Add Circular Buffer Exercise (#624)

Author: tomasnorre

config.json

@@ -1171,6 +1171,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 7
+      },
+      {
+        "slug": "circular-buffer",
+        "name": "Circular Buffer",
+        "uuid": "3f077d57-472a-469a-885d-ebc6aaccd3d7",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 6
       }
     ]
   },

exercises/practice/circular-buffer/.docs/instructions.md

@@ -0,0 +1,58 @@
+# Instructions
+
+A circular buffer, cyclic buffer or ring buffer is a data structure that uses a single, fixed-size buffer as if it were connected end-to-end.
+
+A circular buffer first starts empty and of some predefined length.
+For example, this is a 7-element buffer:
+
+```text
+[ ][ ][ ][ ][ ][ ][ ]
+```
+
+Assume that a 1 is written into the middle of the buffer (exact starting location does not matter in a circular buffer):
+
+```text
+[ ][ ][ ][1][ ][ ][ ]
+```
+
+Then assume that two more elements are added — 2 & 3 — which get appended after the 1:
+
+```text
+[ ][ ][ ][1][2][3][ ]
+```
+
+If two elements are then removed from the buffer, the oldest values inside the buffer are removed.
+The two elements removed, in this case, are 1 & 2, leaving the buffer with just a 3:
+
+```text
+[ ][ ][ ][ ][ ][3][ ]
+```
+
+If the buffer has 7 elements then it is completely full:
+
+```text
+[5][6][7][8][9][3][4]
+```
+
+When the buffer is full an error will be raised, alerting the client that further writes are blocked until a slot becomes free.
+
+When the buffer is full, the client can opt to overwrite the oldest data with a forced write.
+In this case, two more elements — A & B — are added and they overwrite the 3 & 4:
+
+```text
+[5][6][7][8][9][A][B]
+```
+
+3 & 4 have been replaced by A & B making 5 now the oldest data in the buffer.
+Finally, if two elements are removed then what would be returned is 5 & 6 yielding the buffer:
+
+```text
+[ ][ ][7][8][9][A][B]
+```
+
+Because there is space available, if the client again uses overwrite to store C & D then the space where 5 & 6 were stored previously will be used not the location of 7 & 8.
+7 is still the oldest element and the buffer is once again full.
+
+```text
+[C][D][7][8][9][A][B]
+```

exercises/practice/circular-buffer/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "tomasnorre"
+  ],
+  "files": {
+    "solution": [
+      "CircularBuffer.php"
+    ],
+    "test": [
+      "CircularBufferTest.php"
+    ],
+    "example": [
+      ".meta/example.php"
+    ]
+  },
+  "blurb": "A data structure that uses a single, fixed-size buffer as if it were connected end-to-end.",
+  "source": "Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Circular_buffer"
+}

exercises/practice/circular-buffer/.meta/example.php

@@ -0,0 +1,104 @@
+<?php
+
+/*
+ * By adding type hints and enabling strict type checking, code can become
+ * easier to read, self-documenting and reduce the number of potential bugs.
+ * By default, type declarations are non-strict, which means they will attempt
+ * to change the original type to match the type specified by the
+ * type-declaration.
+ *
+ * In other words, if you pass a string to a function requiring a float,
+ * it will attempt to convert the string value to a float.
+ *
+ * To enable strict mode, a single declare directive must be placed at the top
+ * of the file.
+ * This means that the strictness of typing is configured on a per-file basis.
+ * This directive not only affects the type declarations of parameters, but also
+ * a function's return type.
+ *
+ * For more info review the Concept on strict type checking in the PHP track
+ * <link>.
+ *
+ * To disable strict typing, comment out the directive below.
+ */
+
+declare(strict_types=1);
+
+class BufferFullError extends Exception
+{
+}
+
+class BufferEmptyError extends Exception
+{
+}
+
+class CircularBuffer
+{
+    private int $capacity;
+    private array $buffer;
+    private int $readPosition;
+    private int $writePosition;
+
+    public function __construct($capacity)
+    {
+        $this->capacity = $capacity;
+        $this->buffer = array_fill(0, $capacity, null);
+        $this->readPosition = 0;
+        $this->writePosition = 0;
+    }
+
+    /**
+     * @throws BufferEmptyError
+     */
+    public function read()
+    {
+        if ($this->isEmpty()) {
+            throw new BufferEmptyError();
+        }
+        $value = $this->buffer[$this->readPosition];
+        $this->buffer[$this->readPosition] = null;
+        $this->readPosition = ($this->readPosition + 1) % $this->capacity;
+        return $value;
+    }
+
+    /**
+     * @throws BufferFullError
+     */
+    public function write($item): void
+    {
+        if ($this->isFull()) {
+            throw new BufferFullError();
+        }
+        $this->buffer[$this->writePosition] = $item;
+        $this->writePosition = ($this->writePosition + 1) % $this->capacity;
+    }
+
+    /**
+     * @throws BufferEmptyError
+     * @throws BufferFullError
+     */
+    public function forceWrite($item): void
+    {
+        if ($this->isFull()) {
+            $this->read();
+        }
+        $this->write($item);
+    }
+
+    public function clear(): void
+    {
+        $this->buffer = array_fill(0, $this->capacity, null);
+        $this->readPosition = 0;
+        $this->writePosition = 0;
+    }
+
+    private function isEmpty(): bool
+    {
+        return $this->buffer[$this->readPosition] === null;
+    }
+
+    private function isFull(): bool
+    {
+        return $this->buffer[$this->writePosition] !== null;
+    }
+}

exercises/practice/circular-buffer/.meta/tests.toml

@@ -0,0 +1,52 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[28268ed4-4ff3-45f3-820e-895b44d53dfa]
+description = "reading empty buffer should fail"
+
+[2e6db04a-58a1-425d-ade8-ac30b5f318f3]
+description = "can read an item just written"
+
+[90741fe8-a448-45ce-be2b-de009a24c144]
+description = "each item may only be read once"
+
+[be0e62d5-da9c-47a8-b037-5db21827baa7]
+description = "items are read in the order they are written"
+
+[2af22046-3e44-4235-bfe6-05ba60439d38]
+description = "full buffer can't be written to"
+
+[547d192c-bbf0-4369-b8fa-fc37e71f2393]
+description = "a read frees up capacity for another write"
+
+[04a56659-3a81-4113-816b-6ecb659b4471]
+description = "read position is maintained even across multiple writes"
+
+[60c3a19a-81a7-43d7-bb0a-f07242b1111f]
+description = "items cleared out of buffer can't be read"
+
+[45f3ae89-3470-49f3-b50e-362e4b330a59]
+description = "clear frees up capacity for another write"
+
+[e1ac5170-a026-4725-bfbe-0cf332eddecd]
+description = "clear does nothing on empty buffer"
+
+[9c2d4f26-3ec7-453f-a895-7e7ff8ae7b5b]
+description = "overwrite acts like write on non-full buffer"
+
+[880f916b-5039-475c-bd5c-83463c36a147]
+description = "overwrite replaces the oldest item on full buffer"
+
+[bfecab5b-aca1-4fab-a2b0-cd4af2b053c3]
+description = "overwrite replaces the oldest item remaining in buffer following a read"
+
+[9cebe63a-c405-437b-8b62-e3fdc1ecec5a]
+description = "initial clear does not affect wrapping around"

exercises/practice/circular-buffer/CircularBuffer.php

@@ -0,0 +1,56 @@
+<?php
+
+/*
+ * By adding type hints and enabling strict type checking, code can become
+ * easier to read, self-documenting and reduce the number of potential bugs.
+ * By default, type declarations are non-strict, which means they will attempt
+ * to change the original type to match the type specified by the
+ * type-declaration.
+ *
+ * In other words, if you pass a string to a function requiring a float,
+ * it will attempt to convert the string value to a float.
+ *
+ * To enable strict mode, a single declare directive must be placed at the top
+ * of the file.
+ * This means that the strictness of typing is configured on a per-file basis.
+ * This directive not only affects the type declarations of parameters, but also
+ * a function's return type.
+ *
+ * For more info review the Concept on strict type checking in the PHP track
+ * <link>.
+ *
+ * To disable strict typing, comment out the directive below.
+ */
+
+declare(strict_types=1);
+
+class BufferFullError extends Exception
+{
+}
+
+class BufferEmptyError extends Exception
+{
+}
+
+class CircularBuffer
+{
+    public function read()
+    {
+        throw new \BadMethodCallException(sprintf('Implement the %s method', __FUNCTION__));
+    }
+
+    public function write($item): void
+    {
+        throw new \BadMethodCallException(sprintf('Implement the %s method', __FUNCTION__));
+    }
+
+    public function forceWrite($item): void
+    {
+        throw new \BadMethodCallException(sprintf('Implement the %s method', __FUNCTION__));
+    }
+
+    public function clear(): void
+    {
+        throw new \BadMethodCallException(sprintf('Implement the %s method', __FUNCTION__));
+    }
+}