using System.Text.RegularExpressions;
using Agent.Core.Skills;
using YamlDotNet.Serialization;
using YamlDotNet.Serialization.NamingConventions;

namespace Agent.App.Configuration;

public static class SkillLoader
{
    private static readonly Regex NamePattern = new("^[a-z0-9]([a-z0-9-]*[a-z0-9])?$", RegexOptions.Compiled);

    public static IReadOnlyList<AgentSkill> Load(string directory)
    {
        if (!Directory.Exists(directory)) return [];

        var skills = new List<AgentSkill>();
        var names = new HashSet<string>(StringComparer.Ordinal);

        foreach (string child in Directory.EnumerateDirectories(directory, "*", SearchOption.TopDirectoryOnly))
        {
            var info = new DirectoryInfo(child);
            if ((info.Attributes & FileAttributes.ReparsePoint) != 0) continue;

            string skillMd = Path.Combine(info.FullName, "SKILL.md");
            if (!File.Exists(skillMd)) continue;

            string dirName = info.Name;
            string content = File.ReadAllText(skillMd);
            var (frontmatter, body) = SplitFrontmatter(content);

            if (frontmatter is null)
                throw new SkillLoadException($"Skill '{dirName}': SKILL.md is missing YAML frontmatter.");

            SkillFrontmatter fm = ParseFrontmatter(frontmatter);

            ValidateSkill(dirName, fm, names);

            skills.Add(new AgentSkill(
                RootPath: info.FullName,
                Name: fm.Name,
                Description: fm.Description,
                Instructions: body.Trim(),
                License: fm.License,
                Compatibility: fm.Compatibility,
                Metadata: fm.Metadata,
                AllowedTools: fm.AllowedTools));
        }

        return skills;
    }

    private static void ValidateSkill(string dirName, SkillFrontmatter fm, HashSet<string> names)
    {
        if (string.IsNullOrWhiteSpace(fm.Name))
            throw new SkillLoadException($"Skill '{dirName}': name is required.");
        if (fm.Name.Length < 1 || fm.Name.Length > 64)
            throw new SkillLoadException($"Skill '{dirName}': name must be 1-64 characters.");
        if (!NamePattern.IsMatch(fm.Name))
            throw new SkillLoadException($"Skill '{dirName}': name must be lowercase alphanumeric with hyphens.");
        if (!fm.Name.Equals(dirName, StringComparison.Ordinal))
            throw new SkillLoadException($"Skill '{dirName}': name '{fm.Name}' does not match directory name.");
        if (string.IsNullOrWhiteSpace(fm.Description))
            throw new SkillLoadException($"Skill '{dirName}': description is required.");
        if (fm.Description.Length > 1024)
            throw new SkillLoadException($"Skill '{dirName}': description must be 1-1024 characters.");
        if (!names.Add(fm.Name))
            throw new SkillLoadException($"Duplicate skill name '{fm.Name}'.");
    }

    private static (string? Frontmatter, string Body) SplitFrontmatter(string content)
    {
        if (!content.StartsWith("---", StringComparison.Ordinal)) return (null, content);
        int end = content.IndexOf("\n---", 3, StringComparison.Ordinal);
        if (end < 0) return (null, content);
        string frontmatter = content[3..end].Trim();
        int bodyStart = end + 4;
        if (bodyStart < content.Length && content[bodyStart] == '\r') bodyStart++;
        if (bodyStart < content.Length && content[bodyStart] == '\n') bodyStart++;
        string body = bodyStart < content.Length ? content[bodyStart..] : string.Empty;
        return (frontmatter, body);
    }

    private static SkillFrontmatter ParseFrontmatter(string yaml)
    {
        try
        {
            var deserializer = new DeserializerBuilder()
                .WithNamingConvention(UnderscoredNamingConvention.Instance)
                .Build();
            return deserializer.Deserialize<SkillFrontmatter>(yaml);
        }
        catch (Exception ex)
        {
            throw new SkillLoadException($"Malformed YAML frontmatter: {ex.Message}");
        }
    }

    private sealed class SkillFrontmatter
    {
        public string Name { get; set; } = "";
        public string Description { get; set; } = "";
        public string? License { get; set; }
        public string? Compatibility { get; set; }
        public Dictionary<string, string>? Metadata { get; set; }
        public string? AllowedTools { get; set; }
    }
}

public sealed class SkillLoadException(string message) : Exception(message);