head: support -n -NUM and -c -NUM

function                                             old     new   delta
head_main                                            406     832    +426
packed_usage                                       29234   29252     +18
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 444/0)             Total: 444 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 128 insertions, 22 deletions

diff --git a/coreutils/head.c b/coreutils/head.c
index 598fccb64..291e1ce37 100644
--- a/coreutils/head.c
+++ b/coreutils/head.c
@@ -21,7 +21,8 @@
 //usage:       "With more than one FILE, precede each with a filename header.\n"
 //usage:     "\n        -n N[kbm]       Print first N lines"
 //usage:        IF_FEATURE_FANCY_HEAD(
-//usage:     "\n        -c N[kbm]       Print first N bytes"
+//usage:     "\n        -n -N[kbm]      Print all except N last lines"
+//usage:     "\n        -c [-]N[kbm]    Print first N bytes"
 //usage:     "\n        -q              Never print headers"
 //usage:     "\n        -v              Always print headers"
 //usage:        )
@@ -38,6 +39,110 @@
 
 /* This is a NOEXEC applet. Be very careful! */
 
+#if !ENABLE_FEATURE_FANCY_HEAD
+# define print_first_N(fp,count,bytes) print_first_N(fp,count)
+#endif
+static void
+print_first_N(FILE *fp, unsigned long count, bool count_bytes)
+{
+#if !ENABLE_FEATURE_FANCY_HEAD
+        const int count_bytes = 0;
+#endif
+        while (count) {
+                int c = getc(fp);
+                if (c == EOF)
+                        break;
+                if (count_bytes || (c == '\n'))
+                        --count;
+                putchar(c);
+        }
+}
+
+#if ENABLE_FEATURE_FANCY_HEAD
+static void
+print_except_N_last_bytes(FILE *fp, unsigned count)
+{
+        unsigned char *circle = xmalloc(++count);
+        unsigned head = 0;
+        for(;;) {
+                int c;
+                c = getc(fp);
+                if (c == EOF)
+                        goto ret;
+                circle[head++] = c;
+                if (head == count)
+                        break;
+        }
+        for (;;) {
+                int c;
+                if (head == count)
+                        head = 0;
+                putchar(circle[head]);
+                c = getc(fp);
+                if (c == EOF)
+                        goto ret;
+                circle[head] = c;
+                head++;
+        }
+ ret:
+        free(circle);
+}
+
+static void
+print_except_N_last_lines(FILE *fp, unsigned count)
+{
+        char **circle = xzalloc((++count) * sizeof(circle[0]));
+        unsigned head = 0;
+        for(;;) {
+                char *c;
+                c = xmalloc_fgets(fp);
+                if (!c)
+                        goto ret;
+                circle[head++] = c;
+                if (head == count)
+                        break;
+        }
+        for (;;) {
+                char *c;
+                if (head == count)
+                        head = 0;
+                fputs(circle[head], stdout);
+                c = xmalloc_fgets(fp);
+                if (!c)
+                        goto ret;
+                free(circle[head]);
+                circle[head++] = c;
+        }
+ ret:
+        head = 0;
+        for(;;) {
+                free(circle[head++]);
+                if (head == count)
+                        break;
+        }
+        free(circle);
+}
+#else
+/* Must never be called */
+void print_except_N_last_bytes(FILE *fp, unsigned count);
+void print_except_N_last_lines(FILE *fp, unsigned count);
+#endif
+
+#if !ENABLE_FEATURE_FANCY_HEAD
+# define eat_num(negative_N,p) eat_num(p)
+#endif
+static unsigned long
+eat_num(bool *negative_N, const char *p)
+{
+#if ENABLE_FEATURE_FANCY_HEAD
+        if (*p == '-') {
+                *negative_N = 1;
+                p++;
+        }
+#endif
+        return xatoul_sfx(p, head_tail_suffixes);
+}
+
 static const char head_opts[] ALIGN1 =
         "n:"
 #if ENABLE_FEATURE_FANCY_HEAD
@@ -52,8 +157,13 @@ int head_main(int argc, char **argv)
 {
         unsigned long count = 10;
 #if ENABLE_FEATURE_FANCY_HEAD
-        int count_bytes = 0;
         int header_threshhold = 1;
+        bool count_bytes = 0;
+        bool negative_N = 0;
+#else
+# define header_threshhold 1
+# define count_bytes       0
+# define negative_N        0
 #endif
         FILE *fp;
         const char *fmt;
@@ -68,7 +178,7 @@ int head_main(int argc, char **argv)
         ) {
                 --argc;
                 ++argv;
-                p = (*argv) + 1;
+                p = argv[0] + 1;
                 goto GET_COUNT;
         }
 #endif
@@ -92,7 +202,7 @@ int head_main(int argc, char **argv)
 #if ENABLE_INCLUDE_SUSv2 || ENABLE_FEATURE_FANCY_HEAD
  GET_COUNT:
 #endif
-                        count = xatoul_sfx(p, head_tail_suffixes);
+                        count = eat_num(&negative_N, p);
                         break;
                 default:
                         bb_show_usage();
@@ -105,39 +215,35 @@ int head_main(int argc, char **argv)
                 *--argv = (char*)"-";
 
         fmt = header_fmt_str + 1;
-#if ENABLE_FEATURE_FANCY_HEAD
         if (argc <= header_threshhold) {
+#if ENABLE_FEATURE_FANCY_HEAD
                 header_threshhold = 0;
-        }
 #else
-        if (argc <= 1) {
                 fmt += 11; /* "" */
-        }
-        /* Now define some things here to avoid #ifdefs in the code below.
-         * These should optimize out of the if conditions below. */
-#define header_threshhold   1
-#define count_bytes         0
 #endif
+        }
+        if (negative_N) {
+                if (count >= INT_MAX / sizeof(char*))
+                        bb_error_msg("count is too big: %lu", count);
+        }
 
         do {
                 fp = fopen_or_warn_stdin(*argv);
                 if (fp) {
-                        unsigned long i;
-
                         if (fp == stdin) {
                                 *argv = (char *) bb_msg_standard_input;
                         }
                         if (header_threshhold) {
                                 printf(fmt, *argv);
                         }
-                        i = count;
-                        while (i) {
-                                int c = getc(fp);
-                                if (c == EOF)
-                                        break;
-                                if (count_bytes || (c == '\n'))
-                                        --i;
-                                putchar(c);
+                        if (negative_N) {
+                                if (count_bytes) {
+                                        print_except_N_last_bytes(fp, count);
+                                } else {
+                                        print_except_N_last_lines(fp, count);
+                                }
+                        } else {
+                                print_first_N(fp, count, count_bytes);
                         }
                         die_if_ferror_stdout();
                         if (fclose_if_not_stdin(fp)) {