package main

import (
	"fmt"
	"os"
	"strings"
)

func applyMarkerPatch(targetFilePath string, modifications []string) error {
	// Parse patch content
	var marker string
	var entriesToAdd []string
	var entriesToRemove []string
	collecting := false

	for _, line := range modifications {
		switch {
		case strings.HasPrefix(line, "#"):
			// Capture marker line (first # line only)
			if marker == "" {
				marker = strings.TrimSpace(strings.TrimPrefix(line, "#"))
				collecting = true
			}
		case collecting && strings.HasPrefix(line, "+"):
			entriesToAdd = append(entriesToAdd, strings.TrimPrefix(line, "+"))
		case collecting && strings.HasPrefix(line, "-"):
			entriesToRemove = append(entriesToRemove, strings.TrimPrefix(line, "-"))
		}
	}

	// Validate input
	if marker == "" {
		return fmt.Errorf("missing marker line starting with #")
	}

	// Read entire file
	content, err := os.ReadFile(targetFilePath)
	if err != nil {
		return fmt.Errorf("failed to read file: %v", err)
	}

	lines := strings.Split(string(content), "\n")
	var modifiedLines []string
	markerFound := false

	for _, line := range lines {
		// Check if current line is our marker
		if !markerFound && strings.TrimSpace(line) == marker {
			markerFound = true
			modifiedLines = append(modifiedLines, line)
			// Add new entries immediately after marker
			modifiedLines = append(modifiedLines, entriesToAdd...)
			continue
		}

		// Check if line should be removed
		keep := true
		for _, removeLine := range entriesToRemove {
			if line == removeLine {
				keep = false
				break
			}
		}

		if keep {
			modifiedLines = append(modifiedLines, line)
		}
	}

	if !markerFound {
		return fmt.Errorf("marker line not found: %q", marker)
	}

	// Write directly to target file
	err = os.WriteFile(targetFilePath, []byte(strings.Join(modifiedLines, "\n")), 0644)
	if err != nil {
		return fmt.Errorf("failed to write changes: %v", err)
	}

	return nil
}