82 lines
2.4 KiB
Go
82 lines
2.4 KiB
Go
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/*
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Copyright 2014 Google Inc. All rights reserved.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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*/
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// Package bytesource provides a rand.Source64 that is determined by a slice of bytes.
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package bytesource
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import (
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"bytes"
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"encoding/binary"
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"io"
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"math/rand"
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)
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// ByteSource implements rand.Source64 determined by a slice of bytes. The random numbers are
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// generated from each 8 bytes in the slice, until the last bytes are consumed, from which a
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// fallback pseudo random source is created in case more random numbers are required.
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// It also exposes a `bytes.Reader` API, which lets callers consume the bytes directly.
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type ByteSource struct {
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*bytes.Reader
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fallback rand.Source
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}
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// New returns a new ByteSource from a given slice of bytes.
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func New(input []byte) *ByteSource {
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s := &ByteSource{
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Reader: bytes.NewReader(input),
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fallback: rand.NewSource(0),
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}
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if len(input) > 0 {
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s.fallback = rand.NewSource(int64(s.consumeUint64()))
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}
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return s
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}
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func (s *ByteSource) Uint64() uint64 {
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// Return from input if it was not exhausted.
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if s.Len() > 0 {
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return s.consumeUint64()
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}
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// Input was exhausted, return random number from fallback (in this case fallback should not be
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// nil). Try first having a Uint64 output (Should work in current rand implementation),
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// otherwise return a conversion of Int63.
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if s64, ok := s.fallback.(rand.Source64); ok {
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return s64.Uint64()
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}
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return uint64(s.fallback.Int63())
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}
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func (s *ByteSource) Int63() int64 {
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return int64(s.Uint64() >> 1)
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}
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func (s *ByteSource) Seed(seed int64) {
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s.fallback = rand.NewSource(seed)
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s.Reader = bytes.NewReader(nil)
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}
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// consumeUint64 reads 8 bytes from the input and convert them to a uint64. It assumes that the the
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// bytes reader is not empty.
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func (s *ByteSource) consumeUint64() uint64 {
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var bytes [8]byte
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_, err := s.Read(bytes[:])
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if err != nil && err != io.EOF {
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panic("failed reading source") // Should not happen.
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}
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return binary.BigEndian.Uint64(bytes[:])
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}
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