Fix imei test and validate check digit

src/at_command/gsn.rs

@@ -30,10 +30,56 @@ impl AtParseLine for Imei {
             return Err("Contains non-digit character".into());
         }
 
+        let provided_check_digit = (line.chars().next_back())
+            .expect("string is not empty")
+            .to_digit(10)
+            .expect("all chars are ascii digits");
+
+        let expected_check_digit = calculate_check_digit(&line[..line.len() - 1]);
+        if (provided_check_digit as u8) != expected_check_digit {
+            return Err("Imei number has invalid check digit".into());
+        }
+
         Ok(Imei { imei: line.into() })
     }
 }
 
+/// Calculate the IMEI check digit from an IMEI string
+///
+/// NOTE: the provided string must not already contain the check digit.
+///
+/// NOTE: the provided string must contain only ascii digits.
+fn calculate_check_digit(imei: &str) -> u8 {
+    // the check digit is calculated by iterating over each digit and
+    // 1. doubling every other digit
+    // 2. summing all digits
+    // (if doubling a digit generated two new digits, sum those as well)
+    // check digit is (10 - (sum % 10)) % 10
+
+    fn is_even(n: usize) -> bool {
+        (n & 1) == 0
+    }
+
+    let sum: u32 = imei
+        .chars()
+        .flat_map(|d| d.to_digit(10))
+        .enumerate()
+        .map(|(i, d)| {
+            if is_even(i) {
+                d
+            } else {
+                let mut doubled = d * 2;
+                if doubled >= 10 {
+                    doubled = (doubled % 10) + 1
+                }
+                doubled
+            }
+        })
+        .sum();
+
+    ((10 - (sum % 10)) % 10) as u8
+}
+
 impl AtResponse for Imei {
     fn from_generic(code: ResponseCode) -> Result<Self, ResponseCode> {
         match code {
@@ -47,11 +93,16 @@ impl AtResponse for Imei {
 mod test {
     use super::*;
     #[test]
-    fn parse_cpsi() {
-        let valid_imeis = ["49015420323751"];
+    fn parse_imei() {
+        let valid_imeis = [
+            "490154203237518",
+            "869951035460918",
+            "869931033480910",
+            "869951035458235",
+        ];
 
         for valid in valid_imeis {
-            assert!(Imei::from_line(valid).is_ok());
+            let _ = Imei::from_line(valid).expect("failed to parse imei");
         }
     }
 }