package review

import (
	"bytes"
	"embed"
	"encoding/json"
	"fmt"
	"os"
	"sort"
	"strings"
	"unicode/utf8"

	"gopkg.in/yaml.v3"
)

//go:embed personas/*.yaml
var embeddedPersonas embed.FS

// MaxPersonaFileSize is the maximum size for persona files (64 KB).
// This prevents denial-of-service via excessively large files.
const MaxPersonaFileSize = 64 * 1024

// MaxYAMLDepth is the maximum nesting depth allowed in YAML persona files.
// This prevents stack exhaustion from deeply nested structures.
const MaxYAMLDepth = 20

// MaxYAMLNodes is the maximum number of YAML nodes allowed in persona files.
// This prevents DoS via wide-but-shallow structures that bypass depth limits.
const MaxYAMLNodes = 1000

// Persona defines a specialized review role with focused expertise.
type Persona struct {
	Name         string   `json:"name" yaml:"name"`
	DisplayName  string   `json:"display_name" yaml:"display_name"`
	ModelPref    string   `json:"model_preference,omitempty" yaml:"model_preference,omitempty"`
	Identity     string   `json:"identity" yaml:"identity"`
	Focus        []string `json:"focus" yaml:"focus"`
	Ignore       []string `json:"ignore" yaml:"ignore"`
	Severity     Severity `json:"severity" yaml:"severity"`
	OutputFormat string   `json:"output_format,omitempty" yaml:"output_format,omitempty"`
}

// Severity defines what constitutes each severity level for this persona.
// These are prompt guidance for the LLM, not output format changes.
type Severity struct {
	Major string `json:"major" yaml:"major"`
	Minor string `json:"minor" yaml:"minor"`
	Nit   string `json:"nit" yaml:"nit"`
}

// LoadPersona loads a persona from a JSON or YAML file path.
// Format is detected by file extension: .yaml/.yml for YAML, .json or other for JSON.
// Files larger than MaxPersonaFileSize are rejected.
//
// Symlinks are supported: os.Stat follows symlinks, so a symlink pointing to
// a regular file will pass the IsRegular() check. Symlinks to non-regular files
// (directories, FIFOs, devices) are still rejected.
func LoadPersona(path string) (*Persona, error) {
	// os.Stat follows symlinks, so symlinks to regular files are supported.
	// The IsRegular() check operates on the target, not the symlink itself.
	info, err := os.Stat(path)
	if err != nil {
		return nil, fmt.Errorf("read persona file %s: %w", path, err)
	}
	if !info.Mode().IsRegular() {
		return nil, fmt.Errorf("persona file %s is not a regular file", path)
	}
	if info.Size() > MaxPersonaFileSize {
		return nil, fmt.Errorf("persona file %s exceeds maximum size (%d bytes)", path, MaxPersonaFileSize)
	}
	data, err := os.ReadFile(path)
	if err != nil {
		return nil, fmt.Errorf("read persona file %s: %w", path, err)
	}
	// Re-check size after read to defend against TOCTOU races where file
	// grows between stat and read (e.g., appending process, replaced file).
	if len(data) > MaxPersonaFileSize {
		return nil, fmt.Errorf("persona file %s exceeds maximum size (%d bytes)", path, MaxPersonaFileSize)
	}
	return parsePersona(data, path)
}

// LoadBuiltinPersona loads a built-in persona by name.
// Returns an error if the persona doesn't exist.
// Built-in personas are stored in YAML format only (see embed directive).
func LoadBuiltinPersona(name string) (*Persona, error) {
	yamlFile := name + ".yaml"
	data, err := embeddedPersonas.ReadFile("personas/" + yamlFile)
	if err != nil {
		available := ListBuiltinPersonas()
		return nil, fmt.Errorf("unknown built-in persona %q (available: %s)", name, strings.Join(available, ", "))
	}
	return parsePersona(data, "builtin:"+yamlFile)
}

// ListBuiltinPersonas returns the names of all built-in personas in sorted order.
// Returns an empty slice if the embedded directory cannot be read.
func ListBuiltinPersonas() []string {
	entries, err := embeddedPersonas.ReadDir("personas")
	if err != nil {
		return []string{}
	}
	seen := make(map[string]bool)
	for _, e := range entries {
		if e.IsDir() {
			continue
		}
		name := e.Name()
		// Strip extension to get persona name
		var personaName string
		switch {
		case strings.HasSuffix(name, ".yaml"):
			personaName = strings.TrimSuffix(name, ".yaml")
		case strings.HasSuffix(name, ".yml"):
			personaName = strings.TrimSuffix(name, ".yml")
		case strings.HasSuffix(name, ".json"):
			personaName = strings.TrimSuffix(name, ".json")
		default:
			continue
		}
		if !seen[personaName] {
			seen[personaName] = true
		}
	}
	names := make([]string, 0, len(seen))
	for name := range seen {
		names = append(names, name)
	}
	sort.Strings(names)
	return names
}

// parsePersona parses persona data from JSON or YAML format.
// Format is detected by the source file extension.
func parsePersona(data []byte, source string) (*Persona, error) {
	lowerSource := strings.ToLower(source)
	isYAML := strings.HasSuffix(lowerSource, ".yaml") || strings.HasSuffix(lowerSource, ".yml")

	var p Persona
	var err error
	if isYAML {
		err = unmarshalYAMLWithDepthLimit(data, &p, MaxYAMLDepth)
	} else {
		// Use json.Decoder with DisallowUnknownFields for consistency with
		// YAML's KnownFields(true) - both reject unknown fields to catch typos.
		dec := json.NewDecoder(bytes.NewReader(data))
		dec.DisallowUnknownFields()
		err = dec.Decode(&p)
	}
	if err != nil {
		return nil, fmt.Errorf("parse persona %s: %w", source, err)
	}
	if err := validatePersona(&p, source); err != nil {
		return nil, err
	}
	return &p, nil
}

// unmarshalYAMLWithDepthLimit unmarshals YAML data with explicit depth limiting
// and strict field checking. This protects against stack exhaustion from deeply
// nested structures and catches typos in field names.
// Multi-document YAML files are rejected to prevent silent data loss.
func unmarshalYAMLWithDepthLimit(data []byte, out any, maxDepth int) error {
	// First pass: decode into a yaml.Node to check depth limits and node counts.
	// This prevents stack exhaustion before we attempt to decode into structs.
	var node yaml.Node
	dec := yaml.NewDecoder(bytes.NewReader(data))
	if err := dec.Decode(&node); err != nil {
		return err
	}

	// Reject multi-document YAML files - silently ignoring additional documents
	// could lead to confusing behavior where users think their changes take effect.
	var extra yaml.Node
	if dec.Decode(&extra) == nil {
		return fmt.Errorf("multi-document YAML is not supported; only single-document files are allowed")
	}

	nodeCount := 0
	if err := checkYAMLDepth(&node, 0, maxDepth, MaxYAMLNodes, make(map[*yaml.Node]struct{}), &nodeCount); err != nil {
		return err
	}

	// Second pass: decode with strict field checking enabled.
	// KnownFields(true) rejects unknown keys, catching typos like "focuss" or "identiy".
	// We must re-decode from the original data because yaml.Node.Decode() doesn't
	// support the KnownFields option.
	strictDec := yaml.NewDecoder(bytes.NewReader(data))
	strictDec.KnownFields(true)
	return strictDec.Decode(out)
}

// checkYAMLDepth recursively checks that YAML nodes don't exceed the depth limit
// or the total node count limit. It also detects alias cycles to prevent infinite
// recursion from crafted YAML with self-referential aliases.
func checkYAMLDepth(node *yaml.Node, depth, maxDepth, maxNodes int, seen map[*yaml.Node]struct{}, nodeCount *int) error {
	if depth > maxDepth {
		return fmt.Errorf("YAML nesting depth exceeds maximum (%d)", maxDepth)
	}

	// Track total nodes visited as defense-in-depth against wide-but-shallow attacks.
	*nodeCount++
	if *nodeCount > maxNodes {
		return fmt.Errorf("YAML node count exceeds maximum (%d)", maxNodes)
	}

	// Cycle detection: if we've seen this node before, we're in a cycle.
	if _, ok := seen[node]; ok {
		return nil // Already validated this subtree, skip to avoid infinite recursion.
	}
	seen[node] = struct{}{}

	// Handle alias nodes: follow the alias to its anchor target.
	// Increment depth when following aliases since they expand the effective structure.
	if node.Kind == yaml.AliasNode && node.Alias != nil {
		return checkYAMLDepth(node.Alias, depth+1, maxDepth, maxNodes, seen, nodeCount)
	}

	for _, child := range node.Content {
		if err := checkYAMLDepth(child, depth+1, maxDepth, maxNodes, seen, nodeCount); err != nil {
			return err
		}
	}
	return nil
}

func validatePersona(p *Persona, source string) error {
	if p.Name == "" {
		return fmt.Errorf("persona %s: name is required", source)
	}
	if p.Identity == "" {
		return fmt.Errorf("persona %s: identity is required", source)
	}
	// DisplayName defaults to Name if not set
	if p.DisplayName == "" {
		p.DisplayName = p.Name
	}
	return nil
}

// CapitalizeFirst capitalizes the first rune of a string in a Unicode-safe way.
// Returns the original string if it's empty.
func CapitalizeFirst(s string) string {
	if s == "" {
		return s
	}
	r, size := utf8.DecodeRuneInString(s)
	if r == utf8.RuneError {
		return s
	}
	return strings.ToUpper(string(r)) + s[size:]
}