Implement Spreadsheet Cells with Dependencies

Problem Statement

Implement a basic Spreadsheet system that supports setting cell values and defining dependencies. A cell can either contain a raw numeric value or a formula. A formula is defined as a simple addition operation between two other cells (e.g., "A1 + B2").

When the value of a cell changes, all cells that depend on it must be re-evaluated. If a circular dependency is introduced (e.g., A depends on B, and B depends on A), the system should detect it and raise an error.

Requirements

Implement the Spreadsheet class with the following methods:

• set_value(cell: str, value: float): Sets a cell to a static number.
• set_formula(cell: str, formula: str): Sets a cell to a formula (e.g., "A1 + B2").
• get_value(cell: str) -> float: Returns the current computed value of the cell. If it has a formula, it computes it based on the current values of its dependencies.

Constraints

• Cell names follow the format [Letter][Number] (e.g., "A1", "B2").
• Formulas only support the format "[Cell1] + [Cell2]".
• The spreadsheet should handle transitive dependencies (e.g., C depends on B, B depends on A).

Examples

s = Spreadsheet()
s.set_value("A1", 10.0)
s.set_value("B1", 20.0)
s.set_formula("C1", "A1 + B1")
# s.get_value("C1") returns 30.0

s.set_value("A1", 5.0)
# s.get_value("C1") returns 25.0

Edge Cases

• Circular references: set_formula("A1", "B1 + C1") followed by set_formula("B1", "A1 + D1") should raise an error.
• Updating a dependency should propagate changes through multiple layers of formulas.