From 22a2bb388b3a42cd25cd493c180e0193d6196332 Mon Sep 17 00:00:00 2001
From: Zeda Thomas <xedaelnara@gmail.com>
Date: Fri, 21 Feb 2020 21:14:23 -0500
Subject: [PATCH 1/2] fixed overflow bug with f24amean and f24geomean

---
 f24/f24amean.z80   | 36 +++++++++++++++++++++++++++++++++++-
 f24/f24geomean.z80 | 40 +++++++++++++++++++++++++++++++++++++++-
 2 files changed, 74 insertions(+), 2 deletions(-)

diff --git a/f24/f24amean.z80 b/f24/f24amean.z80
index 7430f8f..33519d3 100644
--- a/f24/f24amean.z80
+++ b/f24/f24amean.z80
@@ -2,7 +2,24 @@
 #define included_f24amean
 
 f24amean:
-;(AHL+BDE) ==> AHL
+;(AHL+CDE) ==> AHL
+
+;Make sure x+y won't overflow
+;save A
+  ld b,a
+  and $7F
+  jr z,f24amean_nooverflow
+  cp $7E
+  jr z,f24amean_overflow
+
+  ld a,c
+  and $7F
+  jr z,f24amean_nooverflow
+  cp $7E
+  jr z,f24amean_overflow
+
+f24amean_nooverflow:
+  ld a,b
   call f24add
 #ifdef included_f24div2
   jp f24div2
@@ -22,4 +39,21 @@ f24div2:
   dec a
   ret
 #endif
+
+f24amean_overflow:
+;need to decrement the exponents first
+  ld a,b
+  push de
+  push bc
+  call f24div2
+  pop bc
+  ex (sp),hl
+  push af
+  ld a,c
+  call f24div2
+  pop bc
+  pop de
+  ld c,b
+  jp f24add
+
 #endif
diff --git a/f24/f24geomean.z80 b/f24/f24geomean.z80
index 97a03b7..31fc8bb 100644
--- a/f24/f24geomean.z80
+++ b/f24/f24geomean.z80
@@ -11,7 +11,45 @@
 #include "f24sqrt.z80"
 
 f24geomean:
-;sqrt(AHL*BDE) ==> AHL
+;sqrt(AHL*CDE) ==> AHL
+
+;return NaN if the product is negative
+  xor c
+  jp p,+_
+  ld a,$7F
+  ld h,a
+  ret
+_:
+
+;the product is positive, so we can just set the inputs to positive
+  xor c
+  and $7F
+  ret z   ;may as well exit if the input is 0
+  cp $7F
+  jr z,f24geomean_sub
+  ld b,a
+
+  ld a,c
+  and $7F
+  ret z
+  cp $7F
+  jr z,f24geomean_sub
+
+;now calculate the output exponent
+  add a,b
+  rra
+  push af
+
+  ld a,63
+  ld c,a
+  adc a,0
+  call f24geomean_sub
+  pop bc
+  add a,b
+  sub 63
+  ret
+
+f24geomean_sub:
   call f24mul
   jp f24sqrt
 #endif

From 131ab9a8973c6b44c0d03435a6e156e3b20b4dcf Mon Sep 17 00:00:00 2001
From: Zeda Thomas <xedaelnara@gmail.com>
Date: Fri, 21 Feb 2020 22:55:43 -0500
Subject: [PATCH 2/2] removed warning now that it is fixed

---
 f24/f24geomean.z80 | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/f24/f24geomean.z80 b/f24/f24geomean.z80
index 31fc8bb..5bd1dd2 100644
--- a/f24/f24geomean.z80
+++ b/f24/f24geomean.z80
@@ -1,9 +1,3 @@
-;!! Note !!
-; This has numerical issues in that it is directly computing the the product,
-; then the square root. This can cause overflow when it shouldn't (note that as
-; long as both inputs are finite, the output should be strictly finite). There
-; is a slower and more difficult algorithm that can and probably should be made.
-
 #ifndef included_f24geomean
 #define included_f24geomean