From 7bd9934a4be7c22b8f464ab7a1cc243801b5957c Mon Sep 17 00:00:00 2001
From: Tom Willemse
Date: Mon, 11 Oct 2021 00:13:15 -0700
Subject: Chapter 21.3

---
 clox/src/chunk.h    |  1 +
 clox/src/compiler.c | 12 +++++++++++-
 clox/src/debug.c    |  2 ++
 clox/src/vm.c       | 10 ++++++++++
 4 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/clox/src/chunk.h b/clox/src/chunk.h
index 4194e8c..dda2b31 100644
--- a/clox/src/chunk.h
+++ b/clox/src/chunk.h
@@ -10,6 +10,7 @@ typedef enum {
   OP_TRUE,
   OP_FALSE,
   OP_POP,
+  OP_GET_GLOBAL,
   OP_DEFINE_GLOBAL,
   OP_EQUAL,
   OP_GREATER,
diff --git a/clox/src/compiler.c b/clox/src/compiler.c
index 9dd7927..f125c10 100644
--- a/clox/src/compiler.c
+++ b/clox/src/compiler.c
@@ -138,6 +138,7 @@ static ParseRule *getRule(TokenType type);
 static void parsePrecedence(Precedence precedence);
 static uint8_t parseVariable(const char *name);
 static void defineVariable(uint8_t global);
+static uint8_t identifierConstant(Token *name);
 
 static void binary() {
   TokenType operatorType = parser.previous.type;
@@ -276,6 +277,15 @@ static void string() {
       copyString(parser.previous.start + 1, parser.previous.length - 2)));
 }
 
+static void namedVariable(Token name) {
+  uint8_t arg = identifierConstant(&name);
+  emitBytes(OP_GET_GLOBAL, arg);
+}
+
+static void variable() {
+  namedVariable(parser.previous);
+}
+
 static void unary() {
   TokenType operatorType = parser.previous.type;
 
@@ -320,7 +330,7 @@ ParseRule rules[] = {
     [TOKEN_GREATER_EQUAL] = {NULL, binary, PREC_COMPARISON},
     [TOKEN_LESS] = {NULL, binary, PREC_COMPARISON},
     [TOKEN_LESS_EQUAL] = {NULL, binary, PREC_COMPARISON},
-    [TOKEN_IDENTIFIER] = {NULL, NULL, PREC_NONE},
+    [TOKEN_IDENTIFIER] = {variable, NULL, PREC_NONE},
     [TOKEN_STRING] = {string, NULL, PREC_NONE},
     [TOKEN_NUMBER] = {number, NULL, PREC_NONE},
     [TOKEN_AND] = {NULL, NULL, PREC_NONE},
diff --git a/clox/src/debug.c b/clox/src/debug.c
index 5663091..669fc78 100644
--- a/clox/src/debug.c
+++ b/clox/src/debug.c
@@ -44,6 +44,8 @@ int disassembleInstruction(Chunk *chunk, int offset) {
     return simpleInstruction("OP_FALSE", offset);
   case OP_POP:
     return simpleInstruction("OP_POP", offset);
+  case OP_GET_GLOBAL:
+    return constantInstruction("OP_GET_GLOBAL", chunk, offset);
   case OP_DEFINE_GLOBAL:
     return constantInstruction("OP_DEFINE_GLOBAL", chunk, offset);
   case OP_EQUAL:
diff --git a/clox/src/vm.c b/clox/src/vm.c
index 709f3d1..5e35abd 100644
--- a/clox/src/vm.c
+++ b/clox/src/vm.c
@@ -115,6 +115,16 @@ static InterpretResult run() {
     case OP_POP:
       pop();
       break;
+    case OP_GET_GLOBAL: {
+      ObjString *name = READ_STRING();
+      Value value;
+      if (!tableGet(&vm.globals, name, &value)) {
+	runtimeError("Undefined variable '%s'.", name->chars);
+	return INTERPRET_RUNTIME_ERROR;
+      }
+      push(value);
+      break;
+    }
     case OP_DEFINE_GLOBAL: {
       ObjString *name = READ_STRING();
       tableSet(&vm.globals, name, peek(0));
-- 
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