From 6b775194811823b84e2694c20c036506d5855215 Mon Sep 17 00:00:00 2001
From: Peter Mikkelsen <petermikkelsen10@gmail.com>
Date: Sat, 29 Jan 2022 23:04:14 +0000
Subject: Add two very incomplete implementations of factorial and binomial (!Y
 and X!Y)

---
 apl9.h      |  2 ++
 functions.c | 47 +++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/apl9.h b/apl9.h
index 0bfa6e9..cf2c352 100644
--- a/apl9.h
+++ b/apl9.h
@@ -290,6 +290,7 @@ Array *fnRecip(Array *);
 Array *fnExponent(Array *);
 Array *fnNaturalLog(Array *);
 Array *fnPiTimes(Array *);
+Array *fnFactorial(Array *);
 Array *fnRoll(Array *);
 Array *fnMagnitude(Array *);
 Array *fnCeiling(Array *);
@@ -326,6 +327,7 @@ Array *fnTimes(Array *, Array *);
 Array *fnDivide(Array *, Array *);
 Array *fnPower(Array *, Array *);
 Array *fnLogarithm(Array *, Array *);
+Array *fnBinomial(Array *, Array *);
 Array *fnDeal(Array *, Array *);
 Array *fnResidue(Array *, Array *);
 Array *fnMaximum(Array *, Array *);
diff --git a/functions.c b/functions.c
index 1645e77..0f40a9a 100644
--- a/functions.c
+++ b/functions.c
@@ -15,7 +15,7 @@ fnmonad monadfunctiondefs[] = {
 	fnNaturalLog, /* ⍟ */
 	0, /* ⌹ */
 	fnPiTimes, /* ○ */
-	0, /* ! */
+	fnFactorial, /* ! */
 	fnRoll, /* ? */
 	fnMagnitude, /* | */
 	fnCeiling, /* ⌈ */
@@ -71,7 +71,7 @@ fndyad dyadfunctiondefs[] = {
 	fnLogarithm, /* ⍟ */
 	0, /* ⌹ */
 	0, /* ○ */
-	0, /* ! */
+	fnBinomial, /* ! */
 	fnDeal, /* ? */
 	fnResidue, /* | */
 	fnMaximum, /* ⌈ */
@@ -323,6 +323,32 @@ fnPiTimes(Array *right)
 	return res;
 }
 
+Array *
+fnFactorial(Array *right)
+{
+	Array *result = nil;
+	if(right->type == AtypeArray){
+		result = duparrayshape(right, AtypeArray);
+		for(int i = 0; i < right->size; i++)
+			result->arraydata[i] = fnFactorial(right->arraydata[i]);
+	}else if(right->type == AtypeInt){
+		result = duparrayshape(right, AtypeFloat);
+		for(int i = 0; i < result->size; i++){
+			double x = 1;
+			vlong n = right->intdata[i];
+			if(n < 0)
+				throwerror(nil, EDomain);
+			for(int j = 1; j <= n; j++)
+				x *= j;
+			result->floatdata[i] = x;
+		}
+	}else if(right->type == AtypeFloat)
+		throwerror(L"Factorial of floating values", ENotImplemented);
+	else
+		throwerror(nil, EType);
+	return result;
+}
+
 Array *
 fnRoll(Array *right)
 {
@@ -906,6 +932,23 @@ SCALAR_FUNCTION_2(fnLogarithm, 1, left->type,
 		break;
 )
 
+SCALAR_FUNCTION_2(fnBinomial, 0, AtypeFloat,
+	case AtypeInt:
+		if(left->intdata[i] > right->intdata[i])
+			res->floatdata[i] = 0;
+		else{
+			Array *a = mkscalarint(left->intdata[i]);
+			Array *w = mkscalarint(right->intdata[i]);
+			Array *x = rundfn(L"(!⍵)÷(!⍺)×!⍵-⍺", nil, nil, a, w);
+			res->floatdata[i] = x->floatdata[0];
+			freearray(a);
+			freearray(w);
+			freearray(x);
+			/* TODO horrible */
+		}
+		break;
+)
+
 Array *
 fnDeal(Array *left, Array *right)
 {
-- 
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