C:リングバッファを逆向きに読み出す

/*
 * read reverse ring-buffer
 */
#include <stdio.h>
#include <string.h>

#define MAX_BUF (16)

struct ring_t {
    int  head;          // now data
    int  tail;          // next target data
    char data[MAX_BUF];
};

void p_data(char *data){
    int i;
    for (i=0; i<MAX_BUF; i++){
        printf("%c ", data[i]);
    }
    printf("\n");
}

main(){
    struct ring_t rbuf;
    char data[MAX_BUF];

    int i, index;

    // set ring-buffer
    memset(&rbuf, 0, sizeof(rbuf));
    memset(rbuf.data, '-', sizeof(rbuf.data));
    for (i=0; i<20; i++){
        rbuf.data[rbuf.tail]='A'+i;  // set test data
        rbuf.head=rbuf.tail;
        rbuf.tail=(i+1) % MAX_BUF;
    }
    printf("rbuf.head=%d\n   ", rbuf.head);
    p_data(rbuf.data);               // test

    // data copy (read ring-buffer)
    memset(data, 0, sizeof(data));
    for (i=0; i<MAX_BUF; i++){
        index = (rbuf.head + i) % MAX_BUF;
        data[i] = rbuf.data[index];
    }
    printf("f: ");
    p_data(data);                    // test

    // data copy (read reverse ring-buffer)
    memset(data, 0, sizeof(data));
    for (i=0; i<MAX_BUF; i++){
        index = (rbuf.head + MAX_BUF - i) % MAX_BUF;
        data[i] = rbuf.data[index];
    }
    printf("r: ");
    p_data(data);                    // test
}

$ a.out
rbuf.head=3
   Q R S T E F G H I J K L M N O P
f: T E F G H I J K L M N O P Q R S
r: T S R Q P O N M L K J I H G F E