Finder over a file descriptor

find.c

#include <stdio.h>
#include <string.h>
#include <unistd.h>

struct finder {
    long long lineno;
    unsigned char *needle;
    size_t nlen, noff;
    size_t hlen, hoff;
    int fd;
    unsigned char haystack[1<<14];  // may be smaller than the needle
};

static void
finder_init(struct finder *f, int fd, const void *needle, size_t len)
{
    f->needle = (unsigned char *)needle;
    f->nlen = len;
    f->noff = 0;
    f->hlen = 0;
    f->hoff = 0;
    f->lineno = 0;
    f->fd = fd;
}

static unsigned char
xlower(unsigned char c)
{
    return c + ((c >= 'A' && c <= 'Z') << 5);
}

// Return the line number of the next instance of the needle, zero for
// EOF, and -1 on error.
static long long
finder_next(struct finder *f)
{
    for (;;) {
        while (f->hoff < f->hlen) {
            unsigned char c = xlower(f->haystack[f->hoff++]);
            if (c == '\n') {
                int ismatch = f->noff == f->nlen;
                f->lineno++;
                f->noff = 0;
                if (ismatch) {
                    return f->lineno;
                }
            } else if (f->noff < f->nlen && c == xlower(f->needle[f->noff])) {
                f->noff++;
            } else {
                f->noff = (size_t)-1;
            }
        }

        int r = read(f->fd, f->haystack, sizeof(f->haystack));
        switch (r) {
        case -1: return -1;
        case  0: return  0;
        default: f->hoff = 0;
                 f->hlen = r;
        }
    }
}

int
main(int argc, char **argv)
{
    struct finder f[1];
    finder_init(f, 0, argv[argc-1], strlen(argv[argc-1]));
    for (;;) {
        long long lineno = finder_next(f);
        switch (lineno) {
        case -1: return 1;
        case  0: fflush(stdout);
                 return ferror(stdout);
        default: printf("%lld\n", lineno);
        }
    }
}