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Source file src/reflect/deepequal.go

Documentation: reflect

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Deep equality test via reflection
     6  
     7  package reflect
     8  
     9  import "unsafe"
    10  
    11  // During deepValueEqual, must keep track of checks that are
    12  // in progress. The comparison algorithm assumes that all
    13  // checks in progress are true when it reencounters them.
    14  // Visited comparisons are stored in a map indexed by visit.
    15  type visit struct {
    16  	a1  unsafe.Pointer
    17  	a2  unsafe.Pointer
    18  	typ Type
    19  }
    20  
    21  // Tests for deep equality using reflected types. The map argument tracks
    22  // comparisons that have already been seen, which allows short circuiting on
    23  // recursive types.
    24  func deepValueEqual(v1, v2 Value, visited map[visit]bool, depth int) bool {
    25  	if !v1.IsValid() || !v2.IsValid() {
    26  		return v1.IsValid() == v2.IsValid()
    27  	}
    28  	if v1.Type() != v2.Type() {
    29  		return false
    30  	}
    31  
    32  	// if depth > 10 { panic("deepValueEqual") }	// for debugging
    33  
    34  	// We want to avoid putting more in the visited map than we need to.
    35  	// For any possible reference cycle that might be encountered,
    36  	// hard(t) needs to return true for at least one of the types in the cycle.
    37  	hard := func(k Kind) bool {
    38  		switch k {
    39  		case Map, Slice, Ptr, Interface:
    40  			return true
    41  		}
    42  		return false
    43  	}
    44  
    45  	if v1.CanAddr() && v2.CanAddr() && hard(v1.Kind()) {
    46  		addr1 := unsafe.Pointer(v1.UnsafeAddr())
    47  		addr2 := unsafe.Pointer(v2.UnsafeAddr())
    48  		if uintptr(addr1) > uintptr(addr2) {
    49  			// Canonicalize order to reduce number of entries in visited.
    50  			// Assumes non-moving garbage collector.
    51  			addr1, addr2 = addr2, addr1
    52  		}
    53  
    54  		// Short circuit if references are already seen.
    55  		typ := v1.Type()
    56  		v := visit{addr1, addr2, typ}
    57  		if visited[v] {
    58  			return true
    59  		}
    60  
    61  		// Remember for later.
    62  		visited[v] = true
    63  	}
    64  
    65  	switch v1.Kind() {
    66  	case Array:
    67  		for i := 0; i < v1.Len(); i++ {
    68  			if !deepValueEqual(v1.Index(i), v2.Index(i), visited, depth+1) {
    69  				return false
    70  			}
    71  		}
    72  		return true
    73  	case Slice:
    74  		if v1.IsNil() != v2.IsNil() {
    75  			return false
    76  		}
    77  		if v1.Len() != v2.Len() {
    78  			return false
    79  		}
    80  		if v1.Pointer() == v2.Pointer() {
    81  			return true
    82  		}
    83  		for i := 0; i < v1.Len(); i++ {
    84  			if !deepValueEqual(v1.Index(i), v2.Index(i), visited, depth+1) {
    85  				return false
    86  			}
    87  		}
    88  		return true
    89  	case Interface:
    90  		if v1.IsNil() || v2.IsNil() {
    91  			return v1.IsNil() == v2.IsNil()
    92  		}
    93  		return deepValueEqual(v1.Elem(), v2.Elem(), visited, depth+1)
    94  	case Ptr:
    95  		if v1.Pointer() == v2.Pointer() {
    96  			return true
    97  		}
    98  		return deepValueEqual(v1.Elem(), v2.Elem(), visited, depth+1)
    99  	case Struct:
   100  		for i, n := 0, v1.NumField(); i < n; i++ {
   101  			if !deepValueEqual(v1.Field(i), v2.Field(i), visited, depth+1) {
   102  				return false
   103  			}
   104  		}
   105  		return true
   106  	case Map:
   107  		if v1.IsNil() != v2.IsNil() {
   108  			return false
   109  		}
   110  		if v1.Len() != v2.Len() {
   111  			return false
   112  		}
   113  		if v1.Pointer() == v2.Pointer() {
   114  			return true
   115  		}
   116  		for _, k := range v1.MapKeys() {
   117  			val1 := v1.MapIndex(k)
   118  			val2 := v2.MapIndex(k)
   119  			if !val1.IsValid() || !val2.IsValid() || !deepValueEqual(val1, val2, visited, depth+1) {
   120  				return false
   121  			}
   122  		}
   123  		return true
   124  	case Func:
   125  		if v1.IsNil() && v2.IsNil() {
   126  			return true
   127  		}
   128  		// Can't do better than this:
   129  		return false
   130  	default:
   131  		// Normal equality suffices
   132  		return valueInterface(v1, false) == valueInterface(v2, false)
   133  	}
   134  }
   135  
   136  // DeepEqual reports whether x and y are ``deeply equal,'' defined as follows.
   137  // Two values of identical type are deeply equal if one of the following cases applies.
   138  // Values of distinct types are never deeply equal.
   139  //
   140  // Array values are deeply equal when their corresponding elements are deeply equal.
   141  //
   142  // Struct values are deeply equal if their corresponding fields,
   143  // both exported and unexported, are deeply equal.
   144  //
   145  // Func values are deeply equal if both are nil; otherwise they are not deeply equal.
   146  //
   147  // Interface values are deeply equal if they hold deeply equal concrete values.
   148  //
   149  // Map values are deeply equal when all of the following are true:
   150  // they are both nil or both non-nil, they have the same length,
   151  // and either they are the same map object or their corresponding keys
   152  // (matched using Go equality) map to deeply equal values.
   153  //
   154  // Pointer values are deeply equal if they are equal using Go's == operator
   155  // or if they point to deeply equal values.
   156  //
   157  // Slice values are deeply equal when all of the following are true:
   158  // they are both nil or both non-nil, they have the same length,
   159  // and either they point to the same initial entry of the same underlying array
   160  // (that is, &x[0] == &y[0]) or their corresponding elements (up to length) are deeply equal.
   161  // Note that a non-nil empty slice and a nil slice (for example, []byte{} and []byte(nil))
   162  // are not deeply equal.
   163  //
   164  // Other values - numbers, bools, strings, and channels - are deeply equal
   165  // if they are equal using Go's == operator.
   166  //
   167  // In general DeepEqual is a recursive relaxation of Go's == operator.
   168  // However, this idea is impossible to implement without some inconsistency.
   169  // Specifically, it is possible for a value to be unequal to itself,
   170  // either because it is of func type (uncomparable in general)
   171  // or because it is a floating-point NaN value (not equal to itself in floating-point comparison),
   172  // or because it is an array, struct, or interface containing
   173  // such a value.
   174  // On the other hand, pointer values are always equal to themselves,
   175  // even if they point at or contain such problematic values,
   176  // because they compare equal using Go's == operator, and that
   177  // is a sufficient condition to be deeply equal, regardless of content.
   178  // DeepEqual has been defined so that the same short-cut applies
   179  // to slices and maps: if x and y are the same slice or the same map,
   180  // they are deeply equal regardless of content.
   181  //
   182  // As DeepEqual traverses the data values it may find a cycle. The
   183  // second and subsequent times that DeepEqual compares two pointer
   184  // values that have been compared before, it treats the values as
   185  // equal rather than examining the values to which they point.
   186  // This ensures that DeepEqual terminates.
   187  func DeepEqual(x, y interface{}) bool {
   188  	if x == nil || y == nil {
   189  		return x == y
   190  	}
   191  	v1 := ValueOf(x)
   192  	v2 := ValueOf(y)
   193  	if v1.Type() != v2.Type() {
   194  		return false
   195  	}
   196  	return deepValueEqual(v1, v2, make(map[visit]bool), 0)
   197  }
   198  

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