func rewrite()

in edit/buildozer.go [917:1044]
115 lines of code
30 McCabe index (conditional complexity)

func rewrite(opts *Options, commandsForFile commandsForFile) *rewriteResult {
	name := commandsForFile.file
	var data []byte
	var err error
	var fi os.FileInfo
	records := []*apipb.Output_Record{}
	if name == stdinPackageName { // read on stdin
		data, err = ioutil.ReadAll(os.Stdin)
		if err != nil {
			return &rewriteResult{file: name, errs: []error{err}}
		}
	} else {
		origName := name
		for _, suffix := range BuildFileNames {
			if strings.HasSuffix(name, "/"+suffix) {
				name = strings.TrimSuffix(name, suffix)
				break
			}
		}
		for _, suffix := range BuildFileNames {
			name = name + suffix
			data, fi, err = file.ReadFile(name)
			if err == nil {
				break
			}
			name = strings.TrimSuffix(name, suffix)
		}
		if err != nil {
			data, fi, err = file.ReadFile(name)
		}
		if err != nil {
			err = errors.New("file not found or not readable")
			return &rewriteResult{file: origName, errs: []error{err}}
		}
	}

	f, err := build.ParseBuild(name, data)
	if err != nil {
		return &rewriteResult{file: name, errs: []error{err}}
	}
	f.WorkspaceRoot, f.Pkg, f.Label = wspace.SplitFilePath(name)

	vars := map[string]*build.AssignExpr{}
	if opts.EditVariables {
		vars = getGlobalVariables(f.Stmt)
	}
	var errs []error
	changed := false
	for _, commands := range commandsForFile.commands {
		target := commands.target
		commands := commands.commands
		_, absPkg, rule := InterpretLabelForWorkspaceLocation(opts.RootDir, target)
		if label := labels.Parse(target); label.Package == stdinPackageName {
			// Special-case: This is already absolute
			absPkg = stdinPackageName
		}
		if strings.HasSuffix(absPkg, "...") {
			// Special case: the provided target contains an ellipsis, use the file package
			absPkg = f.Pkg
		}

		targets, err := expandTargets(f, rule)
		if err != nil {
			cerr := commandError(commands, target, err)
			errs = append(errs, cerr)
			if !opts.KeepGoing {
				return &rewriteResult{file: name, errs: errs, records: records}

			}
		}
		targets = filterRules(opts, targets)
		for _, cmd := range commands {
			cmdInfo := AllCommands[cmd.tokens[0]]
			// Depending on whether a transformation is rule-specific or not, it should be applied to
			// every rule that satisfies the filter or just once to the file.
			cmdTargets := targets
			if !cmdInfo.PerRule {
				cmdTargets = []*build.Rule{nil}
			}
			for _, r := range cmdTargets {
				record := &apipb.Output_Record{}
				newf, err := cmdInfo.Fn(opts, CmdEnvironment{f, r, vars, absPkg, cmd.tokens[1:], record})
				if len(record.Fields) != 0 {
					records = append(records, record)
				}
				if err != nil {
					cerr := commandError([]command{cmd}, target, err)
					if opts.KeepGoing {
						errs = append(errs, cerr)
					} else {
						return &rewriteResult{file: name, errs: []error{cerr}, records: records}
					}
				}
				if newf != nil {
					changed = true
					f = newf
				}
			}
		}
	}
	if !changed {
		return &rewriteResult{file: name, errs: errs, records: records}
	}
	f = RemoveEmptyPackage(f)
	ndata, err := buildifier.Buildify(opts, f)
	if err != nil {
		return &rewriteResult{file: name, errs: []error{fmt.Errorf("running buildifier: %v", err)}, records: records}
	}

	if opts.Stdout || name == stdinPackageName {
		opts.OutWriter.Write(ndata)
		return &rewriteResult{file: name, errs: errs, records: records}
	}

	if bytes.Equal(data, ndata) {
		return &rewriteResult{file: name, errs: errs, records: records}
	}

	if err := EditFile(fi, name); err != nil {
		return &rewriteResult{file: name, errs: []error{err}, records: records}
	}

	if err := file.WriteFile(name, ndata); err != nil {
		return &rewriteResult{file: name, errs: []error{err}, records: records}
	}

	return &rewriteResult{file: name, errs: errs, modified: true, records: records}
}