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Add support for comparing graphs #85

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Dec 16, 2019
Merged

Add support for comparing graphs #85

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53 changes: 45 additions & 8 deletions cmp/compare.go
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Original file line number Diff line number Diff line change
Expand Up @@ -80,6 +80,11 @@ import (
// Pointers and interfaces are equal if they are both nil or both non-nil,
// where they have the same underlying concrete type and recursively
// calling Equal on the underlying values reports equal.
//
// Before recursing into a pointer, slice element, or map, the current path
// is checked to detect whether the address has already been visited.
// If there is a cycle, then the pointed at values are considered equal
// only if both addresses were previously visited in the same path step.
func Equal(x, y interface{}, opts ...Option) bool {
vx := reflect.ValueOf(x)
vy := reflect.ValueOf(y)
Expand Down Expand Up @@ -137,6 +142,7 @@ type state struct {
// Calling statelessCompare must not result in observable changes to these.
result diff.Result // The current result of comparison
curPath Path // The current path in the value tree
curPtrs pointerPath // The current set of visited pointers
reporters []reporter // Optional reporters

// recChecker checks for infinite cycles applying the same set of
Expand All @@ -155,6 +161,7 @@ type state struct {
func newState(opts []Option) *state {
// Always ensure a validator option exists to validate the inputs.
s := &state{opts: Options{validator{}}}
s.curPtrs.Init()
s.processOption(Options(opts))
return s
}
Expand Down Expand Up @@ -187,9 +194,9 @@ func (s *state) processOption(opt Option) {
// This function is stateless in that it does not alter the current result,
// or output to any registered reporters.
func (s *state) statelessCompare(step PathStep) diff.Result {
// We do not save and restore the curPath because all of the compareX
// methods should properly push and pop from the path.
// It is an implementation bug if the contents of curPath differs from
// We do not save and restore curPath and curPtrs because all of the
// compareX methods should properly push and pop from them.
// It is an implementation bug if the contents of the paths differ from
// when calling this function to when returning from it.

oldResult, oldReporters := s.result, s.reporters
Expand All @@ -211,9 +218,17 @@ func (s *state) compareAny(step PathStep) {
}
s.recChecker.Check(s.curPath)

// Obtain the current type and values.
// Cycle-detection for slice elements (see NOTE in compareSlice).
t := step.Type()
vx, vy := step.Values()
if si, ok := step.(SliceIndex); ok && si.isSlice && vx.IsValid() && vy.IsValid() {
px, py := vx.Addr(), vy.Addr()
if eq, visited := s.curPtrs.Push(px, py); visited {
s.report(eq, reportByCycle)
return
}
defer s.curPtrs.Pop(px, py)
}

// Rule 1: Check whether an option applies on this node in the value tree.
if s.tryOptions(t, vx, vy) {
Expand Down Expand Up @@ -393,9 +408,21 @@ func (s *state) compareSlice(t reflect.Type, vx, vy reflect.Value) {
return
}

// TODO: Support cyclic data structures.
// NOTE: It is incorrect to call curPtrs.Push on the slice header pointer
// since slices represents a list of pointers, rather than a single pointer.
// The pointer checking logic must be handled on a per-element basis
// in compareAny.
//
// A slice header (see reflect.SliceHeader) in Go is a tuple of a starting
// pointer P, a length N, and a capacity C. Supposing each slice element has
// a memory size of M, then the slice is equivalent to the list of pointers:
// [P+i*M for i in range(N)]
//
// For example, v[:0] and v[:1] are slices with the same starting pointer,
// but they are clearly different values. Using the slice pointer alone
// violates the assumption that equal pointers implies equal values.

step := SliceIndex{&sliceIndex{pathStep: pathStep{typ: t.Elem()}}}
step := SliceIndex{&sliceIndex{pathStep: pathStep{typ: t.Elem()}, isSlice: isSlice}}
withIndexes := func(ix, iy int) SliceIndex {
if ix >= 0 {
step.vx, step.xkey = vx.Index(ix), ix
Expand Down Expand Up @@ -472,7 +499,12 @@ func (s *state) compareMap(t reflect.Type, vx, vy reflect.Value) {
return
}

// TODO: Support cyclic data structures.
// Cycle-detection for maps.
if eq, visited := s.curPtrs.Push(vx, vy); visited {
s.report(eq, reportByCycle)
return
}
defer s.curPtrs.Pop(vx, vy)

// We combine and sort the two map keys so that we can perform the
// comparisons in a deterministic order.
Expand Down Expand Up @@ -509,7 +541,12 @@ func (s *state) comparePtr(t reflect.Type, vx, vy reflect.Value) {
return
}

// TODO: Support cyclic data structures.
// Cycle-detection for pointers.
if eq, visited := s.curPtrs.Push(vx, vy); visited {
s.report(eq, reportByCycle)
return
}
defer s.curPtrs.Pop(vx, vy)

vx, vy = vx.Elem(), vy.Elem()
s.compareAny(Indirect{&indirect{pathStep{t.Elem(), vx, vy}}})
Expand Down
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