package eventrules

// go generate -import github.com/mesos/mesos-go/api/v1/lib/scheduler -type E:*scheduler.Event:&scheduler.Event{}
// GENERATED CODE FOLLOWS; DO NOT EDIT.

import (
	"context"
	"fmt"
	"sync"

	"github.com/mesos/mesos-go/api/v1/lib/scheduler"
)

type (
	evaler interface {
		// Eval executes a filter, rule, or decorator function; if the returned event is nil then
		// no additional rule evaluation should be processed for the event.
		// Eval implementations should not modify the given event parameter (to avoid side effects).
		// If changes to the event object are needed, the suggested approach is to make a copy,
		// modify the copy, and pass the copy to the chain.
		// Eval implementations SHOULD be safe to execute concurrently.
		Eval(context.Context, *scheduler.Event, error, Chain) (context.Context, *scheduler.Event, error)
	}

	// Rule is the functional adaptation of evaler.
	// A nil Rule is valid: it is Eval'd as a noop.
	Rule func(context.Context, *scheduler.Event, error, Chain) (context.Context, *scheduler.Event, error)

	// Chain is invoked by a Rule to continue processing an event. If the chain is not invoked,
	// no additional rules are processed.
	Chain func(context.Context, *scheduler.Event, error) (context.Context, *scheduler.Event, error)

	// Rules is a list of rules to be processed, in order.
	Rules []Rule

	// ErrorList accumulates errors that occur while processing a Chain of Rules. Accumulated
	// errors should be appended to the end of the list. An error list should never be empty.
	// Callers should use the package Error() func to properly accumulate (and flatten) errors.
	ErrorList []error
)

var (
	_ = evaler(Rule(nil))
	_ = evaler(Rules{})
)

// ChainIdentity is a Chain that returns the arguments as its results.
func ChainIdentity(ctx context.Context, e *scheduler.Event, err error) (context.Context, *scheduler.Event, error) {
	return ctx, e, err
}

// Eval is a convenience func that processes a nil Rule as a noop.
func (r Rule) Eval(ctx context.Context, e *scheduler.Event, err error, ch Chain) (context.Context, *scheduler.Event, error) {
	if r != nil {
		return r(ctx, e, err, ch)
	}
	return ch(ctx, e, err)
}

// Eval is a Rule func that processes the set of all Rules. If there are no rules in the
// set then control is simply passed to the Chain.
func (rs Rules) Eval(ctx context.Context, e *scheduler.Event, err error, ch Chain) (context.Context, *scheduler.Event, error) {
	return ch(rs.Chain()(ctx, e, err))
}

// Chain returns a Chain that evaluates the given Rules, in order, propagating the (context.Context, *scheduler.Event, error)
// from Rule to Rule. Chain is safe to invoke concurrently.
func (rs Rules) Chain() Chain {
	if len(rs) == 0 {
		return ChainIdentity
	}
	return func(ctx context.Context, e *scheduler.Event, err error) (context.Context, *scheduler.Event, error) {
		return rs[0].Eval(ctx, e, err, rs[1:].Chain())
	}
}

// New is the semantic equivalent of Rules{r1, r2, ..., rn}.Eval and exists purely for convenience.
func New(rs ...Rule) Rule { return Rules(rs).Eval }

const msgNoErrors = "no errors"

// Error implements error; returns the message of the first error in the list.
func (es ErrorList) Error() string {
	switch len(es) {
	case 0:
		return msgNoErrors
	case 1:
		return es[0].Error()
	default:
		return fmt.Sprintf("%s (and %d more errors)", es[0], len(es)-1)
	}
}

// Error2 aggregates the given error params, returning nil if both are nil.
// Use Error2 to avoid the overhead of creating a slice when aggregating only 2 errors.
func Error2(a, b error) error {
	if a == nil {
		if b == nil {
			return nil
		}
		if list, ok := b.(ErrorList); ok {
			return flatten(list).Err()
		}
		return b
	}
	if b == nil {
		if list, ok := a.(ErrorList); ok {
			return flatten(list).Err()
		}
		return a
	}
	return Error(a, b)
}

// Err reduces an empty or singleton error list
func (es ErrorList) Err() error {
	switch len(es) {
	case 0:
		return nil
	case 1:
		return es[0]
	default:
		return es
	}
}

// IsErrorList returns true if err is a non-nil error list
func IsErrorList(err error) bool {
	if err != nil {
		_, ok := err.(ErrorList)
		return ok
	}
	return false
}

// Error aggregates, and then flattens, a list of errors accrued during rule processing.
// Returns nil if the given list of errors is empty or contains all nil errors.
func Error(es ...error) error {
	return flatten(es).Err()
}

func flatten(errors []error) ErrorList {
	if errors == nil || len(errors) == 0 {
		return nil
	}
	result := make([]error, 0, len(errors))
	for _, err := range errors {
		if err != nil {
			if multi, ok := err.(ErrorList); ok {
				result = append(result, flatten(multi)...)
			} else {
				result = append(result, err)
			}
		}
	}
	return ErrorList(result)
}

// TODO(jdef): other ideas for Rule decorators: When(func() bool), WhenNot(func() bool)

// If only executes the receiving rule if b is true; otherwise, the returned rule is a noop.
func (r Rule) If(b bool) Rule {
	if b {
		return r
	}
	return nil
}

// Unless only executes the receiving rule if b is false; otherwise, the returned rule is a noop.
func (r Rule) Unless(b bool) Rule {
	if !b {
		return r
	}
	return nil
}

// Once returns a Rule that executes the receiver only once.
func (r Rule) Once() Rule {
	if r == nil {
		return nil
	}
	var once sync.Once
	return func(ctx context.Context, e *scheduler.Event, err error, ch Chain) (context.Context, *scheduler.Event, error) {
		ruleInvoked := false
		once.Do(func() {
			ctx, e, err = r(ctx, e, err, ch)
			ruleInvoked = true
		})
		if !ruleInvoked {
			ctx, e, err = ch(ctx, e, err)
		}
		return ctx, e, err
	}
}

// UnlessDone returns a decorated rule that checks context.Done: if the context has been canceled then the rule chain
// is aborted and the context.Err is merged with the current error state.
// Returns nil (noop) if the receiving Rule is nil.
func (r Rule) UnlessDone() Rule {
	if r == nil {
		return nil
	}
	return func(ctx context.Context, e *scheduler.Event, err error, ch Chain) (context.Context, *scheduler.Event, error) {
		select {
		case <-ctx.Done():
			return ctx, e, Error2(err, ctx.Err())
		default:
			return r(ctx, e, err, ch)
		}
	}
}

type Overflow int

const (
	// OverflowWait waits until the rule may execute, or the context is canceled.
	OverflowWait Overflow = iota
	// OverflowOtherwise skips over the decorated rule and invoke an alternative instead.
	OverflowOtherwise
)

// RateLimit invokes the receiving Rule if a read of chan "p" succeeds (closed chan = no rate limit), otherwise proceeds
// according to the specified Overflow policy. May be useful, for example, when rate-limiting logged events.
// Returns nil (noop) if the receiver is nil, otherwise a nil chan will normally trigger an overflow.
// Panics when OverflowWait is specified with a nil chan, in order to prevent deadlock.
// A cancelled context will trigger the "otherwise" rule.
func (r Rule) RateLimit(p <-chan struct{}, over Overflow, otherwise Rule) Rule {
	return limit(r, acquireChan(p), over, otherwise)
}

// acquireChan wraps a signal chan with a func that can be used with rateLimit.
// should only be called by rate limiting funcs (that implement deadlock avoidance).
func acquireChan(tokenCh <-chan struct{}) func(context.Context, bool) bool {
	if tokenCh == nil {
		// always false: acquire never succeeds; panic if told to block (to avoid deadlock)
		return func(ctx context.Context, block bool) bool {
			if block {
				select {
				case <-ctx.Done():
				default:
					panic("deadlock detected: block should never be true when the token chan is nil")
				}
			}
			return false
		}
	}
	return func(ctx context.Context, block bool) bool {
		if block {
			select {
			case <-tokenCh:
				// tie breaker prefers Done
				select {
				case <-ctx.Done():
				default:
					return true
				}
			case <-ctx.Done():
			}
			return false
		}
		select {
		case <-tokenCh:
			return true
		default:
			return false
		}
	}
}

// limit is a generic Rule decorator that limits invocations of said Rule.
// The "acquire" func SHOULD NOT block if the supplied Context is Done.
// MUST only invoke "acquire" once per event.
// TODO(jdef): leaving this as internal for now because the interface still feels too messy.
func limit(r Rule, acquire func(_ context.Context, block bool) bool, over Overflow, otherwise Rule) Rule {
	if r == nil {
		return nil
	}
	if acquire == nil {
		panic("acquire func is not allowed to be nil")
	}
	blocking := false
	switch over {
	case OverflowOtherwise:
	case OverflowWait:
		blocking = true
	default:
		panic(fmt.Sprintf("unexpected Overflow type: %#v", over))
	}
	return func(ctx context.Context, e *scheduler.Event, err error, ch Chain) (context.Context, *scheduler.Event, error) {
		if !acquire(ctx, blocking) {
			return otherwise.Eval(ctx, e, err, ch)
		}
		return r(ctx, e, err, ch)
	}
}

/* TODO(jdef) not sure that this is very useful, leaving out for now...

// EveryN invokes the receiving rule beginning with the first event seen and then every n'th
// time after that. If nthTime is less then 2 then the receiver is returned, undecorated.
// The "otherwise" Rule (may be null) is invoked for every event in between the n'th invocations.
// A cancelled context will trigger the "otherwise" rule.
func (r Rule) EveryN(nthTime int, otherwise Rule) Rule {
	if nthTime < 2 || r == nil {
		return r
	}
	return limit(r, acquireEveryN(nthTime), OverflowOtherwise, otherwise)
}

// acquireEveryN returns an "acquire" func (for use w/ rate-limiting) that returns true every N'th invocation.
// the returned func MUST NOT be used with a potentially blocking Overflow policy (or else it panics).
// nthTime SHOULD be greater than math.MinInt32, values less than 2 probably don't make sense in practice.
func acquireEveryN(nthTime int) func(context.Context, bool) bool {
	var (
		i       = 1 // begin with the first event seen
		m       sync.Mutex
	)
	return func(ctx context.Context, block bool) (result bool) {
		if block {
			panic("acquireEveryN should never be asked to block")
		}
		select {
		case <-ctx.Done():
		default:
			m.Lock()
			i--
			if i <= 0 {
				i = nthTime
				result = true
			}
			m.Unlock()
		}
		return
	}
}

*/

// Drop aborts the Chain and returns the (context.Context, *scheduler.Event, error) tuple as-is.
func Drop() Rule {
	return Rule(nil).ThenDrop()
}

// ThenDrop executes the receiving rule, but aborts the Chain, and returns the (context.Context, *scheduler.Event, error) tuple as-is.
func (r Rule) ThenDrop() Rule {
	return func(ctx context.Context, e *scheduler.Event, err error, _ Chain) (context.Context, *scheduler.Event, error) {
		return r.Eval(ctx, e, err, ChainIdentity)
	}
}

// Fail returns a Rule that injects the given error.
func Fail(injected error) Rule {
	return func(ctx context.Context, e *scheduler.Event, err error, ch Chain) (context.Context, *scheduler.Event, error) {
		return ch(ctx, e, Error2(err, injected))
	}
}

// DropOnError returns a Rule that generates a nil event if the error state != nil
func DropOnError() Rule {
	return Rule(nil).DropOnError()
}

// DropOnError decorates a rule by pre-checking the error state: if the error state != nil then
// the receiver is not invoked and (e, err) is returned; otherwise control passes to the receiving rule.
func (r Rule) DropOnError() Rule {
	return func(ctx context.Context, e *scheduler.Event, err error, ch Chain) (context.Context, *scheduler.Event, error) {
		if err != nil {
			return ctx, e, err
		}
		return r.Eval(ctx, e, err, ch)
	}
}

// AndThen returns a list of rules, beginning with the receiver, followed by DropOnError, and then
// all of the rules specified by the next parameter. The net effect is: execute the receiver rule
// and only if there is no error state, continue processing the next rules, in order.
func (r Rule) AndThen(next ...Rule) Rule {
	return append(Rules{r, DropOnError()}, next...).Eval
}

func DropOnSuccess() Rule {
	return Rule(nil).DropOnSuccess()
}

func (r Rule) DropOnSuccess() Rule {
	return func(ctx context.Context, e *scheduler.Event, err error, ch Chain) (context.Context, *scheduler.Event, error) {
		if err == nil {
			// bypass remainder of chain
			return ctx, e, err
		}
		return r.Eval(ctx, e, err, ch)
	}
}

func (r Rule) OnFailure(next ...Rule) Rule {
	return append(Rules{r, DropOnSuccess()}, next...).Eval
}