Programming exercise - Routing of telephone calls

Description

Some telephone operators have submitted their price lists including price per minute for different phone number prefixes. The price lists look like this:

Operator A:

1          0.9

268      5.1

46        0.17

4620    0.0

468      0.15

4631    0.15

4673    0.9

46732  1.1

Operator B:

1          0.92

44        0.5

46        0.2

467      1.0

48        1.2

And so on...

The left column represents the telephone prefix (country + area code) and the right column represents the operators price per minute for a number starting with that prefix. When several prefixes match the same number, the longest one should be used. If you, for example, dial +46-xx-21xxxxx you will have to pay $ 1.1/min with Operator A and $ 1.0/min with Operator B.

If a price list does not include a certain prefix you cannot use that operator to dial numbers starting with that prefix. For example it is not possible to dial +44 numbers with operator A but it is possible with Operator B.

The goal with this exercise is to write a program that can handle any number of price lists (operators) and then can calculate which operator that is cheapest for a certain number. You can assume that each price list can have thousands of entries but they will all fit together in memory.

Telephone numbers should be inputted in the same format as in price lists, for example "68123456789". The challenge is to find the cheapest operator for that number.

Requirements

Please use primarily PHP or Python to solve the exercise and create a simple and responsive GUI, using HTML, CSS and JavaScript.Do not use a database.

Put your focus on the following:

? code design

? efficient data structures and algorithms

? code readability

The solution should be running with no errors or warnings when unpacked and copied into webserver's root.  Both front- and backend code should be unit tested and the unit tests should also be running with no errors or warnings.