From e1992f60a5c7642055fad42104c03ee2b098326e Mon Sep 17 00:00:00 2001
From: Jack Kelly <jack@jackkelly.name>
Date: Sun, 28 May 2023 09:06:23 +1000
Subject: [PATCH] zlox: `while` statements

---
 zlox/src/chunk.h    |  1 +
 zlox/src/compiler.c | 27 +++++++++++++++++++++++++++
 zlox/src/debug.c    |  2 ++
 zlox/src/vm.c       |  6 ++++++
 4 files changed, 36 insertions(+)

diff --git a/zlox/src/chunk.h b/zlox/src/chunk.h
index 608ce26..6b53b4f 100644
--- a/zlox/src/chunk.h
+++ b/zlox/src/chunk.h
@@ -27,6 +27,7 @@ typedef enum {
   OP_PRINT,
   OP_JUMP,
   OP_JUMP_IF_FALSE,
+  OP_LOOP,
   OP_RETURN,
 } OpCode;
 
diff --git a/zlox/src/compiler.c b/zlox/src/compiler.c
index 692465e..370bcf2 100644
--- a/zlox/src/compiler.c
+++ b/zlox/src/compiler.c
@@ -138,6 +138,16 @@ static void emitBytes(uint8_t byte1, uint8_t byte2) {
   emitByte(byte2);
 }
 
+static void emitLoop(int loopStart) {
+  emitByte(OP_LOOP);
+
+  int offset = currentChunk()->count - loopStart + 2;
+  if (offset > UINT16_MAX) error("Loop body too large.");
+
+  emitByte((offset >> 8) & 0xff);
+  emitByte(offset & 0xff);
+}
+
 static int emitJump(uint8_t instruction) {
   emitByte(instruction);
   emitByte(0xff);
@@ -496,6 +506,21 @@ static void printStatement() {
   emitByte(OP_PRINT);
 }
 
+static void whileStatement() {
+  int loopStart = currentChunk()->count;
+  consume(TOKEN_LEFT_PAREN, "Expect '(' after 'while'.");
+  expression();
+  consume(TOKEN_RIGHT_PAREN, "Expect ')' after condition.");
+
+  int exitJump = emitJump(OP_JUMP_IF_FALSE);
+  emitByte(OP_POP);
+  statement();
+  emitLoop(loopStart);
+
+  patchJump(exitJump);
+  emitByte(OP_POP);
+}
+
 static void synchronize() {
   parser.panicMode = false;
 
@@ -535,6 +560,8 @@ static void statement() {
     printStatement();
   } else if (match(TOKEN_IF)) {
     ifStatement();
+  } else if (match(TOKEN_WHILE)) {
+    whileStatement();
   } else if (match(TOKEN_LEFT_BRACE)) {
     beginScope();
     block();
diff --git a/zlox/src/debug.c b/zlox/src/debug.c
index 57b0e89..c7c94da 100644
--- a/zlox/src/debug.c
+++ b/zlox/src/debug.c
@@ -97,6 +97,8 @@ int disassembleInstruction(Chunk *chunk, int offset) {
     return jumpInstruction("OP_JUMP", 1, chunk, offset);
   case OP_JUMP_IF_FALSE:
     return jumpInstruction("OP_JUMP_IF_FALSE", 1, chunk, offset);
+  case OP_LOOP:
+    return jumpInstruction("OP_LOOP", -1, chunk, offset);
   case OP_RETURN:
     return simpleInstruction("OP_RETURN", offset);
   default:
diff --git a/zlox/src/vm.c b/zlox/src/vm.c
index 64d8ef5..a1cf358 100644
--- a/zlox/src/vm.c
+++ b/zlox/src/vm.c
@@ -195,6 +195,12 @@ static InterpretResult run() {
         if (isFalsey(peek(0))) vm.ip += offset;
         break;
       }
+    case OP_LOOP:
+      {
+        uint16_t offset = READ_SHORT();
+        vm.ip -= offset;
+        break;
+      }
     case OP_RETURN:
       /* Exit interpreter. */
       return INTERPRET_OK;
-- 
2.45.2