From 72986869976a912b2e6bda6994f39afcae9211cb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?L=C3=A9ana=20=E6=B1=9F?= <leana.jiang+git@icloud.com>
Date: Fri, 26 Sep 2025 16:51:18 +0800
Subject: [PATCH] feat(lexer): implement numericEntity lexer

---
 src/Lexer.hs | 28 +++++++++++++++++++++++++++-
 test/Spec.hs | 13 +++++++++++++
 2 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/src/Lexer.hs b/src/Lexer.hs
index 4a85fb5..47c4e1c 100644
--- a/src/Lexer.hs
+++ b/src/Lexer.hs
@@ -8,10 +8,11 @@ module Lexer (
 where
 
 import Control.Monad (mfilter, void)
+import Data.Char (ord, toLower)
 import Data.Functor (($>))
 import Data.Text (Text, intercalate)
 import Data.Text qualified as Text
-import GHC.Unicode (isAlphaNum, isControl, isPrint, isSpace, isUpper)
+import GHC.Unicode (isAlphaNum, isControl, isDigit, isPrint, isSpace, isUpper)
 import ParserMonad (Parser, initialParserState)
 import Text.Parsec
 import Text.Parsec qualified as Parsec
@@ -94,6 +95,7 @@ lexText = go
                   , labeledLink
                   , module_
                   , anchor
+                  , numericEntity
                   , textElement
                   , quotes
                   , birdTrack
@@ -255,6 +257,30 @@ bold = delimitedSymmetric "__" BoldOpen BoldClose
 monospace :: Lexer
 monospace = delimitedSymmetric "@" MonospaceOpen MonospaceClose
 
+decimal :: Parser Int
+decimal = read . Text.unpack <$> takeWhile1_ isDigit
+
+hexadecimal :: Parser Int
+hexadecimal = "x" *> (convert 0 . fmap (normalise . toLower) <$> many1 hexDigit)
+ where
+  normalise :: Char -> Int
+  normalise c
+    | ord '0' <= n && n <= ord '9' = n - ord '0'
+    | ord 'A' <= n && n <= ord 'F' = n - ord 'A' + 10
+    | ord 'a' <= n && n <= ord 'f' = n - ord 'a' + 10
+    | otherwise = error "unexpected: invalid hex number"
+   where
+    n = ord c
+
+  convert :: Int -> [Int] -> Int
+  convert acc [] = acc
+  convert acc (x : xs) = convert (acc * 16 + x) xs
+
+numericEntity :: Lexer
+numericEntity = do
+  x <- located $ between "&#" ";" (NumericEntity <$> (hexadecimal <|> decimal))
+  pure [x]
+
 other :: Lexer
 other = do
   pos <- getPosition
diff --git a/test/Spec.hs b/test/Spec.hs
index 7258a2d..03e1b81 100644
--- a/test/Spec.hs
+++ b/test/Spec.hs
@@ -30,6 +30,7 @@ main = hspec $ do
       it "bird tracks" birdTracks
       it "module names" modules
       it "quotes" quotes
+      it "numeric entity" numericEntity
       it "ignores nesting" ignoreNesting
 
   describe "Parser" do
@@ -152,6 +153,18 @@ space = do
                   , (1, 2, Newline)
                   ]
 
+numericEntity :: Expectation
+numericEntity = do
+  "&#65; &#955;"
+    `shouldLexTo` [ (1, 1, NumericEntity 65)
+                  , (1, 6, Space)
+                  , (1, 7, NumericEntity 955) -- lambda
+                  ]
+  -- Hex
+  "&#x65;"
+    `shouldLexTo` [ (1, 1, NumericEntity 101)
+                  ]
+
 monospace :: Expectation
 monospace =
   "@mono@"