From 192b637c101e41d52d56c7c576ac94a75c6855df Mon Sep 17 00:00:00 2001
From: Andrew Johnson <andrewjohnson@Andrews-MacBook-Pro.local>
Date: Thu, 24 May 2018 21:50:31 -0600
Subject: [PATCH] debugging asserts

---
 Chapter09/Cargo.toml          |  5 +++++
 Chapter09/debugging_assert.rs | 37 +++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)
 create mode 100644 Chapter09/debugging_assert.rs

diff --git a/Chapter09/Cargo.toml b/Chapter09/Cargo.toml
index fa96ea8..9b7d8b3 100644
--- a/Chapter09/Cargo.toml
+++ b/Chapter09/Cargo.toml
@@ -79,3 +79,8 @@ path = "debugging_heartbeat.rs"
 [[bin]]
 name = "debugging_buggy_worker"
 path = "debugging_buggy_worker.rs"
+
+[[bin]]
+name = "debugging_assert"
+path = "debugging_assert.rs"
+
diff --git a/Chapter09/debugging_assert.rs b/Chapter09/debugging_assert.rs
new file mode 100644
index 0000000..68c7d3a
--- /dev/null
+++ b/Chapter09/debugging_assert.rs
@@ -0,0 +1,37 @@
+use std::io;
+
+fn debug_precondition(n: u64) -> u64 {
+   debug_assert!(n < 100);
+   n * n
+}
+
+fn debug_postcondition(n: u64) -> u64 {
+   let r = n * n;
+   debug_assert!(r > 10);
+   r
+}
+
+fn runtime_precondition(n: u64) -> Result<u64,()> {
+   if !(n<100) { return Err(()) };
+   Ok(n * n)
+}
+
+fn runtime_postcondition(n: u64) -> Result<u64,()> {
+   let r = n * n;
+   if !(r>10) { return Err(()) };
+   Ok(r)
+}
+
+fn main() {
+   //inward facing code should assert expectations
+   debug_precondition(5);
+   debug_postcondition(5);
+
+   //outward facing code should handle errors
+   let mut s = String::new();
+   println!("Please input a positive integer greater or equal to 4:");
+   io::stdin().read_line(&mut s).expect("error reading input");
+   let i = s.trim().parse::<u64>().expect("error parsing input as integer");
+   runtime_precondition(i).expect("runtime precondition violated");
+   runtime_postcondition(i).expect("runtime postcondition violated");
+}