From 0ba76a120350076071a56b183774e6cd4b7a3915 Mon Sep 17 00:00:00 2001
From: Florian Schroedl <flo.schroedl@gmail.com>
Date: Sun, 28 Aug 2022 18:19:53 +0200
Subject: [PATCH] Add peek functions

---
 src_v2/parser/parser_api.nim | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/src_v2/parser/parser_api.nim b/src_v2/parser/parser_api.nim
index cf9d384..0ff4f19 100644
--- a/src_v2/parser/parser_api.nim
+++ b/src_v2/parser/parser_api.nim
@@ -185,6 +185,27 @@ proc ignore*(parserFn: parserFnT): parserFnT {.inline.} =
       tokens: parser.tokens,
     ))
 
+proc peek*(amount: int, parserFn: parserFnT): parserFnT {.inline.} =
+  ## Creates parser function with a nested `parserFn`:
+  ## Parses using the `parserFn` but dont capture the resulting tokens and keep the current position.
+  return proc(parser: Parser): ParserResult =
+    let state = parser.state
+    let newIndex = state.position + amount
+
+    let newParser = initParser(
+      stream = state.stream,
+      tokens = parser.tokens,
+      position = newIndex,
+      lastPosition = state.position,
+    )
+
+    parserFn(newParser)
+    .map(p => parser)
+
+proc peek1*(parserFn: parserFnT): parserFnT = peek(1, parserFn)
+proc peekCurrent*(parserFn: parserFnT): parserFnT = peek(0, parserFn)
+proc peekBack1*(parserFn: parserFnT): parserFnT = peek(-1, parserFn)
+
 proc manyUntil*(acceptFn: parserFnT, stopFn: parserFnT): parserFnT {.inline.} =
   ## Creates parser function with a nested `acceptFn` parser function until the `stopFn` parserFunction is met:
   ## Parses until the `stopFn` is reached or on an errror.
@@ -330,6 +351,12 @@ when isMainModule:
     # ignore
     assert testParser123.flatMap(ignore(ch('1'))).tokensToString() == ""
 
+    # peek
+    assert testParser123.flatMap(ch('1') + peekCurrent(ch('2')) + ch('2')).tokensToString() == "12"
+    assert testParser123.flatMap(ch('1') + peekCurrent(ch('1'))).isErr() == true
+    assert testParser123.flatMap(peekCurrent(startOfStream)).isOk() == true
+    assert testParser123.flatMap(ch('1') + peekBack1(startOfStream)).isOk() == true
+
     # manyUntil
     assert testAbc1Parser.flatMap(manyUntil(anyCh, digit)).tokensToString() == "abc"