math/big: implemented Float.Int64, simplified Float.Uint64
Change-Id: Ic270ffa7ec6f6dd4b0a951c64ad965447cce1417
Reviewed-on: https://go-review.googlesource.com/4571
Reviewed-by: Alan Donovan <adonovan@google.com>
diff --git a/src/math/big/float.go b/src/math/big/float.go
index 3464192..06b3ae2 100644
--- a/src/math/big/float.go
+++ b/src/math/big/float.go
@@ -240,13 +240,7 @@
return len(x.mant) == 0 && x.exp != infExp
}
// x.exp > 0
- if uint(x.exp) >= x.prec {
- return true // not enough precision for fractional mantissa
- }
- // 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
+ return x.prec <= uint(x.exp) || x.minPrec() <= uint(x.exp) // not enough bits for fractional mantissa
}
// IsInf reports whether x is an infinity, according to sign.
@@ -660,66 +654,104 @@
return v
}
+// minPrec returns the minimum precision required to represent
+// x without loss of accuracy.
+// TODO(gri) this might be useful to export, perhaps under a better name
+func (x *Float) minPrec() uint {
+ return uint(len(x.mant))*_W - x.mant.trailingZeroBits()
+}
+
// Uint64 returns the unsigned integer resulting from truncating x
-// towards zero. If 0 <= x < 2**64, the result is Exact if x is an
-// integer; and Below if x has a fractional part. The result is (0,
-// Above) for x < 0, and (math.MaxUint64, Below) for x > math.MaxUint64.
+// towards zero. If 0 <= x <= math.MaxUint64, the result is Exact
+// if x is an integer and Below otherwise.
+// The result is (0, Above) for x < 0, and (math.MaxUint64, Below)
+// for x > math.MaxUint64.
func (x *Float) Uint64() (uint64, Accuracy) {
- // TODO(gri) there ought to be an easier way to implement this efficiently
if debugFloat {
x.validate()
}
- // pick off easy cases
- if x.exp <= 0 {
- // |x| < 1 || |x| == Inf
- if x.exp == infExp {
- // ±Inf
- if x.neg {
- return 0, Above // -Inf
- }
- return math.MaxUint64, Below // +Inf
- }
- if len(x.mant) == 0 {
- return 0, Exact // ±0
- }
- // 0 < |x| < 1
- if x.neg {
- return 0, Above
- }
- return 0, Below
- }
- // x.exp > 0
- if x.neg {
+ switch x.ord() {
+ case -2, -1:
+ // x < 0
return 0, Above
- }
- // x > 0
- if x.exp <= 64 {
- // u = trunc(x) fits into a uint64
- u := high64(x.mant) >> (64 - uint32(x.exp))
- // x.mant[len(x.mant)-1] != 0
- // determine minimum required precision for x
- minPrec := uint(len(x.mant))*_W - x.mant.trailingZeroBits()
- if minPrec <= 64 {
- return u, Exact
+ case 0:
+ // x == 0 || x == -0
+ return 0, Exact
+ case 1:
+ // 0 < x < +Inf
+ if x.exp <= 0 {
+ // 0 < x < 1
+ return 0, Below
}
- return u, Below
+ // 1 <= x < +Inf
+ if x.exp <= 64 {
+ // u = trunc(x) fits into a uint64
+ u := high64(x.mant) >> (64 - uint32(x.exp))
+ if x.minPrec() <= 64 {
+ return u, Exact
+ }
+ return u, Below // x truncated
+ }
+ fallthrough // x too large
+ case 2:
+ // x == +Inf
+ return math.MaxUint64, Below
}
- // x is too large
- return math.MaxUint64, Below
+ panic("unreachable")
}
-// TODO(gri) FIX THIS (inf, rounding mode, errors, etc.)
-func (x *Float) Int64() int64 {
- m := high64(x.mant)
- s := x.exp
- var i int64
- if s >= 0 {
- i = int64(m >> (64 - uint(s)))
+// Int64 returns the integer resulting from truncating x towards zero.
+// If math.MinInt64 <= x <= math.MaxInt64, the result is Exact if x is
+// an integer, and Above (x < 0) or Below (x > 0) otherwise.
+// The result is (math.MinInt64, Above) for x < math.MinInt64, and
+// (math.MaxInt64, Below) for x > math.MaxInt64.
+func (x *Float) Int64() (int64, Accuracy) {
+ if debugFloat {
+ x.validate()
}
- if x.neg {
- return -i
+
+ switch x.ord() {
+ case -2:
+ // x == -Inf
+ return math.MinInt64, Above
+ case 0:
+ // x == 0 || x == -0
+ return 0, Exact
+ case -1, 1:
+ // 0 < |x| < +Inf
+ acc := Below
+ if x.neg {
+ acc = Above
+ }
+ if x.exp <= 0 {
+ // 0 < |x| < 1
+ return 0, acc
+ }
+ // 1 <= |x| < +Inf
+ if x.exp <= 63 {
+ // i = trunc(x) fits into an int64 (excluding math.MinInt64)
+ i := int64(high64(x.mant) >> (64 - uint32(x.exp)))
+ if x.neg {
+ i = -i
+ }
+ if x.minPrec() <= 63 {
+ return i, Exact
+ }
+ return i, acc // x truncated
+ }
+ if x.neg {
+ // check for special case x == math.MinInt64 (i.e., x == -(0.5 << 64))
+ if x.exp == 64 && x.minPrec() == 1 {
+ acc = Exact
+ }
+ return math.MinInt64, acc
+ }
+ fallthrough
+ case 2:
+ // x == +Inf
+ return math.MaxInt64, Below
}
- return i
+ panic("unreachable")
}
// Float64 returns the closest float64 value of x
@@ -776,9 +808,8 @@
// x.mant[len(x.mant)-1] != 0
// determine minimum required precision for x
allBits := uint(len(x.mant)) * _W
- minPrec := allBits - x.mant.trailingZeroBits()
exp := uint(x.exp)
- if exp >= minPrec {
+ if x.minPrec() <= exp {
acc = Exact
}
// shift mantissa as needed
@@ -1199,8 +1230,8 @@
y.validate()
}
- mx := x.mag()
- my := y.mag()
+ mx := x.ord()
+ my := y.ord()
switch {
case mx < my:
return -1
@@ -1227,17 +1258,17 @@
return y
}
-// mag returns:
+// ord classifies x and returns:
//
-// -2 if x == -Inf
-// -1 if x < 0
-// 0 if x == -0 or x == +0
-// +1 if x > 0
+// -2 if -Inf == x
+// -1 if -Inf < x < 0
+// 0 if x == 0 (signed or unsigned)
+// +1 if 0 < x < +Inf
// +2 if x == +Inf
//
-// mag is a helper function for Cmp.
-func (x *Float) mag() int {
- m := 1
+// TODO(gri) export (and remove IsInf)?
+func (x *Float) ord() int {
+ m := 1 // common case
if len(x.mant) == 0 {
m = 0
if x.exp == infExp {
diff --git a/src/math/big/float_test.go b/src/math/big/float_test.go
index 3ec8e83..dd059ba 100644
--- a/src/math/big/float_test.go
+++ b/src/math/big/float_test.go
@@ -21,6 +21,14 @@
return u
}
+func (x *Float) int64() int64 {
+ i, acc := x.Int64()
+ if acc != Exact {
+ panic(fmt.Sprintf("%s is not an int64", x.Format('g', 10)))
+ }
+ return i
+}
+
func TestFloatZeroValue(t *testing.T) {
// zero (uninitialized) value is a ready-to-use 0.0
var x Float
@@ -69,7 +77,7 @@
test.op(z, make(test.x), make(test.y))
got := 0
if !z.IsInf(0) {
- got = int(z.Int64())
+ got = int(z.int64())
}
if got != test.want {
t.Errorf("%d %c %d = %d; want %d", test.x, test.opname, test.y, got, test.want)
@@ -257,7 +265,7 @@
f.Round(f, prec, mode)
// check result
- r1 := f.Int64()
+ r1 := f.int64()
p1 := f.Precision()
a1 := f.Accuracy()
if r1 != r || p1 != prec || a1 != a {
@@ -430,7 +438,7 @@
}
var f Float
f.SetInt64(want)
- if got := f.Int64(); got != want {
+ if got := f.int64(); got != want {
t.Errorf("got %#x (%s); want %#x", got, f.Format('p', 0), want)
}
}
@@ -440,7 +448,7 @@
const x int64 = 0x7654321076543210 // 63 bits needed
for prec := uint(1); prec <= 63; prec++ {
f := NewFloat(0, prec, ToZero).SetInt64(x)
- got := f.Int64()
+ got := f.int64()
want := x &^ (1<<(63-prec) - 1) // cut off (round to zero) low 63-prec bits
if got != want {
t.Errorf("got %#x (%s); want %#x", got, f.Format('p', 0), want)
@@ -571,17 +579,21 @@
out uint64
acc Accuracy
}{
- {"0", 0, Exact},
- {"-0", 0, Exact},
- {"-1", 0, Above},
{"-Inf", 0, Above},
+ {"-1", 0, Above},
{"-1e-1000", 0, Above},
+ {"-0", 0, Exact},
+ {"0", 0, Exact},
{"1e-1000", 0, Below},
+ {"1", 1, Exact},
+ {"1.000000000000000000001", 1, Below},
{"12345.0", 12345, Exact},
- {"12345.6", 12345, Below},
+ {"12345.000000000000000000001", 12345, Below},
{"18446744073709551615", 18446744073709551615, Exact},
{"18446744073709551615.000000000000000000001", math.MaxUint64, Below},
+ {"18446744073709551616", math.MaxUint64, Below},
{"1e10000", math.MaxUint64, Below},
+ {"+Inf", math.MaxUint64, Below},
} {
x := makeFloat(test.x)
out, acc := x.Uint64()
@@ -591,6 +603,44 @@
}
}
+func TestFloatInt64(t *testing.T) {
+ for _, test := range []struct {
+ x string
+ out int64
+ acc Accuracy
+ }{
+ {"-Inf", math.MinInt64, Above},
+ {"-1e10000", math.MinInt64, Above},
+ {"-9223372036854775809", math.MinInt64, Above},
+ {"-9223372036854775808.000000000000000000001", math.MinInt64, Above},
+ {"-9223372036854775808", -9223372036854775808, Exact},
+ {"-9223372036854775807.000000000000000000001", -9223372036854775807, Above},
+ {"-9223372036854775807", -9223372036854775807, Exact},
+ {"-12345.000000000000000000001", -12345, Above},
+ {"-12345.0", -12345, Exact},
+ {"-1.000000000000000000001", -1, Above},
+ {"-1", -1, Exact},
+ {"-1e-1000", 0, Above},
+ {"0", 0, Exact},
+ {"1e-1000", 0, Below},
+ {"1", 1, Exact},
+ {"1.000000000000000000001", 1, Below},
+ {"12345.0", 12345, Exact},
+ {"12345.000000000000000000001", 12345, Below},
+ {"9223372036854775807", 9223372036854775807, Exact},
+ {"9223372036854775807.000000000000000000001", math.MaxInt64, Below},
+ {"9223372036854775808", math.MaxInt64, Below},
+ {"1e10000", math.MaxInt64, Below},
+ {"+Inf", math.MaxInt64, Below},
+ } {
+ x := makeFloat(test.x)
+ out, acc := x.Int64()
+ if out != test.out || acc != test.acc {
+ t.Errorf("%s: got %d (%s); want %d (%s)", test.x, out, acc, test.out, test.acc)
+ }
+ }
+}
+
func TestFloatInt(t *testing.T) {
for _, test := range []struct {
x string
