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<!DOCTYPE document PUBLIC "-//APACHE//DTD Documentation V1.3//EN" "../dtd/document-v13.dtd">

<document>
    <header>
        <title>Formula Support</title>
        <authors>
            <person email="avik@apache.org" name="Avik Sengupta" id="AS"/>
        </authors>
    </header>
    <body>
        <section><title>Introduction</title>
            <p>
                This document describes the current state of formula support in POI. 
                The information in this document currently applies to the 3.11 version of POI. 
                Since this area is a work in progress, this document will be updated with new features as and 
                when they are added. 
            </p>
           
        </section>
        <section><title>The basics</title>
              <p>
                In org.apache.poi.ss.usermodel.Cell
                    <strong> setCellFormula(&quot;formulaString&quot;) </strong> is used to add a formula to a sheet and 
                    <strong> getCellFormula() </strong> is used to retrieve the string representation of a formula. 
                </p>
            <p>
                We aim to support the complete excel grammar for formulas. Thus, the string that you pass in 
                to the <em> setCellFormula </em> call should be what you expect to type into excel. Also, note
                that you should NOT add a "=" to the front of the string.  
            </p>
            <p>
                Please note that localized versions of Excel allow to enter localized function-names. However internally
                Excel stores the English names and thus POI only supports these and not the localized ones.
            </p>
        </section>
        <section><title>Supported Features</title>
        <ul>
              <li>References: single cell &amp; area, 2D &amp; 3D, relative &amp; absolute</li>
              <li>Literals: number, text, boolean, error and array</li>
              <li>Operators: arithmetic and logical, some region operators</li>
              <li>Built-in functions: over 350 recognised, 280 evaluatable</li>
              <li>Add-in functions: 24 from Analysis Toolpack</li>
              <li>Array Formulas: via Sheet.setArrayFormula() and Sheet.removeArrayFormula()</li>
        </ul>
        </section>
        <section><title>Not yet supported</title>
            <ul>
                <li>Manipulating table formulas (In Excel, formulas that look like "{=...}" as opposed to "=...")</li>
                <li>Region operators: union, intersection</li>
                <li>Parsing of previously uncalled add-in functions</li>
                <li>Preservation of whitespace in formulas (when POI manipulates them)</li>
            </ul>
        </section>

        <section><title>Internals</title>
            <p>
                Formulas in Excel are stored as  sequences of tokens in Reverse Polish Notation order. The 
                <link href="http://sc.openoffice.org/excelfileformat.pdf">open office XLS spec</link> is the best
                documentation you will find for the format.
            </p>

            <p>
                The tokens used by excel are modeled as individual *Ptg classes in the <strong>
                org.apache.poi.hssf.record.formula</strong> package. 
            </p>
            <p>
                The task of parsing a formula string into an array of RPN ordered tokens is done by the <strong>
                org.apache.poi.ss.formula.FormulaParser</strong> class. This class implements a hand 
                written recursive descent parser. 
            </p>
            <p>
                Formula tokens in Excel are stored in one of three possible <em> operand classes </em>:
                Reference, Value and Array. Based on the location of a token, its class can change
                in complicated and undocumented ways. While we have support for most cases, we
                are not sure if we have covered all bases (since there is no documentation for this area.)
                We would therefore like you to report any
                occurrence of #VALUE! in a cell upon opening a POI generated workbook in excel. (Check that
                typing the formula into Excel directly gives a valid result.)
            </p>
            <p>Check out the <link href="http://poi.apache.org/apidocs/">javadocs </link> for details. 
            </p>
        </section>
    </body>
</document>
