Snippet to discover the BOM of a text encoding

gistfile1.m

NSData *bom = nil;
NSStringEncoding encoding = ...;
NSData *a = [@"a" dataUsingEncoding:encoding];
NSData *aa = [@"aa" dataUsingEncoding:encoding];
if ([a length] * 2 != [aa length]) {
  NSUInteger characterLength = [aa length] - [a length];
  bom = [a subdataWithRange:NSMakeRange(0, [a length]-characterLength)];
}