diff --git a/zig/src/vector.zig b/zig/src/vector.zig
index 1d002b3..bb088ab 100644
--- a/zig/src/vector.zig
+++ b/zig/src/vector.zig
@@ -1,11 +1,15 @@
 const std = @import("std");
 
+/// return empty instance of `Vector`
+/// use `new()` or `new_with_capacity()` to
+/// get an initialised instance of `Vector`
 pub fn vector(comptime T: type) type
 {
     return struct
     {
-        const Self: type = @This();
+        const Vector: type = @This();
         const VectorType: type = T;
+        const DEFAULT_SIZE: usize = 16;
 
         const VectorError: type = error {
             OutOfBounds,
@@ -16,52 +20,73 @@ pub fn vector(comptime T: type) type
         array: []VectorType,
         length: usize,
 
-        pub fn new(allocator: std.mem.Allocator) !Self
+        /// create new vector with the default capacity
+        ///
+        /// returns initialised instance of `Vector`
+        pub fn new(allocator: std.mem.Allocator) !Vector
         {
-            const DEFAULT_SIZE: usize = 16;
-            return Self
+            return Vector
             {
                 .allocator = allocator,
-                .array = try allocator.alloc(VectorType, DEFAULT_SIZE),
+                .array = allocator.alloc(VectorType, DEFAULT_SIZE) catch return VectorError.FailedToAllocateMemory,
                 .length = 0,
             };
         }
 
-        pub fn new_with_capacity(allocator: std.mem.Allocator, capacity: usize) !Self
+        /// create new vector with the given capacity
+        ///
+        /// returns initialised instance of `Vector`
+        pub fn new_with_capacity(allocator: std.mem.Allocator, size: usize) VectorError!Vector
         {
-            return Self
+            return Vector
             {
                 .allocator = allocator,
-                .array = try allocator.alloc(VectorType, capacity),
+                .array = allocator.alloc(VectorType, size) catch return VectorError.FailedToAllocateMemory,
                 .length = 0,
             };
         }
 
-        pub fn len(self: *Self) usize
+        /// get the vectors length
+        ///
+        /// **note:** length != capacity
+        /// length   = number of items in the vector
+        /// capacity = maximum number of items the vector can hold
+        pub fn len(self: *Vector) usize
         {
             return self.length;
         }
 
-        pub fn push(self: *Self, item: VectorType) !void
+        /// get the vectors capacity
+        ///
+        /// **note:** length != capacity
+        /// length   = number of items in the vector
+        /// capacity = maximum number of items the vector can hold
+        pub fn capacity(self: *Vector) usize
         {
-            if (self.length >= self.array.len)
-            {
-                const new_buffer: []VectorType = try self.allocator.alloc(VectorType, 2 * self.array.len);
-                @memcpy(new_buffer, self.array);
-                self.allocator.free(self.array);
-                self.array = new_buffer;
-            }
+            return self.array.len;
+        }
+
+        /// add one item at the end of the vector
+        pub fn push(self: *Vector, item: VectorType) VectorError!void
+        {
+            if (self.length >= self.array.len) try self.double_capacity();
+
             self.array[self.length] = item;
             self.length += 1;
         }
 
-        pub fn pop(self: *Self) VectorType
+        /// remove and return last item of the vector
+        pub fn pop(self: *Vector) VectorType
         {
             self.length -= 1;
             return self.array[self.length];
         }
 
-        pub fn drain(self: *Self, range: [2]usize) VectorError![]VectorType
+        /// remove and return a range of elements
+        ///
+        /// range[0] has to be less than range[1]
+        /// the range bounds are **both** included
+        pub fn drain(self: *Vector, range: [2]usize) VectorError![]VectorType
         {
             if (2 != range.len
             or range[1] <= range[0]
@@ -80,11 +105,46 @@ pub fn vector(comptime T: type) type
 
             // TODO: return array instead of pointer
             @memcpy(slice.ptr, self.array[range[0]..range[1] + 1]);
-            @memmove(self.array.ptr + range[0], self.array[range[1]..self.length]);
+            @memmove(self.array.ptr + range[0], self.array[range[1] + 1..self.length]);
             self.length -= slice_length;
 
             return slice;
         }
+
+        /// add slice of `VectorType` to the end of the vector
+        pub fn extend(self: *Vector, slice: []VectorType) VectorError!void
+        {
+            if (slice.len == 0) return;
+
+            if (slice.len > self.capacity()) {
+                try self.increase_capacity(slice.len + self.capacity());
+            }
+            else if (slice.len > self.capacity() - self.length)
+            {
+                try self.double_capacity();
+            }
+
+            @memcpy(self.array.ptr + self.length, slice);
+        }
+
+        /// double the vectors capacity
+        fn double_capacity(self: *Vector) VectorError!void
+        {
+            try self.increase_capacity(2 * self.capacity());
+        }
+
+        /// increase the vectors capacity to given size
+        fn increase_capacity(self: *Vector, size: usize) VectorError!void
+        {
+            const new_buffer: []VectorType = self.allocator.alloc(VectorType, size)
+            catch
+            {
+                return VectorError.FailedToAllocateMemory;
+            };
+            @memcpy(new_buffer, self.array);
+            self.allocator.free(self.array);
+            self.array = new_buffer;
+        }
     };
 }