Extract multiple substrings from same string efficiently - scala

Question

I have a large dataset of URL strings containing key-value pairs, and I want to capture a list of values from that string. One example of a string is below:
"GET /no_cache/bi_page?Log=1&pg_inst=600474500174606089&pg=mdot_fyc_pnt&platform=mdot&ver=10.c110&pid=157876860906745096&rid=157876731027276387&srch_id=-2&row=7&seq=1&tot=1&tsp=1&test_name=m_control&logDomain=http%3A%2F%2Fwww.xyz.com&ref_url=http%3A%2F%2Fm.xyz.com%2F&z=44134 HTTP/1.1"
So if my list of values to return come from keys: "pg","test_name","some_other_key" ... I'd want the function to return ("mdot_fyc","m_control","NA") for this row.
I could just write three separate regex lines to capture each value. But some of these strings are long and I could have dozens of these values to extract instead of just three.
What's the most efficient way to extract multiple values from the same string?

Answer 1

Here is a simple 1-pass solution. Let me know if it's fast enough.
I'm no expert in URLs, so it might need tuning. Basically it assumes that there are no unescaped spaces, '?', '&' or '=' characters.
It can be further smoothed with low-level opti.
def extractParams(params: List[String], from: String): Map[String, String] = {
val a = from.toCharArray
val len = a.length
import scala.annotation.tailrec
#tailrec
def extract(p: Set[String], start: Int, results: Map[String, String]): Map[String, String] = {
var paramStart = start
var nextEquals = -1
var nextAmpersand = -1
if (start == 0) { // find start of params
var i = 0
while (i < len && a(i) != '?') {
i += 1
}
if (i == len) return results
paramStart = i
}
{ // find equals
var i = paramStart
while (i < len && a(i) != '=') {
i += 1
}
if (i == len) return results
nextEquals = i
}
{ // find nextAmpersand or end
var i = nextEquals
while (i < len && !(a(i) == '&' || a(i) == ' ')) {
i += 1
}
nextAmpersand = i
}
val paramNameArr = new Array[Char](nextEquals - paramStart - 1)
System.arraycopy(a, paramStart + 1, paramNameArr, 0, nextEquals - paramStart - 1)
val paramName = new String(paramNameArr)
var newResults = results
var newP = p
if (p.contains(paramName)) { // find param value
val paramValueArr = new Array[Char](nextAmpersand - nextEquals - 1)
System.arraycopy(a, nextEquals + 1, paramValueArr, 0, nextAmpersand - nextEquals - 1)
val paramValue = new String(paramValueArr)
newResults = newResults + (paramName -> paramValue)
newP = p - (paramName)
}
if (nextAmpersand == len || a(nextAmpersand) == ' ') { // check for end
return newResults
} else {
return extract(newP, nextAmpersand, newResults)
}
}
extract(params.toSet, "GET ".length, Map.empty)
}