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Efficiently Process a Huge XML File

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Demonstrates a technique for processing a huge XML file (can be any size, even many gigabytes).

Note: This example requires Chilkat v9.5.0.80 or greater.

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const std = @import("std");
const chilkat = @import("chilkat");

pub fn main(init: std.process.Init) !void {
    const alloc = init.arena.allocator();

    // This example shows a way to efficiently process a gigantic XML file -- one that may be too large
    // to fit in memory.
    //
    // Two types of XML parsers exist: DOM parsers and SAX parsers.

    // A DOM parser is a Document Object Model parser, where the entire XML is loaded into memory
    // and the application has the luxury of interacting with the XML in a convenient, random-access
    // way.  The Chilkat Xml class is a DOM parser.  Because the entire XML is loaded into memory,
    // huge XML files (on the order of gigabytes) are usually not loadable for memory constraints.

    // A SAX parser is such that the XML file is parsed as an input stream.  No DOM exists.
    // Using a SAX parser is generally less palatable than using a DOM parser, for many reasons.
    //
    // The technique described here is a hybrid.  It streams the XML file as unstructured text
    // to extract fragments that are individually treated as separate XML documents loaded into
    // the Chilkat Xml parser.
    //
    // For example, imagine your XML file is several GBs in size, but has a relatively simple structure, such as:
    //
    // <Transactions>
    //     <Transaction id="1">
    //          ...
    //     </Transaction>
    //     <Transaction id="2">
    //          ...
    //     </Transaction>
    //     <Transaction id="3">
    //          ...
    //     </Transaction>
    // ...
    // </Transactions>

    // In the following code, each <Transaction ...> ... </Transaction>
    // is extracted and loaded separately into an Xml object, where it can be manipulated
    // independently.  The entire XML file is never entirely loaded into memory.

    const fac = try chilkat.FileAccess.init();
    defer fac.deinit();

    fac.openForRead("qa_data/xml/transactions.xml") catch {
        std.debug.print("{s}\n", .{try fac.getLastErrorText(alloc)});
        return;
    };

    const xml = try chilkat.Xml.init();
    defer xml.deinit();
    const sb = try chilkat.StringBuilder.init();
    defer sb.deinit();
    var first_iteration: bool = true;
    var retval: i32 = 1;
    var num_transactions: i32 = 0;

    // The begin marker is "XML tag aware".  If the begin marker begins with "<"
    // and ends with ">", then it is assumed to be an XML tag and it will also match
    // substrings where the ">" can be a whitespace char.
    const begin_marker = "<Transaction>";
    const end_marker = "</Transaction>";

    while (retval == 1) {
        sb.clear();
        // The retval can have the following values:
        // 0: No more fragments exist.
        // 1: Captured the next fragment.  The text from beginMarker to endMarker, including the markers, are returned in sb.
        // -1: Error.
        retval = fac.readNextFragment(first_iteration, begin_marker, end_marker, "utf-8", sb);
        first_iteration = false;

        if (retval == 1) {
            num_transactions = num_transactions + 1;
            try xml.loadSb(sb, true);
            // Your application may now do what it needs with this particular XML fragment...
        }
    }

    if (retval < 0) {
        std.debug.print("{s}\n", .{try fac.getLastErrorText(alloc)});
    }

    std.debug.print("numTransactions: {d}\n", .{num_transactions});
}