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.md file.

package cmp

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

type (
	// 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.
	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.
	PathStep interface {
		String() string
		Type() reflect.Type // Resulting type after performing the path step
		isPathStep()
	}

	// StructField represents a struct field access on a field called Name.
	StructField interface {
		PathStep
		Name() string
		Index() int
		isStructField()
	}
	// SliceIndex is an index operation on a slice or array at some index Key.
	SliceIndex interface {
		PathStep
		Key() int // May return -1 if in a split state

		// SplitKeys returns 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.
		SplitKeys() (x int, y int)

		isSliceIndex()
	}
	// MapIndex is an index operation on a map at some index Key.
	MapIndex interface {
		PathStep
		Key() reflect.Value
		isMapIndex()
	}
	// Indirect represents pointer indirection on the parent type.
	Indirect interface {
		PathStep
		isIndirect()
	}
	// TypeAssertion represents a type assertion on an interface.
	TypeAssertion interface {
		PathStep
		isTypeAssertion()
	}
	// Transform is a transformation from the parent type to the current type.
	Transform interface {
		PathStep
		Name() string
		Func() reflect.Value

		// Option returns the originally constructed Transformer option.
		// The == operator can be used to detect the exact option used.
		Option() Option

		isTransform()
	}
)

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
		case *typeAssertion:
			// As a special-case, elide type assertions on anonymous types
			// since they are typically generated dynamically and can be very
			// verbose. For example, some transforms return interface{} because
			// of Go's lack of generics, but typically take in and return the
			// exact same concrete type.
			if s.Type().PkgPath() == "" {
				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
	}

	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
		force      bool                // Forcibly allow visibility
		pvx, pvy   reflect.Value       // Parent values
		field      reflect.StructField // Field information
	}
	sliceIndex struct {
		pathStep
		xkey, ykey int
	}
	mapIndex struct {
		pathStep
		key reflect.Value
	}
	indirect struct {
		pathStep
	}
	typeAssertion struct {
		pathStep
	}
	transform struct {
		pathStep
		trans *transformer
	}
)

func (ps pathStep) Type() reflect.Type { return ps.typ }
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)
}
func (ps pathStep) isPathStep() {}

func (sf structField) String() string { return fmt.Sprintf(".%s", sf.name) }
func (sf structField) Name() string   { return sf.name }
func (sf structField) Index() int     { return sf.idx }
func (sf structField) isStructField() {}

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)
	}
}
func (si sliceIndex) Key() int {
	if si.xkey != si.ykey {
		return -1
	}
	return si.xkey
}
func (si sliceIndex) SplitKeys() (x, y int) { return si.xkey, si.ykey }
func (si sliceIndex) isSliceIndex()         {}

func (mi mapIndex) String() string     { return fmt.Sprintf("[%#v]", mi.key) }
func (mi mapIndex) Key() reflect.Value { return mi.key }
func (mi mapIndex) isMapIndex()        {}

func (in indirect) String() string { return "*" }
func (in indirect) isIndirect()    {}

func (ta typeAssertion) String() string   { return fmt.Sprintf(".(%v)", ta.typ) }
func (ta typeAssertion) isTypeAssertion() {}

func (tf transform) String() string      { return fmt.Sprintf("%s()", tf.trans.name) }
func (tf transform) Name() string        { return tf.trans.name }
func (tf transform) Func() reflect.Value { return tf.trans.fnc }
func (tf transform) Option() Option      { return tf.trans }
func (tf transform) isTransform()        {}

var (
	_ PathStep = StructField(structField{})
	_ PathStep = SliceIndex(sliceIndex{})
	_ PathStep = MapIndex(mapIndex{})
	_ PathStep = Indirect(indirect{})
	_ PathStep = TypeAssertion(typeAssertion{})
	_ PathStep = Transform(transform{})
)

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