From be69ac321da25bdd29034ce7bc12eb034d5d699b Mon Sep 17 00:00:00 2001
From: Nemo <me@captnemo.in>
Date: Mon, 1 Jun 2020 23:05:03 +0530
Subject: [PATCH] Reduce stack manipulation (use M instead of A)

---
 .../StackArithmetic/SimpleAdd/SimpleAdd.asm   | 20 ++------
 vm/VMTranslator.php                           | 49 +++++++++++--------
 2 files changed, 33 insertions(+), 36 deletions(-)

diff --git a/projects/07/StackArithmetic/SimpleAdd/SimpleAdd.asm b/projects/07/StackArithmetic/SimpleAdd/SimpleAdd.asm
index 094f2cb..9307c85 100644
--- a/projects/07/StackArithmetic/SimpleAdd/SimpleAdd.asm
+++ b/projects/07/StackArithmetic/SimpleAdd/SimpleAdd.asm
@@ -4,10 +4,8 @@ D=A
 @SP
 A=M
 M=D
-A=A+1
-D=A
 @SP
-M=D
+M=M+1
 // end push constant 7
 // push constant 8
 @8
@@ -15,25 +13,17 @@ D=A
 @SP
 A=M
 M=D
-A=A+1
-D=A
 @SP
-M=D
+M=M+1
 // end push constant 8
 // ==== add ====
 // pop starts
 @SP
-A=M
-A=A-1
+A=M-1
 D=M
 // pop ends
 // inner add starts
 A=A-1
-D=D+M
-// inner add ends
-M=D
-A=A+1
-D=A
+M=D+M
 @SP
-M=D
-// ==== add ====
+M=M-1
diff --git a/vm/VMTranslator.php b/vm/VMTranslator.php
index 0c7d170..8781a4a 100644
--- a/vm/VMTranslator.php
+++ b/vm/VMTranslator.php
@@ -68,50 +68,63 @@ class Parser {
 
 class CodeWriter {
   function __construct($outputFile) {
+    $this->ic = 0;
     $this->file = fopen($outputFile, "w");
   }
 
+  function __destruct() {
+    fclose($this->file);
+  }
+
   function writeArithmetic(String $command ) {
     // Read top of stack to D
     $this->write([
       "// ==== $command ====",
       "// pop starts",
       "@SP",
-      "A=M",
-      "A=A-1",
+      "A=M-1",
       "D=M",
       "// pop ends",
+      "// inner $command starts",
+      'A=A-1',
     ]);
     switch ($command) {
       case 'add':
         // But add it to previous D this time
         $this->write([
-          "// inner add starts",
-          "A=A-1",
-          "D=D+M",
-          "// inner add ends",
+          "M=D+M"
         ]);
         break;
 
+      case 'eq':
+        $jumpPointer = $this->ic+5;
+        $this->write([
+          'D=M-D', // ic
+          'M=0', // set M=0, in case they aren't equal
+          "@{jumpPointer}",
+          'D;JEQ', //ic+2
+          'M=0',   //ic+3
+          'M=M-1', //ic+4
+          // set M=-1 (TRUE), in case they are equal
+          // *SP=-1 = true
+        ]);
+
       default:
         # code...
         break;
     }
 
     $this->write([
-      // And then write the result back to M
-      "M=D",
-      // Now we write our current A to SP
-      "A=A+1",
-      "D=A",
-      "@SP",
-      "M=D",
-      "// ==== $command ===="
+      '@SP',
+      'M=M-1'
     ]);
   }
 
   private function write(Array $lines) {
     foreach ($lines as $line) {
+      if (substr($line, 0, 2) !== "//") {
+        $this->ic += 1;
+      }
       fwrite($this->file, "$line\n");
     }
   }
@@ -131,10 +144,8 @@ class CodeWriter {
           // Write D to SP
           "M=D",
           // Bump Stack Pointer
-          "A=A+1",
-          "D=A",
           "@SP",
-          "M=D",
+          "M=M+1",
           "// end push $segment $index"
         ]);
         break;
@@ -159,10 +170,6 @@ class CodeWriter {
         break;
     }
   }
-
-  function close() {
-    throw new \Exception("Not yet Implemented");
-  }
 }