From 1bf60c0cfc8e26d4f9e92e044d37169f55ac170b Mon Sep 17 00:00:00 2001
From: Peter Mikkelsen <petermikkelsen10@gmail.com>
Date: Tue, 25 Jan 2022 17:05:41 +0000
Subject: Get ready for inverse of functions

---
 apl9.h      |  3 +++
 inverse.c   | 13 +++++++++++++
 mkfile      |  1 +
 operators.c |  8 +++++++-
 4 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 inverse.c

diff --git a/apl9.h b/apl9.h
index c275d73..7c03918 100644
--- a/apl9.h
+++ b/apl9.h
@@ -264,6 +264,9 @@ void throwerror(Rune *, int);
 		memcpy(globalerror.jmp, old, sizeof(jmp_buf));\
 }
 
+/* inverse.c */
+Function inverse(Function);
+
 /* Monadic functions from function.c */
 Array *fnNegate(Array *);
 Array *fnSign(Array *);
diff --git a/inverse.c b/inverse.c
new file mode 100644
index 0000000..b37664b
--- /dev/null
+++ b/inverse.c
@@ -0,0 +1,13 @@
+#include <u.h>
+#include <libc.h>
+#include <bio.h>
+
+#include "apl9.h"
+
+Function
+inverse(Function f)
+{
+	Rune *msg = runesmprint("No inverse defined for %S", ppfunction(f));
+	throwerror(msg, EDomain);
+	return f;
+}
\ No newline at end of file
diff --git a/mkfile b/mkfile
index ab00568..017cc1a 100644
--- a/mkfile
+++ b/mkfile
@@ -14,6 +14,7 @@ OFILES=\
 	quadnames.$O\
 	error.$O\
 	hybrids.$O\
+	inverse.$O\
 
 HFILES=\
 	apl9.h\
diff --git a/operators.c b/operators.c
index 17ba76e..1ecb8aa 100644
--- a/operators.c
+++ b/operators.c
@@ -101,8 +101,14 @@ opPower(Datum *lefto, Datum *righto, Array *left, Array *right)
 		else
 			code = L"next←⍶⍵ ⋄ next⍹⍵:⍵ ⋄ ∇next";
 	}else if(righto->tag == ArrayTag){
-		if(righto->array->type != AtypeInt || righto->array->rank != 0 || righto->array->size != 1 || righto->array->intdata[0] < 0)
+		if(righto->array->type != AtypeInt || righto->array->rank != 0 || righto->array->size != 1)
 			throwerror(L"right operand to ⍣", EDomain);
+		vlong times = righto->array->intdata[0];
+		if(times < 0){
+			lefto->func = inverse(lefto->func);
+			times = -times;
+		}
+		righto->array->intdata[0] = times;
 		if(left)
 			code = L"⍹=0:⍵ ⋄ ⍺ ⍶⍙(⍹-1)⊢⍺⍶⍵";
 		else
-- 
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