annotate base/StorageAdviser.h @ 1752:6d09d68165a4 by-id

Further review of ById: make IDs only available when adding a model to the ById store, not by querying the item directly. This means any id encountered in the wild must have been added to the store at some point (even if later released), which simplifies reasoning about lifecycles

/* -*- c-basic-offset: 4 indent-tabs-mode: nil -*-  vi:set ts=8 sts=4 sw=4: */

/*
    Sonic Visualiser
    An audio file viewer and annotation editor.
    Centre for Digital Music, Queen Mary, University of London.
    This file copyright 2006 QMUL.
    
    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License as
    published by the Free Software Foundation; either version 2 of the
    License, or (at your option) any later version.  See the file
    COPYING included with this distribution for more information.
*/

#ifndef SV_STORAGE_ADVISER_H
#define SV_STORAGE_ADVISER_H

#include <cstdlib>

#include <QString>

/**
 * A utility class designed to help decide whether to store cache data
 * (for example FFT outputs) in memory or on disk in the TempDirectory.
 * This is basically a compendium of simple rules of thumb.
 */

class StorageAdviser
{
public:
    // pass to recommend() zero or more of these OR'd together
    enum Criteria {
        NoCriteria           = 0,
        SpeedCritical        = 1,
        PrecisionCritical    = 2,
        LongRetentionLikely  = 4,
        FrequentLookupLikely = 8
    };

    // recommend() returns one or two of these OR'd together
    enum Recommendation {
        NoRecommendation   = 0,
        UseMemory          = 1, // Disc is strongly contraindicated
        PreferMemory       = 2, // Either would do; memory probably better
        PreferDisc         = 4, // Either would do; disc probably better
        UseDisc            = 8, // Probably won't fit in memory
        ConserveSpace      = 16,// Whatever you choose, keep it compact
        UseAsMuchAsYouLike = 32 // Take my advice and there'll be space for all
    };

    /**
     * Recommend where to store some data, given certain storage and
     * recall criteria.  The minimum size is the approximate amount of
     * data in kilobytes that will be stored if the recommendation is
     * to ConserveSpace; the maximum size is approximately the amount
     * that will be used if UseAsMuchAsYouLike is returned.
     *
     * May throw InsufficientDiscSpace exception if there appears to
     * be nowhere the minimum amount of data can be stored.
     */
    static Recommendation recommend(Criteria criteria,
                                    size_t minimumSize,
                                    size_t maximumSize);

    enum AllocationArea {
        MemoryAllocation,
        DiscAllocation
    };

    /**
     * Specify that we are planning to use a given amount of storage
     * (in kilobytes), but haven't allocated it yet.
     */
    static void notifyPlannedAllocation(AllocationArea area, size_t size);

    /**
     * Specify that we have now allocated, or abandoned the allocation
     * of, the given amount (in kilobytes) of a storage area that was
     * previously notified using notifyPlannedAllocation.
     */
    static void notifyDoneAllocation(AllocationArea area, size_t size);

    /**
     * Force all subsequent recommendations to use the (perhaps
     * partial) specification given here.  If NoRecommendation given
     * here, this will reset to the default free behaviour.
     */
    static void setFixedRecommendation(Recommendation recommendation);

private:
    static size_t m_discPlanned;
    static size_t m_memoryPlanned;
    static Recommendation m_baseRecommendation;

    enum StorageStatus {
        Unknown,
        Insufficient,
        Marginal,
        Sufficient
    };

    static QString criteriaToString(int);
    static QString recommendationToString(int);
    static QString storageStatusToString(StorageStatus);
};

#endif