Question

I’m looking for the fastest method I can use to search a body of text for the indexes of multiple characters.

For example:

searchString = 'abcdefabcdef'; 
searchChars  = ['a','b'];
// returns {'a':[0,6], 'b':[1,7]}

Answer 1

After timing a few single pass algorithms and Guffa’s regex, I ended up going with this:

function findIndexesMultiPass(str,find) {
  var x, output = {};

  for (var i = 0; i < find.length; i++) {
    output[find[i]] = [];
    x = 0;
    while ((x = str.indexOf(find[i], x)) > -1) {
      output[find[i]].push(x++);
    }
  }

  return output;
}

var searchString = "abcd abcd abcd";
var searchChars  = ['a', 'b'];
var result = findIndexesMultiPass(searchString, searchChars);
// {'a':[0,5,10], 'b':[1,6,11]}

This turned out to be pretty slow:

function findIndexesOnePass(str,find) {
  var output = {};

  for (var i = 0; i < find.length; i++) {
    output[find[i]] = [];
  }

  for (var i = 0; i < str.length; i++) {
    var currentChar = str.charAt(i);
    if (output[currentChar] !== undefined) {
      output[currentChar].push(i);
    }
  }

  return output;
}

var searchString = "abcd abcd abcd";
var searchChars  = ['a', 'b'];
var result = findIndexesOnePass(searchString, searchChars);
// {'a':[0,5,10], 'b':[1,6,11]}

Rough times (indexes of 3 characters)

Google Chrome (Mac)
findIndexesMultiPass: 44ms
findIndexesOnePass: 799ms
findIndexesRegEx: 95ms

Safari
findIndexesMultiPass: 48ms
findIndexesOnePass: 325ms
findIndexesRegEx: 293ms

Firefox
findIndexesMultiPass: 56ms
findIndexesOnePass: 369ms
findIndexesRegEx: 786ms