interviews

METAR reports

A METAR report is a loose international semi-standard used by airports for reporting information about wind speeds, humidity, and weather conditions. We’re going to write a program that parses a subset of these reports from a stream and keeps some running aggregates.

METAR Format

The report format looks like this:

<ICAO Code> <Timestamp> <Wind Info>

Which breaks down into this:

ICAO Code

This is a string in the ASCII range of upper-case letters. It is at least one such character. It is terminated by a space after the final character.

Examples:

• YYZ
• A
• LAX
• BIRK

We’re not concerned about verifying the validity of these codes in this exercise. Parsing them is enough.

Timestamp

This is a string in the format of:

<day of month><hours><minutes>Z

Where:

• day of month: 2 digits, the parsed number is in the range of 1-31 inclusive
• hours: 2 digits, the parsed number is in the range of 0-23 inclusive
• minutes: 2 digits, the parsed number in the range of 0-59

Wind Info

This one is a little tricky. The METAR format specifies wind speeds in two different units: knots or meters per second. To complicate matters there is an optional gusts value.

<direction><speed><gusts?><unit>

Eg:

18027KT
180120MPS
01323G30MPS

The components of the format can be parsed as follows:

• direction: 3 digits
• speed: 2-3 digits, minimum 00
• gusts?: 2 digits, optional. When it appears, parsed as G23
• unit: Either KT or MPS

Example Reports

YYZ 122201Z 12023MPS
LAX 022355Z 09332G78KT
FR 110232Z 001100G12MPS

Requirements

At minimum we require an interface that lets us:

• parse a METAR format record as described above
  • the returned data structure should normalize speeds to MPS
  • you can normalize a speed value in KT by dividing the value by 2
• Read a stream of records, one per line
  • and keep a running average wind speed per airport seen
  • and the current wind speed of each airport seen

Write a script to generate a few hundred thousand random reports one per line. Your program should be able to read the entire stream without failure and be able to either:

• display the running tally
• print a final report after reading the entire stream