Remove usages of cons #5
Description
When we use a lot of cons (e.g. in many) this will slow down the code, because a copy of the array has to be made and inserting at index 0 is O(n).
There are a couple of workarounds. The most "brute-force" one would be to rewrite many (and other combinators). I'm sorry, the code is quite ugly, but I think many parts can be factored out nicely to keep everything in a functional style. The key trick is to have one result array that's not shared, so the compiler can modify it without making copies.
public func many<In, Out, Outs: RangeReplaceableCollectionType where Outs.Generator.Element == Out>(p: Parser<In, Out>) -> Parser<In, Outs>
{
return Parser { input in
var result = Outs()
var remainder = input
while true {
switch parse(p, remainder) {
case .Done(let input, let out):
result.append(out)
remainder = input
case .Fail(_, _, _): return .Done(remainder, result)
}
}
}
}This makes it twice as fast on my machine! I think some other cases are manyTill, sepBy1, count, etc.