package calc

import (
	"errors"
	"fmt"
	"math"
	"strconv"
	"strings"
)

// 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 indicator.
// Entry model: classic "in-progress entry buffer" (you see what you are typing).
type Engine struct {
	// committed accumulator (result of previous operations)
	accumulator float64

	// current in-progress entry as string (for display and for next operand)
	entry string

	// pending operator to apply when next operand is committed ("", "+", "-", "*", "/", "mod")
	pendingOp string

	// the left-hand value for the pending operation
	pendingLeft float64

	base Base
}

// NewEngine creates a fresh calculator engine starting in DEC with 0.
func NewEngine() *Engine {
	return &Engine{
		accumulator: 0,
		entry:       "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 *displayed* numeric value is effectively an integer
// (within epsilon to tolerate fp noise from operations like 1/3 or 0.1+0.2).
// We use the committed accumulator when there is no active entry, otherwise the parsed entry.
func (e *Engine) IsInteger() bool {
	val := e.currentNumericValue()
	_, frac := math.Modf(val)
	return math.Abs(frac) < 1e-10
}

// currentNumericValue returns the number that should be considered "current"
// for operations and for BASE cycling / formatting.
func (e *Engine) currentNumericValue() float64 {
	if e.entry != "" && e.entry != "0" {
		if v, err := strconv.ParseFloat(e.entry, 64); err == nil {
			return v
		}
	}
	return e.accumulator
}

// CycleBase advances to the next base in the cycle (DEC→HEX→BIN→OCT→DEC).
// Returns ErrConversionNotPossible (and does not change base) if the current
// numeric value has a fractional part. The 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
		}
	}
	e.base = BaseDEC
	return nil
}

// FormatForDisplay returns the string to render in the large number area
// according to the current base and the current numeric value.
func (e *Engine) FormatForDisplay() string {
	val := e.currentNumericValue()

	// If we have an active entry string (user is typing), prefer showing the
	// literal entry in DEC (classic calculator behavior). For other bases we
	// still format the numeric value (after commit it will be consistent).
	if e.base == BaseDEC {
		if e.entry != "" {
			return e.entry
		}
		// For DEC committed results, show a reasonable representation.
		if math.Abs(val) < 1e12 {
			return strconv.FormatFloat(val, 'f', -1, 64)
		}
		return strconv.FormatFloat(val, 'e', -1, 64)
	}

	// Non-DEC bases: only make sense for integers. The caller should have
	// already guarded via CycleBase / IsInteger, but we coerce to int64 here.
	// Negative numbers in hex/bin/oct are shown as two's complement style for
	// programmer feel is out of MVP scope — we just show the positive integer part.
	i := int64(val)
	if i < 0 {
		i = 0 // MVP: treat negative as 0 for non-DEC display
	}

	switch e.base {
	case BaseHEX:
		return strings.ToUpper(strconv.FormatInt(i, 16))
	case BaseBIN:
		return strconv.FormatInt(i, 2)
	case BaseOCT:
		return strconv.FormatInt(i, 8)
	default:
		return strconv.FormatFloat(val, 'f', -1, 64)
	}
}

// EnterDigit appends a decimal digit to the current entry buffer.
func (e *Engine) EnterDigit(d rune) {
	if e.entry == "0" || e.entry == "" {
		e.entry = string(d)
	} else {
		e.entry += string(d)
	}
}

// EnterDecimalPoint adds a '.' to the entry if one is not already present.
func (e *Engine) EnterDecimalPoint() {
	if !strings.Contains(e.entry, ".") {
		if e.entry == "" {
			e.entry = "0."
		} else {
			e.entry += "."
		}
	}
}

// SetOperator commits the current entry (if any) as the left operand and
// records the operator for the next Equals / next operator.
func (e *Engine) SetOperator(op string) {
	// Commit whatever is in the entry as the left side for this op.
	left := e.currentNumericValue()
	e.pendingLeft = left
	e.pendingOp = op
	e.entry = "0" // prepare for right-hand side entry
	e.accumulator = left
}

// Equals / commit the pending operation using the current entry as right-hand side.
func (e *Engine) Equals() {
	right := e.currentNumericValue()
	result := e.applyPending(e.pendingLeft, right, e.pendingOp)
	e.accumulator = result
	e.entry = formatResultEntry(result)
	e.pendingOp = ""
	e.pendingLeft = 0
}

// applyPending performs the arithmetic for the given operator.
func (e *Engine) applyPending(left, right float64, op string) float64 {
	switch op {
	case "+":
		return left + right
	case "-":
		return left - right
	case "*":
		return left * right
	case "/":
		if right == 0 {
			return 0 // MVP: avoid panic, could later surface "ERR"
		}
		return left / right
	case "mod", "MOD":
		return math.Mod(left, right)
	default:
		return right
	}
}

// formatResultEntry produces a clean entry string for a committed result.
func formatResultEntry(v float64) string {
	if math.IsInf(v, 0) || math.IsNaN(v) {
		return "0"
	}
	s := strconv.FormatFloat(v, 'f', -1, 64)
	// Trim trailing .000 etc for cleanliness while keeping decimal when needed.
	if strings.Contains(s, ".") {
		s = strings.TrimRight(s, "0")
		s = strings.TrimRight(s, ".")
	}
	if s == "" {
		s = "0"
	}
	return s
}

// Mod performs immediate modulo using the current entry as the right operand
// (useful for a dedicated MOD button that acts like = for modulo).
func (e *Engine) Mod() {
	right := e.currentNumericValue()
	result := math.Mod(e.accumulator, right)
	e.accumulator = result
	e.entry = formatResultEntry(result)
	e.pendingOp = ""
}

// ClearEntry clears only the current entry buffer (C button).
func (e *Engine) ClearEntry() {
	e.entry = "0"
}

// AllClear resets everything (AC button).
func (e *Engine) AllClear() {
	e.accumulator = 0
	e.entry = "0"
	e.pendingOp = ""
	e.pendingLeft = 0
	e.base = BaseDEC // or keep current base? MVP resets for simplicity; can relax later
}

// ChangeSign toggles the sign of the current entry (or accumulator if no entry).
func (e *Engine) ChangeSign() {
	if e.entry != "" && e.entry != "0" {
		if strings.HasPrefix(e.entry, "-") {
			e.entry = strings.TrimPrefix(e.entry, "-")
		} else {
			e.entry = "-" + e.entry
		}
		return
	}
	e.accumulator = -e.accumulator
	e.entry = formatResultEntry(e.accumulator)
}

// Backspace removes the last character from the current entry.
func (e *Engine) Backspace() {
	if len(e.entry) <= 1 {
		e.entry = "0"
		return
	}
	e.entry = e.entry[:len(e.entry)-1]
	if e.entry == "" || e.entry == "-" {
		e.entry = "0"
	}
}

// TODO for future: support for the exact "pending on display" formatting
// when a base change happens mid-operation (reformat the left operand).