diff --git a/src/validate.rs b/src/validate.rs
index 9b90c42..a9d3192 100644
--- a/src/validate.rs
+++ b/src/validate.rs
@@ -5,11 +5,37 @@ use crate::util;
 pub fn validate(verbosity: u8, config: util::config::Config) {
     println!("Validating");
 
+    let in_suffix = "in"; //TODO make this configurable?
+    let out_suffix = "out";
     // get list of all file names in input/output
-    let in_files: Vec<PathBuf>;
-    let out_files: Vec<PathBuf>;
+    let in_files: Vec<PathBuf> = get_dir_files(config.input_path).expect("Failed to read the input directory contents");
+    let out_files: Vec<PathBuf> = get_dir_files(config.output_path).expect("Failed to read the output directory contents");
+
+    let mismatch: Vec<(PathBuf, PathBuf)>;
+    let mut invalid_suffix_in: Vec<PathBuf> = vec![];
+    let mut invalid_suffix_out: Vec<PathBuf> = vec![];
 
     // check if they end in .in or .out respectively
+    for file in in_files {
+        if !file.ends_with(in_suffix) {
+            invalid_suffix_in.push(file);
+        }
+    }
+
+    for file in out_files {
+        if !file.ends_with(out_suffix) {
+            invalid_suffix_out.push(file);
+        }
+    }
+
+    // print the delinquints
+    if invalid_suffix_in.len() > 0 {
+        print_invalid_suffixes(invalid_suffix_in, String::from(in_suffix));
+    }
+
+    if invalid_suffix_out.len() > 0 {
+        print_invalid_suffixes(invalid_suffix_out, String::from(out_suffix));
+    }
 
     // make sure each has a match
 }
@@ -35,15 +61,44 @@ fn get_dir_files(path: PathBuf) -> Result<Vec<PathBuf>, &'static str>  {
     return Ok(res);
 }
 
+fn compare_files(in_files: Vec<PathBuf>, out_files: Vec<PathBuf>) -> Vec<PathBuf> {
+    let mut res: Vec<PathBuf> = vec![];
 
-fn check_suffix(path: PathBuf, suf: String) -> bool {
-    return false;
+    res.append(&mut compare_list(in_files, out_files));
+    res.append(&mut compare_list(out_files, in_files));
+
+    return res;
 }
 
-fn check_match(path1: PathBuf, path2: PathBuf, in_suf: String, out_suf: String) -> bool {
-    return false;
+fn compare_list(from: Vec<PathBuf>, against: Vec<PathBuf>) -> Vec<PathBuf> {
+    let res: Vec<PathBuf> = vec![];
+
+    // Compare each input file to each output file
+    for file in from {
+        let beg = file.file_stem().unwrap(); // ignore suffix
+        for out in against {
+            if beg == out.file_stem().unwrap() {
+                break;
+            }
+        }
+        res.push(file); // if the file does not have a counterpart, then push to the lost files
+    }
+
+    return res;
 }
 
+fn print_invalid_suffixes(files: Vec<PathBuf>, exp: String) {
+    println!("Invalid suffixes detected 😱:" );
+    println!("Current => Suggested");
+    for mut file in files {
+        let initial = match file.to_str() {
+            Some(x) => x,
+            _       => panic!("Failed to load the file"),
+        };
+
+        println!("{} => {}", initial, file.set_extension(exp).to_string());
+    }
+}
 
 #[cfg(test)]
 mod tests {