Add circular buffer (#352)

config.json

@@ -630,6 +630,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 2
+      },
+      {
+        "slug": "circular-buffer",
+        "name": "Circular Buffer",
+        "uuid": "fbf9d4b0-5943-4691-9bb9-f51a69c4c060",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4
       }
     ]
   },

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+3 & 4 have been replaced by A & B making 5 now the oldest data in the buffer.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 +1,17 @@
+{
+  "authors": ["blakelewis"],
+  "files": {
+    "solution": [
+      "circular-buffer.rkt"
+    ],
+    "test": [
+      "circular-buffer-test.rkt"
+    ],
+    "example": [
+      ".meta/example.rkt"
+    ]
+  },
+  "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.rkt

@@ -0,0 +1,32 @@
+#lang racket
+
+(provide circular-buffer%)
+
+(define circular-buffer%
+  (class object%
+    (super-new)
+    (init capacity)
+    (define buf-size capacity)
+    (define buf-modulus (* 2 capacity))
+    (define data (make-vector capacity))
+    (define front 0)
+    (define back 0)
+    (define (buf-empty?) (= front back))
+    (define (buf-full?) (= (modulo (- back front) buf-modulus) buf-size))
+    (define (next-spot s) (modulo (add1 s) buf-modulus))
+    (define (advance-front) (set! front (next-spot front)))
+    (define (advance-back) (set! back (next-spot back)))
+    (define/public (clear) (set! front back))
+    (define/public (read)
+      (if (buf-empty?)
+        (error "buffer is empty")
+        (let ([v (vector-ref data (modulo front buf-size))])
+          (begin (advance-front) v))))
+    (define/public (write v)
+      (if (buf-full?)
+        (error "buffer is full")
+        (begin (vector-set! data (modulo back buf-size) v)
+               (advance-back))))
+    (define/public (overwrite v)
+      (if (buf-full?) (advance-front) null)
+      (write v))))

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-test.rkt

@@ -0,0 +1,114 @@
+#lang racket/base
+
+(require racket/class)
+(require "circular-buffer.rkt")
+
+(module+ test
+  (require rackunit rackunit/text-ui))
+
+(module+ test
+  (define suite
+    (test-suite
+      "circular buffer tests"
+
+      (test-case "reading empty buffer should fail"
+	(define buffer (new circular-buffer% [capacity 1]))
+	(check-exn exn:fail?  (λ () (send buffer read))))
+
+      (test-case "can read an item just written"
+	(define buffer (new circular-buffer% [capacity 1]))
+	(send buffer write 1)
+	(check-equal? (send buffer read) 1))
+
+      (test-case "each item may only be read once"
+	(define buffer (new circular-buffer% [capacity 1]))
+	(send buffer write 1)
+	(check-equal? (send buffer read) 1)
+	(check-exn exn:fail?  (λ () (send buffer read))))
+
+      (test-case "items are read in the order they are written"
+	(define buffer (new circular-buffer% [capacity 2]))
+	(send buffer write 1)
+	(send buffer write 2)
+	(check-equal? (send buffer read) 1)
+	(check-equal? (send buffer read) 2))
+
+      (test-case "full buffer can't be written to"
+	(define buffer (new circular-buffer% [capacity 1]))
+	(send buffer write 1)
+	(check-exn exn:fail?  (λ () (send buffer write 2))))
+
+      (test-case "a read frees up capacity for another write"
+	(define buffer (new circular-buffer% [capacity 1]))
+	(send buffer write 1)
+	(check-equal? (send buffer read) 1)
+	(send buffer write 2)
+	(check-equal? (send buffer read) 2))
+
+      (test-case "read position is maintained even across multiple writes"
+	(define buffer (new circular-buffer% [capacity 3]))
+	(send buffer write 1)
+	(send buffer write 2)
+	(check-equal? (send buffer read) 1)
+	(send buffer write 3)
+	(check-equal? (send buffer read) 2)
+	(check-equal? (send buffer read) 3))
+
+      (test-case "items cleared out of buffer can't be read"
+	(define buffer (new circular-buffer% [capacity 1]))
+	(send buffer write 1)
+	(send buffer clear)
+	(check-exn exn:fail?  (λ () (send buffer read))))
+
+      (test-case "clear frees up capacity for another write"
+	(define buffer (new circular-buffer% [capacity 1]))
+	(send buffer write 1)
+	(send buffer clear)
+	(send buffer write 2)
+	(check-equal? (send buffer read) 2))
+
+      (test-case "clear does nothing on empty buffer"
+	(define buffer (new circular-buffer% [capacity 1]))
+	(send buffer clear)
+	(send buffer write 1)
+	(check-equal? (send buffer read) 1))
+
+      (test-case "overwrite acts like write on non-full buffer"
+	(define buffer (new circular-buffer% [capacity 2]))
+	(send buffer write 1)
+	(send buffer overwrite 2)
+	(check-equal? (send buffer read) 1)
+	(check-equal? (send buffer read) 2))
+
+      (test-case "overwrite replaces the oldest item on full buffer"
+	(define buffer (new circular-buffer% [capacity 2]))
+	(send buffer write 1)
+	(send buffer write 2)
+	(send buffer overwrite 3)
+	(check-equal? (send buffer read) 2)
+	(check-equal? (send buffer read) 3))
+
+      (test-case "overwrite replaces the oldest item remaining in buffer following a read"
+	(define buffer (new circular-buffer% [capacity 3]))
+	(send buffer write 1)
+	(send buffer write 2)
+	(send buffer write 3)
+	(check-equal? (send buffer read) 1)
+	(send buffer write 4)
+	(send buffer overwrite 5)
+	(check-equal? (send buffer read) 3)
+	(check-equal? (send buffer read) 4)
+	(check-equal? (send buffer read) 5))
+
+      (test-case "initial clear does not affect wrapping around"
+	(define buffer (new circular-buffer% [capacity 2]))
+	(send buffer clear)
+	(send buffer write 1)
+	(send buffer write 2)
+	(send buffer overwrite 3)
+	(send buffer overwrite 4)
+	(check-equal? (send buffer read) 3)
+	(check-equal? (send buffer read) 4)
+	(check-exn exn:fail?  (λ () (send buffer read))))))
+
+  (run-tests suite))

exercises/practice/circular-buffer/circular-buffer.rkt

@@ -0,0 +1,7 @@
+#lang racket
+
+(provide circular-buffer%)
+
+(define circular-buffer%
+  (class object%
+    (error "Not implemented yet")))