// Copyright 2020 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 fillstruct defines an Analyzer that automatically
// fills in a struct declaration with zero value elements for each field.
package fillstruct

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

	"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/go/ast/inspector"
	"golang.org/x/tools/internal/analysisinternal"
	"golang.org/x/tools/internal/span"
	"golang.org/x/tools/internal/typeparams"
)

const Doc = `note incomplete struct initializations

This analyzer provides diagnostics for any struct literals that do not have
any fields initialized. Because the suggested fix for this analysis is
expensive to compute, callers should compute it separately, using the
SuggestedFix function below.
`

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

func run(pass *analysis.Pass) (interface{}, error) {
	inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
	nodeFilter := []ast.Node{(*ast.CompositeLit)(nil)}
	inspect.Preorder(nodeFilter, func(n ast.Node) {
		info := pass.TypesInfo
		if info == nil {
			return
		}
		expr := n.(*ast.CompositeLit)

		var file *ast.File
		for _, f := range pass.Files {
			if f.Pos() <= expr.Pos() && expr.Pos() <= f.End() {
				file = f
				break
			}
		}
		if file == nil {
			return
		}

		typ := info.TypeOf(expr)
		if typ == nil {
			return
		}

		// Ignore types that have type parameters for now.
		// TODO: support type params.
		if typ, ok := typ.(*types.Named); ok {
			if tparams := typeparams.ForNamed(typ); tparams != nil && tparams.Len() > 0 {
				return
			}
		}

		// Find reference to the type declaration of the struct being initialized.
		for {
			p, ok := typ.Underlying().(*types.Pointer)
			if !ok {
				break
			}
			typ = p.Elem()
		}
		typ = typ.Underlying()

		obj, ok := typ.(*types.Struct)
		if !ok {
			return
		}
		fieldCount := obj.NumFields()

		// Skip any struct that is already populated or that has no fields.
		if fieldCount == 0 || fieldCount == len(expr.Elts) {
			return
		}

		var fillable bool
		var fillableFields []string
		for i := 0; i < fieldCount; i++ {
			field := obj.Field(i)
			// Ignore fields that are not accessible in the current package.
			if field.Pkg() != nil && field.Pkg() != pass.Pkg && !field.Exported() {
				continue
			}
			// Ignore structs containing fields that have type parameters for now.
			// TODO: support type params.
			if typ, ok := field.Type().(*types.Named); ok {
				if tparams := typeparams.ForNamed(typ); tparams != nil && tparams.Len() > 0 {
					return
				}
			}
			if _, ok := field.Type().(*typeparams.TypeParam); ok {
				return
			}
			fillable = true
			fillableFields = append(fillableFields, fmt.Sprintf("%s: %s", field.Name(), field.Type().String()))
		}
		if !fillable {
			return
		}
		var name string
		switch typ := expr.Type.(type) {
		case *ast.Ident:
			name = typ.Name
		case *ast.SelectorExpr:
			name = fmt.Sprintf("%s.%s", typ.X, typ.Sel.Name)
		default:
			totalFields := len(fillableFields)
			maxLen := 20
			// Find the index to cut off printing of fields.
			var i, fieldLen int
			for i = range fillableFields {
				if fieldLen > maxLen {
					break
				}
				fieldLen += len(fillableFields[i])
			}
			fillableFields = fillableFields[:i]
			if i < totalFields {
				fillableFields = append(fillableFields, "...")
			}
			name = fmt.Sprintf("anonymous struct { %s }", strings.Join(fillableFields, ", "))
		}
		pass.Report(analysis.Diagnostic{
			Message: fmt.Sprintf("Fill %s", name),
			Pos:     expr.Pos(),
			End:     expr.End(),
		})
	})
	return nil, nil
}

func SuggestedFix(fset *token.FileSet, rng span.Range, content []byte, file *ast.File, pkg *types.Package, info *types.Info) (*analysis.SuggestedFix, error) {
	pos := rng.Start // don't use the end

	// TODO(rstambler): Using ast.Inspect would probably be more efficient than
	// calling PathEnclosingInterval. Switch this approach.
	path, _ := astutil.PathEnclosingInterval(file, pos, pos)
	if len(path) == 0 {
		return nil, fmt.Errorf("no enclosing ast.Node")
	}
	var expr *ast.CompositeLit
	for _, n := range path {
		if node, ok := n.(*ast.CompositeLit); ok {
			expr = node
			break
		}
	}

	if info == nil {
		return nil, fmt.Errorf("nil types.Info")
	}
	typ := info.TypeOf(expr)
	if typ == nil {
		return nil, fmt.Errorf("no composite literal")
	}

	// Find reference to the type declaration of the struct being initialized.
	for {
		p, ok := typ.Underlying().(*types.Pointer)
		if !ok {
			break
		}
		typ = p.Elem()
	}
	typ = typ.Underlying()

	obj, ok := typ.(*types.Struct)
	if !ok {
		return nil, fmt.Errorf("unexpected type %v (%T), expected *types.Struct", typ, typ)
	}
	fieldCount := obj.NumFields()

	// Check which types have already been filled in. (we only want to fill in
	// the unfilled types, or else we'll blat user-supplied details)
	prefilledTypes := map[string]ast.Expr{}
	for _, e := range expr.Elts {
		if kv, ok := e.(*ast.KeyValueExpr); ok {
			if key, ok := kv.Key.(*ast.Ident); ok {
				prefilledTypes[key.Name] = kv.Value
			}
		}
	}

	// Use a new fileset to build up a token.File for the new composite
	// literal. We need one line for foo{, one line for }, and one line for
	// each field we're going to set. format.Node only cares about line
	// numbers, so we don't need to set columns, and each line can be
	// 1 byte long.
	fakeFset := token.NewFileSet()
	tok := fakeFset.AddFile("", -1, fieldCount+2)

	line := 2 // account for 1-based lines and the left brace
	var elts []ast.Expr
	var fieldTyps []types.Type
	for i := 0; i < fieldCount; i++ {
		field := obj.Field(i)
		// Ignore fields that are not accessible in the current package.
		if field.Pkg() != nil && field.Pkg() != pkg && !field.Exported() {
			fieldTyps = append(fieldTyps, nil)
			continue
		}
		fieldTyps = append(fieldTyps, field.Type())
	}
	matches := analysisinternal.FindMatchingIdents(fieldTyps, file, rng.Start, info, pkg)
	for i, fieldTyp := range fieldTyps {
		if fieldTyp == nil {
			continue
		}

		tok.AddLine(line - 1) // add 1 byte per line
		if line > tok.LineCount() {
			panic(fmt.Sprintf("invalid line number %v (of %v) for fillstruct", line, tok.LineCount()))
		}
		pos := tok.LineStart(line)

		kv := &ast.KeyValueExpr{
			Key: &ast.Ident{
				NamePos: pos,
				Name:    obj.Field(i).Name(),
			},
			Colon: pos,
		}
		if expr, ok := prefilledTypes[obj.Field(i).Name()]; ok {
			kv.Value = expr
		} else {
			idents, ok := matches[fieldTyp]
			if !ok {
				return nil, fmt.Errorf("invalid struct field type: %v", fieldTyp)
			}

			// Find the identifier whose name is most similar to the name of the field's key.
			// If we do not find any identifier that matches the pattern, generate a new value.
			// NOTE: We currently match on the name of the field key rather than the field type.
			value := analysisinternal.FindBestMatch(obj.Field(i).Name(), idents)
			if value == nil {
				value = populateValue(file, pkg, fieldTyp)
			}
			if value == nil {
				return nil, nil
			}

			kv.Value = value
		}
		elts = append(elts, kv)
		line++
	}

	// If all of the struct's fields are unexported, we have nothing to do.
	if len(elts) == 0 {
		return nil, fmt.Errorf("no elements to fill")
	}

	// Add the final line for the right brace. Offset is the number of
	// bytes already added plus 1.
	tok.AddLine(len(elts) + 1)
	line = len(elts) + 2
	if line > tok.LineCount() {
		panic(fmt.Sprintf("invalid line number %v (of %v) for fillstruct", line, tok.LineCount()))
	}

	cl := &ast.CompositeLit{
		Type:   expr.Type,
		Lbrace: tok.LineStart(1),
		Elts:   elts,
		Rbrace: tok.LineStart(line),
	}

	// Find the line on which the composite literal is declared.
	split := bytes.Split(content, []byte("\n"))
	lineNumber := fset.Position(expr.Lbrace).Line
	firstLine := split[lineNumber-1] // lines are 1-indexed

	// Trim the whitespace from the left of the line, and use the index
	// to get the amount of whitespace on the left.
	trimmed := bytes.TrimLeftFunc(firstLine, unicode.IsSpace)
	index := bytes.Index(firstLine, trimmed)
	whitespace := firstLine[:index]

	// First pass through the formatter: turn the expr into a string.
	var formatBuf bytes.Buffer
	if err := format.Node(&formatBuf, fakeFset, cl); err != nil {
		return nil, fmt.Errorf("failed to run first format on:\n%s\ngot err: %v", cl.Type, err)
	}
	sug := indent(formatBuf.Bytes(), whitespace)

	if len(prefilledTypes) > 0 {
		// Attempt a second pass through the formatter to line up columns.
		sourced, err := format.Source(sug)
		if err == nil {
			sug = indent(sourced, whitespace)
		}
	}

	return &analysis.SuggestedFix{
		TextEdits: []analysis.TextEdit{
			{
				Pos:     expr.Pos(),
				End:     expr.End(),
				NewText: sug,
			},
		},
	}, nil
}

// indent works line by line through str, indenting (prefixing) each line with
// ind.
func indent(str, ind []byte) []byte {
	split := bytes.Split(str, []byte("\n"))
	newText := bytes.NewBuffer(nil)
	for i, s := range split {
		if len(s) == 0 {
			continue
		}
		// Don't add the extra indentation to the first line.
		if i != 0 {
			newText.Write(ind)
		}
		newText.Write(s)
		if i < len(split)-1 {
			newText.WriteByte('\n')
		}
	}
	return newText.Bytes()
}

// populateValue constructs an expression to fill the value of a struct field.
//
// When the type of a struct field is a basic literal or interface, we return
// default values. For other types, such as maps, slices, and channels, we create
// expressions rather than using default values.
//
// The reasoning here is that users will call fillstruct with the intention of
// initializing the struct, in which case setting these fields to nil has no effect.
func populateValue(f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
	under := typ
	if n, ok := typ.(*types.Named); ok {
		under = n.Underlying()
	}
	switch u := under.(type) {
	case *types.Basic:
		switch {
		case u.Info()&types.IsNumeric != 0:
			return &ast.BasicLit{Kind: token.INT, Value: "0"}
		case u.Info()&types.IsBoolean != 0:
			return &ast.Ident{Name: "false"}
		case u.Info()&types.IsString != 0:
			return &ast.BasicLit{Kind: token.STRING, Value: `""`}
		case u.Kind() == types.UnsafePointer:
			return ast.NewIdent("nil")
		default:
			panic("unknown basic type")
		}
	case *types.Map:
		k := analysisinternal.TypeExpr(f, pkg, u.Key())
		v := analysisinternal.TypeExpr(f, pkg, u.Elem())
		if k == nil || v == nil {
			return nil
		}
		return &ast.CompositeLit{
			Type: &ast.MapType{
				Key:   k,
				Value: v,
			},
		}
	case *types.Slice:
		s := analysisinternal.TypeExpr(f, pkg, u.Elem())
		if s == nil {
			return nil
		}
		return &ast.CompositeLit{
			Type: &ast.ArrayType{
				Elt: s,
			},
		}
	case *types.Array:
		a := analysisinternal.TypeExpr(f, pkg, u.Elem())
		if a == nil {
			return nil
		}
		return &ast.CompositeLit{
			Type: &ast.ArrayType{
				Elt: a,
				Len: &ast.BasicLit{
					Kind: token.INT, Value: fmt.Sprintf("%v", u.Len()),
				},
			},
		}
	case *types.Chan:
		v := analysisinternal.TypeExpr(f, pkg, u.Elem())
		if v == nil {
			return nil
		}
		dir := ast.ChanDir(u.Dir())
		if u.Dir() == types.SendRecv {
			dir = ast.SEND | ast.RECV
		}
		return &ast.CallExpr{
			Fun: ast.NewIdent("make"),
			Args: []ast.Expr{
				&ast.ChanType{
					Dir:   dir,
					Value: v,
				},
			},
		}
	case *types.Struct:
		s := analysisinternal.TypeExpr(f, pkg, typ)
		if s == nil {
			return nil
		}
		return &ast.CompositeLit{
			Type: s,
		}
	case *types.Signature:
		var params []*ast.Field
		for i := 0; i < u.Params().Len(); i++ {
			p := analysisinternal.TypeExpr(f, pkg, u.Params().At(i).Type())
			if p == nil {
				return nil
			}
			params = append(params, &ast.Field{
				Type: p,
				Names: []*ast.Ident{
					{
						Name: u.Params().At(i).Name(),
					},
				},
			})
		}
		var returns []*ast.Field
		for i := 0; i < u.Results().Len(); i++ {
			r := analysisinternal.TypeExpr(f, pkg, u.Results().At(i).Type())
			if r == nil {
				return nil
			}
			returns = append(returns, &ast.Field{
				Type: r,
			})
		}
		return &ast.FuncLit{
			Type: &ast.FuncType{
				Params: &ast.FieldList{
					List: params,
				},
				Results: &ast.FieldList{
					List: returns,
				},
			},
			Body: &ast.BlockStmt{},
		}
	case *types.Pointer:
		switch u.Elem().(type) {
		case *types.Basic:
			return &ast.CallExpr{
				Fun: &ast.Ident{
					Name: "new",
				},
				Args: []ast.Expr{
					&ast.Ident{
						Name: u.Elem().String(),
					},
				},
			}
		default:
			return &ast.UnaryExpr{
				Op: token.AND,
				X:  populateValue(f, pkg, u.Elem()),
			}
		}
	case *types.Interface:
		return ast.NewIdent("nil")
	}
	return nil
}