tail: fix "tail -c 20 /dev/huge_disk" (was taking ages)

tail: a few variables renamed
wc: tiny optimization.

2 files changed, 43 insertions, 38 deletions

diff --git a/coreutils/tail.c b/coreutils/tail.c
index beecbaedf..340697f88 100644
--- a/coreutils/tail.c
+++ b/coreutils/tail.c
@@ -74,7 +74,7 @@ static unsigned eat_num(const char *p)
                 p++;
         else if (*p == '+') {
                 p++;
-                G.status = EXIT_FAILURE;
+                G.status = 1; /* mark that we saw "+" */
         }
         return xatou_sfx(p, tail_suffixes);
 }
@@ -92,7 +92,7 @@ int tail_main(int argc, char **argv)
         char *tailbuf;
         size_t tailbufsize;
         int taillen = 0;
-        int newline = 0;
+        int newlines_seen = 0;
         int nfiles, nread, nwrite, seen, i, opt;
 
         int *fds;
@@ -128,12 +128,12 @@ int tail_main(int argc, char **argv)
 #endif
         argc -= optind;
         argv += optind;
-        from_top = G.status;
+        from_top = G.status; /* 1 if there was "-c +N" or "-n +N" */
+        G.status = EXIT_SUCCESS;
 
         /* open all the files */
         fds = xmalloc(sizeof(int) * (argc + 1));
         nfiles = i = 0;
-        G.status = EXIT_SUCCESS;
         if (argc == 0) {
                 struct stat statbuf;
 
@@ -155,39 +155,49 @@ int tail_main(int argc, char **argv)
         if (!nfiles)
                 bb_error_msg_and_die("no files");
 
+        /* prepare the buffer */
         tailbufsize = BUFSIZ;
-
-        /* tail the files */
         if (!from_top && COUNT_BYTES) {
-                if (tailbufsize < count) {
+                if (tailbufsize < count + BUFSIZ) {
                         tailbufsize = count + BUFSIZ;
                 }
         }
+        tailbuf = xmalloc(tailbufsize);
 
-        buf = tailbuf = xmalloc(tailbufsize);
-
+        /* tail the files */
         fmt = header_fmt + 1;   /* Skip header leading newline on first output. */
         i = 0;
         do {
-                /* Be careful.  It would be possible to optimize the count-bytes
-                 * case if the file is seekable.  If you do though, remember that
-                 * starting file position may not be the beginning of the file.
-                 * Beware of backing up too far.  See example in wc.c.
-                 */
-                if (!(count | from_top) && lseek(fds[i], 0, SEEK_END) >= 0) {
-                        continue;
-                }
+                off_t current;
 
                 if (nfiles > header_threshhold) {
                         tail_xprint_header(fmt, argv[i]);
                         fmt = header_fmt;
                 }
 
+                /* Optimizing count-bytes case if the file is seekable.
+                 * Beware of backing up too far.
+                 * Also we exclude files with size 0 (because of /proc/xxx) */
+                current = lseek(fds[i], 0, SEEK_END);
+                if (current > 0) {
+                        if (!from_top) {
+                                if (count == 0)
+                                        continue; /* showing zero lines is easy :) */
+                                if (COUNT_BYTES) {
+                                        current -= count;
+                                        if (current < 0)
+                                                current = 0;
+                                        xlseek(fds[i], current, SEEK_SET);
+                                        bb_copyfd_size(fds[i], STDOUT_FILENO, count);
+                                        continue;
+                                }
+                        }
+                }
+
                 buf = tailbuf;
                 taillen = 0;
                 seen = 1;
-                newline = 0;
-
+                newlines_seen = 0;
                 while ((nread = tail_read(fds[i], buf, tailbufsize-taillen)) > 0) {
                         if (from_top) {
                                 nwrite = nread;
@@ -215,34 +225,32 @@ int tail_main(int argc, char **argv)
                                         }
                                 } else {
                                         int k = nread;
-                                        int nbuf = 0;
+                                        int newlines_in_buf = 0;
 
-                                        while (k) {
-                                                --k;
+                                        do { /* count '\n' in last read */
+                                                k--;
                                                 if (buf[k] == '\n') {
-                                                        ++nbuf;
+                                                        newlines_in_buf++;
                                                 }
-                                        }
+                                        } while (k);
 
-                                        if (newline + nbuf < count) {
-                                                newline += nbuf;
+                                        if (newlines_seen + newlines_in_buf < count) {
+                                                newlines_seen += newlines_in_buf;
                                                 taillen += nread;
                                         } else {
-                                                int extra = 0;
+                                                int extra = (buf[nread-1] != '\n');
 
-                                                if (buf[nread-1] != '\n')
-                                                        extra = 1;
-                                                k = newline + nbuf + extra - count;
+                                                k = newlines_seen + newlines_in_buf + extra - count;
                                                 s = tailbuf;
                                                 while (k) {
                                                         if (*s == '\n') {
-                                                                --k;
+                                                                k--;
                                                         }
-                                                        ++s;
+                                                        s++;
                                                 }
                                                 taillen += nread - (s - tailbuf);
                                                 memmove(tailbuf, s, taillen);
-                                                newline = count - extra;
+                                                newlines_seen = count - extra;
                                         }
                                         if (tailbufsize < taillen + BUFSIZ) {
                                                 tailbufsize = taillen + BUFSIZ;
@@ -251,13 +259,10 @@ int tail_main(int argc, char **argv)
                                 }
                                 buf = tailbuf + taillen;
                         }
-                }
-
+                } /* while (tail_read() > 0) */
                 if (!from_top) {
                         xwrite(STDOUT_FILENO, tailbuf, taillen);
                 }
-
-                taillen = 0;
         } while (++i < nfiles);
 
         buf = xrealloc(tailbuf, BUFSIZ);
diff --git a/coreutils/wc.c b/coreutils/wc.c
index 291af411f..58ea1c7db 100644
--- a/coreutils/wc.c
+++ b/coreutils/wc.c
@@ -73,7 +73,7 @@ int wc_main(int argc, char **argv)
 {
         FILE *fp;
         const char *s, *arg;
-        const char *start_fmt = "%9"COUNT_FMT;
+        const char *start_fmt = " %9"COUNT_FMT + 1;
         const char *fname_fmt = " %s\n";
         COUNT_T *pcounts;
         COUNT_T counts[4];