add new -count option

this makes it possible to use jobflow to cut a subset of line numbers
like a combination of `head` and `tail` utilities.

e.g.

    seq 100 | jobflow -skip=10 -count=10

will print the lines from 11-20

jobflow.c

@@ -146,6 +146,7 @@ typedef struct {
 	unsigned long numthreads;
 	unsigned long threads_running;
 	unsigned long skip;
+	unsigned long count;
 	unsigned long delayedspinup_interval; /* use a random delay until the queue gets filled for the first time.
 				the top value in ms can be supplied via a command line switch.
 				this option makes only sense if the interval is somewhat smaller than the
@@ -371,13 +372,15 @@ static int syntax(void) {
 		"until EOF is received. we call this 'pipe mode'.\n"
 		"\n"
 		"available options:\n\n"
-		"-skip N -threads N -resume -statefile=/tmp/state -delayedflush\n"
+		"-skip N -count N -threads N -resume -statefile=/tmp/state -delayedflush\n"
 		"-delayedspinup N -buffered -joinoutput -limits mem=16M,cpu=10\n"
 		"-eof=XXX\n"
 		"-exec ./mycommand {}\n"
 		"\n"
 		"-skip N\n"
 		"    N=number of entries to skip\n"
+		"-count N\n"
+		"    N=only process count lines (after skipping)\n"
 		"-threads N (alternative: -j N)\n"
 		"    N=number of parallel processes to spawn\n"
 		"-resume\n"
@@ -453,6 +456,7 @@ static int parse_args(unsigned argc, char** argv) {
 		{"statefile", 0, 's', .dest.s = &prog_state.statefile },
 		{"eof", 0, 's', .dest.s = &prog_state.eof_marker },
 		{"skip", 0, 'i', .dest.i = &prog_state.skip },
+		{"count", 0, 'i', .dest.i = &prog_state.count },
 		{"resume", 0, 'b', .dest.b = &resume },
 		{"delayedflush", 0, 'b', .dest.b = &prog_state.delayedflush },
 		{"delayedspinup", 0, 'i', .dest.i = &prog_state.delayedspinup_interval },
@@ -463,6 +467,7 @@ static int parse_args(unsigned argc, char** argv) {
 	};
 
 	prog_state.numthreads = 1;
+	prog_state.count = -1UL;
 
 	for(i=1; i<argc; ++i) {
 		char *p = argv[i], *q = strchr(p, '=');
@@ -671,7 +676,8 @@ static char* mystrnrchr_chk(const char *in, int ch, size_t end) {
 }
 
 static int need_linecounter(void) {
-	return !!prog_state.skip || prog_state.statefile || prog_state.use_seqnr;
+	return !!prog_state.skip || prog_state.statefile ||
+	       prog_state.use_seqnr || prog_state.count != -1UL;
 }
 static size_t count_linefeeds(const char *buf, size_t len) {
 	const char *p = buf, *e = buf+len;
@@ -723,7 +729,11 @@ static int dispatch_line(char* inbuf, size_t len, char** argv) {
 			}
 			if(!len) return 1;
 		}
+	} else if(prog_state.count != -1UL) {
+		if(!prog_state.count) return -1;
+		--prog_state.count;
 	}
+
 	if(!prog_state.cmd_startarg) {
 		write_all(1, inbuf, len);
 		return 1;

test.sh

@@ -68,6 +68,12 @@ $JF -skip=5 < $(tmp).1 > $(tmp).2
 tail -n 5 < $(tmp).1 > $(tmp).3
 test_equal $(tmp).2 $(tmp).3
 
+dotest "seq 10 catmode skip 5 count 3"
+seq 10 > $(tmp).1
+$JF -skip=5 -count=3 < $(tmp).1 > $(tmp).2
+tail -n 5 < $(tmp).1 | head -n 3 > $(tmp).3
+test_equal $(tmp).2 $(tmp).3
+
 dotest "seq 10000 bulk skip 1337"
 seq 10000 | sort -u > $(tmp).1
 $JF -bulk=4K -skip=1337 -exec tests/stdin_printer.out < $(tmp).1 | sort -u > $(tmp).2