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# std.bigint

Arbitrary-precision ('bignum') arithmetic.
Performance is optimized for numbers below ~1000 decimal digits. For X86 machines, highly optimised assembly routines are used.
The following algorithms are currently implemented:

- Karatsuba multiplication
- Squaring is optimized independently of multiplication
- Divide-and-conquer division
- Binary exponentiation

For very large numbers, consider using the GMP library instead.
License:
Boost License 1.0.
Authors:
Don Clugston

Source std/bigint.d

struct `BigInt`;
  A struct representing an arbitrary precision integer.
  All arithmetic operations are supported, except unsigned shift right (>>>). Bitwise operations (|, &, ^, ~) are supported, and behave as if BigInt was an infinite length 2's complement number.
  BigInt implements value semantics using copy-on-write. This means that assignment is cheap, but operations such as x++ will cause heap allocation. (But note that for most bigint operations, heap allocation is inevitable anyway.)
  Examples:

      BigInt a = "9588669891916142";
      BigInt b = "7452469135154800";
      auto c = a * b;
      writeln(c); // BigInt("71459266416693160362545788781600")
      auto d = b * a;
      writeln(d); // BigInt("71459266416693160362545788781600")
      writeln(d); // c
      d = c * BigInt("794628672112");
      writeln(d); // BigInt("56783581982794522489042432639320434378739200")
      auto e = c + d;
      writeln(e); // BigInt("56783581982865981755459125799682980167520800")
      auto f = d + c;
      writeln(f); // e
      auto g = f - c;
      writeln(g); // d
      g = f - d;
      writeln(g); // c
      e = 12345678;
      g = c + e;
      auto h = g / b;
      auto i = g % b;
      writeln(h); // a
      writeln(i); // e
      BigInt j = "-0x9A56_57f4_7B83_AB78";
      BigInt k = j;
      j ^^= 11;
      writeln(k^^11); // j

  this(Range)(Range `s`)
  if (isBidirectionalRange!Range && isSomeChar!(ElementType!Range) && !isInfinite!Range && !isNarrowString!Range);
  pure this(Range)(Range `s`)
  if (isNarrowString!Range);
    Construct a BigInt from a decimal or hexadecimal string. The number must be in the form of a decimal or hex literal. It may have a leading + or - sign, followed by 0x or 0X if hexadecimal. Underscores are permitted in any location after the 0x and/or the sign of the number.
    Parameters:

    Range `s` | a finite bidirectional range of any character type

    Throws:
    std.conv.ConvException if the string doesn't represent a valid number
  pure nothrow this(T)(T `x`)
  if (isIntegral!T);
    Construct a BigInt from a built-in integral type.
    Examples:

        // @system due to failure in FreeBSD32
        ulong data = 1_000_000_000_000;
        auto bigData = BigInt(data);
        writeln(bigData); // BigInt("1_000_000_000_000")

  pure nothrow this(T)(T `x`)
  if (is(Unqual!T == BigInt));
    Construct a BigInt from another BigInt.
    Examples:

        const(BigInt) b1 = BigInt("1_234_567_890");
        BigInt b2 = BigInt(b1);
        writeln(b2); // BigInt("1_234_567_890")

  pure nothrow BigInt `opAssign`(T)(T `x`)
  if (isIntegral!T);
    Assignment from built-in integer types.
    Examples:

        auto b = BigInt("123");
        b = 456;
        writeln(b); // BigInt("456")

  pure @nogc BigInt `opAssign`(T : BigInt)(T `x`);
    Assignment from another BigInt.
    Examples:

        auto b1 = BigInt("123");
        auto b2 = BigInt("456");
        b2 = b1;
        writeln(b2); // BigInt("123")

  pure nothrow BigInt `opOpAssign`(string op, T)(T `y`)
  if ((op == "+" || op == "-" || op == "*" || op == "/" || op == "%" || op == ">>" || op == "<<" || op == "^^" || op == "|" || op == "&" || op == "^") && isIntegral!T);
    Implements assignment operators from built-in integers of the form BigInt op= integer.
    Examples:

        //@system because opOpAssign is @system
        auto b = BigInt("1_000_000_000");

        b += 12345;
        writeln(b); // BigInt("1_000_012_345")

        b /= 5;
        writeln(b); // BigInt("200_002_469")

  pure nothrow BigInt `opOpAssign`(string op, T)(T `y`)
  if ((op == "+" || op == "-" || op == "*" || op == "|" || op == "&" || op == "^" || op == "/" || op == "%") && is(T : BigInt));
    Implements assignment operators of the form BigInt op= BigInt.
    Examples:

        // @system because opOpAssign is @system
        auto x = BigInt("123");
        auto y = BigInt("321");
        x += y;
        writeln(x); // BigInt("444")

  const pure nothrow BigInt `opBinary`(string op, T)(T `y`)
  if ((op == "+" || op == "*" || op == "-" || op == "|" || op == "&" || op == "^" || op == "/" || op == "%") && is(T : BigInt));
    Implements binary operators between BigInts.
    Examples:

        auto x = BigInt("123");
        auto y = BigInt("456");
        BigInt z = x * y;
        writeln(z); // BigInt("56088")

  const pure nothrow BigInt `opBinary`(string op, T)(T `y`)
  if ((op == "+" || op == "*" || op == "-" || op == "/" || op == "|" || op == "&" || op == "^" || op == ">>" || op == "<<" || op == "^^") && isIntegral!T);
    Implements binary operators between BigInt's and built-in integers.
    Examples:

        auto x = BigInt("123");
        x *= 300;
        writeln(x); // BigInt("36900")

  const pure nothrow auto `opBinary`(string op, T)(T `y`)
  if (op == "%" && isIntegral!T);
    Implements a narrowing remainder operation with built-in integer types.
    This binary operator returns a narrower, built-in integer type where applicable, according to the following table.

    BigInt | % | uint | → | long
    BigInt | % | long | → | long
    BigInt | % | ulong | → | BigInt
    BigInt | % | other type | → | int

    Examples:

        auto  x  = BigInt("1_000_000_500");
        long  l  = 1_000_000L;
        ulong ul = 2_000_000UL;
        int   i  = 500_000;
        short s  = 30_000;

        assert(is(typeof(x % l)  == long)   && x % l  == 500L);
        assert(is(typeof(x % ul) == BigInt) && x % ul == BigInt(500));
        assert(is(typeof(x % i)  == int)    && x % i  == 500);
        assert(is(typeof(x % s)  == int)    && x % s  == 10500);

  const pure nothrow BigInt `opBinaryRight`(string op, T)(T `y`)
  if ((op == "+" || op == "*" || op == "|" || op == "&" || op == "^") && isIntegral!T);
  const pure nothrow BigInt `opBinaryRight`(string op, T)(T `y`)
  if (op == "-" && isIntegral!T);
  const pure nothrow T `opBinaryRight`(string op, T)(T `x`)
  if ((op == "%" || op == "/") && isIntegral!T);
    Implements operators with built-in integers on the left-hand side and BigInt on the right-hand side.
    Examples:

        auto x = BigInt("100");
        BigInt y = 123 + x;
        writeln(y); // BigInt("223")

        BigInt z = 123 - x;
        writeln(z); // BigInt("23")

        // Dividing a built-in integer type by BigInt always results in
        // something that fits in a built-in type, so the built-in type is
        // returned, not BigInt.
        assert(is(typeof(1000 / x) == int));
        writeln(1000 / x); // 10

  const pure nothrow BigInt `opUnary`(string op)()
  if (op == "+" || op == "-" || op == "~");
  pure nothrow BigInt `opUnary`(string op)()
  if (op == "++" || op == "--");
    Implements BigInt unary operators.
    Examples:

        auto x = BigInt("1234");
        writeln(-x); // BigInt("-1234")

        ++x;
        writeln(x); // BigInt("1235")

  const pure @nogc bool `opEquals`()(auto ref const BigInt `y`);
  const pure nothrow @nogc bool `opEquals`(T)(T `y`)
  if (isIntegral!T);
    Implements BigInt equality test with other BigInt's and built-in integer types.
    Examples:

        auto x = BigInt("12345");
        auto y = BigInt("12340");
        int z = 12345;
        int w = 54321;

        writeln(x); // x
        assert(x != y);
        writeln(x); // y + 5
        writeln(x); // z
        assert(x != w);

  const pure nothrow @nogc T `opCast`(T : bool)();
    Implements casting to bool.
    Examples:

        // Non-zero values are regarded as true
        auto x = BigInt("1");
        auto y = BigInt("10");
        assert(x);
        assert(y);

        // Zero value is regarded as false
        auto z = BigInt("0");
        assert(!z);

  const pure T `opCast`(T : ulong)();
    Implements casting to integer types.
    Throws:
    std.conv.ConvOverflowException if the number exceeds the target type's range.
    Examples:

        import std.conv : to, ConvOverflowException;
        import std.exception : assertThrown;

        writeln(BigInt("0").to!int); // 0

        writeln(BigInt("0").to!ubyte); // 0
        writeln(BigInt("255").to!ubyte); // 255
        assertThrown!ConvOverflowException(BigInt("256").to!ubyte);
        assertThrown!ConvOverflowException(BigInt("-1").to!ubyte);

  const pure nothrow @nogc T `opCast`(T)()
  if (is(Unqual!T == BigInt));
    Implements casting to/from qualified BigInt's.

    Warning Casting to/from const or immutable may break type system guarantees. Use with care.

    Examples:

        const(BigInt) x = BigInt("123");
        BigInt y = cast() x;    // cast away const
        writeln(y); // x

  const pure nothrow @nogc int `opCmp`(ref const BigInt `y`);
  const pure nothrow @nogc int `opCmp`(T)(T `y`)
  if (isIntegral!T);
  const pure nothrow @nogc int `opCmp`(T : BigInt)(const T `y`);
    Implements 3-way comparisons of BigInt with BigInt or BigInt with built-in integers.
    Examples:

        auto x = BigInt("100");
        auto y = BigInt("10");
        int z = 50;
        const int w = 200;

        assert(y < x);
        assert(x > z);
        assert(z > y);
        assert(x < w);

  const pure nothrow @nogc @safe long `toLong`();
    Returns:
    The value of this BigInt as a long, or long.max/long.min if outside the representable range.
    Examples:

        auto b = BigInt("12345");
        long l = b.toLong();
        writeln(l); // 12345

  const pure nothrow @nogc @safe int `toInt`();
    Returns:
    The value of this BigInt as an int, or int.max/int.min if outside the representable range.
    Examples:

        auto big = BigInt("5_000_000");
        auto i = big.toInt();
        writeln(i); // 5_000_000

        // Numbers that are too big to fit into an int will be clamped to int.max.
        auto tooBig = BigInt("5_000_000_000");
        i = tooBig.toInt();
        writeln(i); // int.max

  const pure nothrow @nogc @property @safe size_t `uintLength`();
    Number of significant uints which are used in storing this number. The absolute value of this BigInt is always < 232*uintLength
  const pure nothrow @nogc @property @safe size_t `ulongLength`();
    Number of significant ulongs which are used in storing this number. The absolute value of this BigInt is always < 264*ulongLength
  const void `toString`(scope void delegate(const(char)[]) `sink`, string `formatString`);
  const void `toString`(scope void delegate(const(char)[]) `sink`, ref scope const FormatSpec!char `f`);
    Convert the BigInt to string, passing it to the given sink.
    Parameters:

    void delegate(const(char)[]) `sink` | A delegate for accepting possibly piecewise segments of the formatted string.
    string `formatString` | A format string specifying the output format.
    Available output formats:
    "d" | Decimal
    "o" | Octal
    "x" | Hexadecimal, lower case
    "X" | Hexadecimal, upper case
    "s" | Default formatting (same as "d")
    null | Default formatting (same as "d")

    Examples:
    `toString` is rarely directly invoked; the usual way of using it is via std.format.format:

        import std.format : format;

        auto x = BigInt("1_000_000");
        x *= 12345;

        writeln(format("%d", x)); // "12345000000"
        writeln(format("%x", x)); // "2_dfd1c040"
        writeln(format("%X", x)); // "2_DFD1C040"
        writeln(format("%o", x)); // "133764340100"

  const nothrow @safe size_t `toHash`();
    Returns:
    A unique hash of the BigInt's value suitable for use in a hash table.
    Examples:
    `toHash` is rarely directly invoked; it is implicitly used when BigInt is used as the key of an associative array.

        string[BigInt] aa;
        aa[BigInt(123)] = "abc";
        aa[BigInt(456)] = "def";

        writeln(aa[BigInt(123)]); // "abc"
        writeln(aa[BigInt(456)]); // "def"

  const T `getDigit`(T = ulong)(size_t `n`)
  if (is(T == ulong) || is(T == uint));
    Gets the nth number in the underlying representation that makes up the whole BigInt.
    Parameters:

    T | the type to view the underlying representation as
    size_t `n` | The nth number to retrieve. Must be less than ulongLength or uintLength with respect to T.

    Returns:
    The nth ulong in the representation of this BigInt.
    Examples:

        auto a = BigInt("1000");
        writeln(a.ulongLength()); // 1
        writeln(a.getDigit(0)); // 1000

        writeln(a.uintLength()); // 1
        writeln(a.getDigit!uint(0)); // 1000

        auto b = BigInt("2_000_000_000_000_000_000_000_000_000");
        writeln(b.ulongLength()); // 2
        writeln(b.getDigit(0)); // 4584946418820579328
        writeln(b.getDigit(1)); // 108420217

        writeln(b.uintLength()); // 3
        writeln(b.getDigit!uint(0)); // 3489660928
        writeln(b.getDigit!uint(1)); // 1067516025
        writeln(b.getDigit!uint(2)); // 108420217

pure nothrow string `toDecimalString`(const(BigInt) `x`);
  Parameters:

  const(BigInt) `x` | The BigInt to convert to a decimal string.

  Returns:
  A string that represents the BigInt as a decimal number.
  Examples:

      auto x = BigInt("123");
      x *= 1000;
      x += 456;

      auto xstr = x.toDecimalString();
      writeln(xstr); // "123456"

string `toHex`(const(BigInt) `x`);
  Parameters:

  const(BigInt) `x` | The BigInt to convert to a hexadecimal string.

  Returns:
  A string that represents the BigInt as a hexadecimal (base 16) number in upper case.
  Examples:

      auto x = BigInt("123");
      x *= 1000;
      x += 456;

      auto xstr = x.toHex();
      writeln(xstr); // "1E240"

Unsigned!T `absUnsign`(T)(T `x`)
if (isIntegral!T);
  Returns the absolute value of x converted to the corresponding unsigned type.
  Parameters:

  T `x` | The integral value to return the absolute value of.

  Returns:
  The absolute value of x.
  Examples:

      writeln((-1).absUnsign); // 1
      writeln(1.absUnsign); // 1

pure nothrow void `divMod`(const BigInt `dividend`, const BigInt `divisor`, out BigInt `quotient`, out BigInt `remainder`);
  Finds the quotient and remainder for the given dividend and divisor in one operation.
  Parameters:

  BigInt `dividend` | the BigInt to divide
  BigInt `divisor` | the BigInt to divide the dividend by
  BigInt `quotient` | is set to the result of the division
  BigInt `remainder` | is set to the remainder of the division

  Examples:

      auto a = BigInt(123);
      auto b = BigInt(25);
      BigInt q, r;

      divMod(a, b, q, r);

      writeln(q); // 4
      writeln(r); // 23
      writeln(q * b + r); // a

pure nothrow BigInt `powmod`(BigInt `base`, BigInt `exponent`, BigInt `modulus`);
  Fast power modulus calculation for BigInt operands.
  Parameters:

  BigInt `base` | the BigInt is basic operands.
  BigInt `exponent` | the BigInt is power exponent of base.
  BigInt `modulus` | the BigInt is modules to be modular of base ^ exponent.

  Returns:
  The power modulus value of (base ^ exponent) % modulus.
  Examples:
  for powmod

      BigInt base = BigInt("123456789012345678901234567890");
      BigInt exponent = BigInt("1234567890123456789012345678901234567");
      BigInt modulus = BigInt("1234567");

      BigInt result = powmod(base, exponent, modulus);
      writeln(result); // 359079

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