Add stream highlighting function

src/parser/parser_types.nim

@@ -268,6 +268,24 @@ proc `$`*(x: ParserToken): string =
   value: {tokenStringValue(x)},
 )"""
 
+const LEFT_HIGHLIGHT_CHAR = ">>"
+const RIGHT_HIGHLIGHT_CHAR = "<<"
+func highlightStreamPosition(stream: string, position: int): string =
+  if position < 0: return stream
+  if position > stream.len - 1: return stream
+
+  let aIndex = position - 1
+  let a = if aIndex < 0: ""
+          else: stream[0..aIndex]
+
+  let ch = stream[position]
+
+  let bIndex = position + 1
+  let b = if bIndex > stream.len - 1: ""
+          else: stream[bIndex..stream.len - 1]
+
+  return a & LEFT_HIGHLIGHT_CHAR & ch & RIGHT_HIGHLIGHT_CHAR & b
+
 proc `$`*(x: ParserState): string =
   &"""ParserState(
   stream: "{x.stream}",
@@ -311,3 +329,14 @@ proc `$`*(x: ParserError): string =
 {errSpace}^ Expected 'EndOfString' at {index} but got {original.len - 1}"""
 
     else: "ParseError"
+
+when isMainModule:
+  block highlightStreamPosition:
+    let s = "abc"
+    # Out of bounds
+    assert s.highlightStreamPosition(-1) == s
+    assert s.highlightStreamPosition(s.len) == s
+    # Regular highlighting
+    assert s.highlightStreamPosition(0) == LEFT_HIGHLIGHT_CHAR & "a" & RIGHT_HIGHLIGHT_CHAR & "bc"
+    assert s.highlightStreamPosition(1) == "a" & LEFT_HIGHLIGHT_CHAR & "b" & RIGHT_HIGHLIGHT_CHAR & "c"
+    assert s.highlightStreamPosition(2) == "ab" & LEFT_HIGHLIGHT_CHAR & "c" & RIGHT_HIGHLIGHT_CHAR