package calc

import (
	"errors"
	"math"
)

// Base represents the current display formatting mode.
type Base string

const (
	BaseDEC Base = "DEC"
	BaseHEX Base = "HEX"
	BaseBIN Base = "BIN"
	BaseOCT Base = "OCT"
)

var basesCycle = []Base{BaseDEC, BaseHEX, BaseBIN, BaseOCT}

// ErrConversionNotPossible is returned by CycleBase when the current value
// has a fractional part and cannot be cleanly displayed in a non-DEC base.
var ErrConversionNotPossible = errors.New("conversion not possible (CERR)")

// Engine holds the calculator state. All math is decimal (float64).
// Bases affect only formatting via FormatForDisplay and the small row.
type Engine struct {
	value     float64
	base      Base
	pendingOp string // "+", "-", "*", "/", "mod", or ""
	pendingVal float64
	// entry buffer / more state will be added in engine implementation
}

// NewEngine creates a fresh calculator engine starting in DEC.
func NewEngine() *Engine {
	return &Engine{
		value: 0,
		base:  BaseDEC,
	}
}

// CurrentBase returns the active display base for the small row.
func (e *Engine) CurrentBase() Base {
	return e.base
}

// IsInteger reports whether the current value is effectively an integer
// (within epsilon to tolerate fp noise from operations like 1/3).
func (e *Engine) IsInteger() bool {
	_, frac := math.Modf(e.value)
	return math.Abs(frac) < 1e-10
}

// CycleBase advances to the next base in the cycle (DEC→HEX→BIN→OCT→DEC).
// Returns ErrConversionNotPossible (and does not change base) if the value
// has a fractional part. The caller (UI) should trigger a "CERR" flash.
func (e *Engine) CycleBase() error {
	if !e.IsInteger() {
		return ErrConversionNotPossible
	}
	for i, b := range basesCycle {
		if b == e.base {
			e.base = basesCycle[(i+1)%len(basesCycle)]
			return nil
		}
	}
	// fallback
	e.base = BaseDEC
	return nil
}

// FormatForDisplay returns the string to show in the large number area
// according to the current base. For MVP only integer values are expected
// in non-DEC bases (callers should have checked via CycleBase or IsInteger).
func (e *Engine) FormatForDisplay() string {
	// Placeholder implementation. Real version in phase 2 will handle
	// proper integer formatting for hex/bin/oct (no 0x prefix, etc.)
	// and fall back gracefully.
	if e.base == BaseDEC {
		// simple for skeleton
		return "0"
	}
	return "0"
}

// TODO (phase 2): Add EnterDigit, SetOperator, Equals, Mod, ClearEntry,
// AllClear, ChangeSign, Backspace, etc. + full FormatForDisplay.