From ecbc58fb917c292f05bed401afe7be0a80971d80 Mon Sep 17 00:00:00 2001
From: Robin Haberkorn <robin.haberkorn@googlemail.com>
Date: Wed, 23 Sep 2015 17:53:39 +0200
Subject: different operators can have the same precedence now

 * SciTECO now has the same operator precedence table as C.
 * It is numerically important whether different operators
   have the same precedence. E.g. "5*2/4" used to be evaluated
   by SciTECO as "5*(2/4)" since division had a higher precedence
   than multiplication. Within in real (!) numbers this would
   be the expected evaluation order.
   Users of other programming languages however would expect
   the expression to be evaluated as "(5*2)/4" which makes
   a numerical difference when working with integers.
 * Operator precedence has been implemented by encoding it
   into the enumeration values used to represent different
   operators.
   Calculating the precedence of a given operator can then
   be done very efficiently and elegantly (in our case using
   a plain right shift operation).
 * documentation updated. We use a precedence table now.
---
 src/expressions.h | 34 ++++++++++++++++++++++++----------
 1 file changed, 24 insertions(+), 10 deletions(-)

(limited to 'src/expressions.h')

diff --git a/src/expressions.h b/src/expressions.h
index f97ec1c..e49880c 100644
--- a/src/expressions.h
+++ b/src/expressions.h
@@ -150,31 +150,45 @@ public:
 	}
 };
 
-/*
+/**
  * Arithmetic expression stacks
  */
 extern class Expressions {
 public:
-	/* reflects also operator precedence */
+	/**
+	 * Operator type.
+	 * The enumeration value divided by 16 represents
+	 * its precedence (the lower, the higher).
+	 * In other words, the value's lower nibble is
+	 * reserved for enumerating operators of the
+	 * same precedence.
+	 */
 	enum Operator {
-		OP_NIL = 0,
-		OP_POW,		// ^*
-		OP_MOD,		// ^/
+		OP_NIL	= 0,
+		OP_POW	= 0x10,	// ^*
+		OP_MOD	= 0x20,	// ^/
 		OP_DIV,		// /
 		OP_MUL,		// *
-		OP_SUB,		// -
+		OP_SUB	= 0x30,	// -
 		OP_ADD,		// +
-		OP_AND,		// &
-		OP_XOR,		// ^#
-		OP_OR,		// #
+		OP_AND	= 0x40,	// &
+		OP_XOR	= 0x50,	// ^#
+		OP_OR	= 0x60,	// #
 				// pseudo operators:
-		OP_NEW,
+		OP_NEW	= 0xF0,
 		OP_BRACE,
 		OP_LOOP,
 		OP_NUMBER
 	};
 
 private:
+	/** Get operator precedence */
+	inline gint
+	precedence(Operator op)
+	{
+		return op >> 4;
+	}
+
 	ValueStack<tecoInt>	numbers;
 	ValueStack<Operator>	operators;
 
-- 
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