// Copyright 2009 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 ast

import (
	"fmt"
	"iter"
)

// A Visitor's Visit method is invoked for each node encountered by [Walk].
// If the result visitor w is not nil, [Walk] visits each of the children
// of node with the visitor w, followed by a call of w.Visit(nil).
type Visitor interface {
	Visit(node Node) (w Visitor)
}

func walkList[N Node](v Visitor, list []N) {
	for _, node := range list {
		Walk(v, node)
	}
}

// TODO(gri): Investigate if providing a closure to Walk leads to
// simpler use (and may help eliminate Inspect in turn).

// Walk traverses an AST in depth-first order: It starts by calling
// v.Visit(node); node must not be nil. If the visitor w returned by
// v.Visit(node) is not nil, Walk is invoked recursively with visitor
// w for each of the non-nil children of node, followed by a call of
// w.Visit(nil).
func Walk(v Visitor, node Node) {
	if v = v.Visit(node); v == nil {
		return
	}

	// walk children
	// (the order of the cases matches the order
	// of the corresponding node types in ast.go)
	switch n := node.(type) {
	// Comments and fields
	case *Comment:
		// nothing to do

	case *CommentGroup:
		walkList(v, n.List)

	case *Field:
		if n.Doc != nil {
			Walk(v, n.Doc)
		}
		walkList(v, n.Names)
		if n.Type != nil {
			Walk(v, n.Type)
		}
		if n.Tag != nil {
			Walk(v, n.Tag)
		}
		if n.Comment != nil {
			Walk(v, n.Comment)
		}

	case *FieldList:
		walkList(v, n.List)

	// Expressions
	case *BadExpr, *Ident, *BasicLit:
		// nothing to do

	case *Ellipsis:
		if n.Elt != nil {
			Walk(v, n.Elt)
		}

	case *FuncLit:
		Walk(v, n.Type)
		Walk(v, n.Body)

	case *CompositeLit:
		if n.Type != nil {
			Walk(v, n.Type)
		}
		walkList(v, n.Elts)

	case *ParenExpr:
		Walk(v, n.X)

	case *SelectorExpr:
		Walk(v, n.X)
		Walk(v, n.Sel)

	case *IndexExpr:
		Walk(v, n.X)
		Walk(v, n.Index)

	case *IndexListExpr:
		Walk(v, n.X)
		walkList(v, n.Indices)

	case *SliceExpr:
		Walk(v, n.X)
		if n.Low != nil {
			Walk(v, n.Low)
		}
		if n.High != nil {
			Walk(v, n.High)
		}
		if n.Max != nil {
			Walk(v, n.Max)
		}

	case *TypeAssertExpr:
		Walk(v, n.X)
		if n.Type != nil {
			Walk(v, n.Type)
		}

	case *CallExpr:
		Walk(v, n.Fun)
		walkList(v, n.Args)

	case *StarExpr:
		Walk(v, n.X)

	case *UnaryExpr:
		Walk(v, n.X)

	case *BinaryExpr:
		Walk(v, n.X)
		Walk(v, n.Y)

	case *KeyValueExpr:
		Walk(v, n.Key)
		Walk(v, n.Value)

	// Types
	case *ArrayType:
		if n.Len != nil {
			Walk(v, n.Len)
		}
		Walk(v, n.Elt)

	case *StructType:
		Walk(v, n.Fields)

	case *FuncType:
		if n.TypeParams != nil {
			Walk(v, n.TypeParams)
		}
		if n.Params != nil {
			Walk(v, n.Params)
		}
		if n.Results != nil {
			Walk(v, n.Results)
		}

	case *InterfaceType:
		Walk(v, n.Methods)

	case *MapType:
		Walk(v, n.Key)
		Walk(v, n.Value)

	case *ChanType:
		Walk(v, n.Value)

	// Statements
	case *BadStmt:
		// nothing to do

	case *DeclStmt:
		Walk(v, n.Decl)

	case *EmptyStmt:
		// nothing to do

	case *LabeledStmt:
		Walk(v, n.Label)
		Walk(v, n.Stmt)

	case *ExprStmt:
		Walk(v, n.X)

	case *SendStmt:
		Walk(v, n.Chan)
		Walk(v, n.Value)

	case *IncDecStmt:
		Walk(v, n.X)

	case *AssignStmt:
		walkList(v, n.Lhs)
		walkList(v, n.Rhs)

	case *GoStmt:
		Walk(v, n.Call)

	case *DeferStmt:
		Walk(v, n.Call)

	case *ReturnStmt:
		walkList(v, n.Results)

	case *BranchStmt:
		if n.Label != nil {
			Walk(v, n.Label)
		}

	case *BlockStmt:
		walkList(v, n.List)

	case *IfStmt:
		if n.Init != nil {
			Walk(v, n.Init)
		}
		Walk(v, n.Cond)
		Walk(v, n.Body)
		if n.Else != nil {
			Walk(v, n.Else)
		}

	case *CaseClause:
		walkList(v, n.List)
		walkList(v, n.Body)

	case *SwitchStmt:
		if n.Init != nil {
			Walk(v, n.Init)
		}
		if n.Tag != nil {
			Walk(v, n.Tag)
		}
		Walk(v, n.Body)

	case *TypeSwitchStmt:
		if n.Init != nil {
			Walk(v, n.Init)
		}
		Walk(v, n.Assign)
		Walk(v, n.Body)

	case *CommClause:
		if n.Comm != nil {
			Walk(v, n.Comm)
		}
		walkList(v, n.Body)

	case *SelectStmt:
		Walk(v, n.Body)

	case *ForStmt:
		if n.Init != nil {
			Walk(v, n.Init)
		}
		if n.Cond != nil {
			Walk(v, n.Cond)
		}
		if n.Post != nil {
			Walk(v, n.Post)
		}
		Walk(v, n.Body)

	case *RangeStmt:
		if n.Key != nil {
			Walk(v, n.Key)
		}
		if n.Value != nil {
			Walk(v, n.Value)
		}
		Walk(v, n.X)
		Walk(v, n.Body)

	// Declarations
	case *ImportSpec:
		if n.Doc != nil {
			Walk(v, n.Doc)
		}
		if n.Name != nil {
			Walk(v, n.Name)
		}
		Walk(v, n.Path)
		if n.Comment != nil {
			Walk(v, n.Comment)
		}

	case *ValueSpec:
		if n.Doc != nil {
			Walk(v, n.Doc)
		}
		walkList(v, n.Names)
		if n.Type != nil {
			Walk(v, n.Type)
		}
		walkList(v, n.Values)
		if n.Comment != nil {
			Walk(v, n.Comment)
		}

	case *TypeSpec:
		if n.Doc != nil {
			Walk(v, n.Doc)
		}
		Walk(v, n.Name)
		if n.TypeParams != nil {
			Walk(v, n.TypeParams)
		}
		Walk(v, n.Type)
		if n.Comment != nil {
			Walk(v, n.Comment)
		}

	case *BadDecl:
		// nothing to do

	case *GenDecl:
		if n.Doc != nil {
			Walk(v, n.Doc)
		}
		walkList(v, n.Specs)

	case *FuncDecl:
		if n.Doc != nil {
			Walk(v, n.Doc)
		}
		if n.Recv != nil {
			Walk(v, n.Recv)
		}
		Walk(v, n.Name)
		Walk(v, n.Type)
		if n.Body != nil {
			Walk(v, n.Body)
		}

	// Files and packages
	case *File:
		if n.Doc != nil {
			Walk(v, n.Doc)
		}
		Walk(v, n.Name)
		walkList(v, n.Decls)
		// don't walk n.Comments - they have been
		// visited already through the individual
		// nodes

	case *Package:
		for _, f := range n.Files {
			Walk(v, f)
		}

	default:
		panic(fmt.Sprintf("ast.Walk: unexpected node type %T", n))
	}

	v.Visit(nil)
}

type inspector func(Node) bool

func (f inspector) Visit(node Node) Visitor {
	if f(node) {
		return f
	}
	return nil
}

// Inspect traverses an AST in depth-first order: It starts by calling
// f(node); node must not be nil. If f returns true, Inspect invokes f
// recursively for each of the non-nil children of node, followed by a
// call of f(nil).
//
// In many cases it may be more convenient to use [Preorder], which
// returns an iterator over the sqeuence of nodes, or [PreorderStack],
// which (like [Inspect]) provides control over descent into subtrees,
// but additionally reports the stack of enclosing nodes.
func Inspect(node Node, f func(Node) bool) {
	Walk(inspector(f), node)
}

// Preorder returns an iterator over all the nodes of the syntax tree
// beneath (and including) the specified root, in depth-first
// preorder.
//
// For greater control over the traversal of each subtree, use
// [Inspect] or [PreorderStack].
func Preorder(root Node) iter.Seq[Node] {
	return func(yield func(Node) bool) {
		ok := true
		Inspect(root, func(n Node) bool {
			if n != nil {
				// yield must not be called once ok is false.
				ok = ok && yield(n)
			}
			return ok
		})
	}
}

// PreorderStack traverses the tree rooted at root,
// calling f before visiting each node.
//
// Each call to f provides the current node and traversal stack,
// consisting of the original value of stack appended with all nodes
// from root to n, excluding n itself. (This design allows calls
// to PreorderStack to be nested without double counting.)
//
// If f returns false, the traversal skips over that subtree. Unlike
// [Inspect], no second call to f is made after visiting node n.
// (In practice, the second call is nearly always used only to pop the
// stack, and it is surprisingly tricky to do this correctly.)
func PreorderStack(root Node, stack []Node, f func(n Node, stack []Node) bool) {
	before := len(stack)
	Inspect(root, func(n Node) bool {
		if n != nil {
			if !f(n, stack) {
				// Do not push, as there will be no corresponding pop.
				return false
			}
			stack = append(stack, n) // push
		} else {
			stack = stack[:len(stack)-1] // pop
		}
		return true
	})
	if len(stack) != before {
		panic("push/pop mismatch")
	}
}
