Add truthy

Makefile

@@ -7,7 +7,7 @@ DISTMAN = $(DISTDIR)/usr/share/man
 all: djl-utils.deb
 
 dist: bin DEBIAN.control
-bin: line sign pasta suptime countdown stopwatch timestamp
+bin: line sign pasta suptime countdown truthy stopwatch timestamp
 
 distdir:
 	mkdir -p $(DISTBIN)
@@ -24,6 +24,9 @@ sign: $(SRCDIR)/sign $(SRCDIR)/sign.1 distdir
 pasta: $(SRCDIR)/pasta distdir
 	cp $(SRCDIR)/pasta $(DISTBIN)/pasta
 
+truthy: $(SRCDIR)/truthy.c distdir
+	cc -o $(DISTBIN)/truthy $(SRCDIR)/truthy.c
+
 suptime: $(SRCDIR)/suptime.c $(SRCDIR)/suptime.1 distdir
 	cc -o $(DISTBIN)/suptime $(SRCDIR)/suptime.c
 	cp $(SRCDIR)/suptime.1 $(DISTMAN)/man1/suptime.1

src/truthy.c

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2021 Dylan Lom <djl@dylanlom.com>
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+ * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+ * AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+ * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+ * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+ * OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+ * PERFORMANCE OF THIS SOFTWARE.
+ */ 
+
+#include <ctype.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <strings.h>
+
+#include "util.h"
+
+const char *argv0;
+
+/*
+ * Check if s is a valid decimal number.
+ */
+bool
+isnum(const char *s)
+{
+    if (*s == '-' || *s == '+') s++;
+    while (*s) {
+        if (!isdigit(*s)) return false;
+        s++;
+    }
+    return true;
+}
+
+/*
+ * Coalesce s into true/false boolean.
+ * The following strings are considered false:
+ *   false, null, 0 (and its other representations, e.g. 000, -, +)
+ * BUG: Non base-10 representations of 0 are considered truthy
+ */
+bool
+truthy(const char *s)
+{
+    if (isnum(s) && atoi(s) == 0) return false;
+    if (strcasecmp(s, "false") == 0 || strcasecmp(s, "null") == 0) return false;
+
+    return true;
+}
+
+/*
+ * Determine if a value is true-adjacent. Inspired by (but not following)
+ * JavaScript's concept of truthy/falsey-ness.
+ * https://developer.mozilla.org/en-US/docs/Glossary/Falsy
+ */
+int
+main(int argc, char *argv[])
+{
+    SET_ARGV0();
+
+    if (argc < 1 || !truthy(argv[0]))
+        return 1;
+    return 0;
+}