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ip/torrent: use NPROC processes in parallel for calculating initial piece hashes

--- a/sys/src/cmd/ip/torrent.c

+++ b/sys/src/cmd/ip/torrent.c

@@ -49,6 +49,7 @@

 };

 int debug;

+int nproc = 1;

 int killgroup = -1;

 int port = 6881;

 char *deftrack = "http://exodus.desync.com/announce";

@@ -1197,6 +1198,12 @@

 		usage();

 	} ARGEND;

+	if((s = getenv("NPROC")) != 0){

+		if((nproc = atoi(s)) <= 0)

+			nproc = 1;

+		free(s);

+	}

+

 	fd = 0;

 	if(*argv)

 		if((fd = open(*argv, OREAD)) < 0)

@@ -1301,8 +1308,18 @@

 	if(len)

 		sysfatal("pieces do not match file length");

-	for(i = 0; i<npieces; i++)

-		havepiece(i);

+	for(i=0; i<nproc; i++){

+		switch(rfork(RFPROC|RFMEM)){

+		case -1:

+			sysfatal("fork: %r");

+		case 0:

+			for(; i<npieces; i+=nproc)

+				havepiece(i);

+			exits(0);

+		}

+	}

+	while(waitpid() >= 0)

+		;

 	srand(time(0));

 	atnotify(catch, 1);