mirror of
https://github.com/tailscale/tailscale.git
synced 2024-12-04 23:45:34 +00:00
ac0353e982
Signed-off-by: slowy07 <slowy.arfy@gmail.com>
390 lines
11 KiB
Go
390 lines
11 KiB
Go
// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package deephash hashes a Go value recursively, in a predictable order,
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// without looping. The hash is only valid within the lifetime of a program.
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// Users should not store the hash on disk or send it over the network.
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// The hash is sufficiently strong and unique such that
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// Hash(x) == Hash(y) is an appropriate replacement for x == y.
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//
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// The definition of equality is identical to reflect.DeepEqual except:
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// * Floating-point values are compared based on the raw bits,
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// which means that NaNs (with the same bit pattern) are treated as equal.
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// * Types which implement interface { AppendTo([]byte) []byte } use
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// the AppendTo method to produce a textual representation of the value.
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// Thus, two values are equal if AppendTo produces the same bytes.
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//
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// WARNING: This package, like most of the tailscale.com Go module,
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// should be considered Tailscale-internal; we make no API promises.
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package deephash
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import (
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"bufio"
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"crypto/sha256"
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"encoding/binary"
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"encoding/hex"
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"fmt"
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"hash"
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"math"
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"reflect"
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"sync"
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"time"
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"unsafe"
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)
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// There is much overlap between the theory of serialization and hashing.
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// A hash (useful for determining equality) can be produced by printing a value
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// and hashing the output. The format must:
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// * be deterministic such that the same value hashes to the same output, and
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// * be parsable such that the same value can be reproduced by the output.
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//
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// The logic below hashes a value by printing it to a hash.Hash.
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// To be parsable, it assumes that we know the Go type of each value:
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// * scalar types (e.g., bool or int32) are printed as fixed-width fields.
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// * list types (e.g., strings, slices, and AppendTo buffers) are prefixed
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// by a fixed-width length field, followed by the contents of the list.
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// * slices, arrays, and structs print each element/field consecutively.
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// * interfaces print with a 1-byte prefix indicating whether it is nil.
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// If non-nil, it is followed by a fixed-width field of the type index,
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// followed by the format of the underlying value.
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// * pointers print with a 1-byte prefix indicating whether the pointer is
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// 1) nil, 2) previously seen, or 3) newly seen. Previously seen pointers are
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// followed by a fixed-width field with the index of the previous pointer.
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// Newly seen pointers are followed by the format of the underlying value.
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// * maps print with a 1-byte prefix indicating whether the map pointer is
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// 1) nil, 2) previously seen, or 3) newly seen. Previously seen pointers
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// are followed by a fixed-width field of the index of the previous pointer.
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// Newly seen maps are printed as a fixed-width field with the XOR of the
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// hash of every map entry. With a sufficiently strong hash, this value is
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// theoretically "parsable" by looking up the hash in a magical map that
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// returns the set of entries for that given hash.
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const scratchSize = 128
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// hasher is reusable state for hashing a value.
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// Get one via hasherPool.
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type hasher struct {
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h hash.Hash
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bw *bufio.Writer
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scratch [scratchSize]byte
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visitStack visitStack
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}
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func (h *hasher) reset() {
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if h.h == nil {
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h.h = sha256.New()
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}
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if h.bw == nil {
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h.bw = bufio.NewWriterSize(h.h, h.h.BlockSize())
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}
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h.bw.Flush()
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h.h.Reset()
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}
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// Sum is an opaque checksum type that is comparable.
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type Sum struct {
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sum [sha256.Size]byte
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}
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func (s1 *Sum) xor(s2 Sum) {
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for i := 0; i < sha256.Size; i++ {
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s1.sum[i] ^= s2.sum[i]
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}
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}
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func (s Sum) String() string {
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return hex.EncodeToString(s.sum[:])
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}
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var (
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once sync.Once
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seed uint64
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)
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func (h *hasher) sum() (s Sum) {
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h.bw.Flush()
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// Sum into scratch & copy out, as hash.Hash is an interface
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// so the slice necessarily escapes, and there's no sha256
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// concrete type exported and we don't want the 'hash' result
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// parameter to escape to the heap:
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copy(s.sum[:], h.h.Sum(h.scratch[:0]))
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return s
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}
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var hasherPool = &sync.Pool{
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New: func() interface{} { return new(hasher) },
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}
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// Hash returns the hash of v.
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func Hash(v interface{}) (s Sum) {
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h := hasherPool.Get().(*hasher)
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defer hasherPool.Put(h)
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h.reset()
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once.Do(func() {
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seed = uint64(time.Now().UnixNano())
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})
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h.hashUint64(seed)
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h.hashValue(reflect.ValueOf(v))
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return h.sum()
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}
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// Update sets last to the hash of v and reports whether its value changed.
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func Update(last *Sum, v ...interface{}) (changed bool) {
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sum := Hash(v)
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if sum == *last {
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// unchanged.
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return false
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}
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*last = sum
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return true
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}
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var appenderToType = reflect.TypeOf((*appenderTo)(nil)).Elem()
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type appenderTo interface {
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AppendTo([]byte) []byte
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}
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func (h *hasher) hashUint8(i uint8) {
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h.bw.WriteByte(i)
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}
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func (h *hasher) hashUint16(i uint16) {
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binary.LittleEndian.PutUint16(h.scratch[:2], i)
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h.bw.Write(h.scratch[:2])
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}
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func (h *hasher) hashUint32(i uint32) {
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binary.LittleEndian.PutUint32(h.scratch[:4], i)
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h.bw.Write(h.scratch[:4])
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}
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func (h *hasher) hashUint64(i uint64) {
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binary.LittleEndian.PutUint64(h.scratch[:8], i)
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h.bw.Write(h.scratch[:8])
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}
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var uint8Type = reflect.TypeOf(byte(0))
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func (h *hasher) hashValue(v reflect.Value) {
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if !v.IsValid() {
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return
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}
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w := h.bw
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if v.CanInterface() {
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// Use AppendTo methods, if available and cheap.
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if v.CanAddr() && v.Type().Implements(appenderToType) {
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a := v.Addr().Interface().(appenderTo)
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size := h.scratch[:8]
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record := a.AppendTo(size)
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binary.LittleEndian.PutUint64(record, uint64(len(record)-len(size)))
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w.Write(record)
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return
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}
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}
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// TODO(dsnet): Avoid cycle detection for types that cannot have cycles.
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// Generic handling.
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switch v.Kind() {
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default:
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panic(fmt.Sprintf("unhandled kind %v for type %v", v.Kind(), v.Type()))
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case reflect.Ptr:
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if v.IsNil() {
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h.hashUint8(0) // indicates nil
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return
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}
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// Check for cycle.
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ptr := pointerOf(v)
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if idx, ok := h.visitStack.seen(ptr); ok {
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h.hashUint8(2) // indicates cycle
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h.hashUint64(uint64(idx))
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return
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}
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h.visitStack.push(ptr)
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defer h.visitStack.pop(ptr)
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h.hashUint8(1) // indicates visiting a pointer
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h.hashValue(v.Elem())
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case reflect.Struct:
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for i, n := 0, v.NumField(); i < n; i++ {
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h.hashValue(v.Field(i))
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}
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case reflect.Slice, reflect.Array:
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vLen := v.Len()
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if v.Kind() == reflect.Slice {
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h.hashUint64(uint64(vLen))
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}
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if v.Type().Elem() == uint8Type && v.CanInterface() {
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if vLen > 0 && vLen <= scratchSize {
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// If it fits in scratch, avoid the Interface allocation.
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// It seems tempting to do this for all sizes, doing
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// scratchSize bytes at a time, but reflect.Slice seems
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// to allocate, so it's not a win.
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n := reflect.Copy(reflect.ValueOf(&h.scratch).Elem(), v)
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w.Write(h.scratch[:n])
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return
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}
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fmt.Fprintf(w, "%s", v.Interface())
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return
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}
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for i := 0; i < vLen; i++ {
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// TODO(dsnet): Perform cycle detection for slices,
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// which is functionally a list of pointers.
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// See https://github.com/google/go-cmp/blob/402949e8139bb890c71a707b6faf6dd05c92f4e5/cmp/compare.go#L438-L450
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h.hashValue(v.Index(i))
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}
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case reflect.Interface:
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if v.IsNil() {
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h.hashUint8(0) // indicates nil
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return
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}
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v = v.Elem()
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h.hashUint8(1) // indicates visiting interface value
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h.hashType(v.Type())
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h.hashValue(v)
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case reflect.Map:
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// Check for cycle.
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ptr := pointerOf(v)
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if idx, ok := h.visitStack.seen(ptr); ok {
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h.hashUint8(2) // indicates cycle
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h.hashUint64(uint64(idx))
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return
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}
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h.visitStack.push(ptr)
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defer h.visitStack.pop(ptr)
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h.hashUint8(1) // indicates visiting a map
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h.hashMap(v)
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case reflect.String:
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s := v.String()
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h.hashUint64(uint64(len(s)))
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w.WriteString(s)
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case reflect.Bool:
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if v.Bool() {
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h.hashUint8(1)
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} else {
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h.hashUint8(0)
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}
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case reflect.Int8:
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h.hashUint8(uint8(v.Int()))
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case reflect.Int16:
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h.hashUint16(uint16(v.Int()))
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case reflect.Int32:
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h.hashUint32(uint32(v.Int()))
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case reflect.Int64, reflect.Int:
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h.hashUint64(uint64(v.Int()))
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case reflect.Uint8:
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h.hashUint8(uint8(v.Uint()))
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case reflect.Uint16:
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h.hashUint16(uint16(v.Uint()))
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case reflect.Uint32:
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h.hashUint32(uint32(v.Uint()))
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case reflect.Uint64, reflect.Uint, reflect.Uintptr:
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h.hashUint64(uint64(v.Uint()))
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case reflect.Float32:
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h.hashUint32(math.Float32bits(float32(v.Float())))
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case reflect.Float64:
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h.hashUint64(math.Float64bits(float64(v.Float())))
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case reflect.Complex64:
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h.hashUint32(math.Float32bits(real(complex64(v.Complex()))))
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h.hashUint32(math.Float32bits(imag(complex64(v.Complex()))))
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case reflect.Complex128:
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h.hashUint64(math.Float64bits(real(complex128(v.Complex()))))
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h.hashUint64(math.Float64bits(imag(complex128(v.Complex()))))
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}
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}
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type mapHasher struct {
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h hasher
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val valueCache // re-usable values for map iteration
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iter reflect.MapIter // re-usable map iterator
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}
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var mapHasherPool = &sync.Pool{
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New: func() interface{} { return new(mapHasher) },
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}
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type valueCache map[reflect.Type]reflect.Value
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func (c *valueCache) get(t reflect.Type) reflect.Value {
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v, ok := (*c)[t]
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if !ok {
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v = reflect.New(t).Elem()
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if *c == nil {
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*c = make(valueCache)
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}
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(*c)[t] = v
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}
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return v
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}
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// hashMap hashes a map in a sort-free manner.
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// It relies on a map being a functionally an unordered set of KV entries.
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// So long as we hash each KV entry together, we can XOR all
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// of the individual hashes to produce a unique hash for the entire map.
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func (h *hasher) hashMap(v reflect.Value) {
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mh := mapHasherPool.Get().(*mapHasher)
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defer mapHasherPool.Put(mh)
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iter := mapIter(&mh.iter, v)
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defer mapIter(&mh.iter, reflect.Value{}) // avoid pinning v from mh.iter when we return
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var sum Sum
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k := mh.val.get(v.Type().Key())
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e := mh.val.get(v.Type().Elem())
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mh.h.visitStack = h.visitStack // always use the parent's visit stack to avoid cycles
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for iter.Next() {
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key := iterKey(iter, k)
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val := iterVal(iter, e)
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mh.h.reset()
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mh.h.hashValue(key)
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mh.h.hashValue(val)
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sum.xor(mh.h.sum())
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}
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h.bw.Write(append(h.scratch[:0], sum.sum[:]...)) // append into scratch to avoid heap allocation
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}
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// visitStack is a stack of pointers visited.
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// Pointers are pushed onto the stack when visited, and popped when leaving.
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// The integer value is the depth at which the pointer was visited.
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// The length of this stack should be zero after every hashing operation.
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type visitStack map[pointer]int
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func (v visitStack) seen(p pointer) (int, bool) {
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idx, ok := v[p]
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return idx, ok
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}
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func (v *visitStack) push(p pointer) {
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if *v == nil {
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*v = make(map[pointer]int)
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}
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(*v)[p] = len(*v)
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}
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func (v visitStack) pop(p pointer) {
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delete(v, p)
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}
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// pointer is a thin wrapper over unsafe.Pointer.
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// We only rely on comparability of pointers; we cannot rely on uintptr since
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// that would break if Go ever switched to a moving GC.
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type pointer struct{ p unsafe.Pointer }
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func pointerOf(v reflect.Value) pointer {
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return pointer{unsafe.Pointer(v.Pointer())}
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}
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// hashType hashes a reflect.Type.
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// The hash is only consistent within the lifetime of a program.
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func (h *hasher) hashType(t reflect.Type) {
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// This approach relies on reflect.Type always being backed by a unique
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// *reflect.rtype pointer. A safer approach is to use a global sync.Map
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// that maps reflect.Type to some arbitrary and unique index.
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// While safer, it requires global state with memory that can never be GC'd.
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rtypeAddr := reflect.ValueOf(t).Pointer() // address of *reflect.rtype
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h.hashUint64(uint64(rtypeAddr))
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}
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