Count decodings of a numeric string

Given a string s of digits representing an encoded message where '1' maps to 'A', ..., '26' maps to 'Z', and '0' cannot appear alone (it must be part of '10' or '20'), count the number of distinct decodings of s. Design an O(n)-time, O( 1)-extra-space algorithm. Follow-ups: handle very large n by returning the count modulo 1,000,000,007; extend the API to support streaming input where characters arrive one by one and you must update the count online.