Simplest to write?
return (from i in Enumerable.Range(0, 1 + x.Length - y.Length)
where x.Skip(i).Take(y.Length).SequenceEqual(y)
select (int?)i).FirstOrDefault().GetValueOrDefault(-1);
Not quite as efficient, of course… a bit more like it:
private static bool IsSubArrayEqual(int[] x, int[] y, int start) {
for (int i = 0; i < y.Length; i++) {
if (x[start++] != y[i]) return false;
}
return true;
}
public static int StartingIndex(this int[] x, int[] y) {
int max = 1 + x.Length - y.Length;
for(int i = 0 ; i < max ; i++) {
if(IsSubArrayEqual(x,y,i)) return i;
}
return -1;
}