Description
For this assignment, you are to write a decoder for a telegraph-style commercial code.
Commercial entities that communicated by telegram would often use a codebook that represented common phrases or messages with single words. For example, the codebook might specify that MAUVE expands to DELIVER FINE QUALITY COPPER INGOTS, saving 20¢ per use in a telegram costing 5¢ per word. Scans of many such codebooks can be found at https://www.jmcvey.net/cable/scans.htm.
We will not be using a codebook. Instead, each sequence of words contained within a message of the form DEFINE codeword definition STOP will define a new mapping from codeword to definition, so that any subsequent appearance of codeword will be replaced by definition, where definition consists zero or more words. This procedure is not recursive: definition will be reproduced verbatim without further expansion.
For example, if the encoded message is
DEFINE MAUVE DELIVER FINE QUALITY COPPER INGOTS STOP YOU SAID I WILL MAUVE BUT YOU DID NOT MAUVE
then the decoded message would be
YOU SAID I WILL DELIVER FINE QUALITY COPPER INGOTS BUT YOU DID NOT DELIVER FINE QUALITY COPPER INGOTS
In this particular case, the definition didn’t save us any money, but a longer complaint letter might allow for additional uses.
8.8.1 Your task
You are to write a program expand that takes an encoded telegram from stdin, parses any definitions contained in the telegram, and replaces any defined codewords with their definitions, writing the result to stdout.
You may assume that the input consists only of words consisting of upper-case letters and digits, separated by one or more whitespace characters. The behavior or your program for any other input is undefined.
For your output, you should similarly separate words by whitespace. Any choice of whitespace is acceptable, although making the output a single line of words separated by spaces may maximize verisimilitude at the cost of readability.
As described above, definitions are not expanded recursively. The definition of a codeword both at time of definition and at time of expansion is always treated as a sequence of words that are not checked for codewords. Because commercial codes are intended to be usable by humans, you may also assume that definitions are of length O(1) as a function of the length n of the encoded message. You may assume that all definitions in the input are complete: if DEFINE appears in a message without a matching STOP, the behavior or your program is undefined. You may also assume that the user does not attempt to define DEFINE or STOP.
If more than one definition is given for a codeword, the most recent definition applies. For example, DEFINE X Y STOP X DEFINE X Z STOP X expands to Y Z.
