From 93734da352105b05055ed4a0114ab28f01d2108a Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Sat, 23 Dec 2017 11:40:44 +0100
Subject: [PATCH] Solution to day 23, in C.

---
 2017/README.md         |  2 +-
 2017/day-23/.gitignore |  1 +
 2017/day-23/Makefile   |  5 +++
 2017/day-23/input.txt  | 32 ++++++++++++++
 2017/day-23/solution.c | 98 ++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 137 insertions(+), 1 deletion(-)
 create mode 100644 2017/day-23/.gitignore
 create mode 100644 2017/day-23/Makefile
 create mode 100644 2017/day-23/input.txt
 create mode 100644 2017/day-23/solution.c

diff --git a/2017/README.md b/2017/README.md
index 1ac7519..b4a939a 100644
--- a/2017/README.md
+++ b/2017/README.md
@@ -8,7 +8,7 @@ The current plan, in no particular order:
 
 - [x] AWK - [Day 04](./day-04/solution.awk)
 - [x] Bash/shell script - [Day 02](./day-02/solution.sh)
-- [ ] C
+- [c] C - [Day 23](./day-23/solution.c)
 - [x] C++ - [Day 01](./day-01/solution.cpp)
 - [x] C# - [Day 07](./day-07/solution.cs)
 - [x] Clojure - [Day 03](./day-03/solution.clj)
diff --git a/2017/day-23/.gitignore b/2017/day-23/.gitignore
new file mode 100644
index 0000000..a4bd26d
--- /dev/null
+++ b/2017/day-23/.gitignore
@@ -0,0 +1 @@
+solution
diff --git a/2017/day-23/Makefile b/2017/day-23/Makefile
new file mode 100644
index 0000000..15cf9cb
--- /dev/null
+++ b/2017/day-23/Makefile
@@ -0,0 +1,5 @@
+CFLAGS=-Wall -Wextra -pedantic -O2 -g
+LD_LIBS=-lm
+
+all: solution
+	./$<
diff --git a/2017/day-23/input.txt b/2017/day-23/input.txt
new file mode 100644
index 0000000..274c84f
--- /dev/null
+++ b/2017/day-23/input.txt
@@ -0,0 +1,32 @@
+set b 84
+set c b
+jnz a 2
+jnz 1 5
+mul b 100
+sub b -100000
+set c b
+sub c -17000
+set f 1
+set d 2
+set e 2
+set g d
+mul g e
+sub g b
+jnz g 2
+set f 0
+sub e -1
+set g e
+sub g b
+jnz g -8
+sub d -1
+set g d
+sub g b
+jnz g -13
+jnz f 2
+sub h -1
+set g b
+sub g c
+jnz g 2
+jnz 1 3
+sub b -17
+jnz 1 -23
diff --git a/2017/day-23/solution.c b/2017/day-23/solution.c
new file mode 100644
index 0000000..bade8a4
--- /dev/null
+++ b/2017/day-23/solution.c
@@ -0,0 +1,98 @@
+#include <ctype.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define MAX_INSTR 1000
+
+#define SETVAL
+
+typedef struct {
+	char op[4];
+	char op_a[2];
+	char op_b[10];
+} instr_t;
+
+int get_val(const char* identifier, const int* registers)
+{
+	if (isalpha(identifier[0])) {
+		return registers[identifier[0] - 'a'];
+	} else {
+		return atoi(identifier);
+	}
+}
+
+int run_program(const instr_t program[], const int max_instr, int registers[])
+{
+	int iptr = 0;
+	int muls = 0;
+	while (iptr < max_instr) {
+		instr_t const* cur = &program[iptr];
+		int op_b = get_val(cur->op_b, registers);
+
+		int* res = &registers[cur->op_a[0] - 'a'];
+
+		if (!strcmp(cur->op, "set")) {
+			*res = op_b;
+		} else if (!strcmp(cur->op, "sub")) {
+			*res -= op_b;
+		} else if (!strcmp(cur->op, "mul")) {
+			*res *= op_b;
+			++muls;
+		} else if (!strcmp(cur->op, "jnz")) {
+			if (get_val(cur->op_a, registers) != 0) {
+				iptr += op_b;
+				continue;
+			}
+		}
+		iptr += 1;
+	}
+
+	return muls;
+}
+
+int is_prime(long long int num) {
+	for (long long int i = 2; i * i <= num; ++i) {
+		if (num % i == 0) {
+			return 0;
+		}
+	}
+
+	return 1;
+}
+
+int run_optimized()
+{
+	int b = 84 * 100 + 100000;
+	int c = b + 17000;
+	int h = 0;
+
+	for (; b <= c; b += 17) {
+		// Test if B is prime, result = f == 1;
+		if (!is_prime(b)) {
+			h += 1;
+		}
+	}
+
+	return h;
+}
+
+int main()
+{
+	instr_t program[MAX_INSTR];
+	int max_instr = 0;
+
+	int debug_registers[8] = {0};
+
+	while (scanf("%s %s %s", program[max_instr].op, program[max_instr].op_a, program[max_instr].op_b) != EOF) {
+		++max_instr;
+	}
+
+	int muls = run_program(program, max_instr, debug_registers);
+	printf("Muls executed: %d\n", muls);
+
+	printf("Final value of 'h': %d\n", run_optimized());
+
+	return 0;
+}