From 9f5a5c87bf7174620ef51605ffd6e93f828056ac Mon Sep 17 00:00:00 2001
From: Peter Wu <peter@lekensteyn.nl>
Date: Sun, 8 May 2016 19:03:09 +0200
Subject: PicoRec: fully implemented

Removed the LAYOUT matching from tokenRegex, these are not part of the
token and should be removed before. Add an auxilliary function that
tests whether a token that matches ID really is a non-keyword.

Implement remaining expressions.
---
 src/PicoRec.java | 81 ++++++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 67 insertions(+), 14 deletions(-)

diff --git a/src/PicoRec.java b/src/PicoRec.java
index 1d6e16b..d38297f 100644
--- a/src/PicoRec.java
+++ b/src/PicoRec.java
@@ -11,10 +11,9 @@ public class PicoRec {
     /** Regular expression describing the NAT symbol. */
     private final static String natRegex = "0|[1-9][0-9]*";
 
-    /** Regular expression matches all tokens accepted by the language (other
-     * than keywords). */
+    /** Regular expression matches all tokens accepted by the language. */
     private final static String tokenRegex =
-        literalRegex("[\\|,;\\+\\*\\-]|:=|" + idRegex + "|" + natRegex);
+        "[\\|,;\\+\\*\\-]|:=|" + idRegex + "|" + natRegex;
     /** Regular expression matching layout tokens (to be ignored). */
     private final static String layoutRegex = "[ \n]*";
 
@@ -28,12 +27,6 @@ public class PicoRec {
     private String input;
     private int offset;
 
-    /** Auxilliary function to produce a regular expression taking into account
-     * layout considerations for symbols. */
-    private static String literalRegex(String regex) {
-        return layoutRegex + "(" + regex + ")" + layoutRegex;
-    }
-
     public PicoRec() {
         idR = new RunAutomaton(new RegExp(idRegex).toAutomaton());
         natR = new RunAutomaton(new RegExp(natRegex).toAutomaton());
@@ -58,9 +51,17 @@ public class PicoRec {
         match(END_SYMBOL);
     }
 
+    /** Returns true if the next token is a non-keyword identifier. */
+    private boolean isId(String token) {
+        boolean result = !("begin".equals(token) || "declare".equals(token) ||
+                "end".equals(token)) && idR.run(token);
+        //System.err.println("Testing for ID: " + token + ": " + result);
+        return result;
+    }
+
     private void parseIdList() {
         // Parses IDLIST ::= ID "," |
-        while (idR.run(next())) {
+        while (isId(next())) {
             match(next()); // ID
             match(",");
         }
@@ -68,17 +69,68 @@ public class PicoRec {
 
     private void parseStatements() {
         // Parses STATEMENTS ::= STATEMENT ";" |
-        // => STATEMENT ::= ID ":=" EXP ";" |
-        while (idR.run(next())) {
+        // => STATEMENTS ::= ID ":=" EXP ";" |
+        while (isId(next())) {
             match(next()); // ID
             match(":=");
             parseExp();
-            match(",");
+            match(";");
         }
     }
 
     private void parseExp() {
-        // TODO implement me
+        // Parses EXP ::= PROD-EXP PLUS-EXP'
+        parseProdExp();
+        parsePlusExpPrefixed();
+    }
+
+    private void parsePlusExp() {
+        // Parses PLUS-EXP ::= PROD-EXP PLUS-EXP'
+        parseProdExp();
+        parsePlusExpPrefixed();
+    }
+
+    private void parsePlusExpPrefixed() {
+        // Parses PLUS-EXP' ::= "+" PROD-EXP PLUS-EXP' |
+        while ("+".equals(next())) {
+            match("*");
+            parseProdExp();
+            parsePlusExpPrefixed();
+        }
+    }
+
+    private void parseProdExp() {
+        // Parses PROD-EXP ::= SINGLE-EXP PROD-EXP'
+        parseSingleExp();
+        parseProdExpPrefixed();
+    }
+
+    private void parseProdExpPrefixed() {
+        // Parses PROD-EXP' ::= "*" SINGLE-EXP PROD-EXP' |
+        while ("*".equals(next())) {
+            match("*");
+            parseSingleExp();
+            parseProdExpPrefixed();
+        }
+    }
+
+    private void parseSingleExp() {
+        // Parses SINGLE-EXP ::= "-" SINGLE-EXP | ID | NAT | "(" EXP ")"
+        String token = next();
+        if ("-".equals(token)) {
+            match("-");
+            parseSingleExp();
+        } else if (isId(token)) {
+            match(token);
+        } else if (natR.run(token)) {
+            match(token);
+        } else if ("(".equals(token)) {
+            match("(");
+            parseExp();
+            match(")");
+        } else {
+            throw new ParseError("Invalid expression");
+        }
     }
 
     /** Skip layout characters. */
@@ -123,6 +175,7 @@ public class PicoRec {
             }
         } else {
             if (!input.startsWith(symbol, offset)) {
+                //System.err.println("Current input: " + input.substring(offset));
                 throw new ParseError("Cannot match symbol: " + symbol);
             }
             offset += symbol.length();
-- 
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