/*
 * Copyright 2009 Google Inc.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements. See the NOTICE file distributed with this
 * work for additional information regarding copyright ownership. The ASF
 * licenses this file to You under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 * 
 * INCLUDES MODIFICATIONS BY RICHARD ZSCHECH AS WELL AS GOOGLE.
 */
package java.math;

/**
 * Static library that provides the basic arithmetic mutable operations for
 * {@link BigInteger}. The operations provided are listed below. <ul
 * type="circle"> <li>Addition.</li> <li>Subtraction.</li> <li>Comparison.</li>
 * </ul> In addition to this, some <i><b>Inplace</b></i> (mutable) methods are
 * provided.
 */
class Elementary {

  /**
   * @see BigInteger#add(BigInteger) .
   * @param op1
   * @param op2
   * @return
   */
  static BigInteger add(BigInteger op1, BigInteger op2) {
    int resDigits[];
    int resSign;
    int op1Sign = op1.sign;
    int op2Sign = op2.sign;

    if (op1Sign == 0) {
      return op2;
    }
    if (op2Sign == 0) {
      return op1;
    }
    int op1Len = op1.numberLength;
    int op2Len = op2.numberLength;

    if (op1Len + op2Len == 2) {
      long a = (op1.digits[0] & 0xFFFFFFFFL);
      long b = (op2.digits[0] & 0xFFFFFFFFL);
      long res;
      int valueLo;
      int valueHi;

      if (op1Sign == op2Sign) {
        res = a + b;
        valueLo = (int) res;
        valueHi = (int) (res >>> 32);
        return ((valueHi == 0) ? new BigInteger(op1Sign, valueLo)
            : new BigInteger(op1Sign, 2, new int[] {valueLo, valueHi}));
      }
      return BigInteger.valueOf((op1Sign < 0) ? (b - a) : (a - b));
    } else if (op1Sign == op2Sign) {
      resSign = op1Sign;
      // an augend should not be shorter than addend
      resDigits = (op1Len >= op2Len) ? add(op1.digits, op1Len, op2.digits,
          op2Len) : add(op2.digits, op2Len, op1.digits, op1Len);
    } else { // signs are different
      int cmp = ((op1Len != op2Len) ? ((op1Len > op2Len) ? 1 : -1)
          : compareArrays(op1.digits, op2.digits, op1Len));

      if (cmp == BigInteger.EQUALS) {
        return BigInteger.ZERO;
      }
      // a minuend should not be shorter than subtrahend
      if (cmp == BigInteger.GREATER) {
        resSign = op1Sign;
        resDigits = subtract(op1.digits, op1Len, op2.digits, op2Len);
      } else {
        resSign = op2Sign;
        resDigits = subtract(op2.digits, op2Len, op1.digits, op1Len);
      }
    }
    BigInteger res = new BigInteger(resSign, resDigits.length, resDigits);
    res.cutOffLeadingZeroes();
    return res;
  }

  /**
   * Compares two arrays. All elements are treated as unsigned integers. The
   * magnitude is the bit chain of elements in big-endian order.
   * 
   * @param a the first array
   * @param b the second array
   * @param size the size of arrays
   * @return 1 if a > b, -1 if a < b, 0 if a == b
   */
  static int compareArrays(final int[] a, final int[] b, final int size) {
    int i;
    for (i = size - 1; (i >= 0) && (a[i] == b[i]); i--) {
      // empty
    }
    return ((i < 0) ? BigInteger.EQUALS
        : (a[i] & 0xFFFFFFFFL) < (b[i] & 0xFFFFFFFFL) ? BigInteger.LESS
            : BigInteger.GREATER);
  }

  /**
   * Same as @link #inplaceAdd(BigInteger, BigInteger), but without the
   * restriction of non-positive values.
   * 
   * @param op1 any number
   * @param op2 any number
   */
  static void completeInPlaceAdd(BigInteger op1, BigInteger op2) {
    if (op1.sign == 0) {
      System.arraycopy(op2.digits, 0, op1.digits, 0, op2.numberLength);
    } else if (op2.sign == 0) {
      return;
    } else if (op1.sign == op2.sign) {
      add(op1.digits, op1.digits, op1.numberLength, op2.digits,
          op2.numberLength);
    } else {
      int sign = unsignedArraysCompare(op1.digits, op2.digits,
          op1.numberLength, op2.numberLength);
      if (sign > 0) {
        subtract(op1.digits, op1.digits, op1.numberLength, op2.digits,
            op2.numberLength);
      } else {
        inverseSubtract(op1.digits, op1.digits, op1.numberLength, op2.digits,
            op2.numberLength);
        op1.sign = -op1.sign;
      }
    }
    op1.numberLength = Math.max(op1.numberLength, op2.numberLength) + 1;
    op1.cutOffLeadingZeroes();
    op1.unCache();
  }

  /**
   * Same as @link #inplaceSubtract(BigInteger, BigInteger), but without the
   * restriction of non-positive values.
   * 
   * @param op1 should have enough space to save the result
   * @param op2
   */
  static void completeInPlaceSubtract(BigInteger op1, BigInteger op2) {
    int resultSign = op1.compareTo(op2);
    if (op1.sign == 0) {
      System.arraycopy(op2.digits, 0, op1.digits, 0, op2.numberLength);
      op1.sign = -op2.sign;
    } else if (op1.sign != op2.sign) {
      add(op1.digits, op1.digits, op1.numberLength, op2.digits,
          op2.numberLength);
      op1.sign = resultSign;
    } else {
      int sign = unsignedArraysCompare(op1.digits, op2.digits,
          op1.numberLength, op2.numberLength);
      if (sign > 0) {
        subtract(op1.digits, op1.digits, op1.numberLength, op2.digits,
            op2.numberLength); // op1 = op1 - op2
        // op1.sign remains equal
      } else {
        inverseSubtract(op1.digits, op1.digits, op1.numberLength, op2.digits,
            op2.numberLength); // op1 = op2 - op1
        op1.sign = -op1.sign;
      }
    }
    op1.numberLength = Math.max(op1.numberLength, op2.numberLength) + 1;
    op1.cutOffLeadingZeroes();
    op1.unCache();
  }

  /**
   * Performs {@code op1 += op2}. {@code op1} must have enough place to store
   * the result (i.e. {@code op1.bitLength() >= op2.bitLength()}). Both should
   * be positive (i.e. {@code op1 >= op2}).
   * 
   * @param op1 the input minuend, and the output result.
   * @param op2 the addend
   */
  static void inplaceAdd(BigInteger op1, BigInteger op2) {
    // PRE: op1 >= op2 > 0
    add(op1.digits, op1.digits, op1.numberLength, op2.digits, op2.numberLength);
    op1.numberLength = Math.min(
        Math.max(op1.numberLength, op2.numberLength) + 1, op1.digits.length);
    op1.cutOffLeadingZeroes();
    op1.unCache();
  }

  /**
   * Performs: {@code op1 += addend}. The number must to have place to hold a
   * possible carry.
   */
  static void inplaceAdd(BigInteger op1, final int addend) {
    int carry = inplaceAdd(op1.digits, op1.numberLength, addend);
    if (carry == 1) {
      op1.digits[op1.numberLength] = 1;
      op1.numberLength++;
    }
    op1.unCache();
  }

  /**
   * Adds an integer value to the array of integers remembering carry.
   * 
   * @return a possible generated carry (0 or 1)
   */
  static int inplaceAdd(int a[], final int aSize, final int addend) {
    long carry = addend & 0xFFFFFFFFL;

    for (int i = 0; (carry != 0) && (i < aSize); i++) {
      carry += a[i] & 0xFFFFFFFFL;
      a[i] = (int) carry;
      carry >>= 32;
    }
    return (int) carry;
  }

  /**
   * Performs {@code op1 -= op2}. {@code op1} must have enough place to store
   * the result (i.e. {@code op1.bitLength() >= op2.bitLength()}). Both should
   * be positive (what implies that {@code op1 >= op2}).
   * 
   * @param op1 the input minuend, and the output result.
   * @param op2 the subtrahend
   */
  static void inplaceSubtract(BigInteger op1, BigInteger op2) {
    // PRE: op1 >= op2 > 0
    subtract(op1.digits, op1.digits, op1.numberLength, op2.digits,
        op2.numberLength);
    op1.cutOffLeadingZeroes();
    op1.unCache();
  }

  /**
   * @see BigInteger#subtract(BigInteger) .
   * @param op1
   * @param op2
   * @return
   */
  static BigInteger subtract(BigInteger op1, BigInteger op2) {
    int resSign;
    int resDigits[];
    int op1Sign = op1.sign;
    int op2Sign = op2.sign;

    if (op2Sign == 0) {
      return op1;
    }
    if (op1Sign == 0) {
      return op2.negate();
    }
    int op1Len = op1.numberLength;
    int op2Len = op2.numberLength;
    if (op1Len + op2Len == 2) {
      long a = (op1.digits[0] & 0xFFFFFFFFL);
      long b = (op2.digits[0] & 0xFFFFFFFFL);
      if (op1Sign < 0) {
        a = -a;
      }
      if (op2Sign < 0) {
        b = -b;
      }
      return BigInteger.valueOf(a - b);
    }
    int cmp = ((op1Len != op2Len) ? ((op1Len > op2Len) ? 1 : -1)
        : Elementary.compareArrays(op1.digits, op2.digits, op1Len));

    if (cmp == BigInteger.LESS) {
      resSign = -op2Sign;
      resDigits = (op1Sign == op2Sign) ? subtract(op2.digits, op2Len,
          op1.digits, op1Len) : add(op2.digits, op2Len, op1.digits, op1Len);
    } else {
      resSign = op1Sign;
      if (op1Sign == op2Sign) {
        if (cmp == BigInteger.EQUALS) {
          return BigInteger.ZERO;
        }
        resDigits = subtract(op1.digits, op1Len, op2.digits, op2Len);
      } else {
        resDigits = add(op1.digits, op1Len, op2.digits, op2Len);
      }
    }
    BigInteger res = new BigInteger(resSign, resDigits.length, resDigits);
    res.cutOffLeadingZeroes();
    return res;
  }

  /**
   * Addss the value represented by {@code b} to the value represented by
   * {@code a}. It is assumed the magnitude of a is not less than the magnitude
   * of b.
   * 
   * @return {@code a + b}
   */
  private static int[] add(int a[], int aSize, int b[], int bSize) {
    // PRE: a[] >= b[]
    int res[] = new int[aSize + 1];
    add(res, a, aSize, b, bSize);
    return res;
  }

  /**
   * Performs {@code res = a + b}.
   */
  private static void add(int res[], int a[], int aSize, int b[], int bSize) {
    // PRE: a.length < max(aSize, bSize)

    int i;
    long carry = (a[0] & 0xFFFFFFFFL) + (b[0] & 0xFFFFFFFFL);

    res[0] = (int) carry;
    carry >>= 32;

    if (aSize >= bSize) {
      for (i = 1; i < bSize; i++) {
        carry += (a[i] & 0xFFFFFFFFL) + (b[i] & 0xFFFFFFFFL);
        res[i] = (int) carry;
        carry >>= 32;
      }
      for (; i < aSize; i++) {
        carry += a[i] & 0xFFFFFFFFL;
        res[i] = (int) carry;
        carry >>= 32;
      }
    } else {
      for (i = 1; i < aSize; i++) {
        carry += (a[i] & 0xFFFFFFFFL) + (b[i] & 0xFFFFFFFFL);
        res[i] = (int) carry;
        carry >>= 32;
      }
      for (; i < bSize; i++) {
        carry += b[i] & 0xFFFFFFFFL;
        res[i] = (int) carry;
        carry >>= 32;
      }
    }
    if (carry != 0) {
      res[i] = (int) carry;
    }
  }

  /**
   * Performs {@code res = b - a}.
   */
  private static void inverseSubtract(int res[], int a[], int aSize, int b[],
      int bSize) {
    int i;
    long borrow = 0;
    if (aSize < bSize) {
      for (i = 0; i < aSize; i++) {
        borrow += (b[i] & 0xFFFFFFFFL) - (a[i] & 0xFFFFFFFFL);
        res[i] = (int) borrow;
        borrow >>= 32; // -1 or 0
      }
      for (; i < bSize; i++) {
        borrow += b[i] & 0xFFFFFFFFL;
        res[i] = (int) borrow;
        borrow >>= 32; // -1 or 0
      }
    } else {
      for (i = 0; i < bSize; i++) {
        borrow += (b[i] & 0xFFFFFFFFL) - (a[i] & 0xFFFFFFFFL);
        res[i] = (int) borrow;
        borrow >>= 32; // -1 or 0
      }
      for (; i < aSize; i++) {
        borrow -= a[i] & 0xFFFFFFFFL;
        res[i] = (int) borrow;
        borrow >>= 32; // -1 or 0
      }
    }
  }

  /**
   * Subtracts the value represented by {@code b} from the value represented by
   * {@code a}. It is assumed the magnitude of a is not less than the magnitude
   * of b.
   * 
   * @return {@code a - b}
   */
  private static int[] subtract(int a[], int aSize, int b[], int bSize) {
    // PRE: a[] >= b[]
    int res[] = new int[aSize];
    subtract(res, a, aSize, b, bSize);
    return res;
  }

  /**
   * Performs {@code res = a - b}. It is assumed the magnitude of a is not less
   * than the magnitude of b.
   */
  private static void subtract(int res[], int a[], int aSize, int b[], int bSize) {
    // PRE: a[] >= b[]
    int i;
    long borrow = 0;

    for (i = 0; i < bSize; i++) {
      borrow += (a[i] & 0xFFFFFFFFL) - (b[i] & 0xFFFFFFFFL);
      res[i] = (int) borrow;
      borrow >>= 32; // -1 or 0
    }
    for (; i < aSize; i++) {
      borrow += a[i] & 0xFFFFFFFFL;
      res[i] = (int) borrow;
      borrow >>= 32; // -1 or 0
    }
  }

  /**
   * Compares two arrays, representing unsigned integer in little-endian order.
   * Returns +1,0,-1 if a is - respective - greater, equal or lesser then b
   */
  private static int unsignedArraysCompare(int[] a, int[] b, int aSize,
      int bSize) {
    if (aSize > bSize) {
      return 1;
    } else if (aSize < bSize) {
      return -1;
    } else {
      int i;
      for (i = aSize - 1; i >= 0 && a[i] == b[i]; i--) {
        // empty
      }
      return i < 0 ? BigInteger.EQUALS
          : ((a[i] & 0xFFFFFFFFL) < (b[i] & 0xFFFFFFFFL) ? BigInteger.LESS
              : BigInteger.GREATER);
    }
  }

  /**
   * Just to denote that this class can't be instantiated.
   */
  private Elementary() {
  }

}