Modeling State

A tiny state machine

typedef enum { IDLE, RUNNING, PAUSED } State;

State step(State s, int event) {
    switch (s) {
        case IDLE:    return event == 1 ? RUNNING : IDLE;
        case RUNNING: return event == 0 ? PAUSED  : RUNNING;
        case PAUSED:  return event == 1 ? RUNNING : PAUSED;
    }
    return s;
}

The enum names the states, the function names the transition rules, and the types make illegal states obvious.

Modeling records

typedef enum { DOC, VIDEO, QUIZ } Kind;

typedef struct {
    int id;
    Kind kind;
    int minutes;
} Material;

int total_minutes(const Material *ms, int n, Kind k) {
    int t = 0;
    for (int i = 0; i < n; i++)
        if (ms[i].kind == k) t += ms[i].minutes;
    return t;
}

This is the data-modeling layer of every C program: an enum for categories, a struct for records, functions that query and update them.

Design hint

When you catch yourself writing status == 3, stop and invent the enum. The rename costs two minutes and pays off for the life of the code.