fix: devec wrong behaviour when reserve then push (#374)

fix: devec wrong behaviour when reserve then push

devec/devec.mbt

@@ -71,7 +71,7 @@ fn realloc[T](self : Devec[T]) -> Unit {
   let old_cap = self.len
   let new_cap = if old_cap == 0 { 8 } else { old_cap * 2 }
   let new_buf = UninitializedArray::make(new_cap)
-  if self.len > 0 {
+  if old_cap > 0 {
     if self.tail >= self.head {
       for i = self.head, j = 0; i <= self.tail; i = i + 1, j = j + 1 {
         new_buf[j] = self.buf[i]
@@ -87,9 +87,11 @@ fn realloc[T](self : Devec[T]) -> Unit {
         j += 1
       }
     }
+    self.tail = self.len - 1
+  } else {
+    self.tail = 0
   }
   self.head = 0
-  self.tail = self.len - 1
   self.buf = new_buf
 }
 
@@ -166,7 +168,9 @@ pub fn push_front[T](self : Devec[T], value : T) -> Unit {
   if self.len == self.buf.length() {
     self.realloc()
   }
-  self.head = if self.head > 0 { self.head - 1 } else { self.buf.length() - 1 }
+  if self.len != 0 {
+    self.head = (self.head + self.buf.length() - 1) % self.buf.length()
+  }
   self.buf[self.head] = value
   self.len += 1
 }
@@ -184,7 +188,9 @@ pub fn push_back[T](self : Devec[T], value : T) -> Unit {
   if self.len == self.buf.length() {
     self.realloc()
   }
-  self.tail = if self.tail < self.buf.length() - 1 { self.tail + 1 } else { 0 }
+  if self.len != 0 {
+    self.tail = (self.tail + self.buf.length() + 1) % self.buf.length()
+  }
   self.buf[self.tail] = value
   self.len += 1
 }
@@ -774,3 +780,26 @@ test "as_iter" {
     content="6",
   )?
 }
+
+test "reserve and push" {
+  // capacity > 0, len = 0
+  let a : Devec[Option[Int]] = Devec::with_capacity(2)
+  a.push_back(Some(1))
+  inspect(a.front(), content="Some(Some(1))")?
+  inspect(a.back(), content="Some(Some(1))")?
+  // capacity = 0, len = 0
+  let b : Devec[Option[Int]] = Devec::with_capacity(0)
+  b.push_back(Some(1))
+  inspect(b.front(), content="Some(Some(1))")?
+  inspect(b.back(), content="Some(Some(1))")?
+  // capacity > 0, len = 0
+  let c : Devec[Option[Int]] = Devec::with_capacity(2)
+  c.push_front(Some(1))
+  inspect(c.front(), content="Some(Some(1))")?
+  inspect(c.back(), content="Some(Some(1))")?
+  // capacity = 0, len = 0
+  let d : Devec[Option[Int]] = Devec::with_capacity(0)
+  d.push_front(Some(1))
+  inspect(d.front(), content="Some(Some(1))")?
+  inspect(d.back(), content="Some(Some(1))")?
+}

devec/types.mbt

@@ -14,6 +14,7 @@
 
 priv type UninitializedArray[T] Array[UnsafeMaybeUninit[T]]
 
+// head and tail point to non-empty slots. When the buffer is empty, head and tail points to the same slot.
 struct Devec[T] {
   mut buf : UninitializedArray[T]
   mut len : Int