//-----------------------------------------------------------------------------
//
// Copyright (C) Microsoft Corporation.  All Rights Reserved.
//
//-----------------------------------------------------------------------------
using System;
using System.Text;


namespace Microsoft.Basetypes {
  using Microsoft.Contracts;
  using BIM = System.Numerics.BigInteger;
  
  /// <summary>
  /// A thin wrapper around System.Numerics.BigInteger
  /// (to be able to define equality, etc. properly)
  /// </summary>
  public struct BigNum {
    
    // the internal representation
    [Rep] internal readonly System.Numerics.BigInteger val;
    public static readonly BigNum ZERO = new BigNum (BIM.Zero);
    public static readonly BigNum ONE  = new BigNum (BIM.One);
    public static readonly BigNum MINUS_ONE = new BigNum (-BIM.One);

    [Pure]
    public static BigNum FromInt(int v) {
      return new BigNum(new BIM(v));
    }

    [Pure]
    public static BigNum FromUInt(uint v) {
      return new BigNum(new BIM((long)v));
    }

    [Pure]
    public static BigNum FromLong(long v) {
      return new BigNum(new BIM(v));
    }

    [Pure]
    public static BigNum FromBigInt(System.Numerics.BigInteger v) {
      return new BigNum(v);
    }

    [Pure]
    public static BigNum FromULong(ulong v) {
      return FromString("" + v);
    }

    [Pure]
    public static BigNum FromString(string v) {
      try {
        return new BigNum(BIM.Parse(v));
      } catch (System.ArgumentException) {
        throw new FormatException();
      }
    }

    public static bool TryParse(string v, out BigNum res) {
      try {
	    res = BigNum.FromString(v);
        return true;
	  } catch (FormatException) {
	    res = ZERO;
	    return false;
      }
    }

    // Convert to int, without checking whether overflows occur
    public int ToInt {
      get {
        return (int)val;
      }
    }

    // Convert to int; assert that no overflows occur
    public int ToIntSafe {
      get {
        assert this.InInt32;
        return this.ToInt;
      }
    }

    public Rational ToRational {
      get {
        return new Rational (this, BigNum.ONE);
      }
	}

    internal BigNum(System.Numerics.BigInteger val) {
      this.val = val;
    }
    
    public static bool operator==(BigNum x, BigNum y) {
      return (x.val == y.val);
    }

    public static bool operator!=(BigNum x, BigNum y) {
      return !(x.val == y.val);
    }

    [Pure][Reads(ReadsAttribute.Reads.Nothing)]
    public override bool Equals(object obj) {
      if (obj == null) return false;
      if (!(obj is BigNum)) return false;
      
      BigNum other = (BigNum)obj;
      return (this.val == other.val);
    }
    
    [Pure]
    public override int GetHashCode() {
      return this.val.GetHashCode();
    }
    
    [Pure]
    public override string! ToString() {
      return (!)val.ToString();
    }

    //////////////////////////////////////////////////////////////////////////////
    // Very limited support for format strings
    // Note: Negative integers are linearised with a minus "-" in hexadecimal,
    // not in 2-complement notation (in contrast to what the method
    // int32.ToString(format) does) 

    [Pure]
	public string! ToString(string! format) {
        if (format.StartsWith("d") || format.StartsWith("D")) {
            string! res = this.Abs.ToString();
			return addMinus(this.Signum,
                            prefixWithZeros(extractPrecision(format), res));
        } else if (format.StartsWith("x") || format.StartsWith("X")) {
            string! res = this.toHex(format.Substring(0, 1));
			return addMinus(this.Signum,
                            prefixWithZeros(extractPrecision(format), res));
        } else {
			throw new FormatException("Format " + format + " is not supported");
        }
	}

    private static readonly System.Numerics.BigInteger BI_2_TO_24 = new BIM(0x1000000);    

    [Pure]
    private string! toHex(string! format) {
	    string! res = "";
        System.Numerics.BigInteger rem = this.Abs.val;

        while (rem > BIM.Zero) {
          res = ((int)(rem %BI_2_TO_24)).ToString(format) + res;
          rem = rem / BI_2_TO_24;
        }

        return res;
    }

    [Pure]
    private int extractPrecision(string! format) {
        if (format.Length > 1)
          // will throw a FormatException if the precision is invalid;
          // that is ok
          return Int32.Parse(format.Substring(1));
        // always output at least one digit
        return 1;
    }

    [Pure]
    private string! addMinus(int signum, string! suffix) {
        if (signum < 0)
          return "-" + suffix;
        return suffix;
    }

    [Pure]
    private string! prefixWithZeros(int minLength, string! suffix) {
        StringBuilder res = new StringBuilder();
        while (res.Length + suffix.Length < minLength)
          res.Append("0");
        res.Append(suffix);
        return res.ToString();
    }

    ////////////////////////////////////////////////////////////////////////////
    // Basic arithmetic operations
    
    public BigNum Abs {
      get {
        return new BigNum(BIM.Abs(this.val));
      }
    }

    public BigNum Neg {
      get {
        return new BigNum(-this.val);
      }
    }

    [Pure]
    public static BigNum operator-(BigNum x) {
      return x.Neg;
    }

    [Pure]
    public static BigNum operator+(BigNum x, BigNum y) {
      return new BigNum(x.val + y.val);
    }

    [Pure]
    public static BigNum operator-(BigNum x, BigNum y) {
      return new BigNum(x.val - y.val);
    }

    [Pure]
    public static BigNum operator*(BigNum x, BigNum y) {
      return new BigNum(x.val * y.val);
    }

    // TODO: check that this has a proper semantics (which? :-))
    [Pure]
    public static BigNum operator/(BigNum x, BigNum y) {
      return new BigNum(x.val / y.val);
    }

    // TODO: check that this has a proper semantics (which? :-))
    [Pure]
    public static BigNum operator%(BigNum x, BigNum y) {
      return new BigNum(x.val - ((x.val / y.val) * y.val));
    }

    [Pure]
    public BigNum Min(BigNum that) {
      return new BigNum(this.val <= that.val ? this.val : that.val);
    }

    [Pure]
    public BigNum Max(BigNum that) {
      return new BigNum(this.val >= that.val ? this.val : that.val);
    }

    /// <summary>
    /// Returns the greatest common divisor of this and _y.
    /// </summary>
    /// <param name="_y"></param>
    /// <returns></returns>
    public BigNum Gcd(BigNum _y)
        ensures !result.IsNegative;
    {
      BigNum x = this.Abs;
      BigNum y = _y.Abs;

      while (true)
      {
        if (x < y)
        {
          y = y % x;
          if (y.IsZero)
          {
            return x;
          }
        }
        else
        {
          x = x % y;
          if (x.IsZero)
          {
            return y;
          }
        }
      }
      return ZERO; // make compiler shut up
    }

    ////////////////////////////////////////////////////////////////////////////
    // Some basic comparison operations

    public int Signum {
	  get {
		return this.val.Sign;
	  }
    }

    public bool IsPositive {
      get {
        return (this.val > BIM.Zero);
      }
    }

    public bool IsNegative {
      get {
        return (this.val < BIM.Zero);
      }
    }

    public bool IsZero {
      get {
        return this.val.IsZero;
      }
    }

    [Pure]
    public int CompareTo(BigNum that) {
	  if (this.val == that.val) return 0;
	  if (this.val < that.val) return -1;
	  return 1;
    }

    [Pure]
    public static bool operator<(BigNum x, BigNum y) {
      return (x.val < y.val);
    }

    [Pure]
    public static bool operator>(BigNum x, BigNum y) {
      return (x.val > y.val);
    }

    [Pure]
    public static bool operator<=(BigNum x, BigNum y) {
      return (x.val <= y.val);
    }

    [Pure]
    public static bool operator>=(BigNum x, BigNum y) {
      return (x.val >= y.val);
    }

    
    private static readonly System.Numerics.BigInteger MaxInt32 =
      new BIM(Int32.MaxValue);
    private static readonly System.Numerics.BigInteger MinInt32 =
      new BIM(Int32.MinValue);
 
    public bool InInt32 {
      get {
        return (val >= MinInt32) && (val <= MaxInt32);
      }
    }
  }
}