using System.Text;
using Microsoft.CodeAnalysis;

namespace StellaOps.Microservice.SourceGen;

/// <summary>
/// Generates JSON Schema (draft 2020-12) from C# types at compile time.
/// </summary>
internal static class SchemaGenerator
{
    /// <summary>
    /// Generates a JSON Schema string from a type symbol.
    /// </summary>
    /// <param name="typeSymbol">The type to generate schema for.</param>
    /// <param name="compilation">The compilation context.</param>
    /// <returns>A JSON Schema string, or null if generation fails.</returns>
    public static string? GenerateSchema(ITypeSymbol typeSymbol, Compilation compilation)
    {
        if (typeSymbol is null)
            return null;

        try
        {
            var context = new SchemaContext(compilation);
            var schema = GenerateTypeSchema(typeSymbol, context, isRoot: true);
            return schema;
        }
        catch
        {
            return null;
        }
    }

    private static string GenerateTypeSchema(ITypeSymbol typeSymbol, SchemaContext context, bool isRoot)
    {
        var sb = new StringBuilder();
        sb.AppendLine("{");

        if (isRoot)
        {
            sb.AppendLine("  \"$schema\": \"https://json-schema.org/draft/2020-12/schema\",");
        }

        // Check for simple types first
        var simpleType = GetSimpleTypeSchema(typeSymbol);
        if (simpleType is not null)
        {
            sb.Append(simpleType);
            sb.AppendLine();
            sb.Append("}");
            return sb.ToString();
        }

        // Check for nullable
        if (typeSymbol.NullableAnnotation == NullableAnnotation.Annotated ||
            (typeSymbol is INamedTypeSymbol namedType && namedType.OriginalDefinition.SpecialType == SpecialType.System_Nullable_T))
        {
            var underlyingType = GetUnderlyingType(typeSymbol);
            if (underlyingType is not null)
            {
                var underlyingSimple = GetSimpleTypeSchema(underlyingType);
                if (underlyingSimple is not null)
                {
                    // Nullable simple type
                    var nullableSchema = underlyingSimple.Replace("\"type\":", "\"type\": [") + ", \"null\"]";
                    sb.Append(nullableSchema);
                    sb.AppendLine();
                    sb.Append("}");
                    return sb.ToString();
                }
            }
        }

        // Check for arrays/lists
        if (IsCollectionType(typeSymbol, out var elementType))
        {
            sb.AppendLine("  \"type\": \"array\",");
            if (elementType is not null)
            {
                var elementSchema = GetSimpleTypeSchema(elementType);
                if (elementSchema is not null)
                {
                    sb.AppendLine("  \"items\": {");
                    sb.Append("    ");
                    sb.AppendLine(elementSchema);
                    sb.AppendLine("  }");
                }
                else
                {
                    sb.AppendLine("  \"items\": { \"type\": \"object\" }");
                }
            }
            else
            {
                sb.AppendLine("  \"items\": {}");
            }
            sb.Append("}");
            return sb.ToString();
        }

        // Object type
        sb.AppendLine("  \"type\": \"object\",");

        // Get properties
        var properties = GetPublicProperties(typeSymbol);
        var requiredProps = new List<string>();

        if (properties.Count > 0)
        {
            sb.AppendLine("  \"properties\": {");

            var propIndex = 0;
            foreach (var prop in properties.OrderBy(p => p.Name))
            {
                var propSchema = GeneratePropertySchema(prop, context);
                var isRequired = IsPropertyRequired(prop);

                if (isRequired)
                {
                    requiredProps.Add(ToCamelCase(prop.Name));
                }

                sb.Append($"    \"{ToCamelCase(prop.Name)}\": ");
                sb.Append(propSchema);

                if (propIndex < properties.Count - 1)
                {
                    sb.AppendLine(",");
                }
                else
                {
                    sb.AppendLine();
                }
                propIndex++;
            }

            sb.AppendLine("  },");
        }

        // Required array
        if (requiredProps.Count > 0)
        {
            sb.Append("  \"required\": [");
            sb.Append(string.Join(", ", requiredProps.Select(p => $"\"{p}\"")));
            sb.AppendLine("],");
        }

        sb.AppendLine("  \"additionalProperties\": false");
        sb.Append("}");

        return sb.ToString();
    }

    private static string GeneratePropertySchema(IPropertySymbol prop, SchemaContext context)
    {
        var type = prop.Type;

        // Handle nullable types
        var isNullable = type.NullableAnnotation == NullableAnnotation.Annotated;
        if (isNullable)
        {
            type = GetUnderlyingType(type) ?? type;
        }

        var simpleSchema = GetSimpleTypeSchema(type);
        if (simpleSchema is not null)
        {
            if (isNullable)
            {
                // Convert "type": "X" to "type": ["X", "null"]
                return "{ " + MakeTypeNullable(simpleSchema) + " }";
            }
            return "{ " + simpleSchema + " }";
        }

        // Collections
        if (IsCollectionType(type, out var elementType))
        {
            var itemSchema = elementType is not null
                ? GetSimpleTypeSchema(elementType) ?? "\"type\": \"object\""
                : "\"type\": \"object\"";

            return $"{{ \"type\": \"array\", \"items\": {{ {itemSchema} }} }}";
        }

        // Complex object - just use object type for now
        return "{ \"type\": \"object\" }";
    }

    private static string? GetSimpleTypeSchema(ITypeSymbol type)
    {
        var fullName = type.ToDisplayString();

        return fullName switch
        {
            "string" or "System.String" => "\"type\": \"string\"",
            "int" or "System.Int32" => "\"type\": \"integer\"",
            "long" or "System.Int64" => "\"type\": \"integer\"",
            "short" or "System.Int16" => "\"type\": \"integer\"",
            "byte" or "System.Byte" => "\"type\": \"integer\"",
            "uint" or "System.UInt32" => "\"type\": \"integer\"",
            "ulong" or "System.UInt64" => "\"type\": \"integer\"",
            "ushort" or "System.UInt16" => "\"type\": \"integer\"",
            "sbyte" or "System.SByte" => "\"type\": \"integer\"",
            "float" or "System.Single" => "\"type\": \"number\"",
            "double" or "System.Double" => "\"type\": \"number\"",
            "decimal" or "System.Decimal" => "\"type\": \"number\"",
            "bool" or "System.Boolean" => "\"type\": \"boolean\"",
            "System.DateTime" or "System.DateTimeOffset" => "\"type\": \"string\", \"format\": \"date-time\"",
            "System.DateOnly" => "\"type\": \"string\", \"format\": \"date\"",
            "System.TimeOnly" or "System.TimeSpan" => "\"type\": \"string\", \"format\": \"time\"",
            "System.Guid" => "\"type\": \"string\", \"format\": \"uuid\"",
            "System.Uri" => "\"type\": \"string\", \"format\": \"uri\"",
            _ => null
        };
    }

    private static ITypeSymbol? GetUnderlyingType(ITypeSymbol type)
    {
        if (type is INamedTypeSymbol namedType)
        {
            if (namedType.OriginalDefinition.SpecialType == SpecialType.System_Nullable_T &&
                namedType.TypeArguments.Length > 0)
            {
                return namedType.TypeArguments[0];
            }
        }

        // For reference types with nullable annotation, just return the type itself
        return type;
    }

    private static bool IsCollectionType(ITypeSymbol type, out ITypeSymbol? elementType)
    {
        elementType = null;

        if (type is IArrayTypeSymbol arrayType)
        {
            elementType = arrayType.ElementType;
            return true;
        }

        if (type is INamedTypeSymbol namedType)
        {
            // Check for List<T>, IList<T>, IEnumerable<T>, ICollection<T>, etc.
            var fullName = namedType.OriginalDefinition.ToDisplayString();
            if ((fullName.StartsWith("System.Collections.Generic.List") ||
                 fullName.StartsWith("System.Collections.Generic.IList") ||
                 fullName.StartsWith("System.Collections.Generic.IEnumerable") ||
                 fullName.StartsWith("System.Collections.Generic.ICollection") ||
                 fullName.StartsWith("System.Collections.Generic.IReadOnlyList") ||
                 fullName.StartsWith("System.Collections.Generic.IReadOnlyCollection")) &&
                namedType.TypeArguments.Length > 0)
            {
                elementType = namedType.TypeArguments[0];
                return true;
            }
        }

        return false;
    }

    private static List<IPropertySymbol> GetPublicProperties(ITypeSymbol type)
    {
        var properties = new List<IPropertySymbol>();

        foreach (var member in type.GetMembers())
        {
            if (member is IPropertySymbol prop &&
                prop.DeclaredAccessibility == Accessibility.Public &&
                !prop.IsStatic &&
                !prop.IsIndexer &&
                prop.GetMethod is not null)
            {
                properties.Add(prop);
            }
        }

        return properties;
    }

    private static bool IsPropertyRequired(IPropertySymbol prop)
    {
        // Check for [Required] attribute
        foreach (var attr in prop.GetAttributes())
        {
            var attrName = attr.AttributeClass?.ToDisplayString();
            if (attrName == "System.ComponentModel.DataAnnotations.RequiredAttribute")
                return true;
        }

        // Non-nullable reference types are required
        if (prop.Type.IsReferenceType &&
            prop.NullableAnnotation != NullableAnnotation.Annotated &&
            prop.Type.NullableAnnotation != NullableAnnotation.Annotated)
        {
            return true;
        }

        return false;
    }

    private static string MakeTypeNullable(string simpleSchema)
    {
        // Convert "type": "X" to "type": ["X", "null"]
        // Input: "type": "string"
        // Output: "type": ["string", "null"]
        const string typePrefix = "\"type\": \"";
        var idx = simpleSchema.IndexOf(typePrefix, StringComparison.Ordinal);
        if (idx < 0)
            return simpleSchema;

        var startOfType = idx + typePrefix.Length;
        var endOfType = simpleSchema.IndexOf('"', startOfType);
        if (endOfType < 0)
            return simpleSchema;

        var typeName = simpleSchema.Substring(startOfType, endOfType - startOfType);
        var rest = simpleSchema.Substring(endOfType + 1);
        return $"\"type\": [\"{typeName}\", \"null\"]{rest}";
    }

    private static string ToCamelCase(string name)
    {
        if (string.IsNullOrEmpty(name))
            return name;

        if (name.Length == 1)
            return name.ToLowerInvariant();

        return char.ToLowerInvariant(name[0]) + name.Substring(1);
    }

    private sealed class SchemaContext
    {
        public Compilation Compilation { get; }
        public Dictionary<string, string> Definitions { get; } = new();

        public SchemaContext(Compilation compilation)
        {
            Compilation = compilation;
        }
    }
}