package engine

import "fmt"

// Operator is a scalar comparison used by generalised constraints like
// Altitude and Time. A constraint fires when its Operator.Test(value, limit)
// returns true.
type Operator int

const (
	OpLess         Operator = iota // value < limit
	OpLessEqual                    // value ≤ limit
	OpGreater                      // value > limit
	OpGreaterEqual                 // value ≥ limit
	OpEqual                        // value == limit
)

// Test evaluates op(value, limit).
func (o Operator) Test(value, limit float64) bool {
	switch o {
	case OpLess:
		return value < limit
	case OpLessEqual:
		return value <= limit
	case OpGreater:
		return value > limit
	case OpGreaterEqual:
		return value >= limit
	case OpEqual:
		return value == limit
	}
	return false
}

// String returns the symbol "<", "<=", ">", ">=", "==".
func (o Operator) String() string {
	switch o {
	case OpLess:
		return "<"
	case OpLessEqual:
		return "<="
	case OpGreater:
		return ">"
	case OpGreaterEqual:
		return ">="
	case OpEqual:
		return "=="
	}
	return "?"
}

// ParseOperator maps a textual operator to its Operator constant.
// Accepts "<", "<=", "le", ">", ">=", "ge", "==", "eq".
func ParseOperator(s string) (Operator, error) {
	switch s {
	case "<", "lt":
		return OpLess, nil
	case "<=", "le":
		return OpLessEqual, nil
	case ">", "gt":
		return OpGreater, nil
	case ">=", "ge":
		return OpGreaterEqual, nil
	case "==", "eq":
		return OpEqual, nil
	default:
		return 0, fmt.Errorf("unknown operator %q", s)
	}
}