Spreadsheet Formulas, Dependencies, and Circular References

Implement a spreadsheet that stores integer literals or formulas referencing other cells. Updating an input cell must affect later reads of all dependent cells. Detect circular references when a requested value cannot be evaluated. Implement `evaluate_sheet(operations: string[][]) -> string[]`. Each operation is `["set", cell, raw]` or `["get", cell]`. Return one string per operation: `"OK"` for set, the computed integer in canonical decimal notation for a successful get, or `"CYCLE"` for a get whose dependency traversal encounters a cycle. Canonical decimal notation has no leading plus or leading zeros, and zero is `"0"`. ### Practice contract The reports describe different formula and error conventions. This exercise uses an explicit shared practice contract without attributing it to a single company: - Cell names match uppercase letters followed by a positive decimal row number, such as `A1` or `AA27`. Row numbers have no leading zeros. - A set stores a signed integer string, such as `-5`, or a formula starting with `=`. - A formula has grammar `term (('+' | '-') term)*`, where a term is a nonnegative integer or a cell name. Spaces may appear around tokens. Unary signs and parentheses inside formulas are not supported in this version. - Unset cells evaluate to zero. Each occurrence of a referenced cell contributes separately: `=A1+A1` counts it twice. - A set replaces the previous raw value completely. It may introduce a cycle; the error is reported by a get whose reachable dependencies contain that cycle. - A cycle elsewhere in the sheet does not make an unrelated get fail. Replacing a formula can remove a cycle, and subsequent reads must then succeed. - Inputs are syntactically valid. There are at most 500 operations and at most 100 terms in a formula. Numeric tokens, stored integers, and every intermediate arithmetic result of an acyclic evaluation fit in a signed 32-bit integer. ### Examples ```text operations = [ ["set","A1","4"], ["set","B1","=A1+A1-1"], ["get","B1"], ["set","A1","8"], ["get","B1"] ] result = ["OK","OK","7","OK","15"] ``` ```text operations = [ ["set","A1","=B1+1"], ["set","B1","=A1"], ["get","A1"], ["get","C1"], ["set","B1","2"], ["get","A1"] ] result = ["OK","OK","CYCLE","0","OK","3"] ``` ```hint Distinguish active work from completed work A cell reached twice through different dependency paths is not necessarily cyclic. Track whether its value is still being evaluated in the current traversal or has already been completed. ```