commands: add let command which supports proper arithmetic

This command works like the corresponding Unix shell command and
is used for adding, multiplying and much more.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 92 insertions, 0 deletions

diff --git a/include/math.h b/include/math.h
new file mode 100644
index 0000000000..5648e3f9c1
--- /dev/null
+++ b/include/math.h
@@ -0,0 +1,92 @@
+/* math.h - interface to shell math "library" -- this allows shells to share
+ *          the implementation of arithmetic $((...)) expansions.
+ *
+ * This aims to be a POSIX shell math library as documented here:
+ *	http://www.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_04
+ *
+ * See math.c for internal documentation.
+ */
+
+/* The math library has just one function:
+ *
+ * arith_t arith(arith_state_t *state, const char *expr);
+ *
+ * The expr argument is the math string to parse.  All normal expansions must
+ * be done already.  i.e. no dollar symbols should be present.
+ *
+ * The state argument is a pointer to a struct of hooks for your shell (see below),
+ * and an error message string (NULL if no error).
+ *
+ * The function returns the answer to the expression.  So if you called it
+ * with the expression:
+ * "1 + 2 + 3"
+ * you would obviously get back 6.
+ */
+
+/* To add support to a shell, you need to implement three functions:
+ *
+ * lookupvar() - look up and return the value of a variable
+ *
+ *	If the shell does:
+ *		foo=123
+ *	Then the code:
+ *		const char *val = lookupvar("foo");
+ *	will result in val pointing to "123"
+ *
+ * setvar() - set a variable to some value
+ *
+ *	If the arithmetic expansion does something like:
+ *		$(( i = 1))
+ *	then the math code will make a call like so:
+ *		setvar("i", "1", 0);
+ *	The storage for the first two parameters are not allocated, so your
+ *	shell implementation will most likely need to strdup() them to save.
+ *
+ * endofname() - return the end of a variable name from input
+ *
+ *	The arithmetic code does not know about variable naming conventions.
+ *	So when it is given an experession, it knows something is not numeric,
+ *	but it is up to the shell to dictate what is a valid identifiers.
+ *	So when it encounters something like:
+ *		$(( some_var + 123 ))
+ *	It will make a call like so:
+ *		end = endofname("some_var + 123");
+ *	So the shell needs to scan the input string and return a pointer to the
+ *	first non-identifier string.  In this case, it should return the input
+ *	pointer with an offset pointing to the first space.  The typical
+ *	implementation will return the offset of first char that does not match
+ *	the regex (in C locale): ^[a-zA-Z_][a-zA-Z_0-9]*
+ */
+
+#ifndef LIB_MATH_H
+#define LIB_MATH_H 1
+
+#ifdef ENABLE_SH_MATH_SUPPORT_64
+typedef long long arith_t;
+#define ARITH_FMT "%lld"
+#define strto_arith_t simple_strtoull
+#else
+typedef long arith_t;
+#define ARITH_FMT "%ld"
+#define strto_arith_t simple_strtoul
+#endif
+
+# define is_name(c)      (isalpha((unsigned char)(c)))
+# define is_in_name(c)   ((c) == '_' || (c) == '.' || isalnum((unsigned char)(c)))
+const char*  arith_endofname(const char *name);
+
+typedef const char* (*arith_var_lookup_t)(const char *name);
+typedef void        (*arith_var_set_t)(const char *name, const char *val);
+typedef const char* (*arith_var_endofname_t)(const char *name);
+
+typedef struct arith_state_t {
+	const char           *errmsg;
+	arith_var_lookup_t    lookupvar;
+	arith_var_set_t       setvar;
+	arith_var_endofname_t endofname;
+	void                 *list_of_recursed_names;
+} arith_state_t;
+
+arith_t arith(arith_state_t *state, const char *expr);
+
+#endif