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fix: Added 'lookbehind' method in lexer #717

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Sep 18, 2023
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fix: Added 'lookbehind' method in lexer #717

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119 changes: 90 additions & 29 deletions kclvm/parser/src/lexer/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,7 @@ mod tests;
use compiler_base_macros::bug;
use compiler_base_span::{self, span::new_byte_pos, BytePos, Span};
use kclvm_ast::ast::NumberBinarySuffix;
use kclvm_ast::token::{self, CommentKind, Token, TokenKind};
use kclvm_ast::token::{self, BinOpToken, CommentKind, Token, TokenKind};
use kclvm_ast::token_stream::TokenStream;
use kclvm_lexer::Base;
use kclvm_span::symbol::Symbol;
Expand All @@ -40,6 +40,7 @@ pub fn parse_token_streams(sess: &ParseSession, src: &str, start_pos: BytePos) -
sess,
start_pos,
pos: start_pos,
tok_start_pos: start_pos,
end_src_index: src.len(),
src,
token: TokenWithIndents::Token {
Expand Down Expand Up @@ -111,6 +112,9 @@ struct Lexer<'a> {
/// The absolute offset within the source_map of the current character.
pos: BytePos,

/// The start position of the current token.
tok_start_pos: BytePos,

/// Stop reading src at this index.
end_src_index: usize,

Expand Down Expand Up @@ -147,18 +151,23 @@ struct IndentContext {
impl<'a> Lexer<'a> {
fn into_tokens(mut self) -> TokenStream {
let mut buf = TokenStreamBuilder::default();
self.token = self.token();
// In the process of look-behind lexing, it is necessary to check the type of the previous token in 'buf',
// If the previous token and the current token can form a multi-character token,
// then the previous token will be popped from 'buf'.
//
// Therefore, the method 'self.token()' needs to take the mutable reference of 'buf' as an incoming argument.
self.token = self.token(&mut buf);

while !self.token.is_eof() {
self.token.append_to(&mut buf);
self.token = self.token();
self.token = self.token(&mut buf);
}

self.eof(&mut buf);
buf.into_token_stream()
}

fn token(&mut self) -> TokenWithIndents {
fn token(&mut self, tok_stream_builder: &mut TokenStreamBuilder) -> TokenWithIndents {
loop {
let start_src_index = self.src_index(self.pos);
let text: &str = &self.src[start_src_index..self.end_src_index];
Expand All @@ -175,12 +184,19 @@ impl<'a> Lexer<'a> {
// Detect and handle indent cases before lexing on-going token
let indent = self.lex_indent_context(token.kind);

let start = self.pos;
// Because of the 'look-behind', the 'start' of the current token becomes a two-way cursor,
// which can not only move forward, but also move backward when 'look-behind'.
// Therefore, the value of 'self.tok_start_pos' can be changed in 'self.lex_token()'.
self.tok_start_pos = self.pos;
// update pos after token and indent handling
self.pos = self.pos + new_byte_pos(token.len as u32);

if let Some(kind) = self.lex_token(token, start) {
let span = self.span(start, self.pos);
// In the process of look-behind lexing, it is necessary to check the type of the previous token in 'tok_stream_builder',
// If the previous token and the current token can form a multi-character token,
// then the previous token will be popped from 'tok_stream_builder'.
// Therefore, the method 'self.lex_token()' needs to take the mutable reference of 'tok_stream_builder' as an incoming argument.
if let Some(kind) = self.lex_token(token, self.tok_start_pos, tok_stream_builder) {
let span = self.span(self.tok_start_pos, self.pos);

match indent {
Some(iord) => {
Expand All @@ -202,7 +218,12 @@ impl<'a> Lexer<'a> {
}

/// Turns `kclvm_lexer::TokenKind` into a rich `kclvm_ast::TokenKind`.
fn lex_token(&mut self, token: kclvm_lexer::Token, start: BytePos) -> Option<TokenKind> {
fn lex_token(
&mut self,
token: kclvm_lexer::Token,
start: BytePos,
tok_stream_builder: &mut TokenStreamBuilder,
) -> Option<TokenKind> {
Some(match token.kind {
kclvm_lexer::TokenKind::LineComment { doc_style: _ } => {
let s = self.str_from(start);
Expand Down Expand Up @@ -244,22 +265,7 @@ impl<'a> Lexer<'a> {
}
// Binary op
kclvm_lexer::TokenKind::Plus => token::BinOp(token::Plus),
kclvm_lexer::TokenKind::Minus => {
let head = start + new_byte_pos(1);
let tail = start + new_byte_pos(2);
if self.has_next_token(head, tail) {
let next_tkn = self.str_from_to(head, tail);
if next_tkn == ">" {
// waste '>' token
self.pos = self.pos + new_byte_pos(1);
token::RArrow
} else {
token::BinOp(token::Minus)
}
} else {
token::BinOp(token::Minus)
}
}
kclvm_lexer::TokenKind::Minus => token::BinOp(token::Minus),
kclvm_lexer::TokenKind::Star => token::BinOp(token::Star),
kclvm_lexer::TokenKind::Slash => token::BinOp(token::Slash),
kclvm_lexer::TokenKind::Percent => token::BinOp(token::Percent),
Expand Down Expand Up @@ -288,7 +294,13 @@ impl<'a> Lexer<'a> {
kclvm_lexer::TokenKind::BangEq => token::BinCmp(token::NotEq),
kclvm_lexer::TokenKind::Lt => token::BinCmp(token::Lt),
kclvm_lexer::TokenKind::LtEq => token::BinCmp(token::LtEq),
kclvm_lexer::TokenKind::Gt => token::BinCmp(token::Gt),
// If the current token is '>',
// then lexer need to check whether the previous token is '-',
// if yes, return token '->', if not return token '>'.
kclvm_lexer::TokenKind::Gt => match self.look_behind(&token, tok_stream_builder) {
Some(tok_kind) => tok_kind,
None => token::BinCmp(token::Gt),
},
kclvm_lexer::TokenKind::GtEq => token::BinCmp(token::GtEq),
// Structural symbols
kclvm_lexer::TokenKind::At => token::At,
Expand Down Expand Up @@ -431,6 +443,36 @@ impl<'a> Lexer<'a> {
})
}

/// From the lexed tokens stack, check whether the token at the top of the stack and the current character can combine a new token.
/// If yes, lexer will pop the token at the top of the stack and return a new token combined with the token poped and the current character.
/// If not, return None.
fn look_behind(
&mut self,
tok: &kclvm_lexer::Token,
tok_stream_builder: &mut TokenStreamBuilder,
) -> Option<TokenKind> {
match tok.kind {
// Most multi-character tokens are lexed in ['kclvm-lexer'],
// and the multi-character tokens that need to be lexed in ['kclvm-parser/lexer'] are only token '->'.
// If a new multi-character token is added later, the corresponding operation can be added here.
kclvm_lexer::TokenKind::Gt => {
if let Some(_) =
tok_stream_builder.pop_if_tok_kind(&TokenKind::BinOp(BinOpToken::Minus))
{
// After the previous token pops up, 'self.tok_start_pos' needs to be updated.
if self.tok_start_pos >= new_byte_pos(1) {
self.tok_start_pos = self.tok_start_pos - new_byte_pos(1);
return Some(TokenKind::RArrow);
} else {
bug!("Internal Bugs: Please connect us to fix it, invalid token start pos")
}
}
}
_ => return None,
}
None
}

fn lex_literal(
&self,
start: BytePos,
Expand Down Expand Up @@ -646,10 +688,6 @@ impl<'a> Lexer<'a> {
&self.src[self.src_index(start)..self.src_index(end)]
}

fn has_next_token(&self, start: BytePos, end: BytePos) -> bool {
!(self.src_index(start) > self.src_index(end) || self.src_index(end) > self.src.len())
}

fn symbol_from_to(&self, start: BytePos, end: BytePos) -> Symbol {
Symbol::intern(self.str_from_to(start, end))
}
Expand Down Expand Up @@ -682,4 +720,27 @@ impl TokenStreamBuilder {
fn into_token_stream(self) -> TokenStream {
TokenStream::new(self.buf)
}

/// Pop the token at the top of the stack, and return None if the stack is empty.
fn pop(&mut self) -> Option<Token> {
self.buf.pop()
}

/// If the token kind at the top of the stack is 'expected_tok_kind',
/// pop the token and return it, otherwise do nothing and return None.
fn pop_if_tok_kind(&mut self, expected_tok_kind: &TokenKind) -> Option<Token> {
if self.peek_tok_kind() == expected_tok_kind {
self.pop()
} else {
None
}
}

/// Peek the kind of the token on the top of the stack.
fn peek_tok_kind(&self) -> &TokenKind {
match self.buf.last() {
Some(tok) => &tok.kind,
None => &TokenKind::Dummy,
}
}
}
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