// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package linalg

// Numeric is type bound that matches any numeric type.
// It would likely be in a constraints package in the standard library.
type Numeric interface {
	~int | ~int8 | ~int16 | ~int32 | ~int64 |
		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
		~float32 | ~float64 |
		~complex64 | ~complex128
}

func DotProduct[T Numeric](s1, s2 []T) T {
	if len(s1) != len(s2) {
		panic("DotProduct: slices of unequal length")
	}
	var r T
	for i := range s1 {
		r += s1[i] * s2[i]
	}
	return r
}

// NumericAbs matches numeric types with an Abs method.
type NumericAbs[T any] interface {
	Numeric

	Abs() T
}

// AbsDifference computes the absolute value of the difference of
// a and b, where the absolute value is determined by the Abs method.
func AbsDifference[T NumericAbs[T]](a, b T) T {
	d := a - b
	return d.Abs()
}

// OrderedNumeric is a type bound that matches numeric types that support the < operator.
type OrderedNumeric interface {
	~int | ~int8 | ~int16 | ~int32 | ~int64 |
		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
		~float32 | ~float64
}

// Complex is a type bound that matches the two complex types, which do not have a < operator.
type Complex interface {
	~complex64 | ~complex128
}

// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
// // OrderedAbs is a helper type that defines an Abs method for
// // ordered numeric types.
// type OrderedAbs[T OrderedNumeric] T
// 
// func (a OrderedAbs[T]) Abs() OrderedAbs[T] {
// 	if a < 0 {
// 		return -a
// 	}
// 	return a
// }
// 
// // ComplexAbs is a helper type that defines an Abs method for
// // complex types.
// type ComplexAbs[T Complex] T
// 
// func (a ComplexAbs[T]) Abs() ComplexAbs[T] {
// 	r := float64(real(a))
// 	i := float64(imag(a))
// 	d := math.Sqrt(r * r + i * i)
// 	return ComplexAbs[T](complex(d, 0))
// }
// 
// func OrderedAbsDifference[T OrderedNumeric](a, b T) T {
// 	return T(AbsDifference(OrderedAbs[T](a), OrderedAbs[T](b)))
// }
// 
// func ComplexAbsDifference[T Complex](a, b T) T {
// 	return T(AbsDifference(ComplexAbs[T](a), ComplexAbs[T](b)))
// }