ch rune) error.
+// TODO(gri) What should the default precision be?
func (z *Float) Scan(r io.ByteScanner, base int) (f *Float, b int, err error) {
// sign
z.neg, err = scanSign(r)
