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(v1, v2) needs to return true for at least one of the types in the cycle,
    37  	// and it's safe and valid to get Value's internal pointer.
    38  	hard := func(v1, v2 Value) bool {
    39  		switch v1.Kind() {
    40  		case Map, Slice, Ptr, Interface:
    41  			// Nil pointers cannot be cyclic. Avoid putting them in the visited map.
    42  			return !v1.IsNil() && !v2.IsNil()
    43  		}
    44  		return false
    45  	}
    46  
    47  	if hard(v1, v2) {
    48  		addr1 := v1.ptr
    49  		addr2 := v2.ptr
    50  		if uintptr(addr1) > uintptr(addr2) {
    51  			// Canonicalize order to reduce number of entries in visited.
    52  			// Assumes non-moving garbage collector.
    53  			addr1, addr2 = addr2, addr1
    54  		}
    55  
    56  		// Short circuit if references are already seen.
    57  		typ := v1.Type()
    58  		v := visit{addr1, addr2, typ}
    59  		if visited[v] {
    60  			return true
    61  		}
    62  
    63  		// Remember for later.
    64  		visited[v] = true
    65  	}
    66  
    67  	switch v1.Kind() {
    68  	case Array:
    69  		for i := 0; i < v1.Len(); i++ {
    70  			if !deepValueEqual(v1.Index(i), v2.Index(i), visited, depth+1) {
    71  				return false
    72  			}
    73  		}
    74  		return true
    75  	case Slice:
    76  		if v1.IsNil() != v2.IsNil() {
    77  			return false
    78  		}
    79  		if v1.Len() != v2.Len() {
    80  			return false
    81  		}
    82  		if v1.Pointer() == v2.Pointer() {
    83  			return true
    84  		}
    85  		for i := 0; i < v1.Len(); i++ {
    86  			if !deepValueEqual(v1.Index(i), v2.Index(i), visited, depth+1) {
    87  				return false
    88  			}
    89  		}
    90  		return true
    91  	case Interface:
    92  		if v1.IsNil() || v2.IsNil() {
    93  			return v1.IsNil() == v2.IsNil()
    94  		}
    95  		return deepValueEqual(v1.Elem(), v2.Elem(), visited, depth+1)
    96  	case Ptr:
    97  		if v1.Pointer() == v2.Pointer() {
    98  			return true
    99  		}
   100  		return deepValueEqual(v1.Elem(), v2.Elem(), visited, depth+1)
   101  	case Struct:
   102  		for i, n := 0, v1.NumField(); i < n; i++ {
   103  			if !deepValueEqual(v1.Field(i), v2.Field(i), visited, depth+1) {
   104  				return false
   105  			}
   106  		}
   107  		return true
   108  	case Map:
   109  		if v1.IsNil() != v2.IsNil() {
   110  			return false
   111  		}
   112  		if v1.Len() != v2.Len() {
   113  			return false
   114  		}
   115  		if v1.Pointer() == v2.Pointer() {
   116  			return true
   117  		}
   118  		for _, k := range v1.MapKeys() {
   119  			val1 := v1.MapIndex(k)
   120  			val2 := v2.MapIndex(k)
   121  			if !val1.IsValid() || !val2.IsValid() || !deepValueEqual(val1, val2, visited, depth+1) {
   122  				return false
   123  			}
   124  		}
   125  		return true
   126  	case Func:
   127  		if v1.IsNil() && v2.IsNil() {
   128  			return true
   129  		}
   130  		// Can't do better than this:
   131  		return false
   132  	default:
   133  		// Normal equality suffices
   134  		return valueInterface(v1, false) == valueInterface(v2, false)
   135  	}
   136  }
   137  
   138  // DeepEqual reports whether x and y are ``deeply equal,'' defined as follows.
   139  // Two values of identical type are deeply equal if one of the following cases applies.
   140  // Values of distinct types are never deeply equal.
   141  //
   142  // Array values are deeply equal when their corresponding elements are deeply equal.
   143  //
   144  // Struct values are deeply equal if their corresponding fields,
   145  // both exported and unexported, are deeply equal.
   146  //
   147  // Func values are deeply equal if both are nil; otherwise they are not deeply equal.
   148  //
   149  // Interface values are deeply equal if they hold deeply equal concrete values.
   150  //
   151  // Map values are deeply equal when all of the following are true:
   152  // they are both nil or both non-nil, they have the same length,
   153  // and either they are the same map object or their corresponding keys
   154  // (matched using Go equality) map to deeply equal values.
   155  //
   156  // Pointer values are deeply equal if they are equal using Go's == operator
   157  // or if they point to deeply equal values.
   158  //
   159  // Slice values are deeply equal when all of the following are true:
   160  // they are both nil or both non-nil, they have the same length,
   161  // and either they point to the same initial entry of the same underlying array
   162  // (that is, &x[0] == &y[0]) or their corresponding elements (up to length) are deeply equal.
   163  // Note that a non-nil empty slice and a nil slice (for example, []byte{} and []byte(nil))
   164  // are not deeply equal.
   165  //
   166  // Other values - numbers, bools, strings, and channels - are deeply equal
   167  // if they are equal using Go's == operator.
   168  //
   169  // In general DeepEqual is a recursive relaxation of Go's == operator.
   170  // However, this idea is impossible to implement without some inconsistency.
   171  // Specifically, it is possible for a value to be unequal to itself,
   172  // either because it is of func type (uncomparable in general)
   173  // or because it is a floating-point NaN value (not equal to itself in floating-point comparison),
   174  // or because it is an array, struct, or interface containing
   175  // such a value.
   176  // On the other hand, pointer values are always equal to themselves,
   177  // even if they point at or contain such problematic values,
   178  // because they compare equal using Go's == operator, and that
   179  // is a sufficient condition to be deeply equal, regardless of content.
   180  // DeepEqual has been defined so that the same short-cut applies
   181  // to slices and maps: if x and y are the same slice or the same map,
   182  // they are deeply equal regardless of content.
   183  //
   184  // As DeepEqual traverses the data values it may find a cycle. The
   185  // second and subsequent times that DeepEqual compares two pointer
   186  // values that have been compared before, it treats the values as
   187  // equal rather than examining the values to which they point.
   188  // This ensures that DeepEqual terminates.
   189  func DeepEqual(x, y interface{}) bool {
   190  	if x == nil || y == nil {
   191  		return x == y
   192  	}
   193  	v1 := ValueOf(x)
   194  	v2 := ValueOf(y)
   195  	if v1.Type() != v2.Type() {
   196  		return false
   197  	}
   198  	return deepValueEqual(v1, v2, make(map[visit]bool), 0)
   199  }
   200  

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