compiler.c

//
// Created by digitsu on 2021/08/26.
//
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "common.h"
#include "compiler.h"
#include "memory.h"
#include "scanner.h"

#ifdef DEBUG_PRINT_CODE
#include "debug.h"
#endif

typedef struct {
    Token current;
    Token previous;
    bool hadError;
    bool panicMode;
} Parser;

typedef enum {
    PREC_NONE,
    PREC_ASSIGNMENT,    // =
    PREC_OR,            // or
    PREC_AND,           // and
    PREC_EQUALITY,      // == !=
    PREC_COMPARISON,    // < > <= >=
    PREC_TERM,          // + -
    PREC_FACTOR,        // * /
    PREC_UNARY,         // ! -
    PREC_CALL,          // . ()
    PREC_PRIMARY,
} Precedence;

typedef void (*ParseFn)(bool canAssign);

typedef struct {
    ParseFn prefix;
    ParseFn infix;
    Precedence precedence;
} ParseRule;

typedef struct {
    Token name;
    int depth;
    bool isCaptured;
} Local;

typedef struct {
    uint8_t index;
    bool isLocal;
} Upvalue;

typedef enum {
    TYPE_FUNCTION,
    TYPE_INITIALIZER,
    TYPE_METHOD,
    TYPE_SCRIPT
} FunctionType;

typedef struct Compiler {
    struct Compiler* enclosing;
    ObjFunction* function;
    FunctionType type;

    Local locals[UINT8_COUNT];
    int localCount;
    Upvalue upvalues[UINT8_COUNT];
    int scopeDepth;
} Compiler;

typedef struct ClassCompiler {
    struct ClassCompiler* enclosing;
    bool hasSuperclass;
} ClassCompiler;

Parser parser;
Compiler* current = NULL;
ClassCompiler* currentClass = NULL; // the nearest innermost enclosing class.

static Chunk* currentChunk() {
    return &current->function->chunk;
}

static void errorAt(Token* token, const char* message) {
    if (parser.panicMode) return;
    parser.panicMode = true;
    fprintf(stderr, "[line %d] Error", token->line);

    if (token->type == TOKEN_EOF) {
        fprintf(stderr, " at end");
    } else if (token->type == TOKEN_ERROR) {
        // Nothing
    } else {
        fprintf(stderr, " at '%.*s'", token->length, token->start);
    }

    fprintf(stderr, ": %s\n", message);
    parser.hadError = true;
}

static void error(const char* message) {
    errorAt(&parser.previous, message);
}

static void errorAtCurrent(const char* message) {
    errorAt(&parser.current, message);
}
static void advance() {
    parser.previous = parser.current;

    for (;;) {
        parser.current = scanToken();
        if (parser.current.type != TOKEN_ERROR) break;

        errorAtCurrent(parser.current.start);
    }
}

static void consume(TokenType type, const char* message) {
    if (parser.current.type == type) {
        advance();
        return;
    }

    errorAtCurrent(message);
}

static bool check(TokenType type) {
    return parser.current.type == type;
}

static bool match(TokenType type) {
    if (!check(type)) return false;
    advance();
    return true;
}

static void emitByte(uint8_t byte) {
    writeChunk(currentChunk(), byte, parser.previous.line);
}

static void emitBytes(uint8_t byte1, uint8_t byte2) {
    emitByte(byte1);
    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); // dummy placeholder for offset operand
    emitByte(0xff); // dummy placeholder (16-bit offset operand, allowing for jumping over 65,535 bytes of code)
    return currentChunk()->count - 2; // returns the offset of this jump instr (subtracts out the 2 offset operands)
}

static void emitReturn() {
    if (current->type == TYPE_INITIALIZER) {
        emitBytes(OP_GET_LOCAL, 0); // this will be the initialized instance 'this', as they put themselves into slot 0
    } else {
        emitByte(OP_NIL); // this makes it so that funcs that don't return anything still leave a nil on the VM stack
    }
    emitByte(OP_RETURN);
}

static uint8_t makeConstant(Value value) {
    int constant = addConstant(currentChunk(), value); // returns the index of the constant in the chunk's const array
    if (constant > UINT8_MAX) {
        error("Too many constants in one chunk.");
        return 0;
    }

    return (uint8_t)constant;
}

static void emitConstant(Value value) {
    // only supports 256 constants. an improvement would be to create an OP_CONSTANT_16 that could store 65k
    emitBytes(OP_CONSTANT, makeConstant(value));
}

static void patchJump(int offset) {
    // -2 to adjust for the bytecode for the jump offset itself.
    int jump = currentChunk()->count - offset - 2;

    if (jump > UINT16_MAX) {
        error("Too much code to jump over.");
    }

    currentChunk()->code[offset] = (jump >> 8) & 0xff;
    currentChunk()->code[offset + 1] = jump & 0xff;
}
static void initCompiler(Compiler* compiler, FunctionType type) {
    compiler->enclosing = current;
    compiler->function = NULL; // Garbage collector paranoia
    compiler->type = type;
    compiler->localCount = 0;
    compiler->scopeDepth = 0;
    compiler->function = newFunction(); // Garbage collector paranoia
    current = compiler;
    if (type != TYPE_SCRIPT) {
        current->function->name = copyString(parser.previous.start, parser.previous.length);
    }

    Local* local = &current->locals[current->localCount++];
    local->depth = 0; // claim stack slot zero for VMs use
    local->isCaptured = false;
    if (type != TYPE_FUNCTION) { // this slot on the local stack is saved for 'this' if this is a method compiling
        local->name.start = "this"; // this just puts the name "this" in the first slot of the locals for this compiler, which is compiling a clojure for a method. The VM needs to put the receiver instance in here
        local->name.length = 4;
    } else {
        local->name.start = "";
        local->name.length = 0;
    }
}

static ObjFunction* endCompiler() {
    emitReturn(); // this is how the end of a function call directs the VM back to the enclosing fns code chunk
    ObjFunction* function = current->function;

#ifdef DEBUG_PRINT_CODE
    if (!parser.hadError) {
        disassembleChunk(currentChunk(), function->name != NULL ? function->name->chars : "<script>");
    }
#endif

    current = current->enclosing;
    return function;
}

static void beginScope() {
    current->scopeDepth++;
}
static void endScope() {
    current->scopeDepth--;

    while (current->localCount > 0 && current->locals[current->localCount - 1].depth > current->scopeDepth) {
        if (current->locals[current->localCount - 1].isCaptured) {
            emitByte(OP_CLOSE_UPVALUE);
        } else {
            emitByte(OP_POP);
        }
        current->localCount--;
    }
}

// forward declare to solve some declaration cycles thanks to C compiler limitations
static void expression();
static void statement();
static void declaration();
static ParseRule* getRule(TokenType type);
static void parsePrecedence(Precedence precedence);

static uint8_t identifierConstant(Token* name) {
    return makeConstant(OBJ_VAL(copyString(name->start, name->length)));
}

static bool identifiersEqual(Token* a, Token* b) {
    if (a->length != b->length) return false;
    return memcmp(a->start, b->start, a->length) == 0;
}

static int resolveLocal(Compiler* compiler, Token* name) {
    for (int i = compiler->localCount - 1; i >= 0; i--) {
        Local* local = &compiler->locals[i];
        if (identifiersEqual(name, &local->name)) {
            if (local->depth == -1) {
                error("Can't read local variable in its own initializer. i.e. var a = a; not allowed");
            }
            return i;
        }
    }

    return -1; // couldn't find it in locals, must be a global
}

static int addUpvalue(Compiler* compiler, uint8_t index, bool isLocal) {
    int upvalueCount = compiler->function->upvalueCount;

    for (int i = 0; i < upvalueCount; i++) {
        Upvalue* upvalue = &compiler->upvalues[i];
        if (upvalue->index == index && upvalue->isLocal == isLocal) {
            return i;
        }
    }

    if (upvalueCount == UINT8_COUNT) {
        error("Too many closure variables in function.");
        return 0;
    }

    compiler->upvalues[upvalueCount].isLocal = isLocal;
    compiler->upvalues[upvalueCount].index = index;
    return compiler->function->upvalueCount++;
}

static int resolveUpvalue(Compiler* compiler, Token* name) {
    if (compiler->enclosing == NULL) return -1;

    int local = resolveLocal(compiler->enclosing, name);
    if (local != -1) {
        compiler->enclosing->locals[local].isCaptured = true;
        return addUpvalue(compiler, (uint8_t)local, true);
    }

    int upvalue = resolveUpvalue(compiler->enclosing, name); // recursive call

    if (upvalue != -1) {
        return addUpvalue(compiler, (uint8_t)upvalue, false);
    }

    return -1;
}

static void addLocal(Token name) {
    if (current->localCount == UINT8_COUNT) {
        error("Too many local variables in function.");
        return;
    }

    Local* local = &current->locals[current->localCount++]; // the reason this doesn't need allocation is that locals is fully allocated array in the compiler
    local->name = name;
    local->depth = -1; // sentinel value for variables in uninitialized state
    local->isCaptured = false;
}

static void declareVariable() {
    if (current->scopeDepth == 0) return; // don't do anything for globals

    Token* name = &parser.previous; // this token has a pointer to the lexemes first char and length. so long as the original source string is around for the compilation, we are fine; no seg faults
    for (int i = current->localCount - 1; i >= 0; i--) {
        Local* local = &current->locals[i];
        if (local->depth != -1 && local->depth < current->scopeDepth) {
            break;
        }

        if (identifiersEqual(name, &local->name)) {
            error("Already a variable with this name in this scope.");
        }
    }
    addLocal(*name);
}

static uint8_t parseVariable(const char* errorMessage) {
    consume(TOKEN_IDENTIFIER, errorMessage);

    declareVariable();
    if (current->scopeDepth > 0) return 0; // 0 is a dummy index for local scope, which doesn't store its names in the constant table

    return identifierConstant(&parser.previous);
}

static void markInitialized() {
    if (current->scopeDepth == 0) return; // if called from funDeclaration which can be from global scope, no need to mark anything
    current->locals[current->localCount - 1].depth =
            current->scopeDepth;
}

static void defineVariable(uint8_t global) {
    // in the case of locals
    if (current->scopeDepth > 0) {
        markInitialized();
        return; // this seems magical, because the value is already at the top of the stack in the VM at this point exactly where we want the local var to live
    }
    // in the case of globals, we need to tell the VM to bind it. (globals are late bound)
    emitBytes(OP_DEFINE_GLOBAL, global);
}

static uint8_t argumentList() {
    uint8_t argCount = 0;
    if (!check(TOKEN_RIGHT_PAREN)) {
        do {
            expression();
            if (argCount == 255) {
                // because argCount is a 8-bit operand to the VM code OP_CALL
                error("Can't have more than 255 arguments.");
            }
            argCount++;
        } while (match(TOKEN_COMMA));
    }
    consume(TOKEN_RIGHT_PAREN, "Expect ')' after arguments.");
    return argCount;
}

static void and_(bool canAssign) {
    int endJump = emitJump(OP_JUMP_IF_FALSE);

    emitByte(OP_POP);
    parsePrecedence(PREC_AND);

    patchJump(endJump);
}

static void binary(bool canAssign) {
    TokenType operatorType = parser.previous.type;
    ParseRule* rule = getRule(operatorType);
    parsePrecedence((Precedence)(rule->precedence + 1));

    switch (operatorType) {
        case TOKEN_BANG_EQUAL:      emitBytes(OP_EQUAL, OP_NOT); break;
        case TOKEN_EQUAL_EQUAL:     emitByte(OP_EQUAL); break;
        case TOKEN_GREATER:         emitByte(OP_GREATER); break;
        case TOKEN_GREATER_EQUAL:   emitBytes(OP_LESS, OP_NOT); break;
        case TOKEN_LESS:            emitByte(OP_LESS); break;
        case TOKEN_LESS_EQUAL:      emitBytes(OP_GREATER, OP_NOT); break;
        case TOKEN_PLUS:
            emitByte(OP_ADD); break;
        case TOKEN_MINUS:
            emitByte(OP_SUBTRACT); break;
        case TOKEN_STAR:
            emitByte(OP_MULTIPLY); break;
        case TOKEN_SLASH:
            emitByte(OP_DIVIDE); break;
        default:
            return; // unreachable
    }
}

static void call(bool canAssign) {
    uint8_t argCount = argumentList();
    emitBytes(OP_CALL, argCount);
}

static void dot(bool canAssign) {
    consume(TOKEN_IDENTIFIER, "Expect property name after '.'.");
    uint8_t name = identifierConstant(&parser.previous);

    if (canAssign && match(TOKEN_EQUAL)) {
        expression();
        emitBytes(OP_SET_PROPERTY, name);

    } else if (match(TOKEN_LEFT_PAREN)) {
        uint8_t argCount = argumentList();
        emitBytes(OP_INVOKE, name);
        emitByte(argCount);
    } else {
        emitBytes(OP_GET_PROPERTY, name);
    }
    // canAssign is false for cases like a + b.c = 3, which is an error, so do not allow this case for the dot to match the equals afterwards
}

static void literal(bool canAssign) {
    switch (parser.previous.type) {
        case TOKEN_FALSE:
            emitByte(OP_FALSE); break;
        case TOKEN_NIL:
            emitByte(OP_NIL); break;
        case TOKEN_TRUE:
            emitByte(OP_TRUE); break;
        default: return; //unreachable
    }
}

static void grouping(bool canAssign) {
    expression();
    consume(TOKEN_RIGHT_PAREN, "Expect ')' after expression.");
}

static void number(bool canAssign) {
    double value = strtod(parser.previous.start, NULL);
    emitConstant(NUMBER_VAL(value));
}

static void or_(bool canAssign) {
    // Nystrom admits this is not as efficient as AND but can be improved with its own jump operator (jump_if_true)
    int elseJump = emitJump(OP_JUMP_IF_FALSE);
    int endJump = emitJump(OP_JUMP);

    patchJump(elseJump);
    emitByte(OP_POP);

    parsePrecedence(PREC_OR);
    patchJump(endJump);
}

static void string(bool canAssign) {
    emitConstant(OBJ_VAL(copyString(parser.previous.start + 1, parser.previous.length - 2)));
}

static void namedVariable(Token name, bool canAssign) {
    // this is the branch that gets executed when we start the expression statement with a variable name identifier
    uint8_t getOp, setOp;
    int arg = resolveLocal(current, &name);
    if (arg != -1) {
        getOp = OP_GET_LOCAL;
        setOp = OP_SET_LOCAL;
    } else if ((arg = resolveUpvalue(current, &name)) != -1) {
        getOp = OP_GET_UPVALUE;
        setOp = OP_SET_UPVALUE;
    } else {
        arg = identifierConstant(&name);
        getOp = OP_GET_GLOBAL;
        setOp = OP_SET_GLOBAL;
    }

    if (canAssign && match(TOKEN_EQUAL)) {
        expression(); // this is the part where we evaluate the right of the '=' assignment expression statement
        emitBytes(setOp, (uint8_t)arg);
    } else {
        emitBytes(getOp, (uint8_t)arg);
    }
}

static void variable(bool canAssign) {
    namedVariable(parser.previous, canAssign);
}

static Token syntheticToken(const char* text) {
    Token token;
    token.start = text;
    token.length = (int)strlen(text);
    return token;
}

static void super_(bool canAssign) {
    if (currentClass == NULL) {
        error("Can't use 'super' outside of a class.");
    } else if (!currentClass->hasSuperclass) {
        error("Can't use 'super' in a class with no superclass.");
    }

    consume(TOKEN_DOT, "Expect '.' after 'super'.");
    consume(TOKEN_IDENTIFIER, "Expect superclass method name.");
    uint8_t name = identifierConstant(&parser.previous);

    namedVariable(syntheticToken("this"), false);
    if (match(TOKEN_LEFT_PAREN)) { // if invoking the super method immediately
        uint8_t argCount = argumentList();
        namedVariable(syntheticToken("super"), false); // this puts the superclass onto the VM stack
        emitBytes(OP_SUPER_INVOKE, name);
        emitByte(argCount);
    } else { // if the rare case of just stashing away the method to be called later
        namedVariable(syntheticToken("super"), false);
        emitBytes(OP_GET_SUPER, name);
    }
}

static void this_(bool canAssign) {
    if (currentClass == NULL) {
        error("Can't use 'this' outside of a class.");
        return;
    }
    variable(false);
}

static void unary(bool canAssign) {
    TokenType operatorType = parser.previous.type;

    // Compile the operand
    parsePrecedence(PREC_UNARY);

    // Emit the operator instruction.
    switch (operatorType) {
        case TOKEN_BANG:emitByte(OP_NOT); break;
        case TOKEN_MINUS: emitByte(OP_NEGATE); break;
        default: return; // unreachable
    }
}

// uses the C99 syntax for indexing into a table of [INDEX] = value
ParseRule rules[] = {
    [TOKEN_LEFT_PAREN]      = {grouping,    call, PREC_CALL},
    [TOKEN_RIGHT_PAREN]     = {NULL, NULL, PREC_NONE},
    [TOKEN_LEFT_BRACE]      = {NULL, NULL, PREC_NONE },
    [TOKEN_RIGHT_BRACE]     = {NULL, NULL, PREC_NONE},
    [TOKEN_COMMA]           = {NULL, NULL, PREC_NONE},
    [TOKEN_DOT]             = {NULL, dot, PREC_CALL},
    [TOKEN_MINUS]           = {unary,       binary,     PREC_TERM},
    [TOKEN_PLUS]            = {NULL, binary,     PREC_TERM},
    [TOKEN_SEMICOLON]       = {NULL, NULL, PREC_NONE},
    [TOKEN_SLASH]           = {NULL, binary,     PREC_FACTOR},
    [TOKEN_STAR]            = {NULL, binary,     PREC_FACTOR},
    [TOKEN_BANG]            = {unary, NULL, PREC_NONE},
    [TOKEN_BANG_EQUAL]      = {NULL, binary, PREC_EQUALITY},
    [TOKEN_EQUAL]           = {NULL, NULL, PREC_NONE},
    [TOKEN_EQUAL_EQUAL]     = {NULL, binary, PREC_EQUALITY},
    [TOKEN_GREATER]         = {NULL, binary, PREC_COMPARISON},
    [TOKEN_GREATER_EQUAL]   = {NULL, binary, PREC_COMPARISON},
    [TOKEN_LESS]            = {NULL, binary, PREC_COMPARISON},
    [TOKEN_LESS_EQUAL]      = {NULL, binary, PREC_COMPARISON},
    [TOKEN_IDENTIFIER]      = {variable, NULL, PREC_NONE},
    [TOKEN_STRING]          = {string, NULL, PREC_NONE},
    [TOKEN_NUMBER]          = {number,      NULL, PREC_NONE},
    [TOKEN_AND]             = {NULL, and_, PREC_AND},
    [TOKEN_CLASS]           = {NULL, NULL, PREC_NONE},
    [TOKEN_ELSE]            = {NULL, NULL, PREC_NONE},
    [TOKEN_FALSE]           = {literal, NULL, PREC_NONE},
    [TOKEN_FOR]             = {NULL, NULL, PREC_NONE},
    [TOKEN_FUN]             = {NULL, NULL, PREC_NONE},
    [TOKEN_IF]              = {NULL, NULL, PREC_NONE},
    [TOKEN_NIL]             = {literal, NULL, PREC_NONE},
    [TOKEN_OR]              = {NULL, or_, PREC_OR},
    [TOKEN_PRINT]           = {NULL, NULL, PREC_NONE},
    [TOKEN_RETURN]          = {NULL, NULL, PREC_NONE},
    [TOKEN_SUPER]           = {super_, NULL, PREC_NONE},
    [TOKEN_THIS]            = {this_, NULL, PREC_NONE},
    [TOKEN_TRUE]            = {literal, NULL, PREC_NONE},
    [TOKEN_VAR]             = {NULL, NULL, PREC_NONE},
    [TOKEN_WHILE]           = {NULL, NULL, PREC_NONE},
    [TOKEN_ERROR]           = {NULL, NULL, PREC_NONE},
    [TOKEN_EOF]             = {NULL, NULL, PREC_NONE},
};

static void parsePrecedence(Precedence precedence) {
    advance();
    ParseFn prefixRule = getRule(parser.previous.type)->prefix; // because getRule returns ParseRule*
    if (prefixRule == NULL) {
        error("Expect expression.");
        return;
    }

    bool canAssign = precedence <= PREC_ASSIGNMENT;
    prefixRule(canAssign);

    while (precedence <= getRule(parser.current.type)->precedence) {
        advance();
        ParseFn infixRule = getRule(parser.previous.type)->infix;
        infixRule(canAssign);
    }

    if (canAssign && match(TOKEN_EQUAL)) {
        error("Invalid assignment target.");
    }
}

static ParseRule* getRule(TokenType type) {
    return &rules[type];
}

static void expression() {
    // the secret sauce
    parsePrecedence(PREC_ASSIGNMENT);
}

static void block() {
    while (!check(TOKEN_RIGHT_BRACE) && !check(TOKEN_EOF)) {
        declaration();
    }

    consume(TOKEN_RIGHT_BRACE, "Expect '}' after block.");
}

static void function(FunctionType type) {
    Compiler compiler;
    initCompiler(&compiler, type);
    beginScope();

    consume(TOKEN_LEFT_PAREN, "Expect '(' after function name.");
    if (!check(TOKEN_RIGHT_PAREN)) {
        do {
            current->function->arity++;
            if (current->function->arity > 255) {
                errorAtCurrent("Can't have more than 255 parameters.");
            }
            uint8_t constant = parseVariable("Expect parameter name.");
            defineVariable(constant);
        } while (match(TOKEN_COMMA));
    }
    consume( TOKEN_RIGHT_PAREN, "Expect ')' after parameters.");
    consume( TOKEN_LEFT_BRACE, "Expect '{' before function body.");
    block();

    ObjFunction* function = endCompiler();
    emitBytes(OP_CLOSURE, makeConstant(OBJ_VAL(function)));

    for (int i = 0; i < function->upvalueCount; i++) {
        emitByte(compiler.upvalues[i].isLocal ? 1 : 0);
        emitByte(compiler.upvalues[i].index);
    }
}

static void method() {
    consume(TOKEN_IDENTIFIER, "Expect method name.");
    uint8_t constant = identifierConstant(&parser.previous);

    FunctionType type = TYPE_METHOD;
    if (parser.previous.length == 4 && memcmp(parser.previous.start, "init", 4) == 0) {
        type = TYPE_INITIALIZER;
    }
    function(type);
    emitBytes(OP_METHOD, constant);
}

static void classDeclaration() {
    consume(TOKEN_IDENTIFIER, "Expect class name.");
    Token className = parser.previous; // grab the name of the class
    uint8_t nameConstant = identifierConstant(&parser.previous);
    declareVariable();

    emitBytes(OP_CLASS, nameConstant);
    defineVariable(nameConstant); //  allow the VM to bind to the variable via this name on the constant table

    // string the new compiler into the list of classes
    ClassCompiler classCompiler;
    classCompiler.hasSuperclass = false;
    classCompiler.enclosing = currentClass;
    currentClass = &classCompiler;

    if (match(TOKEN_LESS)) { // inheritance
        consume(TOKEN_IDENTIFIER, "Expect superclass name.");
        variable(false); // load up the previously mentioned identifier ie superclass

        if (identifiersEqual(&className, &parser.previous)) {
            error("A class can't inherit from itself.");
        }

        beginScope(); // put the superclass onto local scope under a token name 'super'
        addLocal(syntheticToken("super")); // a lexeme in the C string literal part of the binary executable constant data section, so no worries about leaking
        defineVariable(0); // push onto current closure's local, it is already loaded due to the variable() call above.

        namedVariable(className, false); // load up the current identifier class
        emitByte(OP_INHERIT);
        classCompiler.hasSuperclass = true;
    }

    namedVariable(className, false); // loads the classname back to the top of the stack
    consume(TOKEN_LEFT_BRACE, "Expect '{' before class body.");
    while (!check(TOKEN_RIGHT_BRACE) && !check(TOKEN_EOF)) {
        method();
    }
    consume(TOKEN_RIGHT_BRACE, "Expect '}' after class body.");
    emitByte(OP_POP); // this is after the VM is done processing all the OP_METHODS, remove the class name from stack

    if (classCompiler.hasSuperclass) {
        endScope();
    }

    // pop back the previous class compiler
    currentClass = currentClass->enclosing;
}

static void funDeclaration() {
    uint8_t global = parseVariable("Expect function name.");
    markInitialized();
    function(TYPE_FUNCTION);
    defineVariable(global);
}

static void varDeclaration() {
    uint8_t global = parseVariable("Expect variable name."); // the variable here is the index in constant table of the chunk for globals, or the local index for locals

    if (match(TOKEN_EQUAL)) {
        expression();
    } else {
        emitByte(OP_NIL); // just assign nil as this var is uninitialized.
    }
    consume(TOKEN_SEMICOLON, "Expect ';' after variable declaration.");

    defineVariable(global); // this emits code to store the result of the expression into the globals table of the VM. (or sets the scope in the case of locals)
}

static void expressionStatement() {
    expression();
    consume(TOKEN_SEMICOLON, "Expect ';' after expression.");
    emitByte(OP_POP);
}

static void forStatement() {
    beginScope();
    consume(TOKEN_LEFT_PAREN, "Expect '(' after 'for'.");
    if (match(TOKEN_SEMICOLON)) {
        // no initializer.
    } else if (match(TOKEN_VAR)) {
        varDeclaration();
    } else {
        expressionStatement();
    }

    int loopStart = currentChunk()->count; // always loop back to here by default
    int exitJump = -1;

    // this is the conditional check (which is optional)
    if (!match(TOKEN_SEMICOLON)) {
        expression();
        consume(TOKEN_SEMICOLON, "Expect ';' after loop condition.");

        // Jump out of the loop if the condition is false.
        exitJump = emitJump(OP_JUMP_IF_FALSE);
        emitByte(OP_POP); // Condition. (remove it before looping around)
    }
    // this is the increment clause
    // jump over the increment, run the body, jump back up to the increment, run it, then go to the next iteration.
    if (!match(TOKEN_RIGHT_PAREN)) {
        int bodyJump = emitJump(OP_JUMP);
        int incrementStart = currentChunk()->count;
        expression();
        emitByte(OP_POP);
        consume(TOKEN_RIGHT_PAREN, "Expect ')' after for clauses.");

        emitLoop(loopStart);
        loopStart = incrementStart; // set the next loop to start here instead
        patchJump(bodyJump);
    }

    statement();
    emitLoop(loopStart);

    if (exitJump != -1) {
        patchJump(exitJump);
        emitByte(OP_POP); // the Condition result.
    }

    endScope();
}

static void ifStatement() {
    consume(TOKEN_LEFT_PAREN, "Expect '(' after 'if'.");
    expression();
    consume(TOKEN_RIGHT_PAREN, "Expect ')' after condition.");

    int thenJump = emitJump(OP_JUMP_IF_FALSE);
    emitByte(OP_POP); // pop off the condition value before the then branch
    statement();

    int elseJump = emitJump(OP_JUMP);

    patchJump(thenJump);
    emitByte(OP_POP); // pop off the condition at the beginning of the else branch (also if there is no else branch this is the fall through pop)

    if (match(TOKEN_ELSE)) statement();
    patchJump(elseJump);
}

static void printStatement() {
    expression();
    consume(TOKEN_SEMICOLON, "Expect ';' after value.");
    emitByte(OP_PRINT);
}

static void returnStatement() {
    if (current->type == TYPE_SCRIPT) {
        // can't return from the top level as a language rule
        error("Can't return from top-level code.");
    }

    if (match(TOKEN_SEMICOLON)) { // if there is nothing returned, ie 'return;'
        emitReturn(); // the default nil return
    } else { // if there is an expression returned ie 'return <expression> ;'
        if (current->type == TYPE_INITIALIZER) {
            error("Can't return a value from an initializer.");
        }

        expression();
        consume(TOKEN_SEMICOLON, "Expect ';' after return value.");
        emitByte(OP_RETURN);
    }
}

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;

    while (parser.current.type != TOKEN_EOF) {
        if (parser.previous.type == TOKEN_SEMICOLON) return;
        switch (parser.current.type) {
            case TOKEN_CLASS:
            case TOKEN_FUN:
            case TOKEN_VAR:
            case TOKEN_FOR:
            case TOKEN_IF:
            case TOKEN_WHILE:
            case TOKEN_PRINT:
            case TOKEN_RETURN:
                return;

            default:
                ; // do nothing
        }

        advance();
    }
}

static void declaration() {
    if (match(TOKEN_CLASS)) {
        classDeclaration();
    } else if (match(TOKEN_FUN)) {
        funDeclaration();
    } else if (match(TOKEN_VAR)) {
        varDeclaration();
    } else {
        statement();
    }

    if (parser.panicMode) synchronize();
}

static void statement() {
    if (match(TOKEN_PRINT)) {
        printStatement();
    } else if (match(TOKEN_FOR)) {
        forStatement();
    } else if (match(TOKEN_IF)) {
        ifStatement();
    } else if (match(TOKEN_RETURN)) {
        returnStatement();
    } else if (match(TOKEN_WHILE)) {
        whileStatement();
    } else if (match(TOKEN_LEFT_BRACE)) {
        beginScope();
        block();
        endScope();
    } else {
        expressionStatement();
    }
}

ObjFunction* compile(const char* source) {
    initScanner(source);
    Compiler compiler;
    initCompiler(&compiler, TYPE_SCRIPT);

    parser.hadError = false;
    parser.panicMode = false;

    advance();

    while (!match(TOKEN_EOF)) {
        declaration();
    }

    ObjFunction* function = endCompiler();
    return parser.hadError ? NULL : function;
}

void markCompilerRoots() {
    Compiler* compiler = current;
    while (compiler != NULL) {
        markObject((Obj*)compiler->function);
        compiler = compiler->enclosing;
    }
}