package p256k1

import (
	"testing"
)

func TestFieldElementBasics(t *testing.T) {
	// Test zero field element
	var zero FieldElement
	zero.setInt(0)
	zero.normalize()
	if !zero.isZero() {
		t.Error("Zero field element should be zero")
	}

	// Test one field element
	var one FieldElement
	one.setInt(1)
	one.normalize()
	if one.isZero() {
		t.Error("One field element should not be zero")
	}

	// Test equality
	var one2 FieldElement
	one2.setInt(1)
	one2.normalize()
	if !one.equal(&one2) {
		t.Error("Two normalized ones should be equal")
	}
}

func TestFieldElementSetB32(t *testing.T) {
	// Test setting from 32-byte array
	testCases := []struct {
		name  string
		bytes [32]byte
	}{
		{
			name:  "zero",
			bytes: [32]byte{},
		},
		{
			name:  "one",
			bytes: [32]byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1},
		},
		{
			name:  "max_value",
			bytes: [32]byte{0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFE, 0xFF, 0xFF, 0xFC, 0x2F},
		},
	}

	for _, tc := range testCases {
		t.Run(tc.name, func(t *testing.T) {
			var fe FieldElement
			fe.setB32(tc.bytes[:])

			// Test round-trip
			var result [32]byte
			fe.normalize()
			fe.getB32(result[:])

			// For field modulus reduction, we need to check if the result is valid
			if tc.name == "max_value" {
				// This should be reduced modulo p
				var expected FieldElement
				expected.setInt(0) // p mod p = 0
				expected.normalize()
				if !fe.equal(&expected) {
					t.Error("Field modulus should reduce to zero")
				}
			}
		})
	}
}

func TestFieldElementArithmetic(t *testing.T) {
	// Test addition
	var a, b, c FieldElement
	a.setInt(5)
	b.setInt(7)
	c = a
	c.add(&b)
	c.normalize()

	var expected FieldElement
	expected.setInt(12)
	expected.normalize()
	if !c.equal(&expected) {
		t.Error("5 + 7 should equal 12")
	}

	// Test negation
	var neg FieldElement
	neg.negate(&a, a.magnitude)
	neg.normalize()

	var sum FieldElement
	sum = a
	sum.add(&neg)
	sum.normalize()

	if !sum.isZero() {
		t.Error("a + (-a) should equal zero")
	}
}

func TestFieldElementMultiplication(t *testing.T) {
	// Test multiplication
	var a, b, c FieldElement
	a.setInt(5)
	b.setInt(7)
	c.mul(&a, &b)
	c.normalize()

	var expected FieldElement
	expected.setInt(35)
	expected.normalize()
	if !c.equal(&expected) {
		t.Error("5 * 7 should equal 35")
	}

	// Test squaring
	var sq FieldElement
	sq.sqr(&a)
	sq.normalize()

	expected.setInt(25)
	expected.normalize()
	if !sq.equal(&expected) {
		t.Error("5^2 should equal 25")
	}
}

func TestFieldElementNormalization(t *testing.T) {
	var fe FieldElement
	fe.setInt(42)

	// Before normalization
	if fe.normalized {
		fe.normalized = false // Force non-normalized state
	}

	// After normalization
	fe.normalize()
	if !fe.normalized {
		t.Error("Field element should be normalized after normalize()")
	}
	if fe.magnitude != 1 {
		t.Error("Normalized field element should have magnitude 1")
	}
}

func TestFieldElementOddness(t *testing.T) {
	var even, odd FieldElement
	even.setInt(4)
	even.normalize()
	odd.setInt(5)
	odd.normalize()

	if even.isOdd() {
		t.Error("4 should be even")
	}
	if !odd.isOdd() {
		t.Error("5 should be odd")
	}
}

func TestFieldElementConditionalMove(t *testing.T) {
	var a, b, original FieldElement
	a.setInt(5)
	b.setInt(10)
	original = a

	// Test conditional move with flag = 0
	a.cmov(&b, 0)
	if !a.equal(&original) {
		t.Error("Conditional move with flag=0 should not change value")
	}

	// Test conditional move with flag = 1
	a.cmov(&b, 1)
	if !a.equal(&b) {
		t.Error("Conditional move with flag=1 should copy value")
	}
}

func TestFieldElementStorage(t *testing.T) {
	var fe FieldElement
	fe.setInt(12345)
	fe.normalize()

	// Convert to storage
	var storage FieldElementStorage
	fe.toStorage(&storage)

	// Convert back
	var restored FieldElement
	restored.fromStorage(&storage)
	restored.normalize()

	if !fe.equal(&restored) {
		t.Error("Storage round-trip should preserve value")
	}
}

func TestFieldElementEdgeCases(t *testing.T) {
	// Test field modulus boundary
	// Set to p-1 (field modulus minus 1)
	// p-1 = FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2E
	p_minus_1 := [32]byte{
		0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
		0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
		0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
		0xFF, 0xFF, 0xFF, 0xFE, 0xFF, 0xFF, 0xFC, 0x2E,
	}

	var fe FieldElement
	fe.setB32(p_minus_1[:])
	fe.normalize()

	// Add 1 should give 0
	var one FieldElement
	one.setInt(1)
	fe.add(&one)
	fe.normalize()

	if !fe.isZero() {
		t.Error("(p-1) + 1 should equal 0 in field arithmetic")
	}
}

func TestFieldElementClear(t *testing.T) {
	var fe FieldElement
	fe.setInt(12345)

	fe.clear()

	// After clearing, should be zero and normalized
	if !fe.isZero() {
		t.Error("Cleared field element should be zero")
	}
	if !fe.normalized {
		t.Error("Cleared field element should be normalized")
	}
}

// TestMontgomery tests Montgomery multiplication (currently disabled due to incomplete implementation)
// TODO: Re-enable once Montgomery multiplication is fully implemented
func TestMontgomery(t *testing.T) {
	t.Skip("Montgomery multiplication implementation is incomplete - see MONTGOMERY_NOTES.md")
	
	// Test Montgomery conversion round-trip
	t.Run("RoundTrip", func(t *testing.T) {
		var a, b FieldElement
		a.setInt(123)
		b.setInt(456)
		a.normalize()
		b.normalize()

		// Convert to Montgomery form
		aMont := a.ToMontgomery()
		bMont := b.ToMontgomery()

		// Convert back
		aBack := aMont.FromMontgomery()
		bBack := bMont.FromMontgomery()

		// Normalize for comparison
		aBack.normalize()
		bBack.normalize()

		if !aBack.equal(&a) {
			t.Errorf("Round-trip conversion failed for a: got %x, want %x", aBack.n, a.n)
		}
		if !bBack.equal(&b) {
			t.Errorf("Round-trip conversion failed for b: got %x, want %x", bBack.n, b.n)
		}
	})

	// Test Montgomery multiplication correctness
	t.Run("Multiplication", func(t *testing.T) {
		testCases := []struct {
			name string
			a, b int
		}{
			{"small", 123, 456},
			{"medium", 1000, 2000},
			{"one", 1, 1},
			{"zero_a", 0, 123},
			{"zero_b", 123, 0},
		}

		for _, tc := range testCases {
			t.Run(tc.name, func(t *testing.T) {
				var a, b FieldElement
				a.setInt(tc.a)
				b.setInt(tc.b)
				a.normalize()
				b.normalize()

				// Standard multiplication
				var stdResult FieldElement
				stdResult.mul(&a, &b)
				stdResult.normalize()

				// Montgomery multiplication
				aMont := a.ToMontgomery()
				bMont := b.ToMontgomery()
				montResult := MontgomeryMul(aMont, bMont)
				montResult = montResult.FromMontgomery()
				montResult.normalize()

				if !montResult.equal(&stdResult) {
					t.Errorf("Montgomery multiplication failed for %d * %d:\nGot:  %x\nWant: %x",
						tc.a, tc.b, montResult.n, stdResult.n)
				}
			})
		}
	})

	// Test Montgomery multiplication with field modulus boundary values
	t.Run("BoundaryValues", func(t *testing.T) {
		// Test with p-1
		pMinus1Bytes := [32]byte{
			0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
			0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
			0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
			0xFF, 0xFF, 0xFF, 0xFE, 0xFF, 0xFF, 0xFC, 0x2E,
		}

		var pMinus1 FieldElement
		pMinus1.setB32(pMinus1Bytes[:])
		pMinus1.normalize()

		// (p-1) * (p-1) should equal 1 mod p
		var expected FieldElement
		expected.setInt(1)
		expected.normalize()

		// Standard multiplication
		var stdResult FieldElement
		stdResult.mul(&pMinus1, &pMinus1)
		stdResult.normalize()

		// Montgomery multiplication
		pMinus1Mont := pMinus1.ToMontgomery()
		montResult := MontgomeryMul(pMinus1Mont, pMinus1Mont)
		montResult = montResult.FromMontgomery()
		montResult.normalize()

		if !montResult.equal(&expected) {
			t.Errorf("Montgomery multiplication failed for (p-1)*(p-1):\nGot:  %x\nWant: %x",
				montResult.n, expected.n)
		}

		if !stdResult.equal(&expected) {
			t.Errorf("Standard multiplication failed for (p-1)*(p-1):\nGot:  %x\nWant: %x",
				stdResult.n, expected.n)
		}
	})

	// Test multiple Montgomery multiplications in sequence
	t.Run("SequentialMultiplications", func(t *testing.T) {
		var a, b, c FieldElement
		a.setInt(123)
		b.setInt(456)
		c.setInt(789)
		a.normalize()
		b.normalize()
		c.normalize()

		// Standard: (a * b) * c
		var stdResult FieldElement
		stdResult.mul(&a, &b)
		stdResult.mul(&stdResult, &c)
		stdResult.normalize()

		// Montgomery: convert once, multiply multiple times
		aMont := a.ToMontgomery()
		bMont := b.ToMontgomery()
		cMont := c.ToMontgomery()

		montResult := MontgomeryMul(aMont, bMont)
		montResult = MontgomeryMul(montResult, cMont)
		montResult = montResult.FromMontgomery()
		montResult.normalize()

		if !montResult.equal(&stdResult) {
			t.Errorf("Sequential Montgomery multiplication failed:\nGot:  %x\nWant: %x",
				montResult.n, stdResult.n)
		}
	})
}