Add Pascal's Triangle (exercism#360)

* Pascal's triangle for Racket

config.json

@@ -686,6 +686,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 7
+      },
+      {
+        "slug": "pascals-triangle",
+        "name": "Pascal's Triangle",
+        "uuid": "3585411e-5c07-4cf7-8cd7-31df595d5735",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 2
       }
     ]
   },

exercises/practice/pascals-triangle/.docs/instructions.md

@@ -0,0 +1,14 @@
+# Instructions
+
+Compute Pascal's triangle up to a given number of rows.
+
+In Pascal's Triangle each number is computed by adding the numbers to the right and left of the current position in the previous row.
+
+```text
+    1
+   1 1
+  1 2 1
+ 1 3 3 1
+1 4 6 4 1
+# ... etc
+```

exercises/practice/pascals-triangle/.meta/config.json

@@ -0,0 +1,17 @@
+{
+  "authors": ["blakelewis"],
+  "files": {
+    "solution": [
+      "pascals-triangle.rkt"
+    ],
+    "test": [
+      "pascals-triangle-test.rkt"
+    ],
+    "example": [
+      ".meta/example.rkt"
+    ]
+  },
+  "blurb": "Compute Pascal's triangle up to a given number of rows.",
+  "source": "Pascal's Triangle at Wolfram Math World",
+  "source_url": "https://www.wolframalpha.com/input/?i=Pascal%27s+triangle"
+}

exercises/practice/pascals-triangle/.meta/example.rkt

@@ -0,0 +1,17 @@
+#lang racket
+
+(provide rows)
+
+(define (next-row bottom)
+  (do ([xs bottom (cdr xs)]
+       [left 0 (car xs)]
+       [row '() (cons (+ left (car xs)) row)])
+    ((null? xs) (cons 1 row))))
+
+(define (rows height)
+  (for/fold ([triangle '(())]
+             #:result (if (null? triangle)
+                        triangle
+                        (cdr (reverse triangle))))
+    ([r (in-range height)])
+    (cons (next-row (car triangle)) triangle)))

exercises/practice/pascals-triangle/.meta/tests.toml

@@ -0,0 +1,34 @@
+# 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.
+
+[9920ce55-9629-46d5-85d6-4201f4a4234d]
+description = "zero rows"
+
+[70d643ce-a46d-4e93-af58-12d88dd01f21]
+description = "single row"
+
+[a6e5a2a2-fc9a-4b47-9f4f-ed9ad9fbe4bd]
+description = "two rows"
+
+[97206a99-79ba-4b04-b1c5-3c0fa1e16925]
+description = "three rows"
+
+[565a0431-c797-417c-a2c8-2935e01ce306]
+description = "four rows"
+
+[06f9ea50-9f51-4eb2-b9a9-c00975686c27]
+description = "five rows"
+
+[c3912965-ddb4-46a9-848e-3363e6b00b13]
+description = "six rows"
+
+[6cb26c66-7b57-4161-962c-81ec8c99f16b]
+description = "ten rows"

exercises/practice/pascals-triangle/pascals-triangle-test.rkt

@@ -0,0 +1,68 @@
+#lang racket/base
+
+(require "pascals-triangle.rkt")
+
+(module+ test
+  (require rackunit rackunit/text-ui))
+
+(module+ test
+  (define suite
+    (test-suite "pascal's triangle tests"
+
+      (test-equal? "zero rows"
+        (rows 0)
+        '())
+
+      (test-equal? "single row"
+        (rows 1)
+        '((1)))
+
+      (test-equal? "two rows"
+        (rows 2)
+        '((1)
+          (1 1)))
+
+      (test-equal? "three rows"
+        (rows 3)
+        '((1)
+          (1 1)
+          (1 2 1)))
+
+      (test-equal? "four rows"
+        (rows 4)
+        '((1)
+          (1 1)
+          (1 2 1)
+          (1 3 3 1)))
+
+      (test-equal? "five rows"
+        (rows 5)
+        '((1)
+          (1 1)
+          (1 2 1)
+          (1 3 3 1)
+          (1 4 6 4 1)))
+
+      (test-equal? "six rows"
+        (rows 6)
+        '((1)
+          (1 1)
+          (1 2 1)
+          (1 3 3 1)
+          (1 4 6 4 1)
+          (1 5 10 10 5 1)))
+
+      (test-equal? "ten rows"
+        (rows 10)
+        '((1)
+          (1 1)
+          (1 2 1)
+          (1 3 3 1)
+          (1 4 6 4 1)
+          (1 5 10 10 5 1)
+          (1 6 15 20 15 6 1)
+          (1 7 21 35 35 21 7 1)
+          (1 8 28 56 70 56 28 8 1)
+          (1 9 36 84 126 126 84 36 9 1)))))
+
+  (run-tests suite))

exercises/practice/pascals-triangle/pascals-triangle.rkt

@@ -0,0 +1,6 @@
+#lang racket
+
+(provide rows)
+
+(define (rows height)
+  (error "Not implemented yet"))