sorta kinda working on strings for cmenu. being lazy

cmenu/str.c

@@ -0,0 +1,43 @@
+#include <stddef.h>
+
+#include "str.h"
+
+static bool intToIndex(String string, int i, size_t *out_index) {
+	int val = 0;
+	bool is_ok = true;
+	if (i >= 0) {
+		val = i;
+	} else {
+		val = string.size + 1 + i;
+	}
+
+	if (val < 0) {
+		val = 0;
+		is_ok = false;
+	} else if (val > string.size) {
+		val = string.size;
+		is_ok = false;
+	}
+
+	*out_index = val;
+	return is_ok;
+}
+
+bool StringSub(String string, int start, int end, String *out_substr) {
+
+	size_t start_index = 0;
+	bool start_ok = intToIndex(string, start, &start_index);
+	size_t end_index = 0;
+	bool end_ok = intToIndex(string, end, &end_index);
+
+	if (string.size == 0 || start_index >= end_index) {
+		out_substr->size=0;
+		out_substr->buffer=NULL;
+		return false;
+	}
+
+	out_substr->buffer = &string.buffer[start_index];
+	out_substr->size = end_index - start_index;
+
+	return start_ok && end_ok;
+}