// 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.
//
// The analyzer's diagnostic is merely a prompt.
// The actual fix is created by a separate direct call from gopls to
// the SuggestedFixes function.
// Tests of Analyzer.Run can be found in ./testdata/src.
// Tests of the SuggestedFixes logic live in ../../testdata/fillstruct.
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/ast/astutil"
	"golang.org/x/tools/go/ast/inspector"
	"golang.org/x/tools/gopls/internal/util/safetoken"
	"golang.org/x/tools/internal/analysisinternal"
	"golang.org/x/tools/internal/fuzzy"
	"golang.org/x/tools/internal/typeparams"
)

// Diagnose computes diagnostics for fillable struct literals overlapping with
// the provided start and end position.
//
// The diagnostic contains a lazy fix; the actual patch is computed
// (via the ApplyFix command) by a call to [SuggestedFix].
//
// If either start or end is invalid, the entire package is inspected.
func Diagnose(inspect *inspector.Inspector, start, end token.Pos, pkg *types.Package, info *types.Info) []analysis.Diagnostic {
	var diags []analysis.Diagnostic
	nodeFilter := []ast.Node{(*ast.CompositeLit)(nil)}
	inspect.Preorder(nodeFilter, func(n ast.Node) {
		expr := n.(*ast.CompositeLit)

		if (start.IsValid() && expr.End() < start) || (end.IsValid() && expr.Pos() > end) {
			return // non-overlapping
		}

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

		// Find reference to the type declaration of the struct being initialized.
		typ = typeparams.Deref(typ)
		tStruct, ok := typeparams.CoreType(typ).(*types.Struct)
		if !ok {
			return
		}
		// Inv: typ is the possibly-named struct type.

		fieldCount := tStruct.NumFields()

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

		// Are any fields in need of filling?
		var fillableFields []string
		for i := 0; i < fieldCount; i++ {
			field := tStruct.Field(i)
			// Ignore fields that are not accessible in the current package.
			if field.Pkg() != nil && field.Pkg() != pkg && !field.Exported() {
				continue
			}
			fillableFields = append(fillableFields, fmt.Sprintf("%s: %s", field.Name(), field.Type().String()))
		}
		if len(fillableFields) == 0 {
			return
		}

		// Derive a name for the struct type.
		var name string
		if typ != tStruct {
			// named struct type (e.g. pkg.S[T])
			name = types.TypeString(typ, types.RelativeTo(pkg))
		} else {
			// anonymous struct type
			totalFields := len(fillableFields)
			const 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, ", "))
		}
		diags = append(diags, analysis.Diagnostic{
			Message:  fmt.Sprintf("%s literal has missing fields", name),
			Pos:      expr.Pos(),
			End:      expr.End(),
			Category: FixCategory,
			SuggestedFixes: []analysis.SuggestedFix{{
				Message: fmt.Sprintf("Fill %s", name),
				// No TextEdits => computed later by gopls.
			}},
		})
	})

	return diags
}

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

// SuggestedFix computes the suggested fix for the kinds of
// diagnostics produced by the Analyzer above.
func SuggestedFix(fset *token.FileSet, start, end token.Pos, content []byte, file *ast.File, pkg *types.Package, info *types.Info) (*token.FileSet, *analysis.SuggestedFix, error) {
	if info == nil {
		return nil, nil, fmt.Errorf("nil types.Info")
	}

	pos := 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, 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
		}
	}

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

	// Find reference to the type declaration of the struct being initialized.
	typ = typeparams.Deref(typ)
	tStruct, ok := typ.Underlying().(*types.Struct)
	if !ok {
		return nil, nil, fmt.Errorf("%s is not a (pointer to) struct type",
			types.TypeString(typ, types.RelativeTo(pkg)))
	}
	// Inv: typ is the possibly-named struct type.

	fieldCount := tStruct.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)
	prefilledFields := map[string]ast.Expr{}
	for _, e := range expr.Elts {
		if kv, ok := e.(*ast.KeyValueExpr); ok {
			if key, ok := kv.Key.(*ast.Ident); ok {
				prefilledFields[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.
	// TODO(adonovan): why is this necessary? The position information
	// is going to be wrong for the existing trees in prefilledFields.
	// Can't the formatter just do its best with an empty fileset?
	fakeFset := token.NewFileSet()
	tok := fakeFset.AddFile("", -1, fieldCount+2)

	line := 2 // account for 1-based lines and the left brace
	var fieldTyps []types.Type
	for i := 0; i < fieldCount; i++ {
		field := tStruct.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.MatchingIdents(fieldTyps, file, start, info, pkg)
	var elts []ast.Expr
	for i, fieldTyp := range fieldTyps {
		if fieldTyp == nil {
			continue // TODO(adonovan): is this reachable?
		}
		fieldName := tStruct.Field(i).Name()

		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:    fieldName,
			},
			Colon: pos,
		}
		if expr, ok := prefilledFields[fieldName]; ok {
			kv.Value = expr
		} else {
			names, ok := matches[fieldTyp]
			if !ok {
				return nil, nil, fmt.Errorf("invalid struct field type: %v", fieldTyp)
			}

			// Find the name most similar to the field name.
			// If no name matches the pattern, generate a zero value.
			// NOTE: We currently match on the name of the field key rather than the field type.
			if best := fuzzy.BestMatch(fieldName, names); best != "" {
				kv.Value = ast.NewIdent(best)
			} else if v := populateValue(file, pkg, fieldTyp); v != nil {
				kv.Value = v
			} else {
				return nil, nil, nil // no fix to suggest
			}
		}
		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, 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 := safetoken.StartPosition(fset, 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, nil, fmt.Errorf("failed to run first format on:\n%s\ngot err: %v", cl.Type, err)
	}
	sug := indent(formatBuf.Bytes(), whitespace)

	if len(prefilledFields) > 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 fset, &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
// empty expressions such as []T{} or make(chan T) 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.
//
// populateValue returns nil if the value cannot be filled.
func populateValue(f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
	switch u := typ.Underlying().(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")
		case u.Kind() == types.Invalid:
			return nil
		default:
			panic(fmt.Sprintf("unknown basic type %v", u))
		}

	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:
			x := populateValue(f, pkg, u.Elem())
			if x == nil {
				return nil
			}
			return &ast.UnaryExpr{
				Op: token.AND,
				X:  x,
			}
		}

	case *types.Interface:
		if param, ok := typ.(*types.TypeParam); ok {
			// *new(T) is the zero value of a type parameter T.
			// TODO(adonovan): one could give a more specific zero
			// value if the type has a core type that is, say,
			// always a number or a pointer. See go/ssa for details.
			return &ast.StarExpr{
				X: &ast.CallExpr{
					Fun: ast.NewIdent("new"),
					Args: []ast.Expr{
						ast.NewIdent(param.Obj().Name()),
					},
				},
			}
		}

		return ast.NewIdent("nil")
	}
	return nil
}