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// Package ecc (Elliptic Curve Cryptography, 楕円曲線暗号)
package ecc

import "fmt"

// FieldElement 有限体の要素
type FieldElement struct {
	Num   int
	Prime int
}

func (fe FieldElement) String() string {
	return fmt.Sprintf("FieldElement_%v(%v)", fe.Prime, fe.Num)
}

// NewFieldElement ...
func NewFieldElement(n, p int) (FieldElement, error) {
	if n >= p || n < 0 {
		return FieldElement{}, fmt.Errorf("%v not in field range 0 to %v", n, p-1)
	}
	return FieldElement{n, p}, nil
}

// Eq ...
func Eq(fe1, fe2 FieldElement) bool {
	if fe1.Prime == 0 || fe2.Prime == 0 {
		return false
	}
	return fe1.Num == fe2.Num && fe1.Prime == fe2.Prime
}

// Ne ...
func Ne(fe1, fe2 FieldElement) bool {
	return !Eq(fe1, fe2)
}

// Add ...
func Add(fe1, fe2 FieldElement) (FieldElement, error) {
	if fe1.Prime != fe2.Prime {
		return FieldElement{}, fmt.Errorf("Cannot add two numbers in diferent Fields")
	}
	return FieldElement{Num: (fe1.Num + fe2.Num) % fe1.Prime, Prime: fe1.Prime}, nil
}

// Sub ...
func Sub(fe1, fe2 FieldElement) (FieldElement, error) {
	if fe1.Prime != fe2.Prime {
		return FieldElement{}, fmt.Errorf("Cannot sub two numbers in diferent Fields")
	}
	num := (fe1.Num - fe2.Num) % fe1.Prime
	if num < 0 {
		num += fe1.Prime
	}
	return FieldElement{Num: num, Prime: fe1.Prime}, nil
}

package ecc

import "testing"

func TestNewFieldElementEq(t *testing.T) {
	a, _ := NewFieldElement(7, 13)
	b, _ := NewFieldElement(6, 13)
	got := Eq(a, b)
	if got {
		t.Errorf("Eq(%v, %v) got %v, want false", a, b, got)
	}
	got = Eq(a, a)
	if !got {
		t.Errorf("Eq(%v, %v) got %v, want true", a, a, got)
	}
}
func TestNewFieldElementNe(t *testing.T) {
	a, _ := NewFieldElement(7, 13)
	b, _ := NewFieldElement(6, 13)
	got := Ne(a, b)
	if !got {
		t.Errorf("Ne(%v, %v) got %v, want true", a, b, got)
	}
	got = Ne(a, a)
	if got {
		t.Errorf("Ne(%v, %v) got %v, want false", a, a, got)
	}
}

func TestAdd(t *testing.T) {
	a, _ := NewFieldElement(7, 13)
	b, _ := NewFieldElement(12, 13)
	c, _ := NewFieldElement(6, 13)
	d, _ := Add(a, b)
	if Ne(d, c) {
		t.Errorf("Add(%v, %v) got %v, want %v", a, b, d, c)
	}
}

func TestSub(t *testing.T) {
	prime := 19
	tests := []struct {
		nums    []int
		wantNum int
	}{
		{[]int{11, 9}, 2},
		{[]int{6, 13}, 12},
	}
	for _, test := range tests {
		a, _ := NewFieldElement(test.nums[0], prime)
		b, _ := NewFieldElement(test.nums[1], prime)
		want, _ := NewFieldElement(test.wantNum, prime)
		got, _ := Sub(a, b)
		if Ne(got, want) {
			t.Errorf("Sub(%v, %v) got %v, want %v", a, b, got, want)
		}
	}
}

% go test
# bitcoin/ecc [bitcoin/ecc.test]
./ecc_test.go:34:10: undefined: Add
FAIL	bitcoin/ecc [build failed]
% go test
# bitcoin/ecc
./ecc_test.go:36:3: Errorf format %v reads arg #4, but call has 3 args
FAIL	bitcoin/ecc [build failed]
% go test
PASS
ok  	bitcoin/ecc	0.496s
% go test
--- FAIL: TestSub (0.00s)
    ecc_test.go:55: Sub(FieldElement_19(11), FieldElement_19(9)) got FieldElement_19(1), want FieldElement_19(2)
    ecc_test.go:55: Sub(FieldElement_19(6), FieldElement_19(13)) got FieldElement_19(0), want FieldElement_19(12)
FAIL
exit status 1
FAIL	bitcoin/ecc	0.301s
% go test
# bitcoin/ecc [bitcoin/ecc.test]
./ecc_test.go:53:13: undefined: Sub
FAIL	bitcoin/ecc [build failed]
% go test
--- FAIL: TestSub (0.00s)
    ecc_test.go:55: Sub(FieldElement_19(6), FieldElement_19(13)) got FieldElement_19(-7), want FieldElement_19(12)
FAIL
exit status 1
FAIL	bitcoin/ecc	0.280s
% go test
PASS
ok  	bitcoin/ecc	0.312s
%