// Copyright 2022 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 stubmethods

import (
	"bytes"
	"fmt"
	"go/ast"
	"go/format"
	"go/token"
	"go/types"
	"strconv"
	"strings"

	"golang.org/x/tools/go/analysis"
	"golang.org/x/tools/go/analysis/passes/inspect"
	"golang.org/x/tools/go/ast/astutil"
	"golang.org/x/tools/internal/analysisinternal"
	"golang.org/x/tools/internal/typesinternal"
)

const Doc = `stub methods analyzer

This analyzer generates method stubs for concrete types
in order to implement a target interface`

var Analyzer = &analysis.Analyzer{
	Name:             "stubmethods",
	Doc:              Doc,
	Requires:         []*analysis.Analyzer{inspect.Analyzer},
	Run:              run,
	RunDespiteErrors: true,
}

func run(pass *analysis.Pass) (interface{}, error) {
	for _, err := range analysisinternal.GetTypeErrors(pass) {
		ifaceErr := strings.Contains(err.Msg, "missing method") || strings.HasPrefix(err.Msg, "cannot convert")
		if !ifaceErr {
			continue
		}
		var file *ast.File
		for _, f := range pass.Files {
			if f.Pos() <= err.Pos && err.Pos < f.End() {
				file = f
				break
			}
		}
		if file == nil {
			continue
		}
		// Get the end position of the error.
		_, _, endPos, ok := typesinternal.ReadGo116ErrorData(err)
		if !ok {
			var buf bytes.Buffer
			if err := format.Node(&buf, pass.Fset, file); err != nil {
				continue
			}
			endPos = analysisinternal.TypeErrorEndPos(pass.Fset, buf.Bytes(), err.Pos)
		}
		path, _ := astutil.PathEnclosingInterval(file, err.Pos, endPos)
		si := GetStubInfo(pass.TypesInfo, path, err.Pos)
		if si == nil {
			continue
		}
		qf := RelativeToFiles(si.Concrete.Obj().Pkg(), file, nil, nil)
		pass.Report(analysis.Diagnostic{
			Pos:     err.Pos,
			End:     endPos,
			Message: fmt.Sprintf("Implement %s", types.TypeString(si.Interface.Type(), qf)),
		})
	}
	return nil, nil
}

// StubInfo represents a concrete type
// that wants to stub out an interface type
type StubInfo struct {
	// Interface is the interface that the client wants to implement.
	// When the interface is defined, the underlying object will be a TypeName.
	// Note that we keep track of types.Object instead of types.Type in order
	// to keep a reference to the declaring object's package and the ast file
	// in the case where the concrete type file requires a new import that happens to be renamed
	// in the interface file.
	// TODO(marwan-at-work): implement interface literals.
	Interface types.Object
	Concrete  *types.Named
	Pointer   bool
}

// GetStubInfo determines whether the "missing method error"
// can be used to deduced what the concrete and interface types are.
func GetStubInfo(ti *types.Info, path []ast.Node, pos token.Pos) *StubInfo {
	for _, n := range path {
		switch n := n.(type) {
		case *ast.ValueSpec:
			return fromValueSpec(ti, n, pos)
		case *ast.ReturnStmt:
			// An error here may not indicate a real error the user should know about, but it may.
			// Therefore, it would be best to log it out for debugging/reporting purposes instead of ignoring
			// it. However, event.Log takes a context which is not passed via the analysis package.
			// TODO(marwan-at-work): properly log this error.
			si, _ := fromReturnStmt(ti, pos, path, n)
			return si
		case *ast.AssignStmt:
			return fromAssignStmt(ti, n, pos)
		}
	}
	return nil
}

// fromReturnStmt analyzes a "return" statement to extract
// a concrete type that is trying to be returned as an interface type.
//
// For example, func() io.Writer { return myType{} }
// would return StubInfo with the interface being io.Writer and the concrete type being myType{}.
func fromReturnStmt(ti *types.Info, pos token.Pos, path []ast.Node, rs *ast.ReturnStmt) (*StubInfo, error) {
	returnIdx := -1
	for i, r := range rs.Results {
		if pos >= r.Pos() && pos <= r.End() {
			returnIdx = i
		}
	}
	if returnIdx == -1 {
		return nil, fmt.Errorf("pos %d not within return statement bounds: [%d-%d]", pos, rs.Pos(), rs.End())
	}
	concObj, pointer := concreteType(rs.Results[returnIdx], ti)
	if concObj == nil || concObj.Obj().Pkg() == nil {
		return nil, nil
	}
	ef := enclosingFunction(path, ti)
	if ef == nil {
		return nil, fmt.Errorf("could not find the enclosing function of the return statement")
	}
	iface := ifaceType(ef.Results.List[returnIdx].Type, ti)
	if iface == nil {
		return nil, nil
	}
	return &StubInfo{
		Concrete:  concObj,
		Pointer:   pointer,
		Interface: iface,
	}, nil
}

// fromValueSpec returns *StubInfo from a variable declaration such as
// var x io.Writer = &T{}
func fromValueSpec(ti *types.Info, vs *ast.ValueSpec, pos token.Pos) *StubInfo {
	var idx int
	for i, vs := range vs.Values {
		if pos >= vs.Pos() && pos <= vs.End() {
			idx = i
			break
		}
	}

	valueNode := vs.Values[idx]
	ifaceNode := vs.Type
	callExp, ok := valueNode.(*ast.CallExpr)
	// if the ValueSpec is `var _ = myInterface(...)`
	// as opposed to `var _ myInterface = ...`
	if ifaceNode == nil && ok && len(callExp.Args) == 1 {
		ifaceNode = callExp.Fun
		valueNode = callExp.Args[0]
	}
	concObj, pointer := concreteType(valueNode, ti)
	if concObj == nil || concObj.Obj().Pkg() == nil {
		return nil
	}
	ifaceObj := ifaceType(ifaceNode, ti)
	if ifaceObj == nil {
		return nil
	}
	return &StubInfo{
		Concrete:  concObj,
		Interface: ifaceObj,
		Pointer:   pointer,
	}
}

// fromAssignStmt returns *StubInfo from a variable re-assignment such as
// var x io.Writer
// x = &T{}
func fromAssignStmt(ti *types.Info, as *ast.AssignStmt, pos token.Pos) *StubInfo {
	idx := -1
	var lhs, rhs ast.Expr
	// Given a re-assignment interface conversion error,
	// the compiler error shows up on the right hand side of the expression.
	// For example, x = &T{} where x is io.Writer highlights the error
	// under "&T{}" and not "x".
	for i, hs := range as.Rhs {
		if pos >= hs.Pos() && pos <= hs.End() {
			idx = i
			break
		}
	}
	if idx == -1 {
		return nil
	}
	// Technically, this should never happen as
	// we would get a "cannot assign N values to M variables"
	// before we get an interface conversion error. Nonetheless,
	// guard against out of range index errors.
	if idx >= len(as.Lhs) {
		return nil
	}
	lhs, rhs = as.Lhs[idx], as.Rhs[idx]
	ifaceObj := ifaceType(lhs, ti)
	if ifaceObj == nil {
		return nil
	}
	concType, pointer := concreteType(rhs, ti)
	if concType == nil || concType.Obj().Pkg() == nil {
		return nil
	}
	return &StubInfo{
		Concrete:  concType,
		Interface: ifaceObj,
		Pointer:   pointer,
	}
}

// RelativeToFiles returns a types.Qualifier that formats package names
// according to the files where the concrete and interface types are defined.
//
// This is similar to types.RelativeTo except if a file imports the package with a different name,
// then it will use it. And if the file does import the package but it is ignored,
// then it will return the original name. It also prefers package names in ifaceFile in case
// an import is missing from concFile but is present in ifaceFile.
//
// Additionally, if missingImport is not nil, the function will be called whenever the concFile
// is presented with a package that is not imported. This is useful so that as types.TypeString is
// formatting a function signature, it is identifying packages that will need to be imported when
// stubbing an interface.
func RelativeToFiles(concPkg *types.Package, concFile, ifaceFile *ast.File, missingImport func(name, path string)) types.Qualifier {
	return func(other *types.Package) string {
		if other == concPkg {
			return ""
		}

		// Check if the concrete file already has the given import,
		// if so return the default package name or the renamed import statement.
		for _, imp := range concFile.Imports {
			impPath, _ := strconv.Unquote(imp.Path.Value)
			isIgnored := imp.Name != nil && (imp.Name.Name == "." || imp.Name.Name == "_")
			if impPath == other.Path() && !isIgnored {
				importName := other.Name()
				if imp.Name != nil {
					importName = imp.Name.Name
				}
				return importName
			}
		}

		// If the concrete file does not have the import, check if the package
		// is renamed in the interface file and prefer that.
		var importName string
		if ifaceFile != nil {
			for _, imp := range ifaceFile.Imports {
				impPath, _ := strconv.Unquote(imp.Path.Value)
				isIgnored := imp.Name != nil && (imp.Name.Name == "." || imp.Name.Name == "_")
				if impPath == other.Path() && !isIgnored {
					if imp.Name != nil && imp.Name.Name != concPkg.Name() {
						importName = imp.Name.Name
					}
					break
				}
			}
		}

		if missingImport != nil {
			missingImport(importName, other.Path())
		}

		// Up until this point, importName must stay empty when calling missingImport,
		// otherwise we'd end up with `import time "time"` which doesn't look idiomatic.
		if importName == "" {
			importName = other.Name()
		}
		return importName
	}
}

// ifaceType will try to extract the types.Object that defines
// the interface given the ast.Expr where the "missing method"
// or "conversion" errors happen.
func ifaceType(n ast.Expr, ti *types.Info) types.Object {
	tv, ok := ti.Types[n]
	if !ok {
		return nil
	}
	typ := tv.Type
	named, ok := typ.(*types.Named)
	if !ok {
		return nil
	}
	_, ok = named.Underlying().(*types.Interface)
	if !ok {
		return nil
	}
	// Interfaces defined in the "builtin" package return nil a Pkg().
	// But they are still real interfaces that we need to make a special case for.
	// Therefore, protect gopls from panicking if a new interface type was added in the future.
	if named.Obj().Pkg() == nil && named.Obj().Name() != "error" {
		return nil
	}
	return named.Obj()
}

// concreteType tries to extract the *types.Named that defines
// the concrete type given the ast.Expr where the "missing method"
// or "conversion" errors happened. If the concrete type is something
// that cannot have methods defined on it (such as basic types), this
// method will return a nil *types.Named. The second return parameter
// is a boolean that indicates whether the concreteType was defined as a
// pointer or value.
func concreteType(n ast.Expr, ti *types.Info) (*types.Named, bool) {
	tv, ok := ti.Types[n]
	if !ok {
		return nil, false
	}
	typ := tv.Type
	ptr, isPtr := typ.(*types.Pointer)
	if isPtr {
		typ = ptr.Elem()
	}
	named, ok := typ.(*types.Named)
	if !ok {
		return nil, false
	}
	return named, isPtr
}

// enclosingFunction returns the signature and type of the function
// enclosing the given position.
func enclosingFunction(path []ast.Node, info *types.Info) *ast.FuncType {
	for _, node := range path {
		switch t := node.(type) {
		case *ast.FuncDecl:
			if _, ok := info.Defs[t.Name]; ok {
				return t.Type
			}
		case *ast.FuncLit:
			if _, ok := info.Types[t]; ok {
				return t.Type
			}
		}
	}
	return nil
}