Parsing data with Clojure, interval problem.

Question

Hello! I'm writing a little parser in clojure for learning purpose. basically is a TSV file parser that need to be put in a database, but I added a complication. The complication itself is that in the same file there are more intervals. The file look like this:

###andreadipersio 2010-03-19 16:10:00###                                                                                
USER     COMM               PID  PPID  %CPU %MEM      TIME  
root     launchd              1     0   0.0  0.0   2:46.97  
root     DirectoryService    11     1   0.0  0.2   0:34.59  
root     notifyd             12     1   0.0  0.0   0:20.83  
root     diskarbitrationd    13     1   0.0  0.0   0:02.84`
....

###andreadipersio 2010-03-19 16:20:00###                                                                                
USER     COMM               PID  PPID  %CPU %MEM      TIME  
root     launchd              1     0   0.0  0.0   2:46.97  
root     DirectoryService    11     1   0.0  0.2   0:34.59  
root     notifyd             12     1   0.0  0.0   0:20.83  
root     diskarbitrationd    13     1   0.0  0.0   0:02.84

I ended up with this code:

(defn is-header? 
  "Return true  if a line is header"
  [line]
  (> (count (re-find #"^\#{3}" line)) 0))

(defn extract-fields
  "Return regex matches"
  [line pattern]
  (rest (re-find pattern line)))

(defn process-lines
  [lines]
  (map process-line lines))

(defn process-line
  [line]
  (if (is-header? line)
    (extract-fields line header-pattern))
  (extract-fields line data-pattern))

My idea is that in 'process-line' interval need to be merged with data so I have something like this:

('andreadipersio', '2010-03-19', '16:10:00', 'root', 'launchd', 1, 0, 0.0, 0.0, '2:46.97')

for every row till the next interval, but I can't figure how to make this happen.

I tried with something like this:

(def process-line
  [line]
  (if is-header? line)
    (def header-data (extract-fields line header-pattern)))
  (cons header-data (extract-fields line data-pattern)))

But this doesn't work as excepted.

Any hints?

Thanks!

Answer 1

I'm not totally sure based on your description, but perhaps you're just slipping up on the syntax. Is this what you want to do?

(def process-line [line]
  (if (is-header? line) ; extra parens here over your version
    (extract-fields line header-pattern) ; returning this result
    (extract-fields line data-pattern))) ; implicit "else"

If the intent of your "cons" is to group together headers with their associated detail data, you'll need some more code to accomplish that, but if it's just an attempt at "coalescing" and returning either a header or detail line depending on which it is, then this should be correct.

Answer 2

A possible approach:

1. Split the input into lines with line-seq. (If you want to test this on a string, you can obtain a line-seq on it by doing (line-seq (java.io.BufferedReader. (java.io.StringReader. test-string))).)

2. Partition it into sub-sequences each of which contains either a single header line or some number of "process lines" with (clojure.contrib.seq/partition-by is-header? your-seq-of-lines).

3. Assuming there's at least one process line after each header, (partition 2 *2) (where *2 is the sequence obtained in step 2 above) will return a sequence of a form resembling the following: (((header-1) (process-line-1 process-line-2)) ((header-2) (process-line-3 process-line-4))). If the input might contain some header lines not followed by any data lines, then the above could look like (((header-1a header-1b) (process-line-1 process-line-2)) ...).

4. Finally, transform the output of step 3 (*3) with the following function:

---

(defn extract-fields-add-headers
  [[headers process-lines]]
  (let [header-fields (extract-fields (last headers) header-pattern)]
    (map #(concat header-fields (extract-fields % data-pattern))
         process-lines)))

(To explain the (last headers) bit: the only case where we'll get multiple headers here is when some of them have no data lines of their own; the one actually attached to the data lines is the last one.)

---

With these example patterns:

(def data-pattern #"(\w+)\s+(\w+)\s+(\d+)\s+(\d+)\s+([0-9.]+)\s+([0-9.]+)\s+([0-9:.]+)")
(def header-pattern #"###(\w+)\s+([0-9-]+)\s+([0-9:]+)###")
;; we'll need to throw out the "USER  COMM  ..." lines,
;; empty lines and the "..." line which I haven't bothered
;; to remove from your sample input
(def discard-pattern #"^USER\s+COMM|^$|^\.\.\.")

the whole 'pipe' might look like this:

;; just a reminder, normally you'd put this in an ns form:
(use '[clojure.contrib.seq :only (partition-by)])

(->> (line-seq (java.io.BufferedReader. (java.io.StringReader. test-data)))
     (remove #(re-find discard-pattern %)) ; throw out "USER  COMM ..."
     (partition-by is-header?)
     (partition 2)
     ;; mapcat performs a map, then concatenates results
     (mapcat extract-fields-add-headers))

(With the line-seq presumably taking input from a different source in your final programme.)

With your example input, the above produces output like this (line breaks added for clarity):

(("andreadipersio" "2010-03-19" "16:10:00" "root" "launchd" "1" "0" "0.0" "0.0" "2:46.97")
 ("andreadipersio" "2010-03-19" "16:10:00" "root" "DirectoryService" "11" "1" "0.0" "0.2" "0:34.59")
 ("andreadipersio" "2010-03-19" "16:10:00" "root" "notifyd" "12" "1" "0.0" "0.0" "0:20.83")
 ("andreadipersio" "2010-03-19" "16:10:00" "root" "diskarbitrationd" "13" "1" "0.0" "0.0" "0:02.84")
 ("andreadipersio" "2010-03-19" "16:20:00" "root" "launchd" "1" "0" "0.0" "0.0" "2:46.97")
 ("andreadipersio" "2010-03-19" "16:20:00" "root" "DirectoryService" "11" "1" "0.0" "0.2" "0:34.59")
 ("andreadipersio" "2010-03-19" "16:20:00" "root" "notifyd" "12" "1" "0.0" "0.0" "0:20.83")
 ("andreadipersio" "2010-03-19" "16:20:00" "root" "diskarbitrationd" "13" "1" "0.0" "0.0" "0:02.84"))

Answer 3

You're doing (> (count (re-find #"^\#{3}" line)) 0), but you can just do (re-find #"^\#{3}" line) and use the result as a boolean. re-find returns nil if the match fails.

If you're iterating over the items in a collection, and you want to skip some items or combine two or more items in the original into one item in the result, then 99% of the time you want reduce. This usually ends up being very straightforward.

;; These two libs are called "io" and "string" in bleeding-edge clojure-contrib
;; and some of the function names are different.
(require '(clojure.contrib [str-utils :as s]
                           [duck-streams :as io])) ; SO's syntax-highlighter still sucks

(defn clean [line]
  (s/re-gsub #"^###|###\s*$" "" line))

(defn interval? [line]
  (re-find #"^#{3}" line))

(defn skip? [line]
  (or (empty? line)
      (re-find #"^USER" line)))

(defn parse-line [line]
  (s/re-split #"\s+" (clean line)))

(defn parse [file]
  (first
   (reduce
    (fn [[data interval] line]
      (cond
       (interval? line) [data (parse-line line)]
       (skip? line)     [data interval]
       :else            [(conj data (concat interval (parse-line line))) interval]))
    [[] nil]
    (io/read-lines file))))