Parser Stack
Simple explanation
While parsing, the engine keeps a stack of "where am I in the grammar" states alongside the half-built tree pieces. Reductions pop finished pieces off and push back a single combined node.
Technical explanation
parser/stack.zig holds two parallel ArrayLists — states: []u16 and values: []u32 (subtree indices) — kept in lockstep by push/pop/popMany. The stack starts with the grammar's start state; a shift pushes one entry and a reduce of N children pops N entries before pushing the goto target. Both lists live on the Parser and are cleared (capacity retained) between parses, which is why parser reuse is cheap.