From 32072c05c8d1a41145702bacbefba6695df04b73 Mon Sep 17 00:00:00 2001
From: David Phillips <david@sighup.nz>
Date: Thu, 5 Jan 2017 22:45:25 +1300
Subject: Add Monty Hall simulator

---
 .gitignore     |  1 +
 monty/Makefile |  1 +
 monty/monty.c  | 79 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 81 insertions(+)
 create mode 100644 monty/Makefile
 create mode 100644 monty/monty.c

diff --git a/.gitignore b/.gitignore
index 7622e04..4db000d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 words-misc/match_pool
+monty/monty
 *.o
diff --git a/monty/Makefile b/monty/Makefile
new file mode 100644
index 0000000..aa5f1de
--- /dev/null
+++ b/monty/Makefile
@@ -0,0 +1 @@
+monty:
diff --git a/monty/monty.c b/monty/monty.c
new file mode 100644
index 0000000..13ab236
--- /dev/null
+++ b/monty/monty.c
@@ -0,0 +1,79 @@
+
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+
+enum Strategy {
+	STRATEGY_STICK,
+	STRATEGY_CHANGE
+};
+
+
+unsigned int monty(int door_count, unsigned long plays, enum Strategy strategy)
+{
+	unsigned int i = 0;
+	int door = 0;
+	unsigned int wins = 0;
+	int prize = -1;
+	int choice = -1;
+	int remain = -1;
+
+	for (i = 0; i < plays; i++) {
+		prize = rand() % door_count;
+		choice = rand() % door_count;
+
+		/* Monty shall open all doors except two; the contestant's choice
+		 * and pick one door to not open */
+		if (choice == prize) {
+			/* contestant was originally correct? Pick any other door aside
+			 * from their choice to remain. Crude loop strategy, but pseudo-
+			 * randomness should help ensure quick termination */
+			do {
+				remain = rand() % door_count;
+			} while (remain == choice);
+		} else {
+			/* contestant never originally picked prize door; the prize door
+			 * remains */
+			remain = prize;
+		}
+
+		switch (strategy) {
+		case STRATEGY_STICK:
+			/* stick with current door */
+			break;
+		case STRATEGY_CHANGE:
+			/* `change` strategy: switch to remaining door */
+			choice = remain;
+			break;
+		default:
+			return -1;
+		}
+		if (choice == prize)
+			wins++;
+	}
+	return wins;
+}
+
+
+int main()
+{
+	double ratio = 0;
+	int runs = 1000000;
+	int wins = 0;
+
+	srand(time(NULL));
+
+	/* "stick/stay/lazy" strategy */
+	wins = monty(3, runs, STRATEGY_STICK);
+	ratio = ((double)wins) / runs;
+	printf("Stick : Won %d of %d (%.5f) success\n", wins, runs, ratio);
+
+	/* "change" strategy */
+	wins = monty(3, runs, STRATEGY_CHANGE);
+	ratio = ((double)wins) / runs;
+	printf("Change: Won %d of %d (%.5f) success\n", wins, runs, ratio);
+
+	return 0;
+}
-- 
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