Time to use the whole engine. Today you build a real sheet with literals, a range sum, a division, and a conditional, recalculate it, and assert every computed value - proving parsing, evaluation, functions, and ordered recalculation all agree.
Build a sheet mixing literals, SUM over a range, a division, and an IF, recalculate, and assert every value.
This is the engine doing everything at once. The sheet has literal inputs
(A1..A3), a range aggregate (A4 sums them to 60), a division that feeds off
that total (B1 is 60/3 = 20), a conditional that tests the result (B2 is IF B1>15, which is 1), and a formula that combines two computed cells (B3 is 20 + 1 = 21). Every one of those values is only correct if the recalculation order is
right: A4 before B1, B1 before both B2 and B3, and B2 before B3.
That the numbers come out exactly 60, 20, 1, 21 is the whole project proving
itself. The parser turned each formula into a tree, precedents found the
dependencies, the graph and Kahn’s algorithm produced the order, and the evaluator
walked each tree reading fresh values. Nothing here is special-cased for this sheet -
it is the same Recalculate any sheet uses. The next lesson does the thing a
spreadsheet is really for: change an input and watch exactly the right cells update.
s := NewSheet()s.SetNumber("A1", 10); s.SetNumber("A2", 20); s.SetNumber("A3", 30)s.SetCell("A4", "=SUM(A1:A3)")s.SetCell("B1", "=A4/3")s.SetCell("B2", "=IF(B1>15, 1, 0)")s.SetCell("B3", "=B1+B2")s.Recalculate()// A4=60, B1=20, B2=1, B3=21