package app

import (
	"sync"
	"time"

	"golang.org/x/net/context"

	"github.com/weaveworks/scope/common/mtime"
	"github.com/weaveworks/scope/report"
)

// Reporter is something that can produce reports on demand. It's a convenient
// interface for parts of the app, and several experimental components.
type Reporter interface {
	Report(context.Context) (report.Report, error)
	WaitOn(context.Context, chan struct{})
	UnWait(context.Context, chan struct{})
}

// Adder is something that can accept reports. It's a convenient interface for
// parts of the app, and several experimental components.
type Adder interface {
	Add(context.Context, report.Report) error
}

// A Collector is a Reporter and an Adder
type Collector interface {
	Reporter
	Adder
}

// Collector receives published reports from multiple producers. It yields a
// single merged report, representing all collected reports.
type collector struct {
	mtx        sync.Mutex
	reports    []report.Report
	timestamps []time.Time
	window     time.Duration
	cached     *report.Report
	merger     Merger
	waitableCondition
}

type waitableCondition struct {
	sync.Mutex
	waiters map[chan struct{}]struct{}
}

func (wc *waitableCondition) WaitOn(_ context.Context, waiter chan struct{}) {
	wc.Lock()
	wc.waiters[waiter] = struct{}{}
	wc.Unlock()
}

func (wc *waitableCondition) UnWait(_ context.Context, waiter chan struct{}) {
	wc.Lock()
	delete(wc.waiters, waiter)
	wc.Unlock()
}

func (wc *waitableCondition) Broadcast() {
	wc.Lock()
	for waiter := range wc.waiters {
		// Non-block write to channel
		select {
		case waiter <- struct{}{}:
		default:
		}
	}
	wc.Unlock()
}

// NewCollector returns a collector ready for use.
func NewCollector(window time.Duration) Collector {
	return &collector{
		window: window,
		waitableCondition: waitableCondition{
			waiters: map[chan struct{}]struct{}{},
		},
		merger: NewSmartMerger(),
	}
}

// Add adds a report to the collector's internal state. It implements Adder.
func (c *collector) Add(_ context.Context, rpt report.Report) error {
	c.mtx.Lock()
	defer c.mtx.Unlock()
	c.reports = append(c.reports, rpt)
	c.timestamps = append(c.timestamps, mtime.Now())

	c.clean()
	c.cached = nil
	if rpt.Shortcut {
		c.Broadcast()
	}
	return nil
}

// Report returns a merged report over all added reports. It implements
// Reporter.
func (c *collector) Report(_ context.Context) (report.Report, error) {
	c.mtx.Lock()
	defer c.mtx.Unlock()

	// If the oldest report is still within range,
	// and there is a cached report, return that.
	if c.cached != nil && len(c.reports) > 0 {
		oldest := mtime.Now().Add(-c.window)
		if c.timestamps[0].After(oldest) {
			return *c.cached, nil
		}
	}

	c.clean()
	return c.merger.Merge(c.reports), nil
}

func (c *collector) clean() {
	var (
		cleanedReports    = make([]report.Report, 0, len(c.reports))
		cleanedTimestamps = make([]time.Time, 0, len(c.timestamps))
		oldest            = mtime.Now().Add(-c.window)
	)
	for i, r := range c.reports {
		if c.timestamps[i].After(oldest) {
			cleanedReports = append(cleanedReports, r)
			cleanedTimestamps = append(cleanedTimestamps, c.timestamps[i])
		}
	}
	c.reports = cleanedReports
	c.timestamps = cleanedTimestamps
}