A queue on an array in C

Two indices chase each other around a fixed buffer.

#define QUEUE_CAP 8

typedef struct {
    int items[QUEUE_CAP];
    size_t head;
    size_t count;
} Queue;

int enqueue(Queue *q, int value) {
    if (q->count == QUEUE_CAP) {
        return 0;
    }
    size_t tail = (q->head + q->count) % QUEUE_CAP;
    q->items[tail] = value;
    q->count++;
    return 1;
}

int dequeue(Queue *q, int *out) {
    if (q->count == 0) {
        return 0;
    }
    *out = q->items[q->head];
    q->head = (q->head + 1) % QUEUE_CAP;
    q->count--;
    return 1;
}

How it works

1. head is where you take from, tail where you add.
2. The modulo makes the buffer circular.
3. Keeping a count distinguishes empty from full.

Keywords and builtins used here

• dequeue
• enqueue
• if
• int
• return
• size_t
• struct
• typedef

The run, in numbers

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Tokens

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Step 5 of 5 in Lists & stacks, step 10 of 20 in Data structures & algorithms.

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