cache.js

var _ = require("./helpers.js");

exports.wrapDriver = function (volume, opts) {
    opts = _.extend({
        maxSectors: 2048
    }, opts);
    
    var cache = {},
        advice = 'NORMAL',
        secSize = volume.sectorSize;
    
    function _freezeBuffer(b) {
        var f = _.allocBuffer(b.length);
        b.copy(f);
        return f;
    }
    
    function addToCache(i, data) {
        if (advice === 'SEQUENTIAL' || advice === 'NOREUSE') return;
        data = _freezeBuffer(data);
        cache[i] = data;
        //if (data.length > secSize) addToCache(i+1, data.slice(secSize));
        while (data.length > secSize) {
            data = data.slice(secSize);
            cache[++i] = data;
        }
        // simple highest-sectors-lose eviction policy for now
        Object.keys(cache).sort().slice(opts.maxSectors).forEach(function (x) {
            delete cache[x];
        });
        _.log(_.log.DBG, "Cache now contains:", Object.keys(cache).join(','));
    }
    
    return {
        sectorSize: volume.sectorSize,
        numSectors: volume.numSectors,
        advice: function (val) {
            if (!arguments.length) return advice;
            else advice = val;
            if (advice === 'SEQUENTIAL' || advice === 'NOREUSE') cache = {};
            return this;
        },
        readSectors: function (i, dest, cb) {
            // TODO: handle having partial parts of dest!
            if (i in cache && dest.length === secSize) {
                cache[i].copy(dest);
                setImmediate(cb);
            } else volume.readSectors(i, dest, function (e) {
                if (e) cb(e);
                else addToCache(i, dest), cb();
            });
        },
        writeSectors: (!volume.writeSectors) ? null : function (i, data, cb) {
            volume.writeSectors(i, data, function (e) {
                if (e) cb(e);
                else addToCache(i, data), cb();
            });
        }
    };
};