improved error handling

src/main.rs

@@ -2,7 +2,9 @@ use std::fs;
 use rand::rngs::OsRng;
 use rand::Rng;
 
-fn random_lower_char(rng: &mut OsRng) -> u8 {
+const FILENAME : &'static str = "state.txt";
+
+fn random_index(rng: &mut OsRng) -> usize {
     rng.gen_range(0..25)
 }
 
@@ -18,30 +20,54 @@ fn serialize_data(data: &[u32]) -> String {
     serialization 
 }
 
+fn deserialize(s: &str, data: &mut [u32]) {
+    println!("deserializing...");
+    // assumes no duplicate (and disgreeing) lines
+    for line in s.split('\n') {
+        if let Some((letter, val)) = line.split_once(' ') {
+            let c = match letter.chars().next() {
+                Some(c) => c,
+                None => {
+                    println!("ERROR: invalid line format, missing character");
+                    continue;
+                }
+            };
+            let index = (c as u32) - 97;
+            let count = match val.parse::<u32>() {
+                Ok(i) => i,
+                Err(_) => {
+                    println!("ERROR: invalid line format, missing count");
+                    continue;
+                } 
+            };
+            data[index as usize] = count;
+        }
+    }
+}
+
 fn main() {
     // considered using mutable static array for data
     // but that requires unsafe, so we just use non-static memory
     let mut data : [u32; 26] = [0; 26];
 
     // deserialize file contents into data structure
-    if let Ok(contents) = fs::read_to_string("state.txt") {
-        println!("deserializing...");
-        for line in contents.split('\n') {
-            if let Some((letter, val)) = line.split_once(' ') {
-                let index = (letter.chars().next().unwrap() as u32) - 97; // todo: handle failure
-                let count = val.parse::<u32>().unwrap(); // todo: handle failure
-                data[index as usize] = count;
-            }
-        }
+    if let Ok(contents) = fs::read_to_string(FILENAME) {
+        deserialize(&contents, &mut data);
+    } else {
+        println!("ERROR: failed to read file");
     } 
 
     // generate random char and give to data structure
     let mut rng = OsRng;
-    let index = random_lower_char(&mut rng) as usize;
+    let index = random_index(&mut rng);
     data[index] += 1;
+    println!("generated new char {}", (index + 97) as u8 as char);
 
     // write data structure to file
     let s_data = serialize_data(&data);
-    fs::write("state.txt", s_data).expect("Should be able to write to `state.txt`");
-    println!("done")
+    println!("writing...");
+    if let Err(_) = fs::write(FILENAME, s_data) {
+        println!("ERROR: Couldn't write to {}", FILENAME);
+        return;
+    }
 }

state.txt

@@ -1,26 +1,26 @@
-a 0
+a 1
 b 2
 c 1
 d 0
-e 0
+e 1
 f 2
 g 2
 h 1
 i 1
 j 0
 k 1
-l 0
+l 1
 m 4
-n 0
-o 2
+n 2
+o 3
 p 1
-q 1
-r 0
-s 1
+q 3
+r 1
+s 2
 t 3
 u 0
 v 1
 w 4
 x 0
-y 0
+y 1
 z 0