Added option TE_POW_FROM_RIGHT

tinyexpr.c

@@ -22,6 +22,14 @@
  * 3. This notice may not be removed or altered from any source distribution.
  */
 
+/* COMPILE TIME OPTIONS */
+
+/* Exponentiation associativity:
+For a^b^c = (a^b)^c and -a^b = (-a)^b do nothing.
+For a^b^c = a^(b^c) and -a^b = -(a^b) uncomment the next line.*/
+/* #define TE_POW_FROM_RIGHT */
+
+
 #include "tinyexpr.h"
 #include <stdlib.h>
 #include <math.h>
@@ -357,7 +365,48 @@ static te_expr *power(state *s) {
     return ret;
 }
 
+#ifdef TE_POW_FROM_RIGHT
+static te_expr *factor(state *s) {
+    /* <factor>    =    <power> {"^" <power>} */
+    te_expr *ret = power(s);
 
+    int neg = 0;
+    te_expr *insertion = 0;
+
+    if (ret->type == (TE_FUNCTION1 | TE_FLAG_PURE) && ret->function == negate) {
+        te_expr *se = ret->parameters[0];
+        if (se->type != TE_CONSTANT) {
+            free(ret);
+            ret = se;
+            neg = 1;
+        }
+    }
+
+    while (s->type == TOK_INFIX && (s->function == pow)) {
+        te_fun2 t = s->function;
+        next_token(s);
+
+        if (insertion) {
+            /* Make exponentiation go right-to-left. */
+            te_expr *insert = NEW_EXPR(TE_FUNCTION2 | TE_FLAG_PURE, insertion->parameters[1], power(s));
+            insert->function = t;
+            insertion->parameters[1] = insert;
+            insertion = insert;
+        } else {
+            ret = NEW_EXPR(TE_FUNCTION2 | TE_FLAG_PURE, ret, power(s));
+            ret->function = t;
+            insertion = ret;
+        }
+    }
+
+    if (neg) {
+        ret = NEW_EXPR(TE_FUNCTION1 | TE_FLAG_PURE, ret);
+        ret->function = negate;
+    }
+
+    return ret;
+}
+#else
 static te_expr *factor(state *s) {
     /* <factor>    =    <power> {"^" <power>} */
     te_expr *ret = power(s);
@@ -371,6 +420,8 @@ static te_expr *factor(state *s) {
 
     return ret;
 }
+#endif
+
 
 
 static te_expr *term(state *s) {