remove start

rust/src/scanner.rs

@@ -2,7 +2,6 @@ use color_eyre::eyre::Result;
 
 pub struct Scanner<'a> {
     source: &'a [u8],
-    start: usize,
     current: usize,
     line: usize,
 }
@@ -11,7 +10,6 @@ impl<'a> Scanner<'a> {
     pub fn new(source: &'a str) -> Self {
         Self {
             source: source.as_bytes(),
-            start: 0,
             current: 0,
             line: 1,
         }
@@ -20,8 +18,6 @@ impl<'a> Scanner<'a> {
     pub fn scan(&mut self) -> Result<Token> {
         self.skip_whitespace();
 
-        self.start = self.current;
-
         if self.is_at_end() {
             return self.make_token(TokenKind::Eof);
         }
@@ -30,7 +26,7 @@ impl<'a> Scanner<'a> {
         if is_alpha(c) {
             return self.identifier();
         }
-        if c.is_digit(10) {
+        if c.is_ascii_digit() {
             return self.number();
         }
 
@@ -86,7 +82,7 @@ impl<'a> Scanner<'a> {
     fn make_token(&self, kind: TokenKind) -> Result<Token> {
         Ok(Token {
             kind,
-            value: std::str::from_utf8(&self.source[self.start..self.current])?,
+            value: std::str::from_utf8(&self.source[..self.current])?,
             line: self.line,
         })
     }
@@ -175,14 +171,14 @@ impl<'a> Scanner<'a> {
     }
 
     fn number(&mut self) -> Result<Token> {
-        while self.peek().is_digit(10) {
+        while self.peek().is_ascii_digit() {
             self.advance();
         }
 
-        if self.peek() == '.' && self.peek_next().is_digit(10) {
+        if self.peek() == '.' && self.peek_next().is_ascii_digit() {
             self.advance();
 
-            while self.peek().is_digit(10) {
+            while self.peek().is_ascii_digit() {
                 self.advance();
             }
         }
@@ -191,7 +187,7 @@ impl<'a> Scanner<'a> {
     }
 
     fn identifier(&mut self) -> Result<Token> {
-        while is_alpha(self.peek()) || self.peek().is_digit(10) {
+        while is_alpha(self.peek()) || self.peek().is_ascii_digit() {
             self.advance();
         }
 
@@ -199,7 +195,7 @@ impl<'a> Scanner<'a> {
     }
 
     fn identifier_type(&self) -> TokenKind {
-        match self.source[self.start] as char {
+        match self.source[0] as char {
             'a' => self.check_keyword(1, "nd", TokenKind::And),
             'c' => self.check_keyword(1, "lass", TokenKind::Class),
             'e' => self.check_keyword(1, "lse", TokenKind::Else),
@@ -212,8 +208,8 @@ impl<'a> Scanner<'a> {
             'v' => self.check_keyword(1, "ar", TokenKind::Var),
             'w' => self.check_keyword(1, "hile", TokenKind::While),
             'f' => {
-                if self.current - self.start > 1 {
-                    match self.source[self.start + 1] as char {
+                if self.current > 1 {
+                    match self.source[1] as char {
                         'a' => self.check_keyword(2, "lse", TokenKind::False),
                         'o' => self.check_keyword(2, "r", TokenKind::For),
                         'u' => self.check_keyword(2, "n", TokenKind::Fun),
@@ -224,8 +220,8 @@ impl<'a> Scanner<'a> {
                 }
             }
             't' => {
-                if self.current - self.start > 1 {
-                    match self.source[self.start + 1] as char {
+                if self.current > 1 {
+                    match self.source[1] as char {
                         'h' => self.check_keyword(2, "is", TokenKind::This),
                         'r' => self.check_keyword(2, "ue", TokenKind::True),
                         _ => TokenKind::Identifier
@@ -239,8 +235,7 @@ impl<'a> Scanner<'a> {
     }
 
     fn check_keyword(&self, start: usize, rest: &str, kind: TokenKind) -> TokenKind {
-        // Do I have an off-by-one error here?
-        if &self.source[self.start + start..self.current] == rest.as_bytes() {
+        if &self.source[start..self.current] == rest.as_bytes() {
             kind
         } else {
             TokenKind::Identifier