Add (net ricketyspace sicp two twelve).

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

1 files changed, 40 insertions, 0 deletions

diff --git a/net/ricketyspace/sicp/two/twelve.scm b/net/ricketyspace/sicp/two/twelve.scm
new file mode 100644
index 0000000..c15614b
--- /dev/null
+++ b/net/ricketyspace/sicp/two/twelve.scm
@@ -0,0 +1,40 @@
+;;;; Under Creative Commons Attribution-ShareAlike 4.0
+;;;; International. See
+;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+
+(define-module (net ricketyspace sicp two twelve)
+  #:use-module (net ricketyspace sicp two seven)
+  #:export (make-center-width
+            make-center-percent
+            center
+            width
+            percent))
+
+(define (make-center-width c w)
+  (make-interval (- c w) (+ c w)))
+
+(define (make-center-percent c p)
+  (define (pof p c) (* c (/ p 100.0)))
+  (make-interval (- c (pof p c))
+                 (+ c (pof p c))))
+
+(define (center i)
+  (/ (+ (lower-bound i) (upper-bound i)) 2))
+(define (width i)
+  (/ (- (upper-bound i) (lower-bound i)) 2))
+
+(define (center i)
+  (/ (+ (lower-bound i) (upper-bound i)) 2))
+
+(define (width i)
+  (/ (- (upper-bound i) (lower-bound i)) 2))
+
+(define (percent i)
+  (* (/ (width i) (center i)) 100))
+
+;;; Guile REPL
+;;;
+;;; scheme@(guile-user)> (percent (make-center-percent 45 15))
+;;; $16 = 15.0
+;;; scheme@(guile-user)> (percent (make-center-percent 40 16))
+;;; $17 = 15.999999999999998