Suffix-Replacement Grammars

As computer programmers, you have likely heard about regular expressions and context-free grammars. These are rich ways of generating sets of strings over a small alphabet (otherwise known as a formal language). There are other, more esoteric ways of generating languages, such as tree-adjoining gram- mars, context-sensitive grammars, and unrestricted grammars. This problem uses a new method for generating a language: a suffixreplacement grammar. A suffix-replacement grammar consists of a starting string S and a set of suffix-replacement rules. Each rule is of the form X → Y , where X and Y are equal-length strings of alphanumeric characters. This rule means that if the suffix (that is, the rightmost characters) of your current string is X, you can replace that suffix with Y . These rules may be applied arbitrarily many times. For example, suppose there are 4 rules A → B, AB → BA, AA → CC, and CC → BB. You can then transform the string AA to BB using three rule applications: AA → AB (using the A → B rule), then AB → BA (using the AB → BA rule), and finally BA → BB (using the A → B rule again). But you can also do the transformation more quickly by applying only 2 rules: AA → CC and then CC → BB. You must write a program that takes a suffix-replacement grammar and a string T and determines whether the grammar’s starting string S can be transformed into the string T. If this is possible, the program must also find the minimal number of rule applications required to do the transformation. Input The input consists of several test cases. Each case starts with a line containing two equal-length alphanumeric strings S and T (each between 1 and 20 characters long, and separated by whitespace), and an integer NR (0 ≤ NR ≤ 100), which is the number of rules. Each of the next NR lines contains two equal-length alphanumeric strings X and Y (each between 1 and 20 characters long, and separated by whitespace), indicating that X → Y is a rule of the grammar. All strings are case-sensitive. The last test case is followed by a line containing a period. Output For each test case, print the case number (beginning with 1) followed by the minimum number of rule applications required to transform S to T. If the transformation is not possible, print ‘No solution’. Follow the format of the sample output. Sample Input AA BB 4 AB AB BA AA CC CC BB AB3 AC BC cB .

2/2 Sample Output Case 1: 2 Case 2: No solution