// Copyright 2023 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/token"
	"strings"

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

// ErrNoLinkname is returned by LinknameDefinition when no linkname
// directive is found at a particular position.
// As such it indicates that other definitions could be worth checking.
var ErrNoLinkname = errors.New("no linkname directive found")

// LinknameDefinition finds the definition of the linkname directive in m at pos.
// If there is no linkname directive at pos, returns ErrNoLinkname.
func LinknameDefinition(ctx context.Context, snapshot Snapshot, m *protocol.Mapper, from protocol.Position) ([]protocol.Location, error) {
	pkgPath, name, _ := parseLinkname(m, from)
	if pkgPath == "" {
		return nil, ErrNoLinkname
	}

	_, pgf, pos, err := findLinkname(ctx, snapshot, PackagePath(pkgPath), name)
	if err != nil {
		return nil, fmt.Errorf("find linkname: %w", err)
	}
	loc, err := pgf.PosLocation(pos, pos+token.Pos(len(name)))
	if err != nil {
		return nil, fmt.Errorf("location of linkname: %w", err)
	}
	return []protocol.Location{loc}, nil
}

// parseLinkname attempts to parse a go:linkname declaration at the given pos.
// If successful, it returns
// - package path referenced
// - object name referenced
// - byte offset in mapped file of the start of the link target
// of the linkname directives 2nd argument.
//
// If the position is not in the second argument of a go:linkname directive,
// or parsing fails, it returns "", "", 0.
func parseLinkname(m *protocol.Mapper, pos protocol.Position) (pkgPath, name string, targetOffset int) {
	lineStart, err := m.PositionOffset(protocol.Position{Line: pos.Line, Character: 0})
	if err != nil {
		return "", "", 0
	}
	lineEnd, err := m.PositionOffset(protocol.Position{Line: pos.Line + 1, Character: 0})
	if err != nil {
		return "", "", 0
	}

	directive := string(m.Content[lineStart:lineEnd])
	// (Assumes no leading spaces.)
	if !strings.HasPrefix(directive, "//go:linkname") {
		return "", "", 0
	}
	// Sometimes source code (typically tests) has another
	// comment after the directive, trim that away.
	if i := strings.LastIndex(directive, "//"); i != 0 {
		directive = strings.TrimSpace(directive[:i])
	}

	// Looking for pkgpath in '//go:linkname f pkgpath.g'.
	// (We ignore 1-arg linkname directives.)
	parts := strings.Fields(directive)
	if len(parts) != 3 {
		return "", "", 0
	}

	// Inside 2nd arg [start, end]?
	// (Assumes no trailing spaces.)
	offset, err := m.PositionOffset(pos)
	if err != nil {
		return "", "", 0
	}
	end := lineStart + len(directive)
	start := end - len(parts[2])
	if !(start <= offset && offset <= end) {
		return "", "", 0
	}
	linkname := parts[2]

	// Split the pkg path from the name.
	dot := strings.LastIndexByte(linkname, '.')
	if dot < 0 {
		return "", "", 0
	}

	return linkname[:dot], linkname[dot+1:], start
}

// findLinkname searches dependencies of packages containing fh for an object
// with linker name matching the given package path and name.
func findLinkname(ctx context.Context, snapshot Snapshot, pkgPath PackagePath, name string) (Package, *ParsedGoFile, token.Pos, error) {
	// Typically the linkname refers to a forward dependency
	// or a reverse dependency, but in general it may refer
	// to any package that is linked with this one.
	var pkgMeta *Metadata
	metas, err := snapshot.AllMetadata(ctx)
	if err != nil {
		return nil, nil, token.NoPos, err
	}
	RemoveIntermediateTestVariants(&metas)
	for _, meta := range metas {
		if meta.PkgPath == pkgPath {
			pkgMeta = meta
			break
		}
	}
	if pkgMeta == nil {
		return nil, nil, token.NoPos, fmt.Errorf("cannot find package %q", pkgPath)
	}

	// When found, type check the desired package (snapshot.TypeCheck in TypecheckFull mode),
	pkgs, err := snapshot.TypeCheck(ctx, pkgMeta.ID)
	if err != nil {
		return nil, nil, token.NoPos, err
	}
	pkg := pkgs[0]

	obj := pkg.GetTypes().Scope().Lookup(name)
	if obj == nil {
		return nil, nil, token.NoPos, fmt.Errorf("package %q does not define %s", pkgPath, name)
	}

	objURI := safetoken.StartPosition(pkg.FileSet(), obj.Pos())
	pgf, err := pkg.File(span.URIFromPath(objURI.Filename))
	if err != nil {
		return nil, nil, token.NoPos, err
	}

	return pkg, pgf, obj.Pos(), nil
}