wavesurfer/src/peakcache.js

Interactive navigable audio visualization using Web Audio and Canvas

'use strict';

WaveSurfer.PeakCache = {
    init: function() {
        this.clearPeakCache();
    },

    clearPeakCache: function() {
	// Flat array with entries that are always in pairs to mark the
	// beginning and end of each subrange.  This is a convenience so we can
	// iterate over the pairs for easy set difference operations.
        this.peakCacheRanges = [];
	// Length of the entire cachable region, used for resetting the cache
	// when this changes (zoom events, for instance).
        this.peakCacheLength = -1;
    },

    addRangeToPeakCache: function(length, start, end) {
        if (length != this.peakCacheLength) {
            this.clearPeakCache();
            this.peakCacheLength = length;
        }

        // Return ranges that weren't in the cache before the call.
        var uncachedRanges = [];
        var i = 0;
        // Skip ranges before the current start.
        while (i < this.peakCacheRanges.length && this.peakCacheRanges[i] < start) {
            i++;
        }
	// If |i| is even, |start| falls after an existing range.  Otherwise,
	// |start| falls between an existing range, and the uncached region
	// starts when we encounter the next node in |peakCacheRanges| or
	// |end|, whichever comes first.
        if (i % 2 == 0) {
            uncachedRanges.push(start);
        }
        while (i < this.peakCacheRanges.length && this.peakCacheRanges[i] <= end) {
            uncachedRanges.push(this.peakCacheRanges[i]);
            i++;
        }
        // If |i| is even, |end| is after all existing ranges.
        if (i % 2 == 0) {
            uncachedRanges.push(end);
        }

        // Filter out the 0-length ranges.
        uncachedRanges = uncachedRanges.filter(function(item, pos, arr) {
            if (pos == 0) {
                return item != arr[pos + 1];
            } else if (pos == arr.length - 1) {
                return item != arr[pos - 1];
            } else {
                return item != arr[pos - 1] && item != arr[pos + 1];
            }
        });

	// Merge the two ranges together, uncachedRanges will either contain
	// wholly new points, or duplicates of points in peakCacheRanges.  If
	// duplicates are detected, remove both and extend the range.
        this.peakCacheRanges = this.peakCacheRanges.concat(uncachedRanges);
        this.peakCacheRanges = this.peakCacheRanges.sort(function(a, b) {
            return a - b;
        }).filter(function(item, pos, arr) {
            if (pos == 0) {
                return item != arr[pos + 1];
            } else if (pos == arr.length - 1) {
                return item != arr[pos - 1];
            } else {
                return item != arr[pos - 1] && item != arr[pos + 1];
            }
        });

	// Push the uncached ranges into an array of arrays for ease of
	// iteration in the functions that call this.
        var uncachedRangePairs = [];
        for (i = 0; i < uncachedRanges.length; i += 2) {
            uncachedRangePairs.push([uncachedRanges[i], uncachedRanges[i+1]]);
        }

        return uncachedRangePairs;
    },

    // For testing
    getCacheRanges: function() {
      var peakCacheRangePairs = [];
      for (var i = 0; i < this.peakCacheRanges.length; i += 2) {
          peakCacheRangePairs.push([this.peakCacheRanges[i], this.peakCacheRanges[i+1]]);
      }
      return peakCacheRangePairs;
    }
};