package skills

import (
	"context"
	"fmt"
	"io"
	"net/http"
	"os"
	"path/filepath"
	"regexp"
	"sort"
	"strings"
	"time"

	"agentu/internal/safeio"
	"agentu/pkg/config"

	"gopkg.in/yaml.v3"
)

// DefaultDir is the default directory for global skills.
const DefaultDir = "~/.agentu/skills"

// SkillFileName is the required instruction file inside a Codex-style skill directory.
const SkillFileName = "SKILL.md"

// maxInstallDownloadBytes caps HTTP downloads at 1 MiB.
const maxInstallDownloadBytes = 1 << 20

// Skill is a single global Markdown skill.
type Skill struct {
	Name        string
	Description string
	Path        string
	Content     string
}

// InstallResult describes a successful skill installation.
type InstallResult struct {
	Skill     Skill
	TargetDir string
}

// validNameRe allows ASCII letters, digits, dots, underscores, and hyphens.
var validNameRe = regexp.MustCompile(`^[A-Za-z0-9._-]+$`)

// validateSkillName rejects empty, path-like, and otherwise unsafe names.
func validateSkillName(name string) error {
	if name == "" || name == "." || name == ".." {
		return fmt.Errorf("invalid skill name %q: use letters, numbers, dots, underscores, or hyphens", name)
	}
	if strings.ContainsAny(name, `/\`) {
		return fmt.Errorf("invalid skill name %q: use letters, numbers, dots, underscores, or hyphens", name)
	}
	if strings.ContainsAny(name, " \t\n\r") {
		return fmt.Errorf("invalid skill name %q: use letters, numbers, dots, underscores, or hyphens", name)
	}
	if !validNameRe.MatchString(name) {
		return fmt.Errorf("invalid skill name %q: use letters, numbers, dots, underscores, or hyphens", name)
	}
	return nil
}

// skillFrontmatter is the decoded YAML frontmatter.
type skillFrontmatter struct {
	Name        string `yaml:"name"`
	Description string `yaml:"description"`
}

// parseSkillFile reads raw bytes and returns the skill body, frontmatter name,
// and frontmatter description. hasFrontmatter indicates whether the source
// bytes started with a --- delimiter pair.
func parseSkillFile(path string, data []byte) (body string, fmName string, fmDesc string, hasFrontmatter bool, err error) {
	normalized := strings.ReplaceAll(string(data), "\r\n", "\n")
	lines := strings.Split(normalized, "\n")
	if len(lines) >= 2 && lines[0] == "---" {
		for i := 1; i < len(lines); i++ {
			if lines[i] == "---" {
				yamlBlock := strings.Join(lines[1:i], "\n")
				var fm skillFrontmatter
				if err := yaml.Unmarshal([]byte(yamlBlock), &fm); err != nil {
					return "", "", "", false, fmt.Errorf("parse skill %s: %w", path, err)
				}
				bodyText := strings.TrimSpace(strings.Join(lines[i+1:], "\n"))
				return bodyText, fm.Name, fm.Description, true, nil
			}
		}
		return "", "", "", false, fmt.Errorf("parse skill %s: missing closing ---", path)
	}
	return strings.TrimSpace(normalized), "", "", false, nil
}

// LoadGlobal loads global skills from DefaultDir.
func LoadGlobal() ([]Skill, error) {
	return LoadDir(DefaultDir)
}

// LoadDir loads skills from a directory. Missing directories return nil, nil.
func LoadDir(dir string) ([]Skill, error) {
	resolved, err := config.ResolvePath(dir)
	if err != nil {
		return nil, err
	}
	resolved, err = filepath.Abs(resolved)
	if err != nil {
		return nil, err
	}

	info, err := os.Stat(resolved)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, nil
		}
		return nil, fmt.Errorf("read skills directory: %w", err)
	}
	if !info.IsDir() {
		return nil, fmt.Errorf("skills path is not a directory: %s", resolved)
	}

	entries, err := os.ReadDir(resolved)
	if err != nil {
		return nil, fmt.Errorf("read skills directory: %w", err)
	}

	var skills []Skill
	for _, entry := range entries {
		name := entry.Name()
		if strings.HasPrefix(name, ".") {
			continue
		}

		entryPath := filepath.Join(resolved, name)

		// Shape 1: Codex-style directory with SKILL.md.
		isDir, err := isDirectory(entryPath)
		if err != nil {
			return nil, err
		}
		if isDir {
			skillFile := filepath.Join(entryPath, SkillFileName)
			fi, err := os.Stat(skillFile)
			if err != nil {
				if os.IsNotExist(err) {
					continue
				}
				return nil, fmt.Errorf("read skill %s: %w", skillFile, err)
			}
			if !fi.Mode().IsRegular() {
				continue
			}
			skill, err := loadSkillFile(name, skillFile, true)
			if err != nil {
				return nil, err
			}
			if skill != nil {
				skills = append(skills, *skill)
			}
			continue
		}

		// Shape 2: Legacy flat .md file.
		if !entry.Type().IsRegular() {
			continue
		}
		if !strings.EqualFold(filepath.Ext(name), ".md") {
			continue
		}
		skill, err := loadSkillFile(name[:len(name)-len(filepath.Ext(name))], entryPath, false)
		if err != nil {
			return nil, err
		}
		if skill != nil {
			skills = append(skills, *skill)
		}
	}

	sort.Slice(skills, func(i, j int) bool {
		if skills[i].Name != skills[j].Name {
			return skills[i].Name < skills[j].Name
		}
		return skills[i].Path < skills[j].Path
	})

	return skills, nil
}

// isDirectory reports whether path is a directory. For symlinks it resolves
// the target and checks the resolved type.
func isDirectory(path string) (bool, error) {
	fi, err := os.Lstat(path)
	if err != nil {
		if os.IsNotExist(err) {
			return false, nil
		}
		return false, fmt.Errorf("read skill %s: %w", path, err)
	}
	if fi.IsDir() {
		return true, nil
	}
	if fi.Mode()&os.ModeSymlink != 0 {
		resolved, err := filepath.EvalSymlinks(path)
		if err != nil {
			return false, nil
		}
		target, err := os.Stat(resolved)
		if err != nil {
			return false, nil
		}
		return target.IsDir(), nil
	}
	return false, nil
}

// loadSkillFile reads one skill markdown file and returns a Skill or nil when
// the instruction body is empty. legacyBaseName is used as the default name
// when frontmatter does not supply one and isDir is false.
func loadSkillFile(legacyBaseName string, path string, isDirSkill bool) (*Skill, error) {
	data, err := os.ReadFile(path)
	if err != nil {
		return nil, fmt.Errorf("read skill %s: %w", path, err)
	}
	body, fmName, fmDesc, _, err := parseSkillFile(path, data)
	if err != nil {
		return nil, err
	}
	if body == "" {
		return nil, nil
	}

	name := fmName
	if name == "" {
		if isDirSkill {
			name = filepath.Base(filepath.Dir(path))
		} else {
			name = legacyBaseName
		}
	}
	if err := validateSkillName(name); err != nil {
		return nil, err
	}

	return &Skill{
		Name:        name,
		Description: strings.TrimSpace(fmDesc),
		Path:        path,
		Content:     body,
	}, nil
}

// FormatForSystemPrompt formats skills for injection into the system prompt.
// Returns an empty string if items is empty.
func FormatForSystemPrompt(items []Skill) string {
	if len(items) == 0 {
		return ""
	}
	var b strings.Builder
	b.WriteString("Global skills:\n")
	b.WriteString("The following global skills are available for every conversation. Apply them when relevant.")
	for _, skill := range items {
		b.WriteString("\n\n### ")
		b.WriteString(skill.Name)
		b.WriteString("\nSource: ")
		b.WriteString(skill.Path)
		if skill.Description != "" {
			b.WriteString("\nDescription: ")
			b.WriteString(skill.Description)
		}
		b.WriteString("\n\n")
		b.WriteString(skill.Content)
	}
	return b.String()
}

// FormatList formats skills for slash-command output.
func FormatList(items []Skill) string {
	if len(items) == 0 {
		return "No global skills installed."
	}
	var b strings.Builder
	b.WriteString("Global skills:\n")
	for _, skill := range items {
		b.WriteString("- ")
		b.WriteString(skill.Name)
		if skill.Description != "" {
			b.WriteString(" — ")
			b.WriteString(skill.Description)
		}
		b.WriteString("\n  Source: ")
		b.WriteString(skill.Path)
		b.WriteString("\n")
	}
	return b.String()
}

// InstallGlobal installs a skill into DefaultDir.
func InstallGlobal(ctx context.Context, source string) (InstallResult, error) {
	return InstallDir(ctx, DefaultDir, source)
}

// InstallDir installs a Codex-style skill directory under the target root.
func InstallDir(ctx context.Context, dir string, source string) (InstallResult, error) {
	source = strings.TrimSpace(source)
	if source == "" {
		return InstallResult{}, fmt.Errorf("skill source is required")
	}

	resolvedRoot, err := config.ResolvePath(dir)
	if err != nil {
		return InstallResult{}, err
	}
	resolvedRoot, err = filepath.Abs(resolvedRoot)
	if err != nil {
		return InstallResult{}, err
	}

	// HTTP(S) source.
	if strings.HasPrefix(source, "http://") || strings.HasPrefix(source, "https://") {
		return installFromHTTP(ctx, resolvedRoot, source)
	}

	// Local source.
	resolvedSource, err := config.ResolvePath(source)
	if err != nil {
		return InstallResult{}, err
	}
	resolvedSource, err = filepath.Abs(resolvedSource)
	if err != nil {
		return InstallResult{}, err
	}

	fi, err := os.Stat(resolvedSource)
	if err != nil {
		return InstallResult{}, fmt.Errorf("read skill source: %w", err)
	}
	if fi.IsDir() {
		return installFromDirectory(ctx, resolvedRoot, resolvedSource)
	}
	if fi.Mode().IsRegular() && strings.EqualFold(filepath.Ext(resolvedSource), ".md") {
		return installFromFile(ctx, resolvedRoot, resolvedSource)
	}
	return InstallResult{}, fmt.Errorf("skill source must be a directory, .md file, or http(s) URL")
}

func installFromHTTP(ctx context.Context, root string, url string) (InstallResult, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return InstallResult{}, fmt.Errorf("download skill: %w", err)
	}
	client := &http.Client{Timeout: 30 * time.Second}
	resp, err := client.Do(req)
	if err != nil {
		return InstallResult{}, fmt.Errorf("download skill: %w", err)
	}
	defer resp.Body.Close()
	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		return InstallResult{}, fmt.Errorf("download skill: HTTP %d", resp.StatusCode)
	}
	limited := io.LimitReader(resp.Body, int64(maxInstallDownloadBytes)+1)
	data, err := io.ReadAll(limited)
	if err != nil {
		return InstallResult{}, fmt.Errorf("download skill: %w", err)
	}
	if len(data) > maxInstallDownloadBytes {
		return InstallResult{}, fmt.Errorf("skill file is too large: max %d bytes", maxInstallDownloadBytes)
	}
	return installMarkdownBytes(root, data, deriveNameFromURL(url))
}

func installFromDirectory(ctx context.Context, root string, sourceDir string) (InstallResult, error) {
	skillFile := filepath.Join(sourceDir, SkillFileName)
	fi, err := os.Stat(skillFile)
	if err != nil {
		return InstallResult{}, fmt.Errorf("read skill source: %w", err)
	}
	if !fi.Mode().IsRegular() {
		return InstallResult{}, fmt.Errorf("skill source must contain a regular %s file", SkillFileName)
	}
	data, err := os.ReadFile(skillFile)
	if err != nil {
		return InstallResult{}, fmt.Errorf("read skill source: %w", err)
	}

	body, fmName, _, _, err := parseSkillFile(skillFile, data)
	if err != nil {
		return InstallResult{}, err
	}
	_ = body
	name := fmName
	if name == "" {
		name = filepath.Base(sourceDir)
	}
	if err := validateSkillName(name); err != nil {
		return InstallResult{}, err
	}

	targetDir := filepath.Join(root, name)
	if err := createSkillTarget(root, targetDir); err != nil {
		return InstallResult{}, err
	}

	// Copy tree.
	if err := copyTree(ctx, sourceDir, targetDir); err != nil {
		os.RemoveAll(targetDir)
		return InstallResult{}, err
	}

	installedPath := filepath.Join(targetDir, SkillFileName)
	installedData, _ := os.ReadFile(installedPath)
	instBody, instName, instDesc, _, _ := parseSkillFile(installedPath, installedData)
	if instName == "" {
		instName = name
	}

	return InstallResult{
		Skill: Skill{
			Name:        instName,
			Description: strings.TrimSpace(instDesc),
			Path:        installedPath,
			Content:     instBody,
		},
		TargetDir: targetDir,
	}, nil
}

func installFromFile(ctx context.Context, root string, sourcePath string) (InstallResult, error) {
	data, err := os.ReadFile(sourcePath)
	if err != nil {
		return InstallResult{}, fmt.Errorf("read skill source: %w", err)
	}
	baseName := filepath.Base(sourcePath)
	baseNameNoExt := baseName[:len(baseName)-len(filepath.Ext(baseName))]
	// When the source is literally SKILL.md, use the parent directory name.
	if strings.EqualFold(baseName, SkillFileName) {
		baseNameNoExt = filepath.Base(filepath.Dir(sourcePath))
	}
	return installMarkdownBytes(root, data, baseNameNoExt)
}

// installMarkdownBytes parses markdown bytes and writes a Codex-style
// SKILL.md under <root>/<name>/. If the source had frontmatter, the original
// bytes are preserved. Otherwise a normalized header is prepended.
func installMarkdownBytes(root string, data []byte, fallbackName string) (InstallResult, error) {
	normalized := strings.ReplaceAll(string(data), "\r\n", "\n")
	lines := strings.Split(normalized, "\n")

	hasFrontmatter := false
	if len(lines) >= 2 && lines[0] == "---" {
		for i := 1; i < len(lines); i++ {
			if lines[i] == "---" {
				hasFrontmatter = true
				break
			}
		}
	}

	var body string
	var fmName string
	var fmDesc string
	var err error

	if hasFrontmatter {
		body, fmName, fmDesc, _, err = parseSkillFile("<source>", data)
		if err != nil {
			return InstallResult{}, err
		}
	} else {
		body = strings.TrimSpace(normalized)
	}
	name := fmName
	if name == "" {
		name = fallbackName
	}
	if err := validateSkillName(name); err != nil {
		return InstallResult{}, err
	}
	if body == "" {
		return InstallResult{}, fmt.Errorf("skill body is empty")
	}

	targetDir := filepath.Join(root, name)
	if err := createSkillTarget(root, targetDir); err != nil {
		return InstallResult{}, err
	}

	var writeData []byte
	if hasFrontmatter {
		writeData = data
	} else {
		var b strings.Builder
		b.WriteString("---\n")
		fmt.Fprintf(&b, "name: %s\n", name)
		b.WriteString("description: \"\"\n")
		b.WriteString("---\n\n")
		b.WriteString(body)
		b.WriteString("\n")
		writeData = []byte(b.String())
	}

	targetFile := filepath.Join(targetDir, SkillFileName)
	if err := safeio.AtomicWriteFile(targetFile, writeData, 0o644); err != nil {
		os.RemoveAll(targetDir)
		return InstallResult{}, err
	}

	return InstallResult{
		Skill: Skill{
			Name:        name,
			Description: strings.TrimSpace(fmDesc),
			Path:        targetFile,
			Content:     body,
		},
		TargetDir: targetDir,
	}, nil
}

func createSkillTarget(root, targetDir string) error {
	if err := os.MkdirAll(root, 0o755); err != nil {
		return fmt.Errorf("create skills directory: %w", err)
	}
	if err := os.Mkdir(targetDir, 0o755); err != nil {
		if os.IsExist(err) {
			return fmt.Errorf("skill already exists: %s", targetDir)
		}
		return fmt.Errorf("create skill directory: %w", err)
	}
	return nil
}

// deriveNameFromURL extracts a filename stem from a URL for use as a fallback name.
func deriveNameFromURL(rawURL string) string {
	// Strip query and fragment.
	u := rawURL
	if idx := strings.IndexAny(u, "?#"); idx >= 0 {
		u = u[:idx]
	}
	base := filepath.Base(u)
	ext := filepath.Ext(base)
	if ext != "" {
		base = base[:len(base)-len(ext)]
	}
	if base == "" || strings.EqualFold(base, "SKILL") {
		return "downloaded-skill"
	}
	return base
}

// copyTree copies srcDir to dstDir, rejecting symlinks inside the source tree.
func copyTree(ctx context.Context, srcDir string, dstDir string) error {
	return filepath.Walk(srcDir, func(path string, info os.FileInfo, err error) error {
		if err != nil {
			return err
		}
		if err := ctx.Err(); err != nil {
			return err
		}
		rel, err := filepath.Rel(srcDir, path)
		if err != nil {
			return err
		}
		target := filepath.Join(dstDir, rel)

		// Reject any symlink inside the source tree.
		lfi, lerr := os.Lstat(path)
		if lerr != nil {
			return lerr
		}
		if lfi.Mode()&os.ModeSymlink != 0 {
			return fmt.Errorf("skill source contains symlink: %s", path)
		}

		if lfi.IsDir() {
			return os.MkdirAll(target, 0o755)
		}
		if !lfi.Mode().IsRegular() {
			return nil
		}
		data, err := os.ReadFile(path)
		if err != nil {
			return err
		}
		return safeio.AtomicWriteFile(target, data, 0o644)
	})
}