// Copyright 2019 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 source

import (
	"context"
	"errors"
	"fmt"
	"go/ast"
	"go/token"
	"go/types"

	"golang.org/x/tools/gopls/internal/lsp/protocol"
	"golang.org/x/tools/gopls/internal/lsp/safetoken"
	"golang.org/x/tools/gopls/internal/lsp/source/methodsets"
	"golang.org/x/tools/gopls/internal/span"
	"golang.org/x/tools/internal/event"
)

var ErrNotAType = errors.New("not a type name or method")

// implementations returns the concrete implementations of the specified
// interface, or the interfaces implemented by the specified concrete type.
// It populates only the definition-related fields of qualifiedObject.
// (Arguably it should return a smaller data type.)
//
// This is the legacy implementation using the type-check-the-world
// strategy.  It is still needed by the 'references' command for now,
// but we are moving one step at a time.  See implementation2.go for
// the incremental algorithm used by the 'implementations' command.
// TODO(adonovan): eliminate this implementation.
func implementations(ctx context.Context, s Snapshot, f FileHandle, pp protocol.Position) ([]qualifiedObject, error) {
	// Find all named types, even local types
	// (which can have methods due to promotion).
	var (
		allNamed []*types.Named
		pkgs     = make(map[*types.Package]Package)
	)
	metas, err := s.AllMetadata(ctx)
	if err != nil {
		return nil, err
	}
	ids := make([]PackageID, len(metas))
	for i, m := range metas {
		ids[i] = m.ID
	}
	packages, err := s.TypeCheck(ctx, TypecheckFull, ids...)
	if err != nil {
		return nil, err
	}
	for _, pkg := range packages {
		pkgs[pkg.GetTypes()] = pkg
		for _, obj := range pkg.GetTypesInfo().Defs {
			obj, ok := obj.(*types.TypeName)
			// We ignore aliases 'type M = N' to avoid duplicate reporting
			// of the Named type N.
			if !ok || obj.IsAlias() {
				continue
			}
			if named, ok := obj.Type().(*types.Named); ok {
				allNamed = append(allNamed, named)
			}
		}
	}

	qos, err := qualifiedObjsAtProtocolPos(ctx, s, f.URI(), pp)
	if err != nil {
		return nil, err
	}
	var (
		impls []qualifiedObject
		seen  = make(map[token.Position]bool)
	)
	for _, qo := range qos {
		// Ascertain the query identifier (type or method).
		var (
			queryType   types.Type
			queryMethod *types.Func
		)
		switch obj := qo.obj.(type) {
		case *types.Func:
			queryMethod = obj
			if recv := obj.Type().(*types.Signature).Recv(); recv != nil {
				queryType = methodsets.EnsurePointer(recv.Type())
			}
		case *types.TypeName:
			queryType = methodsets.EnsurePointer(obj.Type())
		}

		if queryType == nil {
			return nil, ErrNotAType
		}

		if types.NewMethodSet(queryType).Len() == 0 {
			return nil, nil
		}

		// Find all the named types that match our query.
		for _, named := range allNamed {
			var (
				candObj  types.Object = named.Obj()
				candType              = methodsets.EnsurePointer(named)
			)

			if !concreteImplementsIntf(candType, queryType) {
				continue
			}

			ms := types.NewMethodSet(candType)
			if ms.Len() == 0 {
				// Skip empty interfaces.
				continue
			}

			// If client queried a method, look up corresponding candType method.
			if queryMethod != nil {
				sel := ms.Lookup(queryMethod.Pkg(), queryMethod.Name())
				if sel == nil {
					continue
				}
				candObj = sel.Obj()
			}

			pos := safetoken.StartPosition(s.FileSet(), candObj.Pos())
			if candObj == queryMethod || seen[pos] {
				continue
			}

			pkg := pkgs[candObj.Pkg()] // may be nil (e.g. error)

			// TODO(adonovan): the logic below assumes there is only one
			// predeclared (pkg=nil) object of interest, the error type.
			// That could change in a future version of Go.

			var posn token.Position
			if pkg != nil {
				posn = safetoken.StartPosition(pkg.FileSet(), candObj.Pos())
			}
			if seen[posn] {
				continue
			}
			seen[posn] = true

			impls = append(impls, qualifiedObject{
				obj: candObj,
				pkg: pkg,
			})
		}
	}

	return impls, nil
}

// concreteImplementsIntf returns true if a is an interface type implemented by
// concrete type b, or vice versa.
func concreteImplementsIntf(a, b types.Type) bool {
	aIsIntf, bIsIntf := types.IsInterface(a), types.IsInterface(b)

	// Make sure exactly one is an interface type.
	if aIsIntf == bIsIntf {
		return false
	}

	// Rearrange if needed so "a" is the concrete type.
	if aIsIntf {
		a, b = b, a
	}

	// TODO(adonovan): this should really use GenericAssignableTo
	// to report (e.g.) "ArrayList[T] implements List[T]", but
	// GenericAssignableTo doesn't work correctly on pointers to
	// generic named types. Thus the legacy implementation and the
	// "local" part of implementation2 fail to report generics.
	// The global algorithm based on subsets does the right thing.
	return types.AssignableTo(a, b)
}

// A qualifiedObject is the result of resolving a reference from an
// identifier to an object.
type qualifiedObject struct {
	// definition
	obj types.Object // the referenced object
	pkg Package      // the Package that defines the object (nil => universe)

	// reference (optional)
	node      ast.Node // the reference (*ast.Ident or *ast.ImportSpec) to the object
	sourcePkg Package  // the Package containing node
}

var (
	errBuiltin       = errors.New("builtin object")
	errNoObjectFound = errors.New("no object found")
)

// qualifiedObjsAtProtocolPos returns info for all the types.Objects referenced
// at the given position, for the following selection of packages:
//
// 1. all packages (including all test variants), in their workspace parse mode
// 2. if not included above, at least one package containing uri in full parse mode
//
// Finding objects in (1) ensures that we locate references within all
// workspace packages, including in x_test packages. Including (2) ensures that
// we find local references in the current package, for non-workspace packages
// that may be open.
func qualifiedObjsAtProtocolPos(ctx context.Context, s Snapshot, uri span.URI, pp protocol.Position) ([]qualifiedObject, error) {
	fh, err := s.GetFile(ctx, uri)
	if err != nil {
		return nil, err
	}
	content, err := fh.Read()
	if err != nil {
		return nil, err
	}
	m := protocol.NewMapper(uri, content)
	offset, err := m.PositionOffset(pp)
	if err != nil {
		return nil, err
	}
	return qualifiedObjsAtLocation(ctx, s, positionKey{uri, offset}, map[positionKey]bool{})
}

// A positionKey identifies a byte offset within a file (URI).
//
// When a file has been parsed multiple times in the same FileSet,
// there may be multiple token.Pos values denoting the same logical
// position. In such situations, a positionKey may be used for
// de-duplication.
type positionKey struct {
	uri    span.URI
	offset int
}

// qualifiedObjsAtLocation finds all objects referenced at offset in uri,
// across all packages in the snapshot.
func qualifiedObjsAtLocation(ctx context.Context, s Snapshot, key positionKey, seen map[positionKey]bool) ([]qualifiedObject, error) {
	if seen[key] {
		return nil, nil
	}
	seen[key] = true

	// We search for referenced objects starting with all packages containing the
	// current location, and then repeating the search for every distinct object
	// location discovered.
	//
	// In the common case, there should be at most one additional location to
	// consider: the definition of the object referenced by the location. But we
	// try to be comprehensive in case we ever support variations on build
	// constraints.
	metas, err := s.MetadataForFile(ctx, key.uri)
	if err != nil {
		return nil, err
	}
	ids := make([]PackageID, len(metas))
	for i, m := range metas {
		ids[i] = m.ID
	}
	pkgs, err := s.TypeCheck(ctx, TypecheckWorkspace, ids...)
	if err != nil {
		return nil, err
	}

	// In order to allow basic references/rename/implementations to function when
	// non-workspace packages are open, ensure that we have at least one fully
	// parsed package for the current file. This allows us to find references
	// inside the open package. Use WidestPackage to capture references in test
	// files.
	hasFullPackage := false
	for _, pkg := range pkgs {
		if pkg.ParseMode() == ParseFull {
			hasFullPackage = true
			break
		}
	}
	if !hasFullPackage {
		pkg, _, err := PackageForFile(ctx, s, key.uri, TypecheckFull, WidestPackage)
		if err != nil {
			return nil, err
		}
		pkgs = append(pkgs, pkg)
	}

	// report objects in the order we encounter them. This ensures that the first
	// result is at the cursor...
	var qualifiedObjs []qualifiedObject
	// ...but avoid duplicates.
	seenObjs := map[types.Object]bool{}

	for _, searchpkg := range pkgs {
		pgf, err := searchpkg.File(key.uri)
		if err != nil {
			return nil, err
		}
		pos := pgf.Tok.Pos(key.offset)

		// TODO(adonovan): replace this section with a call to objectsAt().
		path := pathEnclosingObjNode(pgf.File, pos)
		if path == nil {
			continue
		}
		var objs []types.Object
		switch leaf := path[0].(type) {
		case *ast.Ident:
			// If leaf represents an implicit type switch object or the type
			// switch "assign" variable, expand to all of the type switch's
			// implicit objects.
			if implicits, _ := typeSwitchImplicits(searchpkg.GetTypesInfo(), path); len(implicits) > 0 {
				objs = append(objs, implicits...)
			} else {
				obj := searchpkg.GetTypesInfo().ObjectOf(leaf)
				if obj == nil {
					return nil, fmt.Errorf("%w for %q", errNoObjectFound, leaf.Name)
				}
				objs = append(objs, obj)
			}
		case *ast.ImportSpec:
			// Look up the implicit *types.PkgName.
			obj := searchpkg.GetTypesInfo().Implicits[leaf]
			if obj == nil {
				return nil, fmt.Errorf("%w for import %s", errNoObjectFound, UnquoteImportPath(leaf))
			}
			objs = append(objs, obj)
		}

		// Get all of the transitive dependencies of the search package.
		pkgs := make(map[*types.Package]Package)
		var addPkg func(pkg Package)
		addPkg = func(pkg Package) {
			pkgs[pkg.GetTypes()] = pkg
			for _, imp := range pkg.Imports() {
				if _, ok := pkgs[imp.GetTypes()]; !ok {
					addPkg(imp)
				}
			}
		}
		addPkg(searchpkg)
		for _, obj := range objs {
			if obj.Parent() == types.Universe {
				return nil, fmt.Errorf("%q: %w", obj.Name(), errBuiltin)
			}
			pkg, ok := pkgs[obj.Pkg()]
			if !ok {
				event.Error(ctx, fmt.Sprintf("no package for obj %s: %v", obj, obj.Pkg()), err)
				continue
			}
			qualifiedObjs = append(qualifiedObjs, qualifiedObject{
				obj:       obj,
				pkg:       pkg,
				sourcePkg: searchpkg,
				node:      path[0],
			})
			seenObjs[obj] = true

			// If the qualified object is in another file (or more likely, another
			// package), it's possible that there is another copy of it in a package
			// that we haven't searched, e.g. a test variant. See golang/go#47564.
			//
			// In order to be sure we've considered all packages, call
			// qualifiedObjsAtLocation recursively for all locations we encounter. We
			// could probably be more precise here, only continuing the search if obj
			// is in another package, but this should be good enough to find all
			// uses.

			if key, found := packagePositionKey(pkg, obj.Pos()); found {
				otherObjs, err := qualifiedObjsAtLocation(ctx, s, key, seen)
				if err != nil {
					return nil, err
				}
				for _, other := range otherObjs {
					if !seenObjs[other.obj] {
						qualifiedObjs = append(qualifiedObjs, other)
						seenObjs[other.obj] = true
					}
				}
			} else {
				return nil, fmt.Errorf("missing file for position of %q in %q", obj.Name(), obj.Pkg().Name())
			}
		}
	}
	// Return an error if no objects were found since callers will assume that
	// the slice has at least 1 element.
	if len(qualifiedObjs) == 0 {
		return nil, errNoObjectFound
	}
	return qualifiedObjs, nil
}

// packagePositionKey finds the positionKey for the given pos.
//
// The second result reports whether the position was found.
func packagePositionKey(pkg Package, pos token.Pos) (positionKey, bool) {
	for _, pgf := range pkg.CompiledGoFiles() {
		offset, err := safetoken.Offset(pgf.Tok, pos)
		if err == nil {
			return positionKey{pgf.URI, offset}, true
		}
	}
	return positionKey{}, false
}

// pathEnclosingObjNode returns the AST path to the object-defining
// node associated with pos. "Object-defining" means either an
// *ast.Ident mapped directly to a types.Object or an ast.Node mapped
// implicitly to a types.Object.
func pathEnclosingObjNode(f *ast.File, pos token.Pos) []ast.Node {
	var (
		path  []ast.Node
		found bool
	)

	ast.Inspect(f, func(n ast.Node) bool {
		if found {
			return false
		}

		if n == nil {
			path = path[:len(path)-1]
			return false
		}

		path = append(path, n)

		switch n := n.(type) {
		case *ast.Ident:
			// Include the position directly after identifier. This handles
			// the common case where the cursor is right after the
			// identifier the user is currently typing. Previously we
			// handled this by calling astutil.PathEnclosingInterval twice,
			// once for "pos" and once for "pos-1".
			found = n.Pos() <= pos && pos <= n.End()
		case *ast.ImportSpec:
			if n.Path.Pos() <= pos && pos < n.Path.End() {
				found = true
				// If import spec has a name, add name to path even though
				// position isn't in the name.
				if n.Name != nil {
					path = append(path, n.Name)
				}
			}
		case *ast.StarExpr:
			// Follow star expressions to the inner identifier.
			if pos == n.Star {
				pos = n.X.Pos()
			}
		}

		return !found
	})

	if len(path) == 0 {
		return nil
	}

	// Reverse path so leaf is first element.
	for i := 0; i < len(path)/2; i++ {
		path[i], path[len(path)-1-i] = path[len(path)-1-i], path[i]
	}

	return path
}