scanner.c

#include <stdio.h>
#include <string.h>

#include "common.h"
#include "scanner.h"

typedef struct {
    const char* start;
    const char* current;
    int line;
} Scanner;

Scanner scanner;

void initScanner(const char* source) {
    scanner.start = source;
    scanner.current = source;
    scanner.line = 1;
}

static bool isAtEnd() {
    return *scanner.current == '\0';
}

static Token makeToken(TokenType type) {
    Token token;
    token.type = type;
    token.start = scanner.start;
    token.length = (int)(scanner.current - scanner.start);
    token.line = scanner.line;
    return token;
}

static Token errorToken(const char* message) {
    Token token;
    token.type = TOKEN_ERROR;
    token.start = message;
    token.length = (int)strlen(message);
    token.line = scanner.line;
    return token;
}

static char advance() {
    scanner.current++;
    return scanner.current[-1];
}

static char peek(int offset) {
    if (isAtEnd()) return '\0';

    if (offset != 0) {
        return scanner.current[offset];
    }

    return *scanner.current;
}

static bool match(char expected) {
    if (isAtEnd()) return false;
    if (*scanner.current != expected) return false;
    scanner.current++;
    return true;
}

static bool isAlpha(char c) {
     return (c >= 'a' && c <= 'z') ||
            (c >= 'A' && c <= 'Z') ||
            c == '_';
}

static bool isDigit(char c) {
    return c >= '0' && c <= '9';
}

static Token number() {
    while (isDigit(peek(0))) advance();

  // Look for a fractional part.
  if (peek(0) == '.' && isDigit(peek(1))) {
    // Consume the ".".
    advance();

    while (isDigit(peek(0))) advance();
  }

  return makeToken(TOKEN_NUMBER);
}

static Token string() {
    while (peek(0) != '"' && !isAtEnd()) {
        if (peek(0) == '\n') scanner.line++;
        advance();
    }

    if (isAtEnd()) return errorToken("Unterminated string.");

    // consume the closing quote.
    advance();

    return makeToken(TOKEN_STRING);
}

static TokenType checkKeyword(int start, int length, const char* rest, TokenType type) {
    bool sameLength = scanner.current - scanner.start == start + length;
    bool cmp = memcmp(scanner.start + start, rest, length) == 0;

    if (sameLength && cmp) return type;
    return TOKEN_IDENTIFIER;
}

static TokenType identifierType() {
    switch (scanner.start[0]) {
        case 'a': return checkKeyword(1, 2, "nd", TOKEN_AND);
        case 'c': return checkKeyword(1, 4, "lass", TOKEN_CLASS);
        case 'e': return checkKeyword(1, 3, "lse", TOKEN_ELSE);
        case 'f': {
            if (scanner.current - scanner.start > 1) {
                switch (scanner.start[1]) {
                    case 'a': return checkKeyword(2, 3, "lse", TOKEN_FALSE);
                    case 'o': return checkKeyword(2, 1, "r", TOKEN_FOR);
                    case 'u': return checkKeyword(2, 1, "n", TOKEN_FUN);
                }
            }
            break;
        }
        case 'i': return checkKeyword(1, 1, "f", TOKEN_IF);
        case 'n': return checkKeyword(1, 2, "il", TOKEN_NIL);
        case 'o': return checkKeyword(1, 1, "r", TOKEN_OR);
        case 'p': return checkKeyword(1, 4, "rint", TOKEN_PRINT);
        case 'r': return checkKeyword(1, 5, "eturn", TOKEN_RETURN);
        case 's': return checkKeyword(1, 4, "uper", TOKEN_SUPER);
        case 't': {
            if (scanner.current - scanner.start > 1) {
                switch (scanner.start[1]) {
                    case 'h': return checkKeyword(2, 2, "is", TOKEN_THIS);
                    case 'r': return checkKeyword(2, 2, "ue", TOKEN_TRUE);
                }
            }
            break;
        }
        case 'v': return checkKeyword(1, 2, "ar", TOKEN_VAR);
        case 'w': return checkKeyword(1, 4, "hile", TOKEN_WHILE);
    }
    return TOKEN_IDENTIFIER;
}

static Token identifier() {
    while (isAlpha(peek(0)) || isDigit(peek(0))) advance();
    return makeToken(identifierType());
}

static void skipWhitespace() {
    for(;;) {
        char c = peek(0);
        switch (c) {
            case ' ':
            case '\r':
            case '\t':
                advance();
                break;
            case '\n':
                scanner.line++;
                advance();
                break;
            case '/':
                // look the next token
                if (peek(1) == '/') {
                    // consume until the end of the line
                    while(peek(0) != '\n' && !isAtEnd()) {
                        advance();
                    }
                } else {
                    return;
                }
                break;
            default:
                return;
        }
    }
}

Token scanToken() {
    skipWhitespace();

    scanner.start = scanner.current;

    if (isAtEnd()) return makeToken(TOKEN_EOF);

    char c = advance();
    if (isAlpha(c)) {
        return identifier();
    }
    if (isDigit(c)) {
        return number();
    }

    switch (c) {
        case '(': return makeToken(TOKEN_LEFT_PAREN);
        case ')': return makeToken(TOKEN_RIGHT_PAREN);
        case '{': return makeToken(TOKEN_LEFT_BRACE);
        case '}': return makeToken(TOKEN_RIGHT_BRACE);
        case ';': return makeToken(TOKEN_SEMICOLON);
        case ',': return makeToken(TOKEN_COMMA);
        case '.': return makeToken(TOKEN_DOT);
        case '-': return makeToken(TOKEN_MINUS);
        case '+': return makeToken(TOKEN_PLUS);
        case '/': return makeToken(TOKEN_SLASH);
        case '*': return makeToken(TOKEN_STAR);
        case '!':
            return makeToken(
                match('=') ? TOKEN_BANG_EQUAL : TOKEN_BANG);
        case '=':
            return makeToken(
                match('=') ? TOKEN_EQUAL_EQUAL: TOKEN_EQUAL);
        case '<':
            return makeToken(
                match('=') ? TOKEN_LESS_EQUAL: TOKEN_LESS);
        case '>':
            return makeToken(
                match('=') ? TOKEN_GREATER_EQUAL: TOKEN_GREATER);
        case '"': return string();
    }

    return errorToken("Unexpected character.");
}