package calc

import (
	"errors"
	"testing"
)

func TestNewEngine_Defaults(t *testing.T) {
	e := NewEngine()
	if e.CurrentBase() != BaseDEC {
		t.Errorf("expected DEC, got %s", e.CurrentBase())
	}
	if got := e.FormatForDisplay(); got != "0" {
		t.Errorf("initial display: want 0, got %s", got)
	}
}

func TestCycleBase_IntegerOK(t *testing.T) {
	e := NewEngine()
	e.accumulator = 42
	e.entry = ""

	if err := e.CycleBase(); err != nil {
		t.Fatalf("unexpected error: %v", err)
	}
	if e.CurrentBase() != BaseHEX {
		t.Errorf("expected HEX, got %s", e.CurrentBase())
	}
	if got := e.FormatForDisplay(); got != "2A" {
		t.Errorf("hex of 42: want 2A, got %s", got)
	}
}

func TestCycleBase_FractionalCERR(t *testing.T) {
	e := NewEngine()
	// 23/6 = 3.8333...
	e.accumulator = 23.0 / 6.0
	e.entry = ""

	err := e.CycleBase()
	if !errors.Is(err, ErrConversionNotPossible) {
		t.Fatalf("expected ErrConversionNotPossible, got %v", err)
	}
	if e.CurrentBase() != BaseDEC {
		t.Error("base must remain DEC after CERR")
	}
}

func TestBasicArithmetic(t *testing.T) {
	e := NewEngine()

	// 2 + 3 =
	e.EnterDigit('2')
	e.SetOperator("+")
	e.EnterDigit('3')
	e.Equals()

	if got := e.FormatForDisplay(); got != "5" {
		t.Errorf("2+3: want 5, got %s", got)
	}

	// * 4 =
	e.SetOperator("*")
	e.EnterDigit('4')
	e.Equals()
	if got := e.FormatForDisplay(); got != "20" {
		t.Errorf("5*4: want 20, got %s", got)
	}
}

func TestMOD(t *testing.T) {
	e := NewEngine()
	e.EnterDigit('2')
	e.EnterDigit('3')
	e.SetOperator("mod")
	e.EnterDigit('6')
	e.Equals()

	if got := e.FormatForDisplay(); got != "5" {
		t.Errorf("23 mod 6: want 5, got %s", got)
	}
}

func TestFractionalThenBASE_CERR(t *testing.T) {
	e := NewEngine()

	// 1 / 3 = 0.333...
	e.EnterDigit('1')
	e.SetOperator("/")
	e.EnterDigit('3')
	e.Equals()

	if e.IsInteger() {
		t.Error("1/3 should not be integer")
	}

	err := e.CycleBase()
	if !errors.Is(err, ErrConversionNotPossible) {
		t.Fatalf("expected CERR on fractional BASE, got %v", err)
	}
}

func TestClearAndBackspace(t *testing.T) {
	e := NewEngine()
	e.EnterDigit('1')
	e.EnterDigit('2')
	e.EnterDigit('3')
	e.Backspace()
	if got := e.FormatForDisplay(); got != "12" {
		t.Errorf("backspace: want 12, got %s", got)
	}
	e.ClearEntry()
	if got := e.FormatForDisplay(); got != "0" {
		t.Errorf("clear entry: want 0, got %s", got)
	}
	e.AllClear()
	if e.CurrentBase() != BaseDEC {
		t.Error("all clear should reset base to DEC for MVP")
	}
}

func TestChangeSign(t *testing.T) {
	e := NewEngine()
	e.EnterDigit('5')
	e.ChangeSign()
	if got := e.FormatForDisplay(); got != "-5" {
		t.Errorf("sign: want -5, got %s", got)
	}
}

// More tests (entry buffer, multi-op, decimal point, etc.) will be added as the
// TUI spike exercises real usage. The CERR path is the critical one for BASE.