add strutils.removePrefix proc (#6473)

lib/pure/strutils.nim

@@ -2345,8 +2345,7 @@ proc removeSuffix*(s: var string, chars: set[char] = Newlines) {.
 
 proc removeSuffix*(s: var string, c: char) {.
   rtl, extern: "nsuRemoveSuffixChar".} =
-  ## Removes a single character (in-place) from a string.
-  ##
+  ## Removes a single character (in-place) from the end of a string.
   ## .. code-block:: nim
   ##   var
   ##     table = "users"
@@ -2368,6 +2367,43 @@ proc removeSuffix*(s: var string, suffix: string) {.
     newLen -= len(suffix)
     s.setLen(newLen)
 
+proc removePrefix*(s: var string, chars: set[char] = Newlines) {.
+  rtl, extern: "nsuRemovePrefixCharSet".} =
+  ## Removes all characters from `chars` from the start of the string `s`
+  ## (in-place).
+  ## .. code-block:: nim
+  ##   var userInput = "\r\n*~Hello World!"
+  ##   userInput.removePrefix
+  ##   doAssert userInput == "*~Hello World!"
+  ##   userInput.removePrefix({'~', '*'})
+  ##   doAssert userInput == "Hello World!"
+  ##
+  ##   var otherInput = "?!?Hello!?!"
+  ##   otherInput.removePrefix({'!', '?'})
+  ##   doAssert otherInput == "Hello!?!"
+  var start = 0
+  while start < s.len and s[start] in chars: start += 1
+  if start > 0: s.delete(0, start - 1)
+
+proc removePrefix*(s: var string, c: char) {.
+  rtl, extern: "nsuRemovePrefixChar".} =
+  ## Removes a single character (in-place) from the start of a string.
+  ## .. code-block:: nim
+  ##   var ident = "pControl"
+  ##   ident.removePrefix('p')
+  ##   doAssert ident == "Control"
+  removePrefix(s, chars = {c})
+
+proc removePrefix*(s: var string, prefix: string) {.
+  rtl, extern: "nsuRemovePrefixString".} =
+  ## Remove the first matching prefix (in-place) from a string.
+  ## .. code-block:: nim
+  ##   var answers = "yesyes"
+  ##   answers.removePrefix("yes")
+  ##   doAssert answers == "yes"
+  if s.startsWith(prefix):
+    s.delete(0, prefix.len - 1)
+
 when isMainModule:
   doAssert align("abc", 4) == " abc"
   doAssert align("a", 0) == "a"

tests/stdlib/tstrutil.nim

@@ -49,9 +49,71 @@ proc testRemoveSuffix =
   s.removeSuffix("")
   assert s == "hello\r\n\r\n"
 
+proc testRemovePrefix =
+  var s = "\n\rhello"
+  s.removePrefix
+  assert s == "hello"
+  s.removePrefix
+  assert s == "hello"
+
+  s = "\n\nhello"
+  s.removePrefix
+  assert s == "hello"
+
+  s = "\rhello"
+  s.removePrefix
+  assert s == "hello"
+
+  s = "hello \n there"
+  s.removePrefix
+  assert s == "hello \n there"
+
+  s = "hello"
+  s.removePrefix("hel")
+  assert s == "lo"
+  s.removePrefix('l')
+  assert s == "o"
+
+  s = "hellos"
+  s.removePrefix({'h','e'})
+  assert s == "llos"
+  s.removePrefix({'l','o'})
+  assert s == "s"
+
+  s = "aeiou"
+  s.removePrefix("")
+  assert s == "aeiou"
+
+  s = ""
+  s.removePrefix("")
+  assert s == ""
+
+  s = "  "
+  s.removePrefix
+  assert s == "  "
+
+  s = "  "
+  s.removePrefix("")
+  assert s == "  "
+
+  s = "    "
+  s.removePrefix(" ")
+  assert s == "   "
+
+  s = "    "
+  s.removePrefix(' ')
+  assert s == ""
+
+  # Contrary to Chomp in other languages
+  # empty string does not change behaviour
+  s = "\r\n\r\nhello"
+  s.removePrefix("")
+  assert s == "\r\n\r\nhello"
+
 proc main() =
   testStrip()
   testRemoveSuffix()
+  testRemovePrefix()
   for p in split("/home/a1:xyz:/usr/bin", {':'}):
     write(stdout, p)