/*
 * Minio Cloud Storage, (C) 2016 Minio, Inc.
 *
 * 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 main

import (
	"sync"
	"time"
)

// commonTime returns a maximally occurring time from a list of time.
func commonTime(modTimes []time.Time) (modTime time.Time) {
	var maxima int // Counter for remembering max occurrence of elements.
	timeOccurenceMap := make(map[time.Time]int)
	// Ignore the uuid sentinel and count the rest.
	for _, time := range modTimes {
		if time == timeSentinel {
			continue
		}
		timeOccurenceMap[time]++
	}
	// Find the common cardinality from previously collected
	// occurrences of elements.
	for time, count := range timeOccurenceMap {
		if count > maxima {
			maxima = count
			modTime = time
		}
	}
	// Return the collected common uuid.
	return modTime
}

var timeSentinel = time.Unix(0, 0).UTC()

// Boot uuids upto disk count, setting the value to UUID sentinel.
func bootModtimes(diskCount int) []time.Time {
	modTimes := make([]time.Time, diskCount)
	// Boots up all the uuids.
	for i := range modTimes {
		modTimes[i] = timeSentinel
	}
	return modTimes
}

// Extracts list of times from xlMetaV1 slice and returns, skips
// slice elements which have errors. As a special error
// errFileNotFound is treated as a initial good condition.
func listObjectModtimes(partsMetadata []xlMetaV1, errs []error) (modTimes []time.Time) {
	modTimes = bootModtimes(len(partsMetadata))
	// Set a new time value, specifically set when
	// error == errFileNotFound (this is needed when this is a
	// fresh PutObject).
	timeNow := time.Now().UTC()
	for index, metadata := range partsMetadata {
		if errs[index] == nil {
			// Once the file is found, save the uuid saved on disk.
			modTimes[index] = metadata.Stat.ModTime
		} else if errs[index] == errFileNotFound {
			// Once the file is not found then the epoch is current time.
			modTimes[index] = timeNow
		}
	}
	return modTimes
}

// Reads all `xl.json` metadata as a xlMetaV1 slice.
// Returns error slice indicating the failed metadata reads.
func readAllXLMetadata(disks []StorageAPI, bucket, object string) ([]xlMetaV1, []error) {
	errs := make([]error, len(disks))
	metadataArray := make([]xlMetaV1, len(disks))
	var wg = &sync.WaitGroup{}
	// Read `xl.json` parallelly across disks.
	for index, disk := range disks {
		if disk == nil {
			errs[index] = errDiskNotFound
			continue
		}
		wg.Add(1)
		// Read `xl.json` in routine.
		go func(index int, disk StorageAPI) {
			defer wg.Done()
			var err error
			metadataArray[index], err = readXLMeta(disk, bucket, object)
			if err != nil {
				errs[index] = err
				return
			}
		}(index, disk)
	}

	// Wait for all the routines to finish.
	wg.Wait()

	// Return all the metadata.
	return metadataArray, errs
}

func (xl xlObjects) shouldHeal(onlineDisks []StorageAPI) (heal bool) {
	onlineDiskCount := diskCount(onlineDisks)
	// If online disks count is lesser than configured disks, most
	// probably we need to heal the file, additionally verify if the
	// count is lesser than readQuorum, if not we throw an error.
	if onlineDiskCount < len(xl.storageDisks) {
		// Online disks lesser than total storage disks, needs to be
		// healed. unless we do not have readQuorum.
		heal = true
		// Verify if online disks count are lesser than readQuorum
		// threshold, return an error.
		if onlineDiskCount < xl.readQuorum {
			errorIf(errXLReadQuorum, "Unable to establish read quorum, disks are offline.")
			return false
		}
	}
	return heal
}

// Returns slice of online disks needed.
// - slice returing readable disks.
// - xlMetaV1
// - bool value indicating if healing is needed.
func (xl xlObjects) listOnlineDisks(partsMetadata []xlMetaV1, errs []error) (onlineDisks []StorageAPI, modTime time.Time) {
	onlineDisks = make([]StorageAPI, len(xl.storageDisks))

	// List all the file commit ids from parts metadata.
	modTimes := listObjectModtimes(partsMetadata, errs)

	// Reduce list of UUIDs to a single common value.
	modTime = commonTime(modTimes)

	// Create a new online disks slice, which have common uuid.
	for index, t := range modTimes {
		if t == modTime {
			onlineDisks[index] = xl.storageDisks[index]
		} else {
			onlineDisks[index] = nil
		}
	}
	return onlineDisks, modTime
}