diff options
Diffstat (limited to 'libgo/go/math/rand/rand.go')
| -rw-r--r-- | libgo/go/math/rand/rand.go | 47 |
1 files changed, 33 insertions, 14 deletions
diff --git a/libgo/go/math/rand/rand.go b/libgo/go/math/rand/rand.go index 6360128e391..d693bfb52f2 100644 --- a/libgo/go/math/rand/rand.go +++ b/libgo/go/math/rand/rand.go @@ -113,19 +113,18 @@ func (r *Rand) Float64() float64 { // // There is one bug in the value stream: r.Int63() may be so close // to 1<<63 that the division rounds up to 1.0, and we've guaranteed - // that the result is always less than 1.0. To fix that, we treat the - // range as cyclic and map 1 back to 0. This is justified by observing - // that while some of the values rounded down to 0, nothing was - // rounding up to 0, so 0 was underrepresented in the results. - // Mapping 1 back to zero restores some balance. - // (The balance is not perfect because the implementation - // returns denormalized numbers for very small r.Int63(), - // and those steal from what would normally be 0 results.) - // The remapping only happens 1/2⁵³ of the time, so most clients + // that the result is always less than 1.0. + // + // We tried to fix this by mapping 1.0 back to 0.0, but since float64 + // values near 0 are much denser than near 1, mapping 1 to 0 caused + // a theoretically significant overshoot in the probability of returning 0. + // Instead of that, if we round up to 1, just try again. + // Getting 1 only happens 1/2⁵³ of the time, so most clients // will not observe it anyway. +again: f := float64(r.Int63()) / (1 << 63) if f == 1 { - f = 0 + goto again // resample; this branch is taken O(never) } return f } @@ -134,13 +133,11 @@ func (r *Rand) Float64() float64 { func (r *Rand) Float32() float32 { // Same rationale as in Float64: we want to preserve the Go 1 value // stream except we want to fix it not to return 1.0 - // There is a double rounding going on here, but the argument for - // mapping 1 to 0 still applies: 0 was underrepresented before, - // so mapping 1 to 0 doesn't cause too many 0s. // This only happens 1/2²⁴ of the time (plus the 1/2⁵³ of the time in Float64). +again: f := float32(r.Float64()) if f == 1 { - f = 0 + goto again // resample; this branch is taken O(very rarely) } return f } @@ -148,6 +145,11 @@ func (r *Rand) Float32() float32 { // Perm returns, as a slice of n ints, a pseudo-random permutation of the integers [0,n). func (r *Rand) Perm(n int) []int { m := make([]int, n) + // In the following loop, the iteration when i=0 always swaps m[0] with m[0]. + // A change to remove this useless iteration is to assign 1 to i in the init + // statement. But Perm also effects r. Making this change will affect + // the final state of r. So this change can't be made for compatibility + // reasons for Go 1. for i := 0; i < n; i++ { j := r.Intn(i + 1) m[i] = m[j] @@ -156,6 +158,19 @@ func (r *Rand) Perm(n int) []int { return m } +// Read generates len(p) random bytes and writes them into p. It +// always returns len(p) and a nil error. +func (r *Rand) Read(p []byte) (n int, err error) { + for i := 0; i < len(p); i += 7 { + val := r.src.Int63() + for j := 0; i+j < len(p) && j < 7; j++ { + p[i+j] = byte(val) + val >>= 8 + } + } + return len(p), nil +} + /* * Top-level convenience functions */ @@ -209,6 +224,10 @@ func Float32() float32 { return globalRand.Float32() } // from the default Source. func Perm(n int) []int { return globalRand.Perm(n) } +// Read generates len(p) random bytes from the default Source and +// writes them into p. It always returns len(p) and a nil error. +func Read(p []byte) (n int, err error) { return globalRand.Read(p) } + // NormFloat64 returns a normally distributed float64 in the range // [-math.MaxFloat64, +math.MaxFloat64] with // standard normal distribution (mean = 0, stddev = 1) |