http://encoding.spec.whatwg.org/#indexes

[[
The index gb18030 pointer for code point is the return value of these steps:

Let offset be the last code point in index gb18030 that is equal to or less than code point and let pointer offset be its corresponding pointer.

Return a pointer whose value is pointer offset + code point − offset.
]]

I'm afraid this may not be enough for this algorithm, since in this case, some code points may return the same pointer.  For instance, the return values for the code points 0xE7C9 and 0xE7E7 are identical: 33471. It seems to me that in reality, 0xE7C9 should not be a valid code point in GB18030.

I suggest the following change:

-----
The index gb18030 pointer for _code point_ is the return value of these steps:

If _code point_ is less than 0x80 or greater than 0x10FFFF, return null.

If _code point_ is greater than or equal to 0x10000, return a _pointer_ whose value is 189000 + _code point_ - 0x10000.

Let _offset_ be the last code point in index gb18030 that is equal to or less than _code point_ and let _pointer offset_ be its corresponding pointer.

Let _next pointer offset_ be the offset in the list that comes after pointer offset.

If _code point_ minus _offset_ is greater than or equal to _next pointer offset_ minus _pointer offset_, return null.

Return a _pointer_ whose value is _pointer offset_ + _code point_ − _offset_.

-----