Reducers

Reducers are devices or functions designed to reduce a physical quantity or to transform a state. In engineering, a gear reducer or gearbox lowers rotational speed while increasing torque. In software, a reducer is a pure function that computes a new application state from the current state and an action. Both uses share the goal of controlled transformation, with different implementations and constraints.

A mechanical reducer consists of gears in a housing and typically two or more shafts: an input shaft driven by a motor and a slower, higher-torque output shaft. The gear ratio, the proportion of teeth between gears, determines how many input revolutions produce one output revolution. Higher ratios yield greater speed reduction and torque multiplication, but increase size, cost, and losses. Common configurations include parallel shaft (spur or helical), bevel, worm, and planetary gear trains. Design factors include maximum torque, allowable backlash, efficiency, lubrication, cooling, and mounting arrangement. Proper selection matches load characteristics and duty cycle to achieve reliable, smooth operation.

Software reducers are pure functions used in unidirectional data flow architectures, notably Redux. A reducer takes a current state and an action object and returns a new state without modifying the old one. They are typically defined with a switch or mapping over action types and must be deterministic and side-effect free. Immutability is a common pattern, with updates expressed as new objects or copies. Reducers enable predictable state management, easier testing, and features like time-travel debugging and undo/redo. They are used to manage UI state, data stores, and application configuration, and can compose with other reducers to form a larger state tree.