net: Add (net ricketyspace sicp two three).

* net/ricketyspace/sicp/two/three.scm: New file.

1 files changed, 80 insertions, 0 deletions

diff --git a/net/ricketyspace/sicp/two/three.scm b/net/ricketyspace/sicp/two/three.scm
new file mode 100644
index 0000000..8b18523
--- /dev/null
+++ b/net/ricketyspace/sicp/two/three.scm
@@ -0,0 +1,80 @@
+;;;; Under Creative Commons Attribution-ShareAlike 4.0
+;;;; International. See
+;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+
+(define-module (net ricketyspace sicp two three)
+  #:use-module (net ricketyspace sicp two two)
+  #:export (rectangle-make
+            rectangle-make-alt
+            rectangle-area
+            rectangle-perimeter))
+
+;;;;
+;;;;
+;;;; Visual representation of the rectangle.
+;;;;
+;;;;    3                2
+;;;;           l2
+;;;;    +----------------+
+;;;;    |                |
+;;;; w2 |                | w1   width
+;;;;    |                |
+;;;;    +----------------+
+;;;;  start    l1
+;;;;    0                1
+;;;;          length
+;;;;
+
+;;; Rectangle constructors.
+
+(define (rectangle-make length width start)
+  (define TYPE 0)
+  (define p0 start)
+  (define p1 (make-point (+ (x-point p0) length)
+                         (y-point p0)))
+  (define p2 (make-point (+ (x-point p0) length)
+                         (+ (y-point p0) width)))
+  (define p3 (make-point (x-point p0)
+                         (+ (y-point p0) width)))
+  (list TYPE
+        (make-segment p0 p1)   ;l1
+        (make-segment p1 p2)   ;w1
+        (make-segment p3 p2)   ;l2
+        (make-segment p0 p3))) ;w2
+
+(define (rectangle-make-alt length width start)
+  (define TYPE 1)
+  (list TYPE length width start))
+
+
+;;; Rectangle selectors.
+
+(define (rectangle-type rect)
+  (car rect))
+
+(define (rectangle-length rect)
+  (let ((type (rectangle-type rect))
+        (l (cadr rect)))
+    (cond ((= type 0) (- (x-point (end-segment l))
+                         (x-point (start-segment l))))
+          ((= type 1) l)
+          (else (error "Unknown rectangle type")))))
+
+(define (rectangle-width rect)
+  (let ((type (rectangle-type rect))
+        (w (caddr rect)))
+    (cond ((= type 0) (- (y-point (end-segment w))
+                         (y-point (start-segment w))))
+          ((= type 1) w)
+          (else (error "Unknown rectangle type")))))
+
+
+;;; Rectangle utils.
+
+(define (rectangle-area rect)
+  "Returns area of rectangle RECT"
+  (* (rectangle-length rect) (rectangle-width rect)))
+
+(define (rectangle-perimeter rect)
+  "Returns perimeter of rectangle RECT"
+  (* 2 (+ (rectangle-length rect) (rectangle-width rect))))