unikernel/duniverse/ocaml-re/lib/slice.ml

@@ -0,0 +1,70 @@
+open Import
+
+type t =
+  { s : string
+  ; pos : int
+  ; len : int
+  }
+
+module L = struct
+  type nonrec t = t list
+
+  let get_substring slices ~start ~stop =
+    if stop = start
+    then ""
+    else (
+      let slices =
+        let rec drop slices remains =
+          if remains = 0
+          then slices
+          else (
+            match slices with
+            | [] -> assert false
+            | ({ s = _; pos; len } as slice) :: xs ->
+              let remains' = remains - len in
+              if remains' >= 0
+              then drop xs remains'
+              else (
+                let pos = pos + remains in
+                let len = len - remains in
+                { slice with pos; len } :: xs))
+        in
+        drop slices start
+      in
+      let buf = Buffer.create (stop - start) in
+      let rec take slices remains =
+        if remains > 0
+        then (
+          match slices with
+          | [] -> assert false
+          | { s; pos; len } :: xs ->
+            let remains' = remains - len in
+            if remains' > 0
+            then (
+              Buffer.add_substring buf s pos len;
+              take xs remains')
+            else Buffer.add_substring buf s pos remains)
+      in
+      take slices (stop - start);
+      Buffer.contents buf)
+  ;;
+
+  let rec drop t remains =
+    if remains = 0
+    then t
+    else (
+      match t with
+      | [] -> []
+      | ({ s = _; pos; len } as slice) :: t ->
+        if remains >= len
+        then drop t (remains - len)
+        else (
+          let delta = len - remains in
+          { slice with pos = pos + delta; len = len - delta } :: t))
+  ;;
+
+  let drop_rev t remains =
+    (* TODO Use a proper functional queue *)
+    if remains = 0 then t else List.rev (drop (List.rev t) remains)
+  ;;
+end