using System;
using System.IO;

using Org.BouncyCastle.Math;
using Org.BouncyCastle.Utilities;

namespace Org.BouncyCastle.Asn1
{
    public class DerInteger
        : Asn1Object
    {
        internal class Meta : Asn1UniversalType
        {
            internal static readonly Asn1UniversalType Instance = new Meta();

            private Meta() : base(typeof(DerInteger), Asn1Tags.Integer) {}

            internal override Asn1Object FromImplicitPrimitive(DerOctetString octetString)
            {
                return CreatePrimitive(octetString.GetOctets());
            }
        }

        public const string AllowUnsafeProperty = "Org.BouncyCastle.Asn1.AllowUnsafeInteger";

        public static readonly DerInteger Zero = new DerInteger(0);
        public static readonly DerInteger One = new DerInteger(1);
        public static readonly DerInteger Two = new DerInteger(2);
        public static readonly DerInteger Three = new DerInteger(3);
        public static readonly DerInteger Four = new DerInteger(4);
        public static readonly DerInteger Five = new DerInteger(5);

        internal static bool AllowUnsafe()
        {
            string allowUnsafeValue = Platform.GetEnvironmentVariable(AllowUnsafeProperty);
            return allowUnsafeValue != null && Platform.EqualsIgnoreCase("true", allowUnsafeValue);
        }

        internal const int SignExtSigned = -1;
        internal const int SignExtUnsigned = 0xFF;

        private readonly byte[] bytes;
        private readonly int start;

        /**
         * return an integer from the passed in object
         *
         * @exception ArgumentException if the object cannot be converted.
         */
        public static DerInteger GetInstance(object obj)
        {
            if (obj == null)
                return null;

            if (obj is DerInteger derInteger)
                return derInteger;

            if (obj is IAsn1Convertible asn1Convertible)
            {
                if (!(obj is Asn1Object) && asn1Convertible.ToAsn1Object() is DerInteger converted)
                    return converted;
            }
            else if (obj is byte[] bytes)
            {
                try
                {
                    return (DerInteger)Meta.Instance.FromByteArray(bytes);
                }
                catch (IOException e)
                {
                    throw new ArgumentException("failed to construct integer from byte[]: " + e.Message);
                }
            }

            throw new ArgumentException("illegal object in GetInstance: " + Platform.GetTypeName(obj));
        }

        /**
         * return an Integer from a tagged object.
         *
         * @param taggedObject the tagged object holding the object we want
         * @param declaredExplicit true if the object is meant to be explicitly tagged false otherwise.
         * @exception ArgumentException if the tagged object cannot  be converted.
         */
        public static DerInteger GetInstance(Asn1TaggedObject taggedObject, bool declaredExplicit)
        {
            return (DerInteger)Meta.Instance.GetContextInstance(taggedObject, declaredExplicit);
        }

        public static DerInteger GetOptional(Asn1Encodable element)
        {
            if (element == null)
                throw new ArgumentNullException(nameof(element));

            if (element is DerInteger existing)
                return existing;

            return null;
        }

        public static DerInteger GetTagged(Asn1TaggedObject taggedObject, bool declaredExplicit)
        {
            return (DerInteger)Meta.Instance.GetTagged(taggedObject, declaredExplicit);
        }

        public DerInteger(int value)
        {
            this.bytes = BigInteger.ValueOf(value).ToByteArray();
            this.start = 0;
        }

        public DerInteger(long value)
        {
            this.bytes = BigInteger.ValueOf(value).ToByteArray();
            this.start = 0;
        }

		public DerInteger(BigInteger value)
        {
            if (value == null)
                throw new ArgumentNullException("value");

			this.bytes = value.ToByteArray();
            this.start = 0;
        }

        public DerInteger(byte[] bytes)
            : this(bytes, true)
        {
        }

        internal DerInteger(byte[] bytes, bool clone)
        {
            if (IsMalformed(bytes))
                throw new ArgumentException("malformed integer", "bytes");

            this.bytes = clone ? Arrays.Clone(bytes) : bytes;
            this.start = SignBytesToSkip(bytes);
        }

        /**
         * in some cases positive values Get crammed into a space,
         * that's not quite big enough...
         */
        public BigInteger PositiveValue
        {
            get { return new BigInteger(1, bytes); }
        }

        public BigInteger Value
        {
            get { return new BigInteger(bytes); }
        }

        public bool HasValue(int x)
        {
            return (bytes.Length - start) <= 4
                && IntValue(bytes, start, SignExtSigned) == x;
        }

        public bool HasValue(long x)
        {
            return (bytes.Length - start) <= 8
                && LongValue(bytes, start, SignExtSigned) == x;
        }

        public bool HasValue(BigInteger x)
        {
            return null != x
                // Fast check to avoid allocation
                && IntValue(bytes, start, SignExtSigned) == x.IntValue
                && Value.Equals(x);
        }

        public int IntPositiveValueExact
        {
            get
            {
                int count = bytes.Length - start;
                if (count > 4 || (count == 4 && 0 != (bytes[start] & 0x80)))
                    throw new ArithmeticException("ASN.1 Integer out of positive int range");

                return IntValue(bytes, start, SignExtUnsigned);
            }
        }

        public int IntValueExact
        {
            get
            {
                int count = bytes.Length - start;
                if (count > 4)
                    throw new ArithmeticException("ASN.1 Integer out of int range");

                return IntValue(bytes, start, SignExtSigned);
            }
        }

        public long LongValueExact
        {
            get
            {
                int count = bytes.Length - start;
                if (count > 8)
                    throw new ArithmeticException("ASN.1 Integer out of long range");

                return LongValue(bytes, start, SignExtSigned);
            }
        }

        public bool TryGetIntPositiveValueExact(out int value)
        {
            int count = bytes.Length - start;
            if (count > 4 || (count == 4 && 0 != (bytes[start] & 0x80)))
            {
                value = default;
                return false;
            }

            value = IntValue(bytes, start, SignExtUnsigned);
            return true;
        }

        public bool TryGetIntValueExact(out int value)
        {
            int count = bytes.Length - start;
            if (count > 4)
            {
                value = default;
                return false;
            }

            value = IntValue(bytes, start, SignExtSigned);
            return true;
        }

        public bool TryGetLongValueExact(out long value)
        {
            int count = bytes.Length - start;
            if (count > 8)
            {
                value = default;
                return false;
            }

            value = LongValue(bytes, start, SignExtSigned);
            return true;
        }

        internal override IAsn1Encoding GetEncoding(int encoding)
        {
            return new PrimitiveEncoding(Asn1Tags.Universal, Asn1Tags.Integer, bytes);
        }

        internal override IAsn1Encoding GetEncodingImplicit(int encoding, int tagClass, int tagNo)
        {
            return new PrimitiveEncoding(tagClass, tagNo, bytes);
        }

        internal sealed override DerEncoding GetEncodingDer()
        {
            return new PrimitiveDerEncoding(Asn1Tags.Universal, Asn1Tags.Integer, bytes);
        }

        internal sealed override DerEncoding GetEncodingDerImplicit(int tagClass, int tagNo)
        {
            return new PrimitiveDerEncoding(tagClass, tagNo, bytes);
        }

        protected override int Asn1GetHashCode()
		{
			return Arrays.GetHashCode(bytes);
        }

		protected override bool Asn1Equals(Asn1Object asn1Object)
		{
			DerInteger other = asn1Object as DerInteger;
			if (other == null)
				return false;

            return Arrays.AreEqual(this.bytes, other.bytes);
        }

		public override string ToString()
		{
			return Value.ToString();
		}

        internal static DerInteger CreatePrimitive(byte[] contents)
        {
            return new DerInteger(contents, false);
        }

        internal static int GetEncodingLength(BigInteger x)
        {
            return Asn1OutputStream.GetLengthOfEncodingDL(Asn1Tags.Integer, BigIntegers.GetByteLength(x));
        }

        internal static int IntValue(byte[] bytes, int start, int signExt)
        {
            int length = bytes.Length;
            int pos = System.Math.Max(start, length - 4);

            int val = (sbyte)bytes[pos] & signExt;
            while (++pos < length)
            {
                val = (val << 8) | bytes[pos];
            }
            return val;
        }

        internal static long LongValue(byte[] bytes, int start, int signExt)
        {
            int length = bytes.Length;
            int pos = System.Math.Max(start, length - 8);

            long val = (sbyte)bytes[pos] & signExt;
            while (++pos < length)
            {
                val = (val << 8) | bytes[pos];
            }
            return val;
        }

        /**
         * Apply the correct validation for an INTEGER primitive following the BER rules.
         *
         * @param bytes The raw encoding of the integer.
         * @return true if the (in)put fails this validation.
         */
        internal static bool IsMalformed(byte[] bytes)
        {
            switch (bytes.Length)
            {
            case 0:
                return true;
            case 1:
                return false;
            default:
                return (sbyte)bytes[0] == ((sbyte)bytes[1] >> 7) && !AllowUnsafe();
            }
        }

        internal static int SignBytesToSkip(byte[] bytes)
        {
            int pos = 0, last = bytes.Length - 1;
            while (pos < last
                && (sbyte)bytes[pos] == ((sbyte)bytes[pos + 1] >> 7))
            {
                ++pos;
            }
            return pos;
        }
    }
}