iconv

/*
 * Document-method: Iconv#iconv
 * call-seq: iconv(str, start=0, length=-1)
 *
 * Converts string and returns the result.
 * * If +str+ is a String, converts <tt>str[start, length]</tt> and returns the converted string.
 * * If +str+ is +nil+, places converter itself into initial shift state and
 *   just returns a string containing the byte sequence to change the output
 *   buffer to its initial shift state.
 * * Otherwise, raises an exception.
 *
 * === Parameters
 *
 * str::    string to be converted, or nil
 * start::  starting offset
 * length:: conversion length; nil or -1 means whole the string from start
 *
 * === Exceptions
 *
 * * IconvIllegalSequence
 * * IconvInvalidCharacter
 * * IconvOutOfRange
 *
 * === Examples
 *
 * See the Iconv documentation.
 */
static VALUE
iconv_iconv
    (argc, argv, self)
    int argc;
    VALUE *argv;
    VALUE self;
{
    VALUE str, n1, n2;
    VALUE cd = check_iconv(self);
    long start = 0, length = 0, slen = 0;

    rb_scan_args(argc, argv, "12", &str, &n1, &n2);
    if (!NIL_P(str)) slen = RSTRING_LEN(StringValue(str));
    if (argc != 2 || !RTEST(rb_range_beg_len(n1, &start, &length, slen, 0))) {
        if (NIL_P(n1) || ((start = NUM2LONG(n1)) < 0 ? (start += slen) >= 0 : start < slen)) {
            if (NIL_P(n2)) {
                length = -1;
            }
            else if ((length = NUM2LONG(n2)) >= slen - start) {
                length = slen - start;
            }
        }
    }

    return iconv_convert(VALUE2ICONV(cd), str, start, length, NULL);
}