Find fair split of a two-color necklace

Given a circular necklace represented by a string s over {'a','b'}, you may cut the necklace at most twice (yielding three contiguous pieces). Can you assign these three pieces to two people so that each person receives exactly the same number of 'a' beads and the same number of 'b' beads? If it is possible, return any valid pair of cut indices on the circle and the assignment of pieces to each person; otherwise return 'impossible'. Explicitly state feasibility conditions (e.g., necessary parity constraints) and design an algorithm that runs in O(n) time with O( 1) or O(n) extra space. Analyze correctness and complexity. Follow-up: extend to three gem types (e.g., {'a','b','c'}) where you may make at most three cuts (producing four pieces) and still split between two people so each receives equal counts of every type. Provide the generalized algorithm, state required conditions, and discuss how your approach scales with the number of gem types.