function isValidDelim(state, pos) {
    var prevChar, nextChar,
        max = state.posMax,
        can_open = true,
        can_close = true;

    prevChar = pos > 0 ? state.src.charCodeAt(pos - 1) : -1;
    nextChar = pos + 1 <= max ? state.src.charCodeAt(pos + 1) : -1;

    if (prevChar === 0x20|| prevChar === 0x09 ||
            (nextChar >= 0x30 && nextChar <= 0x39)) {
        can_close = false;
    }
    if (nextChar === 0x20 || nextChar === 0x09) {
        can_open = false;
    }

    return {
        can_open: can_open,
        can_close: can_close
    };
}

function math_inline(state, silent) {
    var start, match, token, res, pos, esc_count;

    if (state.src[state.pos] !== "$") { return false; }

    res = isValidDelim(state, state.pos);
    if (!res.can_open) {
        if (!silent) { state.pending += "$"; }
        state.pos += 1;
        return true;
    }

    start = state.pos + 1;
    match = start;
    while ( (match = state.src.indexOf("$", match)) !== -1) {
        pos = match - 1;
        while (state.src[pos] === "\\") { pos -= 1; }

        if ( ((match - pos) % 2) == 1 ) { break; }
        match += 1;
    }

    if (match === -1) {
        if (!silent) { state.pending += "$"; }
        state.pos = start;
        return true;
    }

    if (match - start === 0) {
        if (!silent) { state.pending += "$$"; }
        state.pos = start + 1;
        return true;
    }

    res = isValidDelim(state, match);
    if (!res.can_close) {
        if (!silent) { state.pending += "$"; }
        state.pos = start;
        return true;
    }

    if (!silent) {
        token         = state.push('math_inline', 'math', 0);
        token.markup  = "$";
        token.content = state.src.slice(start, match);
    }

    state.pos = match + 1;
    return true;
}

function math_block(state, start, end, silent){
    var firstLine, lastLine, next, lastPos, found = false, token,
        pos = state.bMarks[start] + state.tShift[start],
        max = state.eMarks[start]

    if(pos + 2 > max){ return false; }
    if(state.src.slice(pos,pos+2)!=='$$'){ return false; }

    pos += 2;
    firstLine = state.src.slice(pos,max);

    if(silent){ return true; }
    if(firstLine.trim().slice(-2)==='$$'){
        firstLine = firstLine.trim().slice(0, -2);
        found = true;
    }

    for(next = start; !found; ){

        next++;

        if(next >= end) break;

        pos = state.bMarks[next]+state.tShift[next];
        max = state.eMarks[next];

        if(pos < max && state.tShift[next] < state.blkIndent)break;

        if(state.src.slice(pos,max).trim().slice(-2)==='$$'){
            lastPos = state.src.slice(0,max).lastIndexOf('$$');
            lastLine = state.src.slice(pos,lastPos);
            found = true;
        }

    }

    state.line = next + 1;

    token = state.push('math_block', 'math', 0);
    token.block = true;
    token.content = (firstLine && firstLine.trim() ? firstLine + '\n' : '')
    + state.getLines(start + 1, next, state.tShift[start], true)
    + (lastLine && lastLine.trim() ? lastLine : '');
    token.map = [ start, state.line ];
    token.markup = '$$';
    return true;
}

function escapeHtml(unsafe) {
    return unsafe
        .replace(/&/g, "&amp;")
        .replace(/</g, "&lt;")
        .replace(/>/g, "&gt;")
        .replace(/"/g, "&quot;")
        .replace(/'/g, "&#039;");
}

function math_plugin(md, options) {
    options = options || {};
    var katexInline = function(latex){
        options.displayMode = false;
        try{
            return katex.renderToString(latex, options);
        }
        catch(error){
            if(options.throwOnError){ console.log(error); }
            return `<span class='katex-error' title='${escapeHtml(error.toString())}'>${escapeHtml(latex)}</span>`;
        }
    };

    var inlineRenderer = function(tokens, idx){
        return katexInline(tokens[idx].content);
    };

    var katexBlock = function(latex){
        options.displayMode = true;
        try{
            return "<p class='katex-block'>" + katex.renderToString(latex, options) + "</p>";
        }
        catch(error){
            if(options.throwOnError){ console.log(error); }
            return `<p class='katex-block katex-error' title='${escapeHtml(error.toString())}'>${escapeHtml(latex)}</p>`;
        }
    }

    var blockRenderer = function(tokens, idx){
        return  katexBlock(tokens[idx].content) + '\n';
    }

    md.inline.ruler.after('escape', 'math_inline', math_inline);
    md.block.ruler.after('blockquote', 'math_block', math_block, {
        alt: [ 'paragraph', 'reference', 'blockquote', 'list' ]
    });
    md.renderer.rules.math_inline = inlineRenderer;
    md.renderer.rules.math_block = blockRenderer;
}