Solidity contract that filters out non-ASCII characters and allow only non-boundary, single-spaces

FilterString.sol

pragma solidity ^0.4.11;

contract FilterString {
	// Filters out non-ASCII characters and allow only non-boundary, single-spaces
	function _filter(string str) public returns (string) {
		bytes memory bytesStr = bytes(str);
		bytes memory valid = new bytes(bytesStr.length);
		uint chr = 0;
		uint ltrim = 0;
		for (uint i = 0; i < bytesStr.length; i++) {
			// Only add space characters if they're not on a boundary or duplicates
			if (bytesStr[i] == 32) {
				if (i != 0 && i != bytesStr.length - 1 && chr != 0 && valid[chr - 1] != 32) {
					valid[chr] = bytesStr[i];
					chr++;
				}
			// Filter out control characters
			} else if (bytesStr[i] < 127 && bytesStr[i] > 31) {
				valid[chr] = bytesStr[i];
				chr++;
			}
		}
		// If nothing has been filtered by now we're clean and can return
		if (bytesStr.length == chr) {
			return string(valid);
		}
		// Remove any additional boundary spaces
		while (valid[chr - 1] == 32) {
			chr--;
		}
		while (valid[ltrim] == 32) {
			ltrim++;
		}
		// Create a bytes array at the new length and return as a string
		bytes memory shortened = new bytes(chr - ltrim);
		for (uint c = ltrim; c < chr; c++) {
				shortened[c] = valid[c];
		}
		return string(shortened);
	}
}