From 092cc1614671caf5b5242b3bf42664831561bb95 Mon Sep 17 00:00:00 2001
From: Aadhavan Srinivasan <aadhavan@twomorecents.org>
Date: Tue, 14 Feb 2023 23:06:46 -0600
Subject: [PATCH] Added 'Even Fibonacci' solution

---
 evenFibonacci/README.md |  3 +++
 evenFibonacci/main.c    |  8 ++++++++
 evenFibonacci/main.s    | 35 +++++++++++++++++++++++++++++++++++
 3 files changed, 46 insertions(+)
 create mode 100644 evenFibonacci/README.md
 create mode 100644 evenFibonacci/main.c
 create mode 100644 evenFibonacci/main.s

diff --git a/evenFibonacci/README.md b/evenFibonacci/README.md
new file mode 100644
index 0000000..9d78765
--- /dev/null
+++ b/evenFibonacci/README.md
@@ -0,0 +1,3 @@
+## Multiples of 3 or 5
+
+This directory contains my solution to the ['Even Fibonacci numbers' problem](https://projecteuler.net/problem=2).
diff --git a/evenFibonacci/main.c b/evenFibonacci/main.c
new file mode 100644
index 0000000..ed92874
--- /dev/null
+++ b/evenFibonacci/main.c
@@ -0,0 +1,8 @@
+#include <stdio.h>
+
+int evenFib();
+
+void main() {
+	printf("%d\n",evenFib());
+	return;
+}
diff --git a/evenFibonacci/main.s b/evenFibonacci/main.s
new file mode 100644
index 0000000..b3168b8
--- /dev/null
+++ b/evenFibonacci/main.s
@@ -0,0 +1,35 @@
+.global evenFib
+
+.section .text
+evenFib:
+init:
+	PUSH {R4}		@The ARM calling convention only allows callee functions to use R0-R3. Since I need R4 as well, I am going to push R4 onto the stack, and pop it off when I am done using it.
+	MOV R0,#0		@Return value - where all the addition is going to happen to
+	LDR R4,=#4000000	@ARM doesn't allow you to 'MOV' constants greater than 8 bits. To get around that, we use the LDR instruction, which lets the assembler place the value in memory, and then load it from there.
+	MOV R1,#1		@Main fibonacci number
+	MOV R2,#0		@Used to hold temporary values when incrementing the main fibonacci number
+	MOV R3,#0		@Previous fibonacci number - This will be 0 at the start
+loop:
+
+incr:
+	MOV R2,R1	@Store the current fibonacci number in R2
+	ADD R1,R1,R3	@Add the previous fibonacci number with the current one
+	MOV R3,R2	@Move R2 (the 'old current' fibonacci number) into R3
+checkbnd:
+	CMP R1,R4	@Check if R1 is greater than the upper bound for fibonacci numbers
+	BGE return	@If it is, then we return
+
+checkparity:		@Parity - The fact of being even or odd.
+	TST R1,#0x1	@Equivalent of an ANDS operation, but the result is discarded. In this case, we are checking if R1's parity by checking the last bit, which would be set to 0 if it is even, and 1 if it is odd.
+	BNE loop	@If R1 is odd, don't do the addition. Instead, go back to the loop, where we move on to the next fibonacci number.
+
+
+isEven: @The program goes here if R1 is even
+	ADD R0,R0,R1	@Add the fibonacci number to R0
+	B loop
+
+return:
+	POP {R4}	@Pop the R4 that we pushed onto the stack earlier
+	BX lr
+
+.section .data