zero_copy_buffer_splice.c

#include <stdlib.h>
#include <stdio.h>
#include <assert.h>
#include <string.h>
#include <windows.h>

typedef struct {
    char *start;
    char *current;
    char *end;
} buf_t;

enum { MAX_BUFS = 16 };
buf_t bufs[MAX_BUFS];
buf_t *buf = bufs;
char c;

void push(char *start, char *end) {
    assert(buf != bufs + MAX_BUFS);
    buf++;
    buf->start = start;
    buf->current = start;
    buf->end = end;
    c = *buf->current;
}

void next() {
    while (buf->current == buf->end) {
        assert(buf != bufs);
        --buf;
    }

    buf->current++;
    c = *buf->current;
}

enum { LOOKAHEAD = 7 };

void splice(char *start, char *end) {
    size_t tail_length = end - start >= LOOKAHEAD? LOOKAHEAD : end - start;
    char *tail_start = end - tail_length;
    char *tail_buffer = (char *)malloc(tail_length + LOOKAHEAD);
    memcpy(tail_buffer, tail_start, tail_length);
    memcpy(tail_buffer + tail_length, buf->current, LOOKAHEAD);

    push(tail_buffer, tail_buffer + tail_length);

    if (start != tail_start)
        push(start, tail_start);
}

int main(int argc, char **argv) {
    char eof[LOOKAHEAD] = {0};
    push(eof, eof + LOOKAHEAD);

    char *text0 = "before #include1ude2 after#";
    char *text1 = "world << #incl";
    char *text2 = "and again#";

    enum { NUM_TEXTS = 3 };
    char *texts[NUM_TEXTS][2] = {
        {text0, text0 + strlen(text0)},
        {text1, text1 + strlen(text1)},
        {text2, text2 + strlen(text2)}
    };

    splice(texts[0][0], texts[0][1]);

    while (c) {
        if (c == '#') {
            next();
            if (strncmp(buf->current, "include", 7) == 0) {
                for (int n = 0; n < 7; n++)
                    next();
                int i = c - '0';
                next();
                if (0 <= i && i < NUM_TEXTS)
                    splice(texts[i][0], texts[i][1]);
            }
        } else {
            printf("%c", c);
            next();
        }
    }
    
    return 0;
}