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Basic algorithm

Then, the basic flow of what we will call the ``postdoc" function operates as follows.

For every document in the corpus we will iterate through a loop until we've exhausted every token in that document. So let's call { getNonNoiseToken } a routine which repeatedly builds tokens from the document's stream, does whatever character assessments are required, checks it against a negative dictionary, and returns a token. If stemming is to be applied, we'll stem the word at this point. Then we will save a POSTING for that token's occurrence in that document. A posting is simply a correspondence between a particular word and a particular document, representing the occurrence of that word in that document. That is, we have a document in front of us and it contains a set of tokens. We are now going to build a representation for each token that tells all of the documents in which it occurs. For each keyword we will maintain the token itself as the key used for subsequent access, and also the head of a linked list of all of postings, each containing the document number and the number of occurrences of the keyword in that document. A sketch of these data structures is shown in Figure (figure) .

After having gone through every single document in the corpus in this fashion, we have a large collection of postings. Here we recommend SPLAY TREES as an appropriate data structure for these keywords and their postings. In the C implementation shown in Figure (FOAref) , the InstallTerm() function inserts a new posting into the Terms tree.

$ is the same as the last time, and simply increment the appropriate counter in this case.

Splay trees are appropriate technology because we can count on the many frequency-biased queries to cause the resulting tree to become well-balanced with use.}

During the processing of each document, it will prove important to know how many keywords are extracted from it. This will be known as the DOCUMENT'S LENGTH , denoted $length_d$; this quantity is important when {\em normalizing} documents of different lengths. One way to implement this computation is to maintain a small, separate file {\tt doclend.d} containing only this one number for each document.

When the set of documents has been exhausted, we need to write out this INVERTED representation to a file for subsequent processing. For every token in the splay tree (typically the traversal will be in lexicographic order), we will organize all its postings. First, we count the number of occurrences of the keyword across all the documents in the corpus; we will call this variable $totfreq_k$. A second, less obvious statistic we will also maintain is how many documents contain this keyword; this variable will be called $docfreq_k$. If there is exactly one occurrence of a keyword in each document, then these two numbers will be the same. But typically there are multiple occurrences of the same keyword in a single document and $totfreq_k > docfreq_k$. Both variables will be important to us in determining appropriate weights for the $Index$ relation (cf. Chapter 3.)

After having gone through all of the documents, and accumulating for each one these two statistics, we must sort the postings in decreasing frequency order. The reason for this won't be apparent until we discuss the matching algorithms (cf. Section §3.5 ), but it turns out to be important that documents that use a keyword most often are at the beginning of the list.

Once the documents' postings have been sorted into descending order of frequency, it is likely that several of the documents in this list will happen to have the same frequency, and we can exploit this fact to compress their representation. Figure (figure) . shows the $\mathname{POSTING}$ list broken into a list of $\mathname{FPOST}$ sublists, one for unique frequency count.

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