feat(code): part 3 iterators

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm/src/iterators/two_merge_iterator.rs

@@ -0,0 +1,80 @@
+use anyhow::Result;
+
+use super::impls::StorageIterator;
+
+/// Merges two iterators of different types into one. If the two iterators have the same key, only
+/// produce the key once and prefer the entry from A.
+pub struct TwoMergeIterator<A: StorageIterator, B: StorageIterator> {
+    a: A,
+    b: B,
+    choose_a: bool,
+}
+
+impl<A: StorageIterator, B: StorageIterator> TwoMergeIterator<A, B> {
+    fn choose_a(a: &A, b: &B) -> bool {
+        if !a.is_valid() {
+            return false;
+        }
+        if !b.is_valid() {
+            return true;
+        }
+        a.key() < b.key()
+    }
+
+    fn skip_b(&mut self) -> Result<()> {
+        if self.a.is_valid() {
+            while self.b.is_valid() && self.b.key() == self.a.key() {
+                self.b.next()?;
+            }
+        }
+        Ok(())
+    }
+
+    pub fn create(a: A, b: B) -> Result<Self> {
+        let mut iter = Self {
+            choose_a: false,
+            a,
+            b,
+        };
+        iter.skip_b()?;
+        iter.choose_a = Self::choose_a(&iter.a, &iter.b);
+        Ok(iter)
+    }
+}
+
+impl<A: StorageIterator, B: StorageIterator> StorageIterator for TwoMergeIterator<A, B> {
+    fn key(&self) -> &[u8] {
+        if self.choose_a {
+            self.a.key()
+        } else {
+            self.b.key()
+        }
+    }
+
+    fn value(&self) -> &[u8] {
+        if self.choose_a {
+            self.a.value()
+        } else {
+            self.b.value()
+        }
+    }
+
+    fn is_valid(&self) -> bool {
+        if self.choose_a {
+            self.a.is_valid()
+        } else {
+            self.b.is_valid()
+        }
+    }
+
+    fn next(&mut self) -> Result<()> {
+        if self.choose_a {
+            self.a.next()?;
+        } else {
+            self.b.next()?;
+        }
+        self.skip_b()?;
+        self.choose_a = Self::choose_a(&self.a, &self.b);
+        Ok(())
+    }
+}