Finished writing ssa solver

solve-triangle.lisp

@@ -11,6 +11,10 @@
 ;; SAS -- Law of Cosines
 ;; SSS -- Law of Cosines
 
+(defun between-p (lower number upper)
+  (and (>= number lower)
+     (<= number upper)))
+
 (defun solve-asa (anglea sidea angleb)
   "Solve a ASA triangle using the method of sines.  Angles assumed to be in degrees."
   (let* ((anglec (- 180 anglea angleb))
@@ -29,20 +33,35 @@
 
 (defun solve-ssa (sidea sideb anglea)
   "Solve an SSA triangle using the method of sines.  Angles are assumed to be in degrees."
-  (let ((angleb (asin (/ (* sideb (sin anglea))
-                         sidea))))
-    (cond
-      ((< 1 angleb) nil)
-      (())))
-  (let* ((angleb (asin (/ (* sideb (sin anglea))
-                          sidea)))
-         (anglec (- 180 anglea angleb))
-         (sidec (/ (* (sin anglec)
-                      sidea)
-                   (sin anglea))))
-    (list :angle-a anglea
-          :angle-b angleb
-          :angle-c anglec
-          :side-a sidea
-          :side-b sideb
-          :side-c sidec)))
+  (let ((sinb (/ (* sideb (sin anglea))
+                 sidea)))
+    (if (between-p -1 sinb 1)
+        (let* ((angleb (asin sinb))
+               (anglec (- 180 anglea angleb))
+               (sidec (asin (/ (* (sin anglec) sidea)
+                               (sin anglea)))))
+          (if (< (+ anglea angleb))
+              (let* ((angleb-prime (- 180 angleb))
+                     (anglec-prime (- 180 anglea angleb-prime))
+                     (sidec-prime (/ (* (sin anglec-prime) sidea)
+                                     (sin anglea))))
+                (values (list :angle-a anglea
+                            :angle-b angleb
+                            :angle-c anglec
+                            :side-a sidea
+                            :side-b sideb
+                            :sidec sidec)
+                        (list :angle-a anglea
+                              :angle-b angleb-prime
+                              :angle-c anglec-prime
+                              :sidea sidea
+                              :sideb sideb
+                              :sidec sidec-prime)))
+              (values (list :angle-a anglea
+                            :angle-b angleb
+                            :angle-c anglec
+                            :side-a sidea
+                            :side-b sideb
+                            :sidec sidec)
+                      nil)))
+        (values nil nil))))