implement Path.HasPrefix and Path.Equals

Paths comparisons done with reflect.DeepEqual may erroneously return
false for numeric IndexSteps, due to the differences in the internal
values of the big.Float.

Add Path.Equals method to ensure all IndexSteps compare correctly.
Add Path.HasPrefix to test that one path is a prefix of another.

cty/path.go

@@ -71,6 +71,48 @@ func (p Path) GetAttr(name string) Path {
 	return ret
 }
 
+// Equals compares 2 Paths for exact equality.
+func (p Path) Equals(other Path) bool {
+	if len(p) != len(other) {
+		return false
+	}
+
+	for i := range p {
+		pv := p[i]
+		switch pv := pv.(type) {
+		case GetAttrStep:
+			ov, ok := other[i].(GetAttrStep)
+			if !ok || pv != ov {
+				return false
+			}
+		case IndexStep:
+			ov, ok := other[i].(IndexStep)
+			if !ok {
+				return false
+			}
+
+			if !pv.Key.RawEquals(ov.Key) {
+				return false
+			}
+		default:
+			// Any invalid steps default to evaluating false.
+			return false
+		}
+	}
+
+	return true
+
+}
+
+// HasPrefix determines if the path p contains the provided prefix.
+func (p Path) HasPrefix(prefix Path) bool {
+	if len(prefix) > len(p) {
+		return false
+	}
+
+	return p[:len(prefix)].Equals(prefix)
+}
+
 // GetAttrPath is a convenience method to start a new Path with a GetAttrStep.
 func GetAttrPath(name string) Path {
 	return Path{}.GetAttr(name)

cty/path_test.go

@@ -123,3 +123,134 @@ func TestPathApply(t *testing.T) {
 		})
 	}
 }
+
+func TestPathEquals(t *testing.T) {
+	tests := []struct {
+		A, B   cty.Path
+		Equal  bool
+		Prefix bool
+	}{
+		{
+			A:      nil,
+			B:      nil,
+			Equal:  true,
+			Prefix: true,
+		},
+		{
+			A:      cty.Path{},
+			B:      cty.Path{},
+			Equal:  true,
+			Prefix: true,
+		},
+		{
+			A: cty.Path{nil},
+			B: cty.Path{cty.GetAttrStep{Name: "attr"}},
+		},
+		{
+			A: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.UnknownVal(cty.String)},
+				cty.GetAttrStep{Name: "attr"},
+			},
+			B: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.StringVal("key")},
+				cty.GetAttrStep{Name: "attr"},
+			},
+		},
+		{
+			A: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.ListVal([]cty.Value{cty.UnknownVal(cty.String)})},
+				cty.GetAttrStep{Name: "attr"},
+			},
+			B: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.ListVal([]cty.Value{cty.StringVal("known")})},
+				cty.GetAttrStep{Name: "attr"},
+			},
+		},
+		{
+			A: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.UnknownVal(cty.String)},
+			},
+			B: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.StringVal("known")},
+				cty.GetAttrStep{Name: "attr"},
+			},
+		},
+		{
+			A: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.StringVal("known")},
+			},
+			B: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.StringVal("known")},
+				cty.GetAttrStep{Name: "attr"},
+			},
+		},
+		{
+			A: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.StringVal("known")},
+				cty.GetAttrStep{Name: "attr"},
+			},
+			B: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.StringVal("known")},
+			},
+			Prefix: true,
+		},
+		{
+			A: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.UnknownVal(cty.String)},
+			},
+			B: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.UnknownVal(cty.String)},
+			},
+			Prefix: true,
+			Equal:  true,
+		},
+		{
+			A: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.NumberFloatVal(0)},
+				cty.GetAttrStep{Name: "attr"},
+			},
+			B: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.NumberIntVal(0)},
+				cty.GetAttrStep{Name: "attr"},
+			},
+			Equal: true,
+		},
+		{
+			A: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.NumberIntVal(1)},
+				cty.GetAttrStep{Name: "attr"},
+			},
+			B: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.NumberIntVal(0)},
+				cty.GetAttrStep{Name: "attr"},
+			},
+		},
+	}
+
+	for i, test := range tests {
+		t.Run(fmt.Sprintf("%d-%#v", i, test.A), func(t *testing.T) {
+			if test.Equal != test.A.Equals(test.B) {
+				t.Fatalf("%#v.Equals(%#v) != %t", test.A, test.B, test.Equal)
+			}
+			if test.Prefix && !test.A.HasPrefix(test.B) {
+				t.Fatalf("!%#v.HasPrefix(%#v)", test.A, test.B)
+			}
+		})
+	}
+}