// 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"
	_ "embed"
	"fmt"
	"go/ast"
	"go/format"
	"go/token"
	"go/types"
	"strings"

	"golang.org/x/tools/go/analysis"
	"golang.org/x/tools/go/ast/astutil"
	"golang.org/x/tools/gopls/internal/util/typesutil"
	"golang.org/x/tools/internal/aliases"
	"golang.org/x/tools/internal/analysisinternal"
	"golang.org/x/tools/internal/typesinternal"
)

//go:embed doc.go
var doc string

var Analyzer = &analysis.Analyzer{
	Name:             "stubmethods",
	Doc:              analysisinternal.MustExtractDoc(doc, "stubmethods"),
	Run:              run,
	RunDespiteErrors: true,
	URL:              "https://pkg.go.dev/golang.org/x/tools/gopls/internal/analysis/stubmethods",
}

// TODO(rfindley): remove this thin wrapper around the stubmethods refactoring,
// and eliminate the stubmethods analyzer.
//
// Previous iterations used the analysis framework for computing refactorings,
// which proved inefficient.
func run(pass *analysis.Pass) (interface{}, error) {
	for _, err := range pass.TypeErrors {
		var file *ast.File
		for _, f := range pass.Files {
			if f.Pos() <= err.Pos && err.Pos < f.End() {
				file = f
				break
			}
		}
		// Get the end position of the error.
		_, _, end, ok := typesinternal.ReadGo116ErrorData(err)
		if !ok {
			var buf bytes.Buffer
			if err := format.Node(&buf, pass.Fset, file); err != nil {
				continue
			}
			end = analysisinternal.TypeErrorEndPos(pass.Fset, buf.Bytes(), err.Pos)
		}
		if diag, ok := DiagnosticForError(pass.Fset, file, err.Pos, end, err.Msg, pass.TypesInfo); ok {
			pass.Report(diag)
		}
	}

	return nil, nil
}

// MatchesMessage reports whether msg matches the error message sought after by
// the stubmethods fix.
func MatchesMessage(msg string) bool {
	return strings.Contains(msg, "missing method") || strings.HasPrefix(msg, "cannot convert") || strings.Contains(msg, "not implement")
}

// DiagnosticForError computes a diagnostic suggesting to implement an
// interface to fix the type checking error defined by (start, end, msg).
//
// If no such fix is possible, the second result is false.
func DiagnosticForError(fset *token.FileSet, file *ast.File, start, end token.Pos, msg string, info *types.Info) (analysis.Diagnostic, bool) {
	if !MatchesMessage(msg) {
		return analysis.Diagnostic{}, false
	}

	path, _ := astutil.PathEnclosingInterval(file, start, end)
	si := GetStubInfo(fset, info, path, start)
	if si == nil {
		return analysis.Diagnostic{}, false
	}
	qf := typesutil.FileQualifier(file, si.Concrete.Obj().Pkg(), info)
	iface := types.TypeString(si.Interface.Type(), qf)
	return analysis.Diagnostic{
		Pos:      start,
		End:      end,
		Message:  msg,
		Category: FixCategory,
		SuggestedFixes: []analysis.SuggestedFix{{
			Message: fmt.Sprintf("Declare missing methods of %s", iface),
			// No TextEdits => computed later by gopls.
		}},
	}, true
}

const FixCategory = "stubmethods" // recognized by gopls ApplyFix

// 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.
	Fset      *token.FileSet // the FileSet used to type-check the types below
	Interface *types.TypeName
	Concrete  *types.Named
	Pointer   bool
}

// GetStubInfo determines whether the "missing method error"
// can be used to deduced what the concrete and interface types are.
//
// TODO(adonovan): this function (and its following 5 helpers) tries
// to deduce a pair of (concrete, interface) types that are related by
// an assignment, either explicitly or through a return statement or
// function call. This is essentially what the refactor/satisfy does,
// more generally. Refactor to share logic, after auditing 'satisfy'
// for safety on ill-typed code.
func GetStubInfo(fset *token.FileSet, info *types.Info, path []ast.Node, pos token.Pos) *StubInfo {
	for _, n := range path {
		switch n := n.(type) {
		case *ast.ValueSpec:
			return fromValueSpec(fset, info, 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(fset, info, pos, path, n)
			return si
		case *ast.AssignStmt:
			return fromAssignStmt(fset, info, n, pos)
		case *ast.CallExpr:
			// Note that some call expressions don't carry the interface type
			// because they don't point to a function or method declaration elsewhere.
			// For eaxmple, "var Interface = (*Concrete)(nil)". In that case, continue
			// this loop to encounter other possibilities such as *ast.ValueSpec or others.
			si := fromCallExpr(fset, info, pos, n)
			if si != nil {
				return si
			}
		}
	}
	return nil
}

// fromCallExpr tries to find an *ast.CallExpr's function declaration and
// analyzes a function call's signature against the passed in parameter to deduce
// the concrete and interface types.
func fromCallExpr(fset *token.FileSet, info *types.Info, pos token.Pos, call *ast.CallExpr) *StubInfo {
	// Find argument containing pos.
	argIdx := -1
	var arg ast.Expr
	for i, callArg := range call.Args {
		if callArg.Pos() <= pos && pos <= callArg.End() {
			argIdx = i
			arg = callArg
			break
		}
	}
	if arg == nil {
		return nil
	}

	concType, pointer := concreteType(arg, info)
	if concType == nil || concType.Obj().Pkg() == nil {
		return nil
	}
	tv, ok := info.Types[call.Fun]
	if !ok {
		return nil
	}
	sig, ok := aliases.Unalias(tv.Type).(*types.Signature)
	if !ok {
		return nil
	}
	var paramType types.Type
	if sig.Variadic() && argIdx >= sig.Params().Len()-1 {
		v := sig.Params().At(sig.Params().Len() - 1)
		if s, _ := v.Type().(*types.Slice); s != nil {
			paramType = s.Elem()
		}
	} else if argIdx < sig.Params().Len() {
		paramType = sig.Params().At(argIdx).Type()
	}
	if paramType == nil {
		return nil // A type error prevents us from determining the param type.
	}
	iface := ifaceObjFromType(paramType)
	if iface == nil {
		return nil
	}
	return &StubInfo{
		Fset:      fset,
		Concrete:  concType,
		Pointer:   pointer,
		Interface: iface,
	}
}

// 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(fset *token.FileSet, info *types.Info, pos token.Pos, path []ast.Node, ret *ast.ReturnStmt) (*StubInfo, error) {
	// Find return operand containing pos.
	returnIdx := -1
	for i, r := range ret.Results {
		if r.Pos() <= pos && pos <= r.End() {
			returnIdx = i
			break
		}
	}
	if returnIdx == -1 {
		return nil, fmt.Errorf("pos %d not within return statement bounds: [%d-%d]", pos, ret.Pos(), ret.End())
	}

	concType, pointer := concreteType(ret.Results[returnIdx], info)
	if concType == nil || concType.Obj().Pkg() == nil {
		return nil, nil
	}
	funcType := enclosingFunction(path, info)
	if funcType == nil {
		return nil, fmt.Errorf("could not find the enclosing function of the return statement")
	}
	if len(funcType.Results.List) != len(ret.Results) {
		return nil, fmt.Errorf("%d-operand return statement in %d-result function",
			len(ret.Results),
			len(funcType.Results.List))
	}
	iface := ifaceType(funcType.Results.List[returnIdx].Type, info)
	if iface == nil {
		return nil, nil
	}
	return &StubInfo{
		Fset:      fset,
		Concrete:  concType,
		Pointer:   pointer,
		Interface: iface,
	}, nil
}

// fromValueSpec returns *StubInfo from a variable declaration such as
// var x io.Writer = &T{}
func fromValueSpec(fset *token.FileSet, info *types.Info, spec *ast.ValueSpec, pos token.Pos) *StubInfo {
	// Find RHS element containing pos.
	var rhs ast.Expr
	for _, r := range spec.Values {
		if r.Pos() <= pos && pos <= r.End() {
			rhs = r
			break
		}
	}
	if rhs == nil {
		return nil // e.g. pos was on the LHS (#64545)
	}

	// Possible implicit/explicit conversion to interface type?
	ifaceNode := spec.Type // var _ myInterface = ...
	if call, ok := rhs.(*ast.CallExpr); ok && ifaceNode == nil && len(call.Args) == 1 {
		// var _ = myInterface(v)
		ifaceNode = call.Fun
		rhs = call.Args[0]
	}
	concType, pointer := concreteType(rhs, info)
	if concType == nil || concType.Obj().Pkg() == nil {
		return nil
	}
	ifaceObj := ifaceType(ifaceNode, info)
	if ifaceObj == nil {
		return nil
	}
	return &StubInfo{
		Fset:      fset,
		Concrete:  concType,
		Interface: ifaceObj,
		Pointer:   pointer,
	}
}

// fromAssignStmt returns *StubInfo from a variable assignment such as
// var x io.Writer
// x = &T{}
func fromAssignStmt(fset *token.FileSet, info *types.Info, assign *ast.AssignStmt, pos token.Pos) *StubInfo {
	// The interface conversion error in an assignment is against the RHS:
	//
	//      var x io.Writer
	//      x = &T{} // error: missing method
	//          ^^^^
	//
	// Find RHS element containing pos.
	var lhs, rhs ast.Expr
	for i, r := range assign.Rhs {
		if r.Pos() <= pos && pos <= r.End() {
			if i >= len(assign.Lhs) {
				// This should never happen as we would get a
				// "cannot assign N values to M variables"
				// before we get an interface conversion error.
				// But be defensive.
				return nil
			}
			lhs = assign.Lhs[i]
			rhs = r
			break
		}
	}
	if lhs == nil || rhs == nil {
		return nil
	}

	ifaceObj := ifaceType(lhs, info)
	if ifaceObj == nil {
		return nil
	}
	concType, pointer := concreteType(rhs, info)
	if concType == nil || concType.Obj().Pkg() == nil {
		return nil
	}
	return &StubInfo{
		Fset:      fset,
		Concrete:  concType,
		Interface: ifaceObj,
		Pointer:   pointer,
	}
}

// ifaceType returns the named interface type to which e refers, if any.
func ifaceType(e ast.Expr, info *types.Info) *types.TypeName {
	tv, ok := info.Types[e]
	if !ok {
		return nil
	}
	return ifaceObjFromType(tv.Type)
}

func ifaceObjFromType(t types.Type) *types.TypeName {
	named, ok := aliases.Unalias(t).(*types.Named)
	if !ok {
		return nil
	}
	if !types.IsInterface(named) {
		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(e ast.Expr, info *types.Info) (*types.Named, bool) {
	tv, ok := info.Types[e]
	if !ok {
		return nil, false
	}
	typ := tv.Type
	ptr, isPtr := aliases.Unalias(typ).(*types.Pointer)
	if isPtr {
		typ = ptr.Elem()
	}
	named, ok := aliases.Unalias(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
}