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compiler/erg_common/lazy_buffer.rs

@@ -0,0 +1,70 @@
+// Copied and modified from https://github.com/rust-itertools/itertools/blob/master/src/lazy_buffer.rs
+// MIT license | Apache 2.0 license
+// License files are placed at the root.
+
+use std::ops::Index;
+
+#[derive(Debug, Clone)]
+pub struct LazyBuffer<I: Iterator> {
+    pub it: I,
+    done: bool,
+    buffer: Vec<I::Item>,
+}
+
+impl<I> LazyBuffer<I>
+where
+    I: Iterator,
+{
+    pub fn new(it: I) -> LazyBuffer<I> {
+        LazyBuffer {
+            it,
+            done: false,
+            buffer: Vec::new(),
+        }
+    }
+
+    pub fn len(&self) -> usize {
+        self.buffer.len()
+    }
+
+    pub fn get_next(&mut self) -> bool {
+        if self.done {
+            return false;
+        }
+        let next_item = self.it.next();
+        match next_item {
+            Some(x) => {
+                self.buffer.push(x);
+                true
+            }
+            None => {
+                self.done = true;
+                false
+            }
+        }
+    }
+
+    pub fn prefill(&mut self, len: usize) {
+        let buffer_len = self.buffer.len();
+
+        if !self.done && len > buffer_len {
+            let delta = len - buffer_len;
+
+            self.buffer.extend(self.it.by_ref().take(delta));
+            self.done = self.buffer.len() < len;
+        }
+    }
+}
+
+impl<I, J> Index<J> for LazyBuffer<I>
+where
+    I: Iterator,
+    I::Item: Sized,
+    Vec<I::Item>: Index<J>
+{
+    type Output = <Vec<I::Item> as Index<J>>::Output;
+
+    fn index(&self, _index: J) -> &Self::Output {
+        self.buffer.index(_index)
+    }
+}