// Copyright 2015 Matthew Holt and The Caddy Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package reverseproxy

import (
	"fmt"
	"net/url"
	"sync/atomic"

	"github.com/caddyserver/caddy/v2"
)

// Host represents a remote host which can be proxied to.
// Its methods must be safe for concurrent use.
type Host interface {
	// NumRequests returns the numnber of requests
	// currently in process with the host.
	NumRequests() int

	// Fails returns the count of recent failures.
	Fails() int

	// Unhealthy returns true if the backend is unhealthy.
	Unhealthy() bool

	// CountRequest counts the given number of requests
	// as currently in process with the host. The count
	// should not go below 0.
	CountRequest(int) error

	// CountFail counts the given number of failures
	// with the host. The count should not go below 0.
	CountFail(int) error

	// SetHealthy marks the host as either healthy (true)
	// or unhealthy (false). If the given status is the
	// same, this should be a no-op. It returns true if
	// the given status was different, false otherwise.
	SetHealthy(bool) (bool, error)
}

// UpstreamPool is a collection of upstreams.
type UpstreamPool []*Upstream

// Upstream bridges this proxy's configuration to the
// state of the backend host it is correlated with.
type Upstream struct {
	Host `json:"-"`

	Address     string `json:"address,omitempty"`
	MaxRequests int    `json:"max_requests,omitempty"`

	// TODO: This could be really useful, to bind requests
	// with certain properties to specific backends
	// HeaderAffinity string
	// IPAffinity     string

	healthCheckPolicy *PassiveHealthChecks
	hostURL           *url.URL
}

// Available returns true if the remote host
// is available to receive requests.
func (u *Upstream) Available() bool {
	return u.Healthy() && !u.Full()
}

// Healthy returns true if the remote host
// is currently known to be healthy or "up".
func (u *Upstream) Healthy() bool {
	healthy := !u.Host.Unhealthy()
	if healthy && u.healthCheckPolicy != nil {
		healthy = u.Host.Fails() < u.healthCheckPolicy.MaxFails
	}
	return healthy
}

// Full returns true if the remote host
// cannot receive more requests at this time.
func (u *Upstream) Full() bool {
	return u.MaxRequests > 0 && u.Host.NumRequests() >= u.MaxRequests
}

// URL returns the upstream host's endpoint URL.
func (u *Upstream) URL() *url.URL {
	return u.hostURL
}

// upstreamHost is the basic, in-memory representation
// of the state of a remote host. It implements the
// Host interface.
type upstreamHost struct {
	numRequests int64 // must be first field to be 64-bit aligned on 32-bit systems (see https://golang.org/pkg/sync/atomic/#pkg-note-BUG)
	fails       int64
	unhealthy   int32
}

// NumRequests returns the number of active requests to the upstream.
func (uh *upstreamHost) NumRequests() int {
	return int(atomic.LoadInt64(&uh.numRequests))
}

// Fails returns the number of recent failures with the upstream.
func (uh *upstreamHost) Fails() int {
	return int(atomic.LoadInt64(&uh.fails))
}

// Unhealthy returns whether the upstream is healthy.
func (uh *upstreamHost) Unhealthy() bool {
	return atomic.LoadInt32(&uh.unhealthy) == 1
}

// CountRequest mutates the active request count by
// delta. It returns an error if the adjustment fails.
func (uh *upstreamHost) CountRequest(delta int) error {
	result := atomic.AddInt64(&uh.numRequests, int64(delta))
	if result < 0 {
		return fmt.Errorf("count below 0: %d", result)
	}
	return nil
}

// CountFail mutates the recent failures count by
// delta. It returns an error if the adjustment fails.
func (uh *upstreamHost) CountFail(delta int) error {
	result := atomic.AddInt64(&uh.fails, int64(delta))
	if result < 0 {
		return fmt.Errorf("count below 0: %d", result)
	}
	return nil
}

// SetHealthy sets the upstream has healthy or unhealthy
// and returns true if the value was different from before,
// or an error if the adjustment failed.
func (uh *upstreamHost) SetHealthy(healthy bool) (bool, error) {
	var unhealthy, compare int32 = 1, 0
	if healthy {
		unhealthy, compare = 0, 1
	}
	swapped := atomic.CompareAndSwapInt32(&uh.unhealthy, compare, unhealthy)
	return swapped, nil
}

// hosts is the global repository for hosts that are
// currently in use by active configuration(s). This
// allows the state of remote hosts to be preserved
// through config reloads.
var hosts = caddy.NewUsagePool()