1 files changed, 156 insertions, 0 deletions
diff --git a/getgopt.go b/getgopt.go
new file mode 100644
index 0000000..166430a
--- /dev/null
+++ b/getgopt.go
@@ -0,0 +1,156 @@
+/*
+ * BSD Zero Clause License
+ *
+ * Copyright (c) 2021 Thomas Voss
+ *
+ * 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.
+ */
+
+package getgopt
+
+import (
+	"fmt"
+	"os"
+)
+
+const (
+	errNoArg  = "option requires an argument -- %c\n"
+	errBadArg = "unknown option -- %c\n"
+)
+
+var (
+	opt    = 1
+	optlen int
+	parsed bool
+	opts   [255]option
+)
+
+var (
+	/* The argument to the matched flag */
+	Optarg string
+	/* If true, print error messages */
+	Opterr = true
+	/* During parsing this is the current index into os.Args being parsed. After parsing this is
+	 * the index to the first non-option element of os.Args.
+	 */
+	Optind = 1
+	/* The flag that caused the last parsing error */
+	Optopt byte
+)
+
+type option struct {
+	arg  bool
+	used bool
+}
+
+func parseArgs(optstring string) {
+	length := len(optstring)
+
+	if (length > 0) && (optstring[0] == ':') {
+		Opterr = false
+	}
+
+	for i, c := range optstring {
+		if c != ':' {
+			opts[c].used = true
+			opts[c].arg = ((i < length-1) && (optstring[i+1] == ':'))
+		}
+	}
+}
+
+/* A function to parse command line flags. This function takes as it's arguments from first to last,
+ * the count of command line arguments, the array of command line arguments (os.Args), an option
+ * string, and a pointer to a byte where the current flag can be stored. When called the current
+ * flag will be stored in `optptr` and the global variables `Optarg`, `Opterr`, `Optind`, and
+ * `Optopt` may be set.
+ *
+ * If there are still more arguments to be parsed, the function will return true. Otherwise false is
+ * returned. This makes it very easy to incorperate into a for/while loop.
+ */
+func Getopt(argc int, argv []string, optstring string, optptr *byte) bool {
+	/* If we havem't parsed the optstring yet, parse it */
+	if !parsed {
+		parseArgs(optstring)
+		parsed = true
+	}
+
+	/* Instantly return false if the follow cases are met */
+	if Optind >= argc || argv[Optind] == "" || argv[Optind][0] != '-' ||
+			argv[Optind] == "-" || optstring == "" {
+		return false
+	} else if argv[Optind] == "--" {
+		Optind++
+		return false
+	}
+
+	/* For each element of argv we calculate its length */
+	if opt == 1 {
+		optlen = len(argv[Optind])
+	}
+
+	/* The current flag */
+	currFlag := argv[Optind][opt]
+
+	if opts[currFlag].used {
+		if opts[currFlag].arg {
+			if opt == optlen-1 {
+				Optind += 2
+				if Optind > argc {
+					Optopt = currFlag
+					if Opterr {
+						*optptr = '?'
+						fmt.Fprintf(os.Stderr, errNoArg, Optopt)
+					} else {
+						*optptr = ':'
+					}
+				} else {
+					Optarg = argv[Optind-1]
+					*optptr = currFlag
+				}
+				opt = 1
+				return true
+			}
+
+			/* If the opt takes an argument but it's not the last character in the
+			 * string
+			 */
+			*optptr = currFlag
+			Optarg = string(argv[Optind][opt+1:])
+			Optind++
+			opt = 1
+		} else { /* If the opt doesn't take an argument */
+			if opt == optlen-1 {
+				opt = 1
+				Optind++
+			} else {
+				opt++
+			}
+
+			*optptr = currFlag
+		}
+	} else { /* If the arg isn't in optstring */
+		if Opterr {
+			fmt.Fprintf(os.Stderr, errBadArg, argv[Optind][opt])
+		}
+		Optopt = currFlag
+		*optptr = '?'
+
+		if opt == optlen-1 {
+			Optind++
+			opt = 1
+		} else {
+			opt++
+		}
+	}
+
+	return true
+}