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Diffstat (limited to 'src/math/rand/rand.go')
-rw-r--r-- | src/math/rand/rand.go | 246 |
1 files changed, 246 insertions, 0 deletions
diff --git a/src/math/rand/rand.go b/src/math/rand/rand.go new file mode 100644 index 000000000..3ffb5c4e5 --- /dev/null +++ b/src/math/rand/rand.go @@ -0,0 +1,246 @@ +// Copyright 2009 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +// Package rand implements pseudo-random number generators. +// +// Random numbers are generated by a Source. Top-level functions, such as +// Float64 and Int, use a default shared Source that produces a deterministic +// sequence of values each time a program is run. Use the Seed function to +// initialize the default Source if different behavior is required for each run. +// The default Source is safe for concurrent use by multiple goroutines. +package rand + +import "sync" + +// A Source represents a source of uniformly-distributed +// pseudo-random int64 values in the range [0, 1<<63). +type Source interface { + Int63() int64 + Seed(seed int64) +} + +// NewSource returns a new pseudo-random Source seeded with the given value. +func NewSource(seed int64) Source { + var rng rngSource + rng.Seed(seed) + return &rng +} + +// A Rand is a source of random numbers. +type Rand struct { + src Source +} + +// New returns a new Rand that uses random values from src +// to generate other random values. +func New(src Source) *Rand { return &Rand{src} } + +// Seed uses the provided seed value to initialize the generator to a deterministic state. +func (r *Rand) Seed(seed int64) { r.src.Seed(seed) } + +// Int63 returns a non-negative pseudo-random 63-bit integer as an int64. +func (r *Rand) Int63() int64 { return r.src.Int63() } + +// Uint32 returns a pseudo-random 32-bit value as a uint32. +func (r *Rand) Uint32() uint32 { return uint32(r.Int63() >> 31) } + +// Int31 returns a non-negative pseudo-random 31-bit integer as an int32. +func (r *Rand) Int31() int32 { return int32(r.Int63() >> 32) } + +// Int returns a non-negative pseudo-random int. +func (r *Rand) Int() int { + u := uint(r.Int63()) + return int(u << 1 >> 1) // clear sign bit if int == int32 +} + +// Int63n returns, as an int64, a non-negative pseudo-random number in [0,n). +// It panics if n <= 0. +func (r *Rand) Int63n(n int64) int64 { + if n <= 0 { + panic("invalid argument to Int63n") + } + if n&(n-1) == 0 { // n is power of two, can mask + return r.Int63() & (n - 1) + } + max := int64((1 << 63) - 1 - (1<<63)%uint64(n)) + v := r.Int63() + for v > max { + v = r.Int63() + } + return v % n +} + +// Int31n returns, as an int32, a non-negative pseudo-random number in [0,n). +// It panics if n <= 0. +func (r *Rand) Int31n(n int32) int32 { + if n <= 0 { + panic("invalid argument to Int31n") + } + if n&(n-1) == 0 { // n is power of two, can mask + return r.Int31() & (n - 1) + } + max := int32((1 << 31) - 1 - (1<<31)%uint32(n)) + v := r.Int31() + for v > max { + v = r.Int31() + } + return v % n +} + +// Intn returns, as an int, a non-negative pseudo-random number in [0,n). +// It panics if n <= 0. +func (r *Rand) Intn(n int) int { + if n <= 0 { + panic("invalid argument to Intn") + } + if n <= 1<<31-1 { + return int(r.Int31n(int32(n))) + } + return int(r.Int63n(int64(n))) +} + +// Float64 returns, as a float64, a pseudo-random number in [0.0,1.0). +func (r *Rand) Float64() float64 { + // A clearer, simpler implementation would be: + // return float64(r.Int63n(1<<53)) / (1<<53) + // However, Go 1 shipped with + // return float64(r.Int63()) / (1 << 63) + // and we want to preserve that value stream. + // + // 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 + // will not observe it anyway. + f := float64(r.Int63()) / (1 << 63) + if f == 1 { + f = 0 + } + return f +} + +// Float32 returns, as a float32, a pseudo-random number in [0.0,1.0). +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). + f := float32(r.Float64()) + if f == 1 { + f = 0 + } + return f +} + +// 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) + for i := 0; i < n; i++ { + j := r.Intn(i + 1) + m[i] = m[j] + m[j] = i + } + return m +} + +/* + * Top-level convenience functions + */ + +var globalRand = New(&lockedSource{src: NewSource(1)}) + +// Seed uses the provided seed value to initialize the default Source to a +// deterministic state. If Seed is not called, the generator behaves as +// if seeded by Seed(1). +func Seed(seed int64) { globalRand.Seed(seed) } + +// Int63 returns a non-negative pseudo-random 63-bit integer as an int64 +// from the default Source. +func Int63() int64 { return globalRand.Int63() } + +// Uint32 returns a pseudo-random 32-bit value as a uint32 +// from the default Source. +func Uint32() uint32 { return globalRand.Uint32() } + +// Int31 returns a non-negative pseudo-random 31-bit integer as an int32 +// from the default Source. +func Int31() int32 { return globalRand.Int31() } + +// Int returns a non-negative pseudo-random int from the default Source. +func Int() int { return globalRand.Int() } + +// Int63n returns, as an int64, a non-negative pseudo-random number in [0,n) +// from the default Source. +// It panics if n <= 0. +func Int63n(n int64) int64 { return globalRand.Int63n(n) } + +// Int31n returns, as an int32, a non-negative pseudo-random number in [0,n) +// from the default Source. +// It panics if n <= 0. +func Int31n(n int32) int32 { return globalRand.Int31n(n) } + +// Intn returns, as an int, a non-negative pseudo-random number in [0,n) +// from the default Source. +// It panics if n <= 0. +func Intn(n int) int { return globalRand.Intn(n) } + +// Float64 returns, as a float64, a pseudo-random number in [0.0,1.0) +// from the default Source. +func Float64() float64 { return globalRand.Float64() } + +// Float32 returns, as a float32, a pseudo-random number in [0.0,1.0) +// from the default Source. +func Float32() float32 { return globalRand.Float32() } + +// Perm returns, as a slice of n ints, a pseudo-random permutation of the integers [0,n) +// from the default Source. +func Perm(n int) []int { return globalRand.Perm(n) } + +// NormFloat64 returns a normally distributed float64 in the range +// [-math.MaxFloat64, +math.MaxFloat64] with +// standard normal distribution (mean = 0, stddev = 1) +// from the default Source. +// To produce a different normal distribution, callers can +// adjust the output using: +// +// sample = NormFloat64() * desiredStdDev + desiredMean +// +func NormFloat64() float64 { return globalRand.NormFloat64() } + +// ExpFloat64 returns an exponentially distributed float64 in the range +// (0, +math.MaxFloat64] with an exponential distribution whose rate parameter +// (lambda) is 1 and whose mean is 1/lambda (1) from the default Source. +// To produce a distribution with a different rate parameter, +// callers can adjust the output using: +// +// sample = ExpFloat64() / desiredRateParameter +// +func ExpFloat64() float64 { return globalRand.ExpFloat64() } + +type lockedSource struct { + lk sync.Mutex + src Source +} + +func (r *lockedSource) Int63() (n int64) { + r.lk.Lock() + n = r.src.Int63() + r.lk.Unlock() + return +} + +func (r *lockedSource) Seed(seed int64) { + r.lk.Lock() + r.src.Seed(seed) + r.lk.Unlock() +} |