gcd.h

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Embedded Template Library.
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Copyright(c) 2024 John Wellbelove
 
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#ifndef ETL_GCD_INCLUDED
#define ETL_GCD_INCLUDED
 
#include "platform.h"
#include "absolute.h"
#include "static_assert.h"
#include "type_traits.h"
 
namespace etl
{
  //***************************************************************************
  // Greatest Common Divisor.
  // Compile time.
  //***************************************************************************
  template <intmax_t Value1, intmax_t Value2>
  struct gcd_const
  {
    static ETL_CONSTANT intmax_t value = gcd_const<Value2, Value1 % Value2>::value;
  };
 
  template <intmax_t Value1>
  struct gcd_const<Value1, 0>
  {
    static ETL_CONSTANT intmax_t value = Value1;
  };
 
  //***************************************************************************
  // Greatest Common Divisor.
  // For unsigned types.
  //***************************************************************************
  template <typename T>
  ETL_NODISCARD ETL_CONSTEXPR14 typename etl::enable_if<etl::is_unsigned<T>::value, T>::type gcd(T a, T b) ETL_NOEXCEPT
  {
    ETL_STATIC_ASSERT(etl::is_integral<T>::value, "Integral type required");
 
    if ((a == 0 || b == 0))
    {
      return (a + b);
    }
 
    while (b != 0)
    {
      T t = b;
      b   = a % b;
      a   = t;
    }
 
    return a;
  }
 
  //***************************************************************************
  // Greatest Common Divisor.
  // For signed types.
  //***************************************************************************
  template <typename T>
  ETL_NODISCARD ETL_CONSTEXPR14 typename etl::enable_if<etl::is_signed<T>::value, T>::type gcd(T a, T b) ETL_NOEXCEPT
  {
    ETL_STATIC_ASSERT(etl::is_integral<T>::value, "Integral type required");
 
    typedef typename etl::make_unsigned<T>::type utype;
 
    utype ua = etl::absolute_unsigned(a);
    utype ub = etl::absolute_unsigned(b);
 
    return static_cast<T>(gcd(ua, ub));
  }
 
#if ETL_USING_CPP11
  #if ETL_HAS_INITIALIZER_LIST
  //***************************************************************************
  // Greatest Common Divisor.
  // Non-recursive, using an initializer_list.
  // Top level variadic function.
  //***************************************************************************
  template <typename T, typename... TRest>
  ETL_NODISCARD ETL_CONSTEXPR14 T gcd(T first, TRest... rest) ETL_NOEXCEPT
  {
    T result = first;
 
    for (T value : {rest...})
    {
      result = gcd(result, value);
 
      if (result == 1)
      {
        // Early termination: if the GCD is one, it will remain one
        // no matter what other numbers are processed.
        return 1;
      }
    }
 
    return result;
  }
  #else
  //***************************************************************************
  // Greatest Common Divisor.
  // Recursive.
  // Top level variadic function.
  //***************************************************************************
  template <typename T, typename... TRest>
  ETL_NODISCARD ETL_CONSTEXPR14 T gcd(T a, T b, TRest... rest) ETL_NOEXCEPT
  {
    T gcd_ab = gcd(a, b);
 
    if (gcd_ab == 1)
    {
      // Early termination: if the GCD is one, it will remain one
      // no matter what other numbers are processed.
      return 1;
    }
    else
    {
      return gcd(gcd_ab, rest...);
    }
  }
  #endif
#endif
} // namespace etl
 
#endif