Lightweight Database

Cache::Historical accesses an SQLite database via the DBD::SQLite CPAN module. SQLite is not under the GPL, but is released under a Public Domain license; the CPAN module includes the source code for the file-based database. SQLite supports requests in SQL syntax but does without a database server, writing the results directly to a local file instead.

CachedQuote.pm sets the SQLite database file to /tmp/cached-quote.dat in line 21. If you prefer not to leave the cache in a dangerous temporary directory, you can change the default when you call the Cache::Historical constructor – for example, new(sqlite_file => "filename").

The quote() function (line 34) first tries to retrieve the stock price with get() (lines 39-41). If this action fails, the get() method returns an undefined value, which is recognized in line 44. Then the module calls quote_refresh() to update the cache for the period before and after the requested date. After this, get_interpolated() should return a useful value.

At the same time, the code decides whether the daily price is unavailable because the stock exchange was closed on the day in question or because the range is not yet cached. If the script discovers that the day was a Sunday, the module should not try to retrieve the latest share prices from the server because there will not be any until Monday.

Therefore, the quote_refresh() function calls since_last_update() in line 65 to check the period since the last cache refresh. This value is stored as a DateTime::Duration object, and delta_days converts this to whole days. If the cache is less than one day old, an update is not performed (lines 69-72, 79-83), and the last available share price (typically from Friday) is then used (interpolation method in line 49).

Date and Time

The CPAN DateTime module interface is so convenient that developers normally do not bother using anything else, but the Finance::QuoteHist::Yahoo module insists on dates in US standard format: mm/dd/yyyy. Thus, the date_format() function in line 140 calls the strftime() method to convert DateTime objects.

The reverse case – converting a mm/dd/yyyy date to a DateTime object – is handled by the dt_parse() function beginning in line 148.

The DateTime::Format::Strptime module defines a new format; the module's parse_datetime() method analyzes a string passed in to it and returns a new object if successful.

To calculate the date one year ago from a DateTime object, all you need to do is call the object's subtract() method with the years => 1 parameter. Of course, this modifies the object itself.

If you need the original value, you first need to copy the contents to a new object with clone() (line 91).

Line by Line

The pofo script (Listing 2) accepts a configuration file, such as pofo1.txt in Figure 1, at the command line. The cfg_read() function starting in line 190 works its way through the lines of the file, each of which describes a share transaction. The function ignores comments starting with a pound sign, # (line 203) and lines that contain nothing but blanks and comments.