template-nest-go/template_nest.go

package templatenest

import (
	"fmt"
	"io/ioutil"
	"os"
	"path/filepath"
	"regexp"
	"strings"
	"time"
)

type Hash map[string]interface{}

// Option holds configuration for TemplateNest
type Option struct {
	Delimiters        [2]string
	NameLabel         string
	TemplateDir       string
	TemplateExtension string
	DieOnBadParams    bool
}

type TemplateNest struct {
	option Option
	cache   map[string]TemplateFileIndex
}

// TemplateFileIndex represents an indexed template file.
type TemplateFileIndex struct {
	Contents      string
	LastModified  time.Time              // Last modified timestamp of the template file
	Variables     []TemplateFileVariable // List of variables in the template file
	VariableNames map[string]struct{}    // Set of variable names
}

// TemplateFileVariable represents a variable in a template file.
type TemplateFileVariable struct {
	Name          string
	StartPosition uint // Start position of the complete variable string (including delimiters)
	EndPosition   uint // End position of the complete variable string (including delimiters)
	IndentLevel   uint // Indentation level of the variable
	EscapedToken  bool // Indicates if the variable was escaped with token escape character
}

func New(opts Option) (*TemplateNest, error) {
	// Check if the TemplateDir exists and is a directory.
	templateDirInfo, err := os.Stat(opts.TemplateDir)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, fmt.Errorf("template dir `%s` does not exist", opts.TemplateDir)
		}
		return nil, fmt.Errorf("error checking template dir: %w", err)
	}
	if !templateDirInfo.IsDir() {
		return nil, fmt.Errorf("template dir `%s` is not a directory", opts.TemplateDir)
	}

	// Set defaults for options that the user hasn't provided.
	if opts.Delimiters == [2]string{} {
		opts.Delimiters = [2]string{"<!--%", "%-->"}
	}
	if opts.NameLabel == "" {
		opts.NameLabel = "TEMPLATE"
	}
	if opts.TemplateExtension == "" {
		opts.TemplateExtension = "html"
	}

	// Initialize TemplateNest with the final options.
	nest := &TemplateNest{
		option: opts,
		cache:   make(map[string]TemplateFileIndex),
	}

	// Walk through the template directory and index the templates.
	err = filepath.Walk(opts.TemplateDir, func(path string, info os.FileInfo, err error) error {
		if err != nil {
			return err
		}

		// Check if the file has the correct extension.
		if !strings.HasSuffix(info.Name(), "."+opts.TemplateExtension) {
			return nil
		}

		// Index the template and store it in the cache.
		templateIndex, err := nest.index(path)
		if err != nil {
			return err
		}

		// Get the relative path of the file.
		relPath, err := filepath.Rel(opts.TemplateDir, path)
		if err != nil {
			return err
		}

		// Remove the extension from the relative path.
		templateName := strings.TrimSuffix(relPath, "."+opts.TemplateExtension)
		nest.cache[templateName] = templateIndex
		return nil
	})
	if err != nil {
		return nil, err
	}

	return nest, nil
}

func (nest *TemplateNest) index(filePath string) (TemplateFileIndex, error) {
	contents, err := ioutil.ReadFile(filePath)
	if err != nil {
		return TemplateFileIndex{}, fmt.Errorf("Error reading file (`%s`): %w", filePath, err)
	}
	// Capture last modified time
	fileInfo, err := os.Stat(filePath)
	if err != nil {
		return TemplateFileIndex{}, fmt.Errorf("Error getting file (`%s`) info: %w", filePath, err)
	}

	variableNames := make(map[string]struct{})
	variables := []TemplateFileVariable{}

	delimiterStart := nest.option.Delimiters[0]
	delimiterEnd := nest.option.Delimiters[1]

	re := regexp.MustCompile(fmt.Sprintf(
		"%s\\s*(.+?)\\s*%s", regexp.QuoteMeta(delimiterStart), regexp.QuoteMeta(delimiterEnd),
	))

	matches := re.FindAllStringSubmatchIndex(string(contents), -1)
	for _, match := range matches {
		if len(match) < 4 {
			continue
		}

		startIdx := match[0]
		endIdx := match[1]
		nameStartIdx := match[2]
		nameEndIdx := match[3]

		varName := string(contents[nameStartIdx:nameEndIdx])
		variableNames[varName] = struct{}{}

		variables = append(variables, TemplateFileVariable{
			Name:          varName,
			StartPosition: uint(startIdx),
			EndPosition:   uint(endIdx),
		})
	}

	fileIndex := TemplateFileIndex{
		Contents:      string(contents),
		LastModified:  fileInfo.ModTime(),
		VariableNames: variableNames,
		Variables:     variables,
	}

	return fileIndex, nil
}

// getTemplateFilePath takes a template name and returns the file path for the
// template.
func (nest *TemplateNest) getTemplateFilePath(templateName string) string {
	return filepath.Join(
		nest.option.TemplateDir,
		fmt.Sprintf("%s.%s", templateName, nest.option.TemplateExtension),
	)
}

func (nest *TemplateNest) MustRender(toRender interface{}) string {
	render, err := nest.Render(toRender)
	if err != nil {
		panic(err)
	}
	return render
}

func (nest *TemplateNest) Render(toRender interface{}) (string, error) {
	switch v := toRender.(type) {
	case nil:
		return "", nil

	case bool:
		return fmt.Sprintf("%t", v), nil

	case string:
		return v, nil

	case float64, int, int64:
		return fmt.Sprintf("%v", v), nil

	case []Hash:
		var rendered strings.Builder
		for _, item := range v {
			renderedItem, err := nest.Render(item)
			if err != nil {
				return "", err
			}
			rendered.WriteString(renderedItem)
		}
		return rendered.String(), nil

	case Hash:
		tLabel, ok := v[nest.option.NameLabel]
		if !ok {
			return "", fmt.Errorf("missing template label: %s", nest.option.NameLabel)
		}

		tName, ok := tLabel.(string)
		if !ok {
			return "", fmt.Errorf("invalid template label: %+v", tLabel)
		}

		tFile := nest.getTemplateFilePath(tName)
		fileInfo, err := os.Stat(tFile)
		if err != nil {
			return "", fmt.Errorf("error getting file info: %w", err)
		}

		tIndex, exists := nest.cache[tName]

		// If cache doesn't exist or has expired, re-index the file.
		if !exists || fileInfo.ModTime().After(tIndex.LastModified) {
			newIndex, err := nest.index(tFile)
			if err != nil {
				return "", fmt.Errorf("File index failed: %w", err)
			}
			tIndex = newIndex
		}

		rendered := tIndex.Contents
		for i := len(tIndex.Variables) - 1; i >= 0; i-- {
			variable := tIndex.Variables[i]

			// If the variable doesn't exist in template hash then replace it
			// with an empty string.
			replacement := ""

			if value, exists := v[variable.Name]; exists {
				if text, ok := value.(string); ok {
					replacement = text
				} else {
					subRender, err := nest.Render(value)
					if err != nil {
						return "", err
					}
					replacement = subRender
				}
			}

			// Replace in rendered template
			rendered = rendered[:variable.StartPosition] + replacement + rendered[variable.EndPosition:]
		}

		return strings.TrimSpace(rendered), nil

	default:
		return "", fmt.Errorf("Unsupported template type: %+v", v)
	}
}