Execute a Markov algorithm
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You are encouraged to solve this task according to the task description, using any language you may know.
Create an interpreter for a Markov Algorithm. Rules have the syntax:
<ruleset> ::= ((<comment> | <rule>) <newline>+)*
<comment> ::= # {<any character>}
<rule> ::= <pattern> <whitespace> -> <whitespace> [.] <replacement>
<whitespace> ::= (<tab> | <space>) [<whitespace>]
There is one rule per line. If there is a . present before the <replacement>, then this is a terminating rule in which case the interpreter must halt execution. A ruleset consists of a sequence of rules, with optional comments.
In order to promote flexibility, the interpreter should load the set of rules from one file, take the string to operate on from a second file, and write the output to a third.
Use the following three tests on entries:
Ruleset 1:
# This rules file is extracted from Wikipedia: # http://en.wikipedia.org/wiki/Markov_Algorithm A -> apple B -> bag S -> shop T -> the the shop -> my brother a never used -> .terminating rule
Sample text of:
"I bought a B of As from T S."
Should generate the output:
"I bought a bag of apples from my brother."
Ruleset 2:
A test of the terminating rule
# Slightly modified from the rules on Wikipedia A -> apple B -> bag S -> .shop T -> the the shop -> my brother a never used -> .terminating rule
Sample text of:
"I bought a B of As from T S."
Should generate:
"I bought a bag of apples from T shop."
Ruleset 3:
A stretch goal. This tests for correct substitution order and may trap simple regexp based replacement routines if special regexp characters are not escaped.
# BNF Syntax testing rules A -> apple WWWW -> with Bgage -> ->.* B -> bag ->.* -> money W -> WW S -> .shop T -> the the shop -> my brother a never used -> .terminating rule
Sample text of:
"I bought a B of As W my Bgage from T S."
Should generate:
"I bought a bag of apples with my money from T shop."
C++
Note: Non-use of iswhite is intentional, since depending on the locale, other chars besides space and tab might be detected by that function.
<lang cpp>
- include <cstdlib>
- include <iostream>
- include <fstream>
- include <vector>
- include <string>
struct rule {
std::string pattern;
std::string replacement;
bool terminal;
rule(std::string pat, std::string rep, bool term):
pattern(pat),
replacement(rep),
terminal(term)
{
}
};
std::string const whitespace = " \t"; std::string::size_type const npos = std::string::npos;
bool is_whitespace(char c) {
return whitespace.find(c) != npos;
}
int main(int argc, char* argv[]) {
if (argc != 3)
{
std::cout << "usage:\n " << argv[0] << " rulefile text\n";
return EXIT_FAILURE;
}
std::ifstream rulefile(argv[1]);
std::vector<rule> rules;
std::string line;
while (std::getline(rulefile, line))
{
std::string::size_type pos;
// remove comments
pos = line.find('#');
if (pos != npos)
line.resize(pos);
// ignore lines consisting only of whitespace
if (line.find_first_not_of(whitespace) == npos)
continue;
// find "->" surrounded by whitespace
pos = line.find("->");
while (pos != npos && (pos == 0 || !is_whitespace(line[pos-1])))
pos = line.find("->", pos+1);
if (pos == npos || line.length() < pos+3 || !is_whitespace(line[pos+2]))
{
std::cerr << "invalid rule: " << line << "\n";
return EXIT_FAILURE;
}
std::string pattern = line.substr(0, pos-1); std::string replacement = line.substr(pos+3);
// remove additional separating whitespace pattern.erase(pattern.find_last_not_of(whitespace)+1); replacement.erase(0, replacement.find_first_not_of(whitespace));
// test for terminal rule
bool terminal = !replacement.empty() && replacement[0] == '.';
if (terminal)
replacement.erase(0,1);
rules.push_back(rule(pattern, replacement, terminal)); }
std::string text = argv[2];
std::vector<rule>::iterator iter = rules.begin();
while (iter != rules.end())
{
std::string::size_type pos = text.find(iter->pattern);
if (pos != npos)
{
text.replace(pos, iter->pattern.length(), iter->replacement);
if (iter->terminal)
break;
iter = rules.begin();
}
++iter;
}
std::cout << text << "\n";
} </lang>
Haskell
This program expects a source file as an argument and uses the standard input and output devices for the algorithm's I/O.
<lang haskell>import Data.List (isPrefixOf) import Data.Maybe (catMaybes) import Control.Monad import Text.ParserCombinators.Parsec import System.IO import System.Environment (getArgs)
main = do
args <- getArgs
unless (length args == 1) $
fail "Please provide exactly one source file as an argument."
let sourcePath = head args
source <- readFile sourcePath
input <- getContents
case parse markovParser sourcePath source of
Right rules -> putStrLn $ runMarkov rules input
Left err -> hPutStrLn stderr $ "Parse error at " ++ show err
data Rule = Rule
{from :: String, terminating :: Bool, to :: String}
markovParser :: Parser [Rule] markovParser = liftM catMaybes $
(comment <|> rule) `sepEndBy` many1 newline
where comment = char '#' >> skipMany nonnl >> return Nothing
rule = liftM Just $ liftM3 Rule
(manyTill (nonnl <?> "pattern character") $ try arrow)
(succeeds $ char '.')
(many nonnl)
arrow = ws >> string "->" >> ws <?> "whitespace-delimited arrow"
nonnl = noneOf "\n"
ws = many1 $ oneOf " \t"
succeeds p = option False $ p >> return True
runMarkov :: [Rule] -> String -> String runMarkov rules s = f rules s
where f [] s = s
f (Rule from terminating to : rs) s = g "" s
where g _ "" = f rs s
g before ahead@(a : as) = if from `isPrefixOf` ahead
then let new = reverse before ++ to ++ drop (length from) ahead
in if terminating then new else f rules new
else g (a : before) as</lang>
J
Solution:<lang j>require'strings regex'
NB. Lex a Markov program markovLexer =: verb define rules =. LF cut TAB&=`(,:&' ')}y rules =. a: -.~ (dltb@:{.~ i:&'#')&.> rules 0 _1 {"1 '\s+->\s+' (rxmatch rxcut ])S:0 rules )
NB. Given ruleset and target string, output NB. result. markov =: markovLexer@[ stringreplace^:_ ]
NB. Same as above, but output all intermediate NB. evaluations markovDebug =: markovLexer@[ stringreplace^:a: ]</lang>
Example:<lang j> m1 =. noun define # This rules file is extracted from Wikipedia: # http://en.wikipedia.org/wiki/Markov_Algorithm A -> apple B -> bag S -> shop T -> the the shop -> my brother a never used -> .terminating rule )
m1 markov 'I bought a B of As from T S.'
I bought a bag of apples from my brother.
m1 markovDebug 'I bought a B of As from T S.'
I bought a B of As from T S. I bought a bag of apples from the shop. I bought a bag of apples from my brother.</lang> Discussion: This solution implemented in 20 seconds and doesn't fully implement a Markov algorithm. More details on the talk page.
Python
The example uses a regexp to parse the syntax of the grammar. This regexp is multi-line, verbose and uses named groups to aid in understanding the regexp and to allow more meaningful group names to be used when extracting the replacement data from the grammars in function extractreplacements.
<lang python> import re
syntaxre = r"""(?mx) ^(?:
(?: (?P<comment> \# .* ) ) | (?: (?P<blank> \s* ) (?: \n | $ ) ) | (?: (?P<rule> (?P<pat> .+? ) \s+ -> \s+ (?P<term> \.)? (?P<repl> .+) ) )
)$ """
grammar1 = """\
- This rules file is extracted from Wikipedia:
- http://en.wikipedia.org/wiki/Markov_Algorithm
A -> apple B -> bag S -> shop T -> the the shop -> my brother a never used -> .terminating rule """
grammar2 = \
- Slightly modified from the rules on Wikipedia
A -> apple B -> bag S -> .shop T -> the the shop -> my brother a never used -> .terminating rule
grammar3 = \
- BNF Syntax testing rules
A -> apple WWWW -> with Bgage -> ->.* B -> bag ->.* -> money W -> WW S -> .shop T -> the the shop -> my brother a never used -> .terminating rule
text1 = "I bought a B of As from T S."
text2 = "I bought a B of As W my Bgage from T S."
def extractreplacements(grammar):
return [ (matchobj.group('pat'), matchobj.group('repl'), bool(matchobj.group('term')))
for matchobj in re.finditer(syntaxre, grammar)
if matchobj.group('rule')]
def replace(text, replacements):
terminate = False
while not terminate:
for (pat, repl, isterm) in replacements:
if pat in text:
text = text.replace(pat, repl, 1)
terminate = isterm
break
else:
terminate = True
return text
if __name__ == '__main__':
assert replace(text1, extractreplacements(grammar1)) \
== 'I bought a bag of apples from my brother.'
assert replace(text1, extractreplacements(grammar2)) \
== 'I bought a bag of apples from T shop.'
# Stretch goal
assert replace(text2, extractreplacements(grammar3)) \
== 'I bought a bag of apples with my money from T shop.'
</lang>
Ruby
<lang Ruby>raise "Please input an input code file, an input data file, and an output file." if ARGV.size < 3
rules = File.readlines(ARGV[0]).inject([]) do |rules, line|
if line =~ /^\s*#/
rules
elsif line =~ /^(.+)\s+->\s+(\.?)(.*)$/
rules << [$1, $3, $2 != ""]
else
raise "Syntax error: #{line}"
end
end
File.open(ARGV[2], "w") do |file|
file.write(File.read(ARGV[1]).tap { |input_data|
while (matched = rules.find { |match, replace, term|
input_data[match] and input_data.sub!(match, replace)
}) and !matched[2]
end
})
end</lang>
Tcl
<lang tcl>package require Tcl 8.5 if {$argc < 3} {error "usage: $argv0 ruleFile inputFile outputFile"} lassign $argv ruleFile inputFile outputFile
- Read the file of rules
set rules {} set f [open $ruleFile] foreach line [split [read $f] \n[close $f]] {
if {[string match "#*" $line] || $line eq ""} continue
if {[regexp {^(.+)\s+->\s+(\.?)(.*)$} $line -> from final to]} {
lappend rules $from $to [string equal "." $to] [string length $from]
} else {
error "Syntax error: \"$line\""
}
}
- Apply the rules
set f [open $inputFile] set out [open $outputFile w] foreach line [split [read $f] \n[close $f]] {
set any 1
while {$any} {
set any 0 foreach {from to stop fl} $rules { # If we match the 'from' pattern... if {[set idx [string first $from $line]] < 0} { continue }
# Change for the 'to' replacement set line [string replace $line $idx [expr {$idx+$fl-1}] $to]
# Stop if we terminate, otherwise note that we've more work to do if {$stop} { set any 0 break } else { set any 1 } }
}
# Output the processed line puts $out $line
} close $out</lang> In the case where there are no terminating rules and no overlapping issues, the following is an alternative: <lang tcl>package require Tcl 8.5 if {$argc < 3} {error "usage: $argv0 ruleFile inputFile outputFile"} lassign $argv ruleFile inputFile outputFile
- Read the file of rules
set rules {} set f [open $ruleFile] foreach line [split [read $f] \n[close $f]] {
if {[string match "#*" $line] || $line eq ""} continue
if {[regexp {^(.+)\s+->\s+(.*)$} $line -> from to]} {
dict set rules $from $to
} else {
error "Syntax error: \"$line\""
}
}
- Apply the rules in a simplistic manner
set in [open $inputFile] set out [open $outputFile w] set data [read $in] close $in while 1 {
set newData [string map $rules $data]
if {$newData eq $data} break
set data $newData
} puts $out $data close $out</lang>