cmp/path.go

// Copyright 2017, The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package cmp

import (
	"fmt"
	"reflect"
	"strings"
	"unicode"
	"unicode/utf8"

	"github.com/google/go-cmp/cmp/internal/value"
)

// Path is a list of PathSteps describing the sequence of operations to get
// from some root type to the current position in the value tree.
// The first Path element is always an operation-less PathStep that exists
// simply to identify the initial type.
//
// When traversing structs with embedded structs, the embedded struct will
// always be accessed as a field before traversing the fields of the
// embedded struct themselves. That is, an exported field from the
// embedded struct will never be accessed directly from the parent struct.
type Path []PathStep

// PathStep is a union-type for specific operations to traverse
// a value's tree structure. Users of this package never need to implement
// these types as values of this type will be returned by this package.
//
// Implementations of this interface are
// StructField, SliceIndex, MapIndex, Indirect, TypeAssertion, and Transform.
type PathStep interface {
	String() string

	// Type is the resulting type after performing the path step.
	Type() reflect.Type

	// Values is the resulting values after performing the path step.
	// The type of each valid value is guaranteed to be identical to Type.
	//
	// In some cases, one or both may be invalid or have restrictions:
	//	• For StructField, both are not interface-able if the current field
	//	is unexported and the struct type is not explicitly permitted by
	//	an Exporter to traverse unexported fields.
	//	• For SliceIndex, one may be invalid if an element is missing from
	//	either the x or y slice.
	//	• For MapIndex, one may be invalid if an entry is missing from
	//	either the x or y map.
	//
	// The provided values must not be mutated.
	Values() (vx, vy reflect.Value)
}

var (
	_ PathStep = StructField{}
	_ PathStep = SliceIndex{}
	_ PathStep = MapIndex{}
	_ PathStep = Indirect{}
	_ PathStep = TypeAssertion{}
	_ PathStep = Transform{}
)

func (pa *Path) push(s PathStep) {
	*pa = append(*pa, s)
}

func (pa *Path) pop() {
	*pa = (*pa)[:len(*pa)-1]
}

// Last returns the last PathStep in the Path.
// If the path is empty, this returns a non-nil PathStep that reports a nil Type.
func (pa Path) Last() PathStep {
	return pa.Index(-1)
}

// Index returns the ith step in the Path and supports negative indexing.
// A negative index starts counting from the tail of the Path such that -1
// refers to the last step, -2 refers to the second-to-last step, and so on.
// If index is invalid, this returns a non-nil PathStep that reports a nil Type.
func (pa Path) Index(i int) PathStep {
	if i < 0 {
		i = len(pa) + i
	}
	if i < 0 || i >= len(pa) {
		return pathStep{}
	}
	return pa[i]
}

// String returns the simplified path to a node.
// The simplified path only contains struct field accesses.
//
// For example:
//	MyMap.MySlices.MyField
func (pa Path) String() string {
	var ss []string
	for _, s := range pa {
		if _, ok := s.(StructField); ok {
			ss = append(ss, s.String())
		}
	}
	return strings.TrimPrefix(strings.Join(ss, ""), ".")
}

// GoString returns the path to a specific node using Go syntax.
//
// For example:
//	(*root.MyMap["key"].(*mypkg.MyStruct).MySlices)[2][3].MyField
func (pa Path) GoString() string {
	var ssPre, ssPost []string
	var numIndirect int
	for i, s := range pa {
		var nextStep PathStep
		if i+1 < len(pa) {
			nextStep = pa[i+1]
		}
		switch s := s.(type) {
		case Indirect:
			numIndirect++
			pPre, pPost := "(", ")"
			switch nextStep.(type) {
			case Indirect:
				continue // Next step is indirection, so let them batch up
			case StructField:
				numIndirect-- // Automatic indirection on struct fields
			case nil:
				pPre, pPost = "", "" // Last step; no need for parenthesis
			}
			if numIndirect > 0 {
				ssPre = append(ssPre, pPre+strings.Repeat("*", numIndirect))
				ssPost = append(ssPost, pPost)
			}
			numIndirect = 0
			continue
		case Transform:
			ssPre = append(ssPre, s.trans.name+"(")
			ssPost = append(ssPost, ")")
			continue
		}
		ssPost = append(ssPost, s.String())
	}
	for i, j := 0, len(ssPre)-1; i < j; i, j = i+1, j-1 {
		ssPre[i], ssPre[j] = ssPre[j], ssPre[i]
	}
	return strings.Join(ssPre, "") + strings.Join(ssPost, "")
}

type pathStep struct {
	typ    reflect.Type
	vx, vy reflect.Value
}

func (ps pathStep) Type() reflect.Type             { return ps.typ }
func (ps pathStep) Values() (vx, vy reflect.Value) { return ps.vx, ps.vy }
func (ps pathStep) String() string {
	if ps.typ == nil {
		return "<nil>"
	}
	s := ps.typ.String()
	if s == "" || strings.ContainsAny(s, "{}\n") {
		return "root" // Type too simple or complex to print
	}
	return fmt.Sprintf("{%s}", s)
}

// StructField represents a struct field access on a field called Name.
type StructField struct{ *structField }
type structField struct {
	pathStep
	name string
	idx  int

	// These fields are used for forcibly accessing an unexported field.
	// pvx, pvy, and field are only valid if unexported is true.
	unexported bool
	mayForce   bool                // Forcibly allow visibility
	paddr      bool                // Was parent addressable?
	pvx, pvy   reflect.Value       // Parent values (always addressable)
	field      reflect.StructField // Field information
}

func (sf StructField) Type() reflect.Type { return sf.typ }
func (sf StructField) Values() (vx, vy reflect.Value) {
	if !sf.unexported {
		return sf.vx, sf.vy // CanInterface reports true
	}

	// Forcibly obtain read-write access to an unexported struct field.
	if sf.mayForce {
		vx = retrieveUnexportedField(sf.pvx, sf.field, sf.paddr)
		vy = retrieveUnexportedField(sf.pvy, sf.field, sf.paddr)
		return vx, vy // CanInterface reports true
	}
	return sf.vx, sf.vy // CanInterface reports false
}
func (sf StructField) String() string { return fmt.Sprintf(".%s", sf.name) }

// Name is the field name.
func (sf StructField) Name() string { return sf.name }

// Index is the index of the field in the parent struct type.
// See reflect.Type.Field.
func (sf StructField) Index() int { return sf.idx }

// SliceIndex is an index operation on a slice or array at some index Key.
type SliceIndex struct{ *sliceIndex }
type sliceIndex struct {
	pathStep
	xkey, ykey int
	isSlice    bool // False for reflect.Array
}

func (si SliceIndex) Type() reflect.Type             { return si.typ }
func (si SliceIndex) Values() (vx, vy reflect.Value) { return si.vx, si.vy }
func (si SliceIndex) String() string {
	switch {
	case si.xkey == si.ykey:
		return fmt.Sprintf("[%d]", si.xkey)
	case si.ykey == -1:
		// [5->?] means "I don't know where X[5] went"
		return fmt.Sprintf("[%d->?]", si.xkey)
	case si.xkey == -1:
		// [?->3] means "I don't know where Y[3] came from"
		return fmt.Sprintf("[?->%d]", si.ykey)
	default:
		// [5->3] means "X[5] moved to Y[3]"
		return fmt.Sprintf("[%d->%d]", si.xkey, si.ykey)
	}
}

// Key is the index key; it may return -1 if in a split state
func (si SliceIndex) Key() int {
	if si.xkey != si.ykey {
		return -1
	}
	return si.xkey
}

// SplitKeys are the indexes for indexing into slices in the
// x and y values, respectively. These indexes may differ due to the
// insertion or removal of an element in one of the slices, causing
// all of the indexes to be shifted. If an index is -1, then that
// indicates that the element does not exist in the associated slice.
//
// Key is guaranteed to return -1 if and only if the indexes returned
// by SplitKeys are not the same. SplitKeys will never return -1 for
// both indexes.
func (si SliceIndex) SplitKeys() (ix, iy int) { return si.xkey, si.ykey }

// MapIndex is an index operation on a map at some index Key.
type MapIndex struct{ *mapIndex }
type mapIndex struct {
	pathStep
	key reflect.Value
}

func (mi MapIndex) Type() reflect.Type             { return mi.typ }
func (mi MapIndex) Values() (vx, vy reflect.Value) { return mi.vx, mi.vy }
func (mi MapIndex) String() string                 { return fmt.Sprintf("[%#v]", mi.key) }

// Key is the value of the map key.
func (mi MapIndex) Key() reflect.Value { return mi.key }

// Indirect represents pointer indirection on the parent type.
type Indirect struct{ *indirect }
type indirect struct {
	pathStep
}

func (in Indirect) Type() reflect.Type             { return in.typ }
func (in Indirect) Values() (vx, vy reflect.Value) { return in.vx, in.vy }
func (in Indirect) String() string                 { return "*" }

// TypeAssertion represents a type assertion on an interface.
type TypeAssertion struct{ *typeAssertion }
type typeAssertion struct {
	pathStep
}

func (ta TypeAssertion) Type() reflect.Type             { return ta.typ }
func (ta TypeAssertion) Values() (vx, vy reflect.Value) { return ta.vx, ta.vy }
func (ta TypeAssertion) String() string                 { return fmt.Sprintf(".(%v)", ta.typ) }

// Transform is a transformation from the parent type to the current type.
type Transform struct{ *transform }
type transform struct {
	pathStep
	trans *transformer
}

func (tf Transform) Type() reflect.Type             { return tf.typ }
func (tf Transform) Values() (vx, vy reflect.Value) { return tf.vx, tf.vy }
func (tf Transform) String() string                 { return fmt.Sprintf("%s()", tf.trans.name) }

// Name is the name of the Transformer.
func (tf Transform) Name() string { return tf.trans.name }

// Func is the function pointer to the transformer function.
func (tf Transform) Func() reflect.Value { return tf.trans.fnc }

// Option returns the originally constructed Transformer option.
// The == operator can be used to detect the exact option used.
func (tf Transform) Option() Option { return tf.trans }

// pointerPath represents a dual-stack of pointers encountered when
// recursively traversing the x and y values. This data structure supports
// detection of cycles and determining whether the cycles are equal.
// In Go, cycles can occur via pointers, slices, and maps.
//
// The pointerPath uses a map to represent a stack; where descension into a
// pointer pushes the address onto the stack, and ascension from a pointer
// pops the address from the stack. Thus, when traversing into a pointer from
// reflect.Ptr, reflect.Slice element, or reflect.Map, we can detect cycles
// by checking whether the pointer has already been visited. The cycle detection
// uses a separate stack for the x and y values.
//
// If a cycle is detected we need to determine whether the two pointers
// should be considered equal. The definition of equality chosen by Equal
// requires two graphs to have the same structure. To determine this, both the
// x and y values must have a cycle where the previous pointers were also
// encountered together as a pair.
//
// Semantically, this is equivalent to augmenting Indirect, SliceIndex, and
// MapIndex with pointer information for the x and y values.
// Suppose px and py are two pointers to compare, we then search the
// Path for whether px was ever encountered in the Path history of x, and
// similarly so with py. If either side has a cycle, the comparison is only
// equal if both px and py have a cycle resulting from the same PathStep.
//
// Using a map as a stack is more performant as we can perform cycle detection
// in O(1) instead of O(N) where N is len(Path).
type pointerPath struct {
	// mx is keyed by x pointers, where the value is the associated y pointer.
	mx map[value.Pointer]value.Pointer
	// my is keyed by y pointers, where the value is the associated x pointer.
	my map[value.Pointer]value.Pointer
}

func (p *pointerPath) Init() {
	p.mx = make(map[value.Pointer]value.Pointer)
	p.my = make(map[value.Pointer]value.Pointer)
}

// Push indicates intent to descend into pointers vx and vy where
// visited reports whether either has been seen before. If visited before,
// equal reports whether both pointers were encountered together.
// Pop must be called if and only if the pointers were never visited.
//
// The pointers vx and vy must be a reflect.Ptr, reflect.Slice, or reflect.Map
// and be non-nil.
func (p pointerPath) Push(vx, vy reflect.Value) (equal, visited bool) {
	px := value.PointerOf(vx)
	py := value.PointerOf(vy)
	_, ok1 := p.mx[px]
	_, ok2 := p.my[py]
	if ok1 || ok2 {
		equal = p.mx[px] == py && p.my[py] == px // Pointers paired together
		return equal, true
	}
	p.mx[px] = py
	p.my[py] = px
	return false, false
}

// Pop ascends from pointers vx and vy.
func (p pointerPath) Pop(vx, vy reflect.Value) {
	delete(p.mx, value.PointerOf(vx))
	delete(p.my, value.PointerOf(vy))
}

// isExported reports whether the identifier is exported.
func isExported(id string) bool {
	r, _ := utf8.DecodeRuneInString(id)
	return unicode.IsUpper(r)
}