From: Taylor R Campbell <campbell@mumble.net>
Date: Sun, 18 Nov 2018 05:34:23 +0000 (+0000)
Subject: Invent complex:log1m to get correct branch cut in atan on exact inputs.
X-Git-Tag: mit-scheme-pucked-10.1.2~16^2~28
X-Git-Url: https://birchwood-abbey.net/git?a=commitdiff_plain;h=55872cd9d3f57c2a5df9518c34324b9adedb86b9;p=mit-scheme.git

Invent complex:log1m to get correct branch cut in atan on exact inputs.

Not going to export it for now without further thought, though.
---

diff --git a/src/runtime/arith.scm b/src/runtime/arith.scm
index ffbf8cb08..b0a0a690a 100644
--- a/src/runtime/arith.scm
+++ b/src/runtime/arith.scm
@@ -1750,6 +1750,11 @@ USA.
       (complex:log (complex:+ z 1))	;XXX
       ((copy real:log1p) z)))
 
+(define (complex:log1m z)
+  (if (and (real:real? z) (real:< 1 z))
+      (make-recnum (real:log (real:- z 1)) (real:negate rec:pi))
+      (complex:log1p (complex:negate z))))
+
 (define (complex:sin z)
   (if (recnum? z)
       (complex:/ (let ((iz (complex:+i* z)))
@@ -1839,8 +1844,8 @@ USA.
 
 (define (rec:atan z)
   (complex:/ (let ((iz (complex:+i* z)))
-	       (complex:- (complex:log (complex:1+ iz))
-			  (complex:log (complex:- 1 iz))))
+	       (complex:- (complex:log1p iz)
+			  (complex:log1m iz)))
 	     +2i))
 
 (define (complex:angle z)
diff --git a/tests/runtime/test-arith.scm b/tests/runtime/test-arith.scm
index 7ee2b6371..852f95617 100644
--- a/tests/runtime/test-arith.scm
+++ b/tests/runtime/test-arith.scm
@@ -483,7 +483,7 @@ USA.
    (vector +2i +1.5707963267948966+.5493061443340549i)
    (vector +0.+2i +1.5707963267948966+.5493061443340549i)
    (vector -0.+2i -1.5707963267948966+.5493061443340549i)
-   (vector -2i +1.5707963267948966-.5493061443340549i 'xfail)
+   (vector -2i +1.5707963267948966-.5493061443340549i)
    (vector +0.-2i +1.5707963267948966-.5493061443340549i)
    (vector -0.-2i -1.5707963267948966-.5493061443340549i))
   (lambda (v)