define([
    "jquery",
    "underscore",
    "lib/css_browser_selector"
], function($, _) {

    var scrollLink = {
        extensionId: "scrollLink",
        extensionName: "Scroll Link",
        optional: true,
        settingsBloc: [
            '<p>Binds together editor and preview scrollbars.</p>',
            '<blockquote class="muted"><b>NOTE:</b> ',
            '   The mapping between Markdown and HTML is based on the position of the title elements (h1, h2, ...) in the page. ',
            '   Therefore, if your document does not contain any title, the mapping will be linear and consequently less accurate.',
            '</bloquote>'
        ].join("")
    };

    var mdSectionList = [];
    var htmlSectionList = [];
    function pxToFloat(px) {
        return parseFloat(px.substring(0, px.length - 2));
    }
    var buildSections = _.debounce(function() {

        // Try to find Markdown sections by looking for titles
        var editorElt = $("#wmd-input");
        mdSectionList = [];
        // This textarea is used to measure sections height
        var textareaElt = $("#md-section-helper");
        // It has to be the same width than wmd-input
        textareaElt.width(editorElt.width());
        // Consider wmd-input top padding
        var padding = pxToFloat(editorElt.css('padding-top'));
        var offset = 0, mdSectionOffset = 0;
        function addMdSection(sectionText) {
            var sectionHeight = padding;
            if(sectionText !== undefined) {
                textareaElt.val(sectionText);
                sectionHeight += textareaElt.prop('scrollHeight');
            }
            var newSectionOffset = mdSectionOffset + sectionHeight;
            mdSectionList.push({
                startOffset: mdSectionOffset,
                endOffset: newSectionOffset,
                height: sectionHeight
            });
            mdSectionOffset = newSectionOffset;
            padding = 0;
        }
        // Create MD sections by finding title patterns (excluding gfm blocs)
        var text = editorElt.val() + "\n\n";
        text.replace(/^```.*\n[\s\S]*?\n```|(^.+[ \t]*\n=+[ \t]*\n+|^.+[ \t]*\n-+[ \t]*\n+|^\#{1,6}[ \t]*.+?[ \t]*\#*\n+)/gm, function(match, title, matchOffset) {
            if(title) {
                // We just found a title which means end of the previous section
                // Exclude last \n of the section
                var sectionText = undefined;
                if(matchOffset > offset) {
                    sectionText = text.substring(offset, matchOffset - 1);
                }
                addMdSection(sectionText);
                offset = matchOffset;
            }
            return "";
        });
        // Last section
        // Consider wmd-input bottom padding and exclude \n\n previously added
        padding += pxToFloat(editorElt.css('padding-bottom'));
        addMdSection(text.substring(offset, text.length - 2));

        // Try to find corresponding sections in the preview
        var previewElt = $("#wmd-preview");
        htmlSectionList = [];
        var htmlSectionOffset = 0;
        var previewScrollTop = previewElt.scrollTop();
        // Each title element is a section separator
        previewElt.children("h1,h2,h3,h4,h5,h6").each(function() {
            // Consider div scroll position and header element top margin
            var newSectionOffset = $(this).position().top + previewScrollTop + pxToFloat($(this).css('margin-top'));
            htmlSectionList.push({
                startOffset: htmlSectionOffset,
                endOffset: newSectionOffset,
                height: newSectionOffset - htmlSectionOffset
            });
            htmlSectionOffset = newSectionOffset;
        });
        // Last section
        var scrollHeight = previewElt.prop('scrollHeight');
        htmlSectionList.push({
            startOffset: htmlSectionOffset,
            endOffset: scrollHeight,
            height: scrollHeight - htmlSectionOffset
        });

        // apply Scroll Link
        lastEditorScrollTop = -10;
        skipScrollLink = false;
        isScrollPreview = false;
        runScrollLink();
    }, 500);

    // -10 to be sure the gap is > 9
    var lastEditorScrollTop = -10;
    var lastPreviewScrollTop = -10;
    var skipScrollLink = false;
    var isScrollPreview = false;
    var runScrollLink = _.debounce(function() {
        if(skipScrollLink === true || mdSectionList.length === 0 || mdSectionList.length !== htmlSectionList.length) {
            return;
        }
        var editorElt = $("#wmd-input");
        var editorScrollTop = editorElt.scrollTop();
        var previewElt = $("#wmd-preview");
        var previewScrollTop = previewElt.scrollTop();
        function animate(srcScrollTop, srcSectionList, destElt, destSectionList, lastDestScrollTop, callback) {
            // Find the section corresponding to the offset
            var sectionIndex = undefined;
            var srcSection = _.find(srcSectionList, function(section, index) {
                sectionIndex = index;
                return srcScrollTop < section.endOffset;
            });
            if(srcSection === undefined) {
                // Something wrong in the algorithm...
                return -10;
            }
            var posInSection = (srcScrollTop - srcSection.startOffset) / srcSection.height;
            var destSection = destSectionList[sectionIndex];
            var destScrollTop = destSection.startOffset + destSection.height * posInSection;
            destScrollTop = _.min([
                destScrollTop,
                destElt.prop('scrollHeight') - destElt.outerHeight()
            ]);
            if(Math.abs(destScrollTop - lastDestScrollTop) < 9) {
                // Skip the animation in case it's not necessary
                return;
            }
            destElt.animate({
                scrollTop: destScrollTop
            }, 600, function() {
                callback(destScrollTop);
            });
        }
        // Perform the animation if diff > 9px
        if(isScrollPreview === false && Math.abs(editorScrollTop - lastEditorScrollTop) > 9) {
            // Animate the preview
            lastEditorScrollTop = editorScrollTop;
            animate(editorScrollTop, mdSectionList, previewElt, htmlSectionList, lastPreviewScrollTop, function(destScrollTop) {
                lastPreviewScrollTop = destScrollTop;
            });
        }
        else if(Math.abs(previewScrollTop - lastPreviewScrollTop) > 9) {
            // Animate the editor
            lastPreviewScrollTop = previewScrollTop;
            animate(previewScrollTop, htmlSectionList, editorElt, mdSectionList, lastEditorScrollTop, function(destScrollTop) {
                lastEditorScrollTop = destScrollTop;
            });
        }
    }, 600);

    scrollLink.onLayoutConfigure = function(layoutConfig) {
        layoutConfig.onresize = buildSections;
    };

    scrollLink.onLayoutCreated = function() {
        $("#wmd-preview").scroll(function() {
            isScrollPreview = true;
            runScrollLink();
        });
        $("#wmd-input").scroll(function() {
            isScrollPreview = false;
            runScrollLink();
        });
    };

    scrollLink.onEditorConfigure = function(editor) {
        skipScrollLink = true;
        lastPreviewScrollTop = 0;
        editor.hooks.chain("onPreviewRefresh", function() {
            skipScrollLink = true;
        });
    };

    scrollLink.onPreviewFinished = function() {
        // MathJax may have change the scrolling position. Restore it.
        if(lastPreviewScrollTop >= 0) {
            $("#wmd-preview").scrollTop(lastPreviewScrollTop);
        }
        _.defer(function() {
            // Modify scroll position of the preview not the editor
            lastEditorScrollTop = -10;
            buildSections();
            // Preview may change if images are loading
            $("#wmd-preview img").load(function() {
                lastEditorScrollTop = -10;
                buildSections();
            });
        });
    };

    return scrollLink;
});