docs/pg.1

@@ -11,8 +11,7 @@ pg \(en paginate
 
 .SH DESCRIPTION
 
-Pg prints standard input to standard output, pausing every 22 lines and waiting
-for a line feed from the tty before continuing.
+Pg prints standard input to standard output, accepting commands between pages.
 
 .SH DIAGNOSTICS
 

src/pg.c

@@ -16,11 +16,12 @@
  * along with this program. If not, see https://www.gnu.org/licenses/.
  */
 
+#include <errno.h> /* errno */
 #include <stdio.h> /* fclose(3), feof(3), fgetc(3), fgets(3), fopen(3),
                     * fprintf(3), fputc(3), stderr, stdin, stdout, EOF, FILE,
                     * NULL */
 #include <stdlib.h> /* size_t */
-#include <string.h> /* strchr */
+#include <string.h> /* strchr(3), strcmp(3), strerror(3), strtok(3) */
 
 #if !defined EX_OK || !defined EX_SOFTWARE || !defined EX_USAGE 
 #	include <sysexits.h>
@@ -32,45 +33,140 @@
  * keyboard input without going through stdin. Also, a newline is now printed
  * before querying the keyboard. */
 
-static size_t l = 22; /* plan9 default */
+static char *whitespace = " \n\r\t\v";
+
+static struct {
+	size_t quantity;
+	enum { LINES = 0, BYTES = 1 } type;
+} default_page_unit = { 22, LINES } /* Plan 9 default */;
+static FILE *input;
+static char *prompt = ": ";
 
 static char *program_name = "pg";
 
-static int print_lines(FILE *f, size_t l){
+/* Commands start with cmd_. They take an argc and NULL-terminated argv, like
+ * main, and return one of the following RET_ enums as a status. */
+enum{ RET_ARGSERROR, RET_OK, RET_NOCMD, RET_REQSHUTDOWN, RET_SOFTWARE };
+
+static int page_bytes(FILE *f, size_t l){
+	int c;
+
+	while((c = fgetc(f)) != EOF)
+		if((c = fputc(c, stdout)) == EOF || --l == 0)
+			break;
+
+	return c == EOF
+	    ? RET_REQSHUTDOWN
+	    : RET_OK;
+}
+
+static int page_lines(FILE *f, size_t l){
 	int c;
 
 	while((c = fgetc(f)) != EOF)
 		if((c = fputc(c, stdout)) == EOF || (l -= (c == '\n')) == 0)
 			break;
 
-	return c;
+	return c == EOF
+	    ? RET_REQSHUTDOWN
+	    : RET_OK;
+}
+
+static int cmd_quit(int argc, char **argv){ return RET_REQSHUTDOWN; }
+static int cmd_default_page(int argc, char **argv){
+
+	if(strtok(NULL, whitespace) != NULL)
+		return RET_ARGSERROR; /* This shouldn't be possible. */
+	if(default_page_unit.type == BYTES)
+		return page_bytes(input, default_page_unit.quantity);
+	else if(default_page_unit.type == LINES)
+		return page_lines(input, default_page_unit.quantity);
+	else
+		return RET_SOFTWARE;
+}
+
+/* A CmdMap must be NULL-terminated. */
+static struct CmdMap{ char *name; int (*fn)(int, char **); } cmd_map[] = {
+	{ "", cmd_default_page },
+
+	/* don't make the user feel trapped */
+	{ "exit", cmd_quit }, { "q", cmd_quit }, { ":q", cmd_quit },
+	{ ":q!", cmd_quit }, { "quit", cmd_quit }, { "ZZ", cmd_quit },
+	
+	{ NULL, NULL }
+};
+
+#define ARGV_MAX 10
+/* Find and execute the command in the command map, given a corresponding
+ * command line. */
+static int cmdline_exec(struct CmdMap *map, char *cmdline){
+	static int argc;
+	static char *argv[ARGV_MAX];
+
+	if((argv[(argc = 0)] = strtok(cmdline, whitespace)) == NULL){
+		while(cmdline[0] != '\0')
+			cmdline = &cmdline[1];
+		argv[argc] = cmdline;
+		argv[++argc] = NULL;
+	}else{
+		while((argv[++argc] = strtok(NULL , whitespace)) != NULL
+		      && argc < ARGV_MAX);
+	}
+
+	for(; map->name != NULL; map = &map[1])
+		if(strcmp(map->name, argv[0]) == 0)
+			return map->fn(argc, argv);
+
+	fprintf(stderr, "%s: %s: not found\n", program_name, argv[0]);
+	return RET_NOCMD;
 }
 
 int main(int argc, char *argv[]){
 	unsigned char cmd[99+1];
-	char *p;
+	int r;
 	FILE *t;
 
-	if(argc == 0)
-		argv = &program_name;
+	input = stdin;
+
+	if(argc > 0)
+		program_name = argv[0];
 
 	if(argc > 1){
-		fprintf(stderr, "Usage: %s\n", argv[0]);
+		fprintf(stderr, "Usage: %s\n", program_name);
 		return EX_USAGE;
 	}
 
 	if((t = fopen("/dev/tty", "rb")) == NULL){
-		fprintf(stderr, "%s: Could not open /dev/tty\n", argv[0]);
+		fprintf(stderr, "%s: /dev/tty: %s\n", program_name, strerror(errno));
 		return EX_SOFTWARE;
 	}
 
-	while(print_lines(stdin, l) != EOF){
+	r = 0;
+	for(;;){
+		if(r != RET_REQSHUTDOWN){
+			fputs(prompt, stderr);
 			fgets((char *)cmd, (sizeof cmd) / (sizeof *cmd), t);
-		for(p = (char *)cmd; *p != '\0' && strchr(" \t", *p) != NULL; ++p);
-		if(feof(t) || (*p | 32) == 'q')
+			if(strchr((char *)cmd, '\n') == NULL){ /* fast-forward stream */
+				int c;
+
+				while((c = fgetc(t)) != '\n')
+					if(c == EOF)
 						break;
 			}
-
-exit:	fclose(t);
+		}
+		if(r == RET_REQSHUTDOWN || feof(t)){
+			fclose(t);
 			return EX_OK;
+		}
+
+		switch((r = cmdline_exec(cmd_map, (char *)cmd))){
+		case RET_ARGSERROR: case RET_NOCMD: case RET_OK:
+			break;
+		case RET_REQSHUTDOWN: case RET_SOFTWARE: default:
+			r = RET_REQSHUTDOWN;
+		}
+	}
+
+	/* UNREACHABLE */
+	return EX_SOFTWARE;
 }