fix: substring APIs

src/Init/Data/String/Basic.lean

@@ -9,12 +9,6 @@ import Init.Data.Char.Basic
 import Init.Data.Option.Basic
 universes u
 
-/-- A byte position in a `String`. Internally, `String`s are UTF-8 encoded.
-Codepoint positions (counting the Unicode codepoints rather than bytes)
-are represented by plain `Nat`s instead.
-Indexing a `String` by a byte position is constant-time, while codepoint
-positions need to be translated internally to byte positions in linear-time. -/
-
 def List.asString (s : List Char) : String :=
   ⟨s⟩
 
@@ -332,44 +326,52 @@ namespace Substring
 @[inline] def toIterator : Substring → String.Iterator
   | ⟨s, b, _⟩ => ⟨s, b⟩
 
+/-- Returns the codepoint at the given offset into the substring. -/
 @[inline] def get : Substring → String.Pos → Char
   | ⟨s, b, _⟩, p => s.get (b+p)
 
-@[inline] def next : Substring → String.Pos → String.Pos
+/-- Given an offset of a codepoint into the substring,
+returns the offset there of the next codepoint. -/
+@[inline] private def next : Substring → String.Pos → String.Pos
   | ⟨s, b, e⟩, p =>
-    let p := s.next (b+p)
-    if p > e then e - b else p - b
+    let absP := b+p
+    if absP = e then p else s.next absP - b
 
-@[inline] def prev : Substring → String.Pos → String.Pos
+/-- Given an offset of a codepoint into the substring,
+returns the offset there of the previous codepoint. -/
+@[inline] private def prev : Substring → String.Pos → String.Pos
   | ⟨s, b, _⟩, p =>
-    if p = b then p else s.prev (b+p) - b
+    let absP := b+p
+    if absP = b then p else s.prev absP - b
 
-def nextn : Substring → Nat → String.Pos → String.Pos
+private def nextn : Substring → Nat → String.Pos → String.Pos
   | ss, 0,   p => p
   | ss, i+1, p => ss.nextn i (ss.next p)
 
-def prevn : Substring → String.Pos → Nat → String.Pos
+private def prevn : Substring → String.Pos → Nat → String.Pos
   | ss, 0,   p => p
   | ss, i+1, p => ss.prevn i (ss.prev p)
 
 @[inline] def front (s : Substring) : Char :=
   s.get 0
 
+/-- Returns the offset into `s` of the first occurence of `c` in `s`,
+or `s.bsize` if `c` doesn't occur. -/
 @[inline] def posOf (s : Substring) (c : Char) : String.Pos :=
   match s with
   | ⟨s, b, e⟩ => (String.posOfAux s c e b) - b
 
 @[inline] def drop : Substring → Nat → Substring
-  | ss@⟨s, b, e⟩, n => ⟨s, ss.nextn n b, e⟩
+  | ss@⟨s, b, e⟩, n => ⟨s, b + ss.nextn n 0, e⟩
 
 @[inline] def dropRight : Substring → Nat → Substring
-  | ss@⟨s, b, e⟩, n => ⟨s, b, ss.prevn n e⟩
+  | ss@⟨s, b, e⟩, n => ⟨s, b, b + ss.prevn n ss.bsize⟩
 
 @[inline] def take : Substring → Nat → Substring
-  | ss@⟨s, b, e⟩, n => ⟨s, b, ss.nextn n b⟩
+  | ss@⟨s, b, e⟩, n => ⟨s, b, b + ss.nextn n 0⟩
 
 @[inline] def takeRight : Substring → Nat → Substring
-  | ss@⟨s, b, e⟩, n => ⟨s, ss.prevn n e, e⟩
+  | ss@⟨s, b, e⟩, n => ⟨s, b + ss.prevn n ss.bsize, e⟩
 
 @[inline] def atEnd : Substring → String.Pos → Bool
   | ⟨s, b, e⟩, p => b + p == e
@@ -419,7 +421,7 @@ partial def splitOn (s : Substring) (sep : String := " ") : List Substring :=
 def contains (s : Substring) (c : Char) : Bool :=
   s.any (fun a => a == c)
 
-@[specialize] partial def takeWhileAux (s : String) (stopPos : String.Pos) (p : Char → Bool) (i : String.Pos) : String.Pos :=
+@[specialize] private partial def takeWhileAux (s : String) (stopPos : String.Pos) (p : Char → Bool) (i : String.Pos) : String.Pos :=
   if i == stopPos then i
   else if p (s.get i) then takeWhileAux s stopPos p (s.next i)
   else i
@@ -434,7 +436,7 @@ def contains (s : Substring) (c : Char) : Bool :=
     let b := takeWhileAux s e p b;
     ⟨s, b, e⟩
 
-@[specialize] partial def takeRightWhileAux (s : String) (begPos : String.Pos) (p : Char → Bool) (i : String.Pos) : String.Pos :=
+@[specialize] private partial def takeRightWhileAux (s : String) (begPos : String.Pos) (p : Char → Bool) (i : String.Pos) : String.Pos :=
   if i == begPos then i
   else
     let i' := s.prev i

tests/lean/string_imp2.lean

@@ -49,3 +49,8 @@ it₁.remainingToString ++ "-" ++ it₂.remainingToString
 #eval "abc" == "abd"
 #eval "αβγ".drop 1
 #eval "αβγ".takeRight 1
+
+def ss : Substring := "0123abcdαβγδ".toSubstring
+#eval ss.drop 4 |>.takeRight 4
+#eval ss.drop 4 |>.take 4
+#eval ss.dropRight 4 |>.takeRight 4

tests/lean/string_imp2.lean.expected.out

@@ -37,3 +37,6 @@ true
 false
 "βγ"
 "γ"
+"αβγδ".toSubstring
+"abcd".toSubstring
+"abcd".toSubstring