hosts-go/internal/core/parser.go

package core

import (
	"bufio"
	"fmt"
	"net"
	"os"
	"path/filepath"
	"regexp"
	"strings"
	"time"
)

// ParseWarning represents a warning encountered during parsing
type ParseWarning struct {
	Line    int    // Line number (1-based)
	Message string // Warning message
}

// FormattingStyle represents the detected formatting style of a hosts file
type FormattingStyle struct {
	UseTabs       bool // Whether to use tabs for separation
	SpacesPerTab  int  // Number of spaces per tab if using spaces
	IPWidth       int  // Width for IP column alignment
	HostWidth     int  // Width for hostname column alignment
	AlignComments bool // Whether to align comments
}

// DetectFormattingStyle analyzes the given lines to determine the formatting style
func DetectFormattingStyle(lines []string) FormattingStyle {
	style := FormattingStyle{
		UseTabs:       true,
		SpacesPerTab:  4,
		IPWidth:       15,
		HostWidth:     30,
		AlignComments: true,
	}

	tabCount, spaceCount := 0, 0
	spaceLengths := make(map[int]int)

	for _, line := range lines {
		// Skip empty lines and comments
		line = strings.TrimSpace(line)
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}

		if strings.Contains(line, "\t") {
			tabCount++
		} else {
			// Look for sequences of spaces between non-space characters
			spaceRegex := regexp.MustCompile(`\S\s{2,}\S`)
			if spaceRegex.MatchString(line) {
				spaceCount++

				// Find all space sequences and count their lengths
				allSpaces := regexp.MustCompile(`\s{2,}`)
				matches := allSpaces.FindAllString(line, -1)
				for _, match := range matches {
					spaceLengths[len(match)]++
				}
			}
		}
	}

	// Use spaces if more space-separated lines than tab-separated
	if spaceCount > tabCount {
		style.UseTabs = false

		// Find the greatest common divisor of all space lengths to detect the base unit
		if len(spaceLengths) > 0 {
			var lengths []int
			for length := range spaceLengths {
				if length >= 2 {
					lengths = append(lengths, length)
				}
			}

			if len(lengths) > 0 {
				gcd := lengths[0]
				for i := 1; i < len(lengths); i++ {
					gcd = findGCD(gcd, lengths[i])
				}

				// Use GCD as the base spacing unit, but ensure it's at least 2 and reasonable
				if gcd >= 2 && gcd <= 8 {
					style.SpacesPerTab = gcd
				} else if len(lengths) == 1 {
					// Single space length detected, use it directly
					style.SpacesPerTab = lengths[0]
				} else {
					style.SpacesPerTab = 4 // fallback
				}
			} else {
				style.SpacesPerTab = 4 // fallback
			}
		} else {
			style.SpacesPerTab = 4 // fallback
		}
	}

	return style
}

// findGCD finds the greatest common divisor of two integers
func findGCD(a, b int) int {
	for b != 0 {
		a, b = b, a%b
	}
	return a
}

// ParseHostsFile reads and parses a hosts file from the filesystem
func ParseHostsFile(filepath string) (*HostsFile, []ParseWarning, error) {
	file, err := os.Open(filepath)
	if err != nil {
		return nil, nil, fmt.Errorf("failed to open hosts file: %w", err)
	}
	defer file.Close()

	var lines []string
	scanner := bufio.NewScanner(file)
	for scanner.Scan() {
		lines = append(lines, scanner.Text())
	}

	if err := scanner.Err(); err != nil {
		return nil, nil, fmt.Errorf("failed to read hosts file: %w", err)
	}

	return ParseHostsContent(lines)
}

// ParseHostsContent parses hosts file content from a slice of lines
func ParseHostsContent(lines []string) (*HostsFile, []ParseWarning, error) {
	hostsFile := NewHostsFile()
	var warnings []ParseWarning

	for lineNum, line := range lines {
		lineNum++ // Convert to 1-based indexing

		// Skip empty lines
		if strings.TrimSpace(line) == "" {
			continue
		}

		// Handle comments and disabled entries
		if strings.HasPrefix(strings.TrimSpace(line), "#") {
			if entry, warning := parseCommentLine(line, lineNum); entry != nil {
				hostsFile.AddEntry(entry)
				if warning != nil {
					warnings = append(warnings, *warning)
				}
			} else if comment := parseStandaloneComment(line); comment != "" {
				hostsFile.Comments = append(hostsFile.Comments, comment)
			}
			continue
		}

		// Parse regular entry
		entry, warning := parseEntryLine(line, lineNum)
		if entry != nil {
			hostsFile.AddEntry(entry)
		}
		if warning != nil {
			warnings = append(warnings, *warning)
		}
	}

	return hostsFile, warnings, nil
}

// parseCommentLine parses a commented line, which might be a disabled entry
func parseCommentLine(line string, lineNum int) (*HostEntry, *ParseWarning) {
	// Remove the leading # and any whitespace
	content := strings.TrimSpace(strings.TrimPrefix(strings.TrimSpace(line), "#"))

	// Split by comment first to handle inline comments in disabled entries
	commentParts := strings.SplitN(content, "#", 2)
	entryPart := strings.TrimSpace(commentParts[0])

	var inlineComment string
	if len(commentParts) > 1 {
		inlineComment = strings.TrimSpace(commentParts[1])
	}

	// Try to parse as a disabled entry
	parts := regexp.MustCompile(`\s+`).Split(entryPart, -1)
	if len(parts) < 2 {
		return nil, nil // This is just a standalone comment
	}

	ip := strings.TrimSpace(parts[0])
	if net.ParseIP(ip) == nil {
		return nil, nil // Not a valid IP, treat as standalone comment
	}

	hostname := strings.TrimSpace(parts[1])
	if err := validateHostname(hostname); err != nil {
		warning := &ParseWarning{
			Line:    lineNum,
			Message: fmt.Sprintf("invalid hostname in disabled entry: %v", err),
		}
		return nil, warning
	}

	// Create the entry
	entry := &HostEntry{
		IP:       ip,
		Hostname: hostname,
		Aliases:  make([]string, 0),
		Comment:  inlineComment,
		Active:   false, // Commented out = inactive
		Original: line,
	}

	// Parse aliases
	for i := 2; i < len(parts); i++ {
		alias := strings.TrimSpace(parts[i])
		if alias != "" {
			if err := validateHostname(alias); err == nil {
				entry.Aliases = append(entry.Aliases, alias)
			}
		}
	}

	return entry, nil
}

// parseStandaloneComment extracts a standalone comment
func parseStandaloneComment(line string) string {
	content := strings.TrimSpace(strings.TrimPrefix(strings.TrimSpace(line), "#"))
	return content
}

// parseEntryLine parses a regular (non-commented) entry line
func parseEntryLine(line string, lineNum int) (*HostEntry, *ParseWarning) {
	// Split by comment first
	parts := strings.SplitN(line, "#", 2)
	entryPart := strings.TrimSpace(parts[0])

	var comment string
	if len(parts) > 1 {
		comment = strings.TrimSpace(parts[1])
	}

	// Split the entry part by whitespace
	fields := regexp.MustCompile(`\s+`).Split(entryPart, -1)
	if len(fields) < 2 {
		return nil, &ParseWarning{
			Line:    lineNum,
			Message: "missing hostname",
		}
	}

	ip := strings.TrimSpace(fields[0])
	if net.ParseIP(ip) == nil {
		return nil, &ParseWarning{
			Line:    lineNum,
			Message: fmt.Sprintf("invalid IP address: %s", ip),
		}
	}

	hostname := strings.TrimSpace(fields[1])
	if err := validateHostname(hostname); err != nil {
		return nil, &ParseWarning{
			Line:    lineNum,
			Message: fmt.Sprintf("invalid hostname: %v", err),
		}
	}

	entry := &HostEntry{
		IP:       ip,
		Hostname: hostname,
		Aliases:  make([]string, 0),
		Comment:  comment,
		Active:   true,
		Original: line,
	}

	// Parse aliases
	for i := 2; i < len(fields); i++ {
		alias := strings.TrimSpace(fields[i])
		if alias != "" {
			if err := validateHostname(alias); err == nil {
				entry.Aliases = append(entry.Aliases, alias)
			}
		}
	}

	return entry, nil
}

// FormatHostsFile formats a hosts file with intelligent formatting
func FormatHostsFile(hostsFile *HostsFile) []string {
	var lines []string

	// Add standalone comments first (treating them as header comments)
	for _, comment := range hostsFile.Comments {
		lines = append(lines, "# "+comment)
	}

	// Calculate column widths for alignment
	ipWidth, hostWidth := calculateColumnWidths(hostsFile.Entries)

	// Format entries
	for _, entry := range hostsFile.Entries {
		line := formatEntry(entry, ipWidth, hostWidth)
		lines = append(lines, line)
	}

	return lines
}

// calculateColumnWidths determines optimal column widths for alignment
func calculateColumnWidths(entries []*HostEntry) (int, int) {
	maxIPWidth := 10
	maxHostWidth := 15

	for _, entry := range entries {
		if len(entry.IP) > maxIPWidth {
			maxIPWidth = len(entry.IP)
		}
		if len(entry.Hostname) > maxHostWidth {
			maxHostWidth = len(entry.Hostname)
		}
	}

	return maxIPWidth + 2, maxHostWidth + 2
}

// formatEntry formats a single entry with intelligent alignment
func formatEntry(entry *HostEntry, ipWidth, hostWidth int) string {
	var parts []string

	// Format IP address with padding
	parts = append(parts, fmt.Sprintf("%-*s", ipWidth, entry.IP))

	// Format hostname with padding
	parts = append(parts, fmt.Sprintf("%-*s", hostWidth, entry.Hostname))

	// Add aliases
	for _, alias := range entry.Aliases {
		parts = append(parts, alias)
	}

	line := strings.Join(parts, "\t")

	// Add comment if present
	if entry.Comment != "" {
		line += "\t# " + entry.Comment
	}

	// Add comment prefix if inactive
	if !entry.Active {
		line = "# " + line
	}

	return line
}

// WriteHostsFile writes a hosts file to the filesystem with intelligent formatting
func WriteHostsFile(filepath string, hostsFile *HostsFile) error {
	// Create backup before writing
	if _, err := BackupHostsFile(filepath); err != nil {
		return fmt.Errorf("failed to create backup: %w", err)
	}

	// Format the content
	lines := FormatHostsFile(hostsFile)
	content := strings.Join(lines, "\n") + "\n"

	// Write to temporary file first for atomic operation
	tmpPath := filepath + ".tmp"
	if err := os.WriteFile(tmpPath, []byte(content), 0644); err != nil {
		return fmt.Errorf("failed to write temporary file: %w", err)
	}

	// Atomic move
	if err := os.Rename(tmpPath, filepath); err != nil {
		os.Remove(tmpPath) // Clean up temp file
		return fmt.Errorf("failed to replace hosts file: %w", err)
	}

	return nil
}

// BackupHostsFile creates a backup of the hosts file
func BackupHostsFile(hostsPath string) (string, error) {
	// Create config directory
	homeDir, err := os.UserHomeDir()
	if err != nil {
		return "", fmt.Errorf("failed to get home directory: %w", err)
	}

	configDir := filepath.Join(homeDir, ".config", "hosts-go")
	if err := os.MkdirAll(configDir, 0755); err != nil {
		return "", fmt.Errorf("failed to create config directory: %w", err)
	}

	// Create backup filename with timestamp
	timestamp := time.Now().Format("20060102-150405")
	backupPath := filepath.Join(configDir, fmt.Sprintf("hosts.backup.%s", timestamp))

	// Copy the file
	content, err := os.ReadFile(hostsPath)
	if err != nil {
		return "", fmt.Errorf("failed to read original hosts file: %w", err)
	}

	if err := os.WriteFile(backupPath, content, 0644); err != nil {
		return "", fmt.Errorf("failed to write backup file: %w", err)
	}

	return backupPath, nil
}