diff --git a/.gitignore b/.gitignore
index ea8c4bf..96ef6c0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 /target
+Cargo.lock
diff --git a/Cargo.lock b/Cargo.lock
index ea7e35e..54f5177 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -105,7 +105,7 @@ dependencies = [
  "getrandom",
  "once_cell",
  "version_check",
- "zerocopy 0.7.35",
+ "zerocopy",
 ]
 
 [[package]]
@@ -150,6 +150,12 @@ version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fc7eb209b1518d6bb87b283c20095f5228ecda460da70b44f0802523dea6da04"
 
+[[package]]
+name = "android-tzdata"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e999941b234f3131b00bc13c22d06e8c5ff726d1b6318ac7eb276997bbb4fef0"
+
 [[package]]
 name = "android_system_properties"
 version = "0.1.5"
@@ -159,6 +165,61 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "anstream"
+version = "0.6.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "64e15c1ab1f89faffbf04a634d5e1962e9074f2741eef6d97f3c4e322426d526"
+dependencies = [
+ "anstyle",
+ "anstyle-parse",
+ "anstyle-query",
+ "anstyle-wincon",
+ "colorchoice",
+ "is_terminal_polyfill",
+ "utf8parse",
+]
+
+[[package]]
+name = "anstyle"
+version = "1.0.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1bec1de6f59aedf83baf9ff929c98f2ad654b97c9510f4e70cf6f661d49fd5b1"
+
+[[package]]
+name = "anstyle-parse"
+version = "0.2.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "eb47de1e80c2b463c735db5b217a0ddc39d612e7ac9e2e96a5aed1f57616c1cb"
+dependencies = [
+ "utf8parse",
+]
+
+[[package]]
+name = "anstyle-query"
+version = "1.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6d36fc52c7f6c869915e99412912f22093507da8d9e942ceaf66fe4b7c14422a"
+dependencies = [
+ "windows-sys 0.52.0",
+]
+
+[[package]]
+name = "anstyle-wincon"
+version = "3.0.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5bf74e1b6e971609db8ca7a9ce79fd5768ab6ae46441c572e46cf596f59e57f8"
+dependencies = [
+ "anstyle",
+ "windows-sys 0.52.0",
+]
+
+[[package]]
+name = "anyhow"
+version = "1.0.86"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b3d1d046238990b9cf5bcde22a3fb3584ee5cf65fb2765f454ed428c7a0063da"
+
 [[package]]
 name = "arboard"
 version = "3.4.0"
@@ -671,6 +732,60 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "chrono"
+version = "0.4.38"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a21f936df1771bf62b77f047b726c4625ff2e8aa607c01ec06e5a05bd8463401"
+dependencies = [
+ "android-tzdata",
+ "iana-time-zone",
+ "js-sys",
+ "num-traits",
+ "wasm-bindgen",
+ "windows-targets 0.52.6",
+]
+
+[[package]]
+name = "clap"
+version = "4.5.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0fbb260a053428790f3de475e304ff84cdbc4face759ea7a3e64c1edd938a7fc"
+dependencies = [
+ "clap_builder",
+ "clap_derive",
+]
+
+[[package]]
+name = "clap_builder"
+version = "4.5.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "64b17d7ea74e9f833c7dbf2cbe4fb12ff26783eda4782a8975b72f895c9b4d99"
+dependencies = [
+ "anstream",
+ "anstyle",
+ "clap_lex",
+ "strsim",
+]
+
+[[package]]
+name = "clap_derive"
+version = "4.5.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "501d359d5f3dcaf6ecdeee48833ae73ec6e42723a1e52419c79abf9507eec0a0"
+dependencies = [
+ "heck",
+ "proc-macro2",
+ "quote",
+ "syn 2.0.72",
+]
+
+[[package]]
+name = "clap_lex"
+version = "0.7.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1462739cb27611015575c0c11df5df7601141071f07518d56fcc1be504cbec97"
+
 [[package]]
 name = "clipboard-win"
 version = "5.4.0"
@@ -690,6 +805,12 @@ dependencies = [
  "unicode-width",
 ]
 
+[[package]]
+name = "colorchoice"
+version = "1.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d3fd119d74b830634cea2a0f58bbd0d54540518a14397557951e79340abc28c0"
+
 [[package]]
 name = "com"
 version = "0.6.0"
@@ -1116,9 +1237,9 @@ dependencies = [
 
 [[package]]
 name = "flate2"
-version = "1.0.30"
+version = "1.0.31"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5f54427cfd1c7829e2a139fcefea601bf088ebca651d2bf53ebc600eac295dae"
+checksum = "7f211bbe8e69bbd0cfdea405084f128ae8b4aaa6b0b522fc8f2b009084797920"
 dependencies = [
  "crc32fast",
  "miniz_oxide",
@@ -1424,6 +1545,12 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "heck"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2304e00983f87ffb38b55b444b5e3b60a884b5d30c0fca7d82fe33449bbe55ea"
+
 [[package]]
 name = "hermit-abi"
 version = "0.3.9"
@@ -1457,6 +1584,29 @@ dependencies = [
  "windows-sys 0.52.0",
 ]
 
+[[package]]
+name = "iana-time-zone"
+version = "0.1.60"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e7ffbb5a1b541ea2561f8c41c087286cc091e21e556a4f09a8f6cbf17b69b141"
+dependencies = [
+ "android_system_properties",
+ "core-foundation-sys",
+ "iana-time-zone-haiku",
+ "js-sys",
+ "wasm-bindgen",
+ "windows-core",
+]
+
+[[package]]
+name = "iana-time-zone-haiku"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f31827a206f56af32e590ba56d5d2d085f558508192593743f16b2306495269f"
+dependencies = [
+ "cc",
+]
+
 [[package]]
 name = "icrate"
 version = "0.0.4"
@@ -1520,6 +1670,12 @@ dependencies = [
  "windows-sys 0.48.0",
 ]
 
+[[package]]
+name = "is_terminal_polyfill"
+version = "1.70.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7943c866cc5cd64cbc25b2e01621d07fa8eb2a1a23160ee81ce38704e97b8ecf"
+
 [[package]]
 name = "jni"
 version = "0.21.1"
@@ -1715,11 +1871,25 @@ dependencies = [
  "simd-adler32",
 ]
 
+[[package]]
+name = "mrn-generator"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "05e5415d8d2709e498b06c6b0287f4b1b008b33730dd0c5248dd9e4ae62f5f40"
+dependencies = [
+ "anyhow",
+ "chrono",
+ "clap",
+ "rand",
+ "thiserror",
+]
+
 [[package]]
 name = "mrn-generator-ui"
 version = "0.1.0"
 dependencies = [
  "eframe",
+ "mrn-generator",
 ]
 
 [[package]]
@@ -2122,11 +2292,11 @@ dependencies = [
 
 [[package]]
 name = "ppv-lite86"
-version = "0.2.18"
+version = "0.2.20"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "dee4364d9f3b902ef14fab8a1ddffb783a1cb6b4bba3bfc1fa3922732c7de97f"
+checksum = "77957b295656769bb8ad2b6a6b09d897d94f05c41b069aede1fcdaa675eaea04"
 dependencies = [
- "zerocopy 0.6.6",
+ "zerocopy",
 ]
 
 [[package]]
@@ -2540,6 +2710,12 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6637bab7722d379c8b41ba849228d680cc12d0a45ba1fa2b48f2a30577a06731"
 
+[[package]]
+name = "strsim"
+version = "0.11.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7da8b5736845d9f2fcb837ea5d9e2628564b3b043a70948a3f0b778838c5fb4f"
+
 [[package]]
 name = "syn"
 version = "1.0.109"
@@ -2564,12 +2740,13 @@ dependencies = [
 
 [[package]]
 name = "tempfile"
-version = "3.10.1"
+version = "3.11.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "85b77fafb263dd9d05cbeac119526425676db3784113aa9295c88498cbf8bff1"
+checksum = "b8fcd239983515c23a32fb82099f97d0b11b8c72f654ed659363a95c3dad7a53"
 dependencies = [
  "cfg-if",
  "fastrand 2.1.0",
+ "once_cell",
  "rustix 0.38.34",
  "windows-sys 0.52.0",
 ]
@@ -2784,6 +2961,12 @@ dependencies = [
  "percent-encoding",
 ]
 
+[[package]]
+name = "utf8parse"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "06abde3611657adf66d383f00b093d7faecc7fa57071cce2578660c9f1010821"
+
 [[package]]
 name = "version_check"
 version = "0.9.5"
@@ -3178,11 +3361,11 @@ checksum = "ac3b87c63620426dd9b991e5ce0329eff545bccbbb34f3be09ff6fb6ab51b7b6"
 
 [[package]]
 name = "winapi-util"
-version = "0.1.8"
+version = "0.1.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4d4cc384e1e73b93bafa6fb4f1df8c41695c8a91cf9c4c64358067d15a7b6c6b"
+checksum = "cf221c93e13a30d793f7645a0e7762c55d169dbb0a49671918a2319d289b10bb"
 dependencies = [
- "windows-sys 0.52.0",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -3270,6 +3453,15 @@ dependencies = [
  "windows-targets 0.52.6",
 ]
 
+[[package]]
+name = "windows-sys"
+version = "0.59.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e38bc4d79ed67fd075bcc251a1c39b32a1776bbe92e5bef1f0bf1f8c531853b"
+dependencies = [
+ "windows-targets 0.52.6",
+]
+
 [[package]]
 name = "windows-targets"
 version = "0.42.2"
@@ -3645,34 +3837,14 @@ dependencies = [
  "zvariant",
 ]
 
-[[package]]
-name = "zerocopy"
-version = "0.6.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "854e949ac82d619ee9a14c66a1b674ac730422372ccb759ce0c39cabcf2bf8e6"
-dependencies = [
- "byteorder",
- "zerocopy-derive 0.6.6",
-]
-
 [[package]]
 name = "zerocopy"
 version = "0.7.35"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1b9b4fd18abc82b8136838da5d50bae7bdea537c574d8dc1a34ed098d6c166f0"
 dependencies = [
- "zerocopy-derive 0.7.35",
-]
-
-[[package]]
-name = "zerocopy-derive"
-version = "0.6.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "125139de3f6b9d625c39e2efdd73d41bdac468ccd556556440e322be0e1bbd91"
-dependencies = [
- "proc-macro2",
- "quote",
- "syn 2.0.72",
+ "byteorder",
+ "zerocopy-derive",
 ]
 
 [[package]]
diff --git a/Cargo.toml b/Cargo.toml
index 3589d05..8a68778 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -4,4 +4,5 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
+mrn-generator = "0.3.1"
 eframe = "0.28.1"
diff --git a/src/main.rs b/src/main.rs
index a8edab2..c66b623 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,6 +1,6 @@
 use core::f32;
 use eframe::egui;
-use std::process::Command;
+use mrn_generator::{generate_random_mrn, match_procedure};
 
 fn main() -> eframe::Result {
     eframe::run_native(
@@ -66,55 +66,75 @@ impl eframe::App for MrnGeneratorApp {
             if ui.button("Generate MRNs").clicked() {
                 self.output = generate_mrns(
                     &self.country_code,
-                    &self.number_of_mrns,
-                    &self.procedure_category,
-                    &self.combined,
-                    &self.declaration_office,
+                    self.number_of_mrns.parse::<u32>().unwrap_or(1),
+                    (!self.procedure_category.is_empty()).then_some(&self.procedure_category),
+                    (!self.combined.is_empty()).then_some(&self.combined),
+                    (!self.declaration_office.is_empty()).then_some(&self.declaration_office),
                 );
             }
 
             ui.separator();
             ui.label("Output:");
-            ui.text_edit_multiline(&mut self.output);
+            egui::ScrollArea::vertical().show(ui, |ui| {
+                ui.text_edit_multiline(&mut self.output);
+            });
         });
     }
 }
 
 fn generate_mrns(
     country_code: &str,
-    number_of_mrns: &str,
-    procedure_category: &str,
-    combined: &str,
-    declaration_office: &str,
+    number_of_mrns: u32,
+    procedure_category_opt: Option<&str>,
+    combined_opt: Option<&str>,
+    declaration_office_opt: Option<&str>,
 ) -> String {
-    let mut command = Command::new("mrn-generator.exe");
-
-    command.arg("--country-code").arg(country_code);
+    let mut output = String::new();
 
-    if !number_of_mrns.is_empty() {
-        command.arg("--number-of-mrns").arg(number_of_mrns);
+    if country_code.is_empty() {
+        return "Please input a country code with two characters (e.g. 'IT')".to_string();
     }
 
-    if !procedure_category.is_empty() {
-        command.arg("--procedure-category").arg(procedure_category);
-    }
+    for _ in 0..number_of_mrns {
+        let mut is_err = false;
 
-    if !combined.is_empty() {
-        command.arg("--combined").arg(combined);
-    }
+        let mut procedure_opt = None;
+        if let Some(procedure_category) = procedure_category_opt {
+            let procedure_res = match_procedure(procedure_category, combined_opt);
 
-    if !declaration_office.is_empty() {
-        command.arg("--declaration-office").arg(declaration_office);
-    }
+            match procedure_res {
+                Ok(procedure) => procedure_opt = Some(procedure),
+                Err(e) => {
+                    output = e.to_string();
+                    is_err = true;
+                }
+            }
+        } else if let Some(combined) = combined_opt {
+            output = format!("You can't combine {combined} without providing a procedure category")
+                .to_string();
+            break;
+        };
+
+        if is_err {
+            break;
+        }
+
+        let mrn_res = generate_random_mrn(country_code, procedure_opt, declaration_office_opt);
 
-    match command.output() {
-        Ok(output) => {
-            if output.status.success() {
-                String::from_utf8_lossy(&output.stdout).to_string()
-            } else {
-                format!("Error: {}", String::from_utf8_lossy(&output.stderr))
+        match mrn_res {
+            Ok(mrn) => {
+                if output.is_empty() {
+                    output = mrn
+                } else {
+                    output = [output, mrn].join("\n")
+                };
+            }
+            Err(e) => {
+                output = e.to_string();
+                break;
             }
         }
-        Err(e) => format!("Failed to execute process: {}", e),
     }
+
+    output
 }