// Package sessionlimit defines the domain decision shape used for active
// device-session limit evaluation.
package sessionlimit

import (
	"errors"
	"fmt"
)

// Kind identifies the coarse outcome of evaluating the active-session limit.
type Kind string

const (
	// KindDisabled reports that no configured limit is currently active.
	KindDisabled Kind = "disabled"

	// KindAllowed reports that creating the next session is allowed.
	KindAllowed Kind = "allowed"

	// KindExceeded reports that creating the next session would exceed the
	// configured limit.
	KindExceeded Kind = "exceeded"
)

// IsKnown reports whether Kind is one of the session-limit outcomes supported
// by the current domain model.
func (k Kind) IsKnown() bool {
	switch k {
	case KindDisabled, KindAllowed, KindExceeded:
		return true
	default:
		return false
	}
}

// Decision stores the result of evaluating one possible next session creation.
type Decision struct {
	// Kind reports the coarse decision outcome.
	Kind Kind

	// ConfiguredLimit stores the active configured limit when one exists.
	ConfiguredLimit *int

	// ActiveSessionCount stores the current active-session count before create.
	ActiveSessionCount int

	// NextSessionCount stores the count that would exist after creating the next
	// session.
	NextSessionCount int
}

// Validate reports whether Decision satisfies the Stage-2 structural
// invariants.
func (d Decision) Validate() error {
	if !d.Kind.IsKnown() {
		return fmt.Errorf("session-limit decision kind %q is unsupported", d.Kind)
	}
	if d.ActiveSessionCount < 0 {
		return errors.New("session-limit active session count must not be negative")
	}
	if d.NextSessionCount < 0 {
		return errors.New("session-limit next session count must not be negative")
	}
	if d.NextSessionCount != d.ActiveSessionCount+1 {
		return errors.New("session-limit next session count must equal active session count plus one")
	}

	switch d.Kind {
	case KindDisabled:
		if d.ConfiguredLimit != nil {
			return errors.New("disabled session-limit decision must not contain configured limit")
		}
	case KindAllowed, KindExceeded:
		if d.ConfiguredLimit == nil {
			return errors.New("limited session-limit decision must contain configured limit")
		}
		if *d.ConfiguredLimit <= 0 {
			return errors.New("session-limit configured limit must be positive")
		}
		if d.Kind == KindAllowed && d.NextSessionCount > *d.ConfiguredLimit {
			return errors.New("allowed session-limit decision must not exceed configured limit")
		}
		if d.Kind == KindExceeded && d.NextSessionCount <= *d.ConfiguredLimit {
			return errors.New("exceeded session-limit decision must be above configured limit")
		}
	}

	return nil
}