Add circular-buffer exercise (#193)

config.json

@@ -532,6 +532,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 5
+      },
+      {
+        "slug": "circular-buffer",
+        "name": "Circular Buffer",
+        "uuid": "2442f2fe-3304-4d28-9539-b23a31e24c22",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 7
       }
     ]
   },

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

@@ -0,0 +1,29 @@
+# Instructions append
+
+This exercise introduces a couple of concepts you might not have seen before:
+
+## User-defined namespace
+
+Words in 8th all belong to a namespace, e.g. `a:new` refers to the `new` word in the `a` namespace.
+One can also define custom namespaces, which is what we do at the top of the `circular-buffer.8th` file:
+
+```8th
+ns: cb
+```
+
+This means that all words within that file can be accessed via `cb:<word>`, which is a great way to group related functionality.
+
+For more information, check out the [namespaces][namespaces] documentation.
+
+## Exceptions
+
+The `read` and `write` words are both expected to throw an exception when they're called on a circular buffer with an invalid state.
+For more information, check the [exceptions and error handling][exceptions-and-error-handling] documentation.
+
+## Objects
+
+While you're free to implement the circular buffer as you like, you could consider using 8th's [object support][objects].
+
+[namespaces]: https://8th-dev.com/manual.html#namespaces
+[exceptions-and-error-handling]: https://8th-dev.com/manual.html#exceptions-and-error-handling
+[objects]: https://8th-dev.com/manual.html#objects

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": [
+    "erikschierboom"
+  ],
+  "files": {
+    "solution": [
+      "circular-buffer.8th"
+    ],
+    "test": [
+      "test.8th"
+    ],
+    "example": [
+      ".meta/example.8th"
+    ]
+  },
+  "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.8th

@@ -0,0 +1,68 @@
+ns: cb
+
+: count \ o -- n
+  "count" o:@ nip
+;
+
+: capacity \ o -- n
+  "capacity" o:@ nip
+;
+
+: items \ o -- a
+  "items" o:@ nip
+;
+
+: head \ o -- n
+  "head" o:@ nip
+;
+
+: tail \ o -- n
+  "tail" o:@ nip
+;
+
+: read \ o -- o x
+  >r
+  r@ count 0 n:= if r> "circular buffer empty" throw ;then
+  r@ items r@ tail a:_@
+  r@ tail n:1+ r@ capacity n:/mod drop r@ "tail" rot o:! drop
+  r@ count n:1- r@ "count" rot o:!
+  swap
+;
+
+: overwrite \ o x -- o
+  swap >r
+  r@ items r@ head rot a:! r@ "items" rot o:!
+  r@ head n:1+ r@ capacity n:/mod drop "head" swap o:!
+  r@ count r@ capacity n:=
+  if
+    r@ head "tail" swap o:!
+  else
+    r@ count n:1+ "count" swap o:!
+  then
+;
+
+: write \ o x -- o
+  swap >r
+  r@ capacity r@ count n:= if drop r> "circular buffer full" throw ;then
+  r> swap cb:overwrite
+;
+
+: clear \ o  
+  "head"  0 o:!
+  "tail"  0 o:!
+  "count" 0 o:!
+;
+
+: new \ n -- o
+  {
+    class: "circular-buffer",
+    methods: { 
+      read: ' read,
+      write: ' write,
+      overwrite: ' overwrite
+    }
+  } o:new swap 
+  2dup "capacity" swap o:! drop
+  n:1- ' noop 0 rot a:generate "items" swap o:!
+  cb:clear
+;

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/circular-buffer.8th

@@ -0,0 +1,17 @@
+ns: cb
+
+: read \ x -- x x'
+
+;
+
+: write \ x x' -- x
+
+;
+
+: overwrite \ x x' -- x
+
+;
+
+: new \ n -- x
+
+;