Remove the dependency on clap

All three binaries (i.e. readrom, nasm and xa65) used it, and to be
honest rolling our own argument parsing was actually pretty easy to
achieve. This frees us from a big dependency and it also frees us from
inner dependencies such as 'clap_derive' which prevented us from being
able to build binaries purely statically.

As a side effect, we can now display help/version messages which are
more like I'm used to, and certain checks can be moved in the parsing
directly instead of having to be done later. As a cherry on top, there
is some type masturbation that gets removed along the way.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

7 files changed, 260 insertions, 301 deletions

diff --git a/Cargo.lock b/Cargo.lock
index ab5483d..15586ea 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3,101 +3,6 @@
 version = 3
 
 [[package]]
-name = "anstream"
-version = "0.6.17"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "23a1e53f0f5d86382dafe1cf314783b2044280f406e7e1506368220ad11b1338"
-dependencies = [
- "anstyle",
- "anstyle-parse",
- "anstyle-query",
- "anstyle-wincon",
- "colorchoice",
- "is_terminal_polyfill",
- "utf8parse",
-]
-
-[[package]]
-name = "anstyle"
-version = "1.0.9"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8365de52b16c035ff4fcafe0092ba9390540e3e352870ac09933bebcaa2c8c56"
-
-[[package]]
-name = "anstyle-parse"
-version = "0.2.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3b2d16507662817a6a20a9ea92df6652ee4f94f914589377d69f3b21bc5798a9"
-dependencies = [
- "utf8parse",
-]
-
-[[package]]
-name = "anstyle-query"
-version = "1.1.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "79947af37f4177cfead1110013d678905c37501914fba0efea834c3fe9a8d60c"
-dependencies = [
- "windows-sys",
-]
-
-[[package]]
-name = "anstyle-wincon"
-version = "3.0.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2109dbce0e72be3ec00bed26e6a7479ca384ad226efdd66db8fa2e3a38c83125"
-dependencies = [
- "anstyle",
- "windows-sys",
-]
-
-[[package]]
-name = "clap"
-version = "4.5.20"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b97f376d85a664d5837dbae44bf546e6477a679ff6610010f17276f686d867e8"
-dependencies = [
- "clap_builder",
- "clap_derive",
-]
-
-[[package]]
-name = "clap_builder"
-version = "4.5.20"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "19bc80abd44e4bed93ca373a0704ccbd1b710dc5749406201bb018272808dc54"
-dependencies = [
- "anstream",
- "anstyle",
- "clap_lex",
- "strsim",
-]
-
-[[package]]
-name = "clap_derive"
-version = "4.5.18"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4ac6a0c7b1a9e9a5186361f67dfa1b88213572f427fb9ab038efb2bd8c582dab"
-dependencies = [
- "heck",
- "proc-macro2",
- "quote",
- "syn",
-]
-
-[[package]]
-name = "clap_lex"
-version = "0.7.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1462739cb27611015575c0c11df5df7601141071f07518d56fcc1be504cbec97"
-
-[[package]]
-name = "colorchoice"
-version = "1.0.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5b63caa9aa9397e2d9480a9b13673856c78d8ac123288526c37d7839f2a86990"
-
-[[package]]
 name = "equivalent"
 version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -114,28 +19,16 @@ name = "header"
 version = "0.1.0"
 
 [[package]]
-name = "heck"
-version = "0.5.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2304e00983f87ffb38b55b444b5e3b60a884b5d30c0fca7d82fe33449bbe55ea"
-
-[[package]]
 name = "indexmap"
-version = "2.7.0"
+version = "2.7.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "62f822373a4fe84d4bb149bf54e584a7f4abec90e072ed49cda0edea5b95471f"
+checksum = "8c9c992b02b5b4c94ea26e32fe5bccb7aa7d9f390ab5c1221ff895bc7ea8b652"
 dependencies = [
  "equivalent",
  "hashbrown",
 ]
 
 [[package]]
-name = "is_terminal_polyfill"
-version = "1.70.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7943c866cc5cd64cbc25b2e01621d07fa8eb2a1a23160ee81ce38704e97b8ecf"
-
-[[package]]
 name = "lazy_static"
 version = "1.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -151,7 +44,6 @@ checksum = "78ca9ab1a0babb1e7d5695e3530886289c18cf2f87ec19a575a0abdce112e3a3"
 name = "nasm"
 version = "0.1.0"
 dependencies = [
- "clap",
  "xixanta",
 ]
 
@@ -171,18 +63,18 @@ dependencies = [
 
 [[package]]
 name = "proc-macro2"
-version = "1.0.89"
+version = "1.0.93"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f139b0662de085916d1fb67d2b4169d1addddda1919e696f3252b740b629986e"
+checksum = "60946a68e5f9d28b0dc1c21bb8a97ee7d018a8b322fa57838ba31cc878e22d99"
 dependencies = [
  "unicode-ident",
 ]
 
 [[package]]
 name = "quote"
-version = "1.0.37"
+version = "1.0.38"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b5b9d34b8991d19d98081b46eacdd8eb58c6f2b201139f7c5f643cc155a633af"
+checksum = "0e4dccaaaf89514f546c693ddc140f729f958c247918a13380cccc6078391acc"
 dependencies = [
  "proc-macro2",
 ]
@@ -191,24 +83,23 @@ dependencies = [
 name = "readrom"
 version = "1.0.0"
 dependencies = [
- "clap",
  "header",
 ]
 
 [[package]]
 name = "serde"
-version = "1.0.216"
+version = "1.0.217"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0b9781016e935a97e8beecf0c933758c97a5520d32930e460142b4cd80c6338e"
+checksum = "02fc4265df13d6fa1d00ecff087228cc0a2b5f3c0e87e258d8b94a156e984c70"
 dependencies = [
  "serde_derive",
 ]
 
 [[package]]
 name = "serde_derive"
-version = "1.0.216"
+version = "1.0.217"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "46f859dbbf73865c6627ed570e78961cd3ac92407a2d117204c49232485da55e"
+checksum = "5a9bf7cf98d04a2b28aead066b7496853d4779c9cc183c440dbac457641e19a0"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -225,16 +116,10 @@ dependencies = [
 ]
 
 [[package]]
-name = "strsim"
-version = "0.11.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7da8b5736845d9f2fcb837ea5d9e2628564b3b043a70948a3f0b778838c5fb4f"
-
-[[package]]
 name = "syn"
-version = "2.0.85"
+version = "2.0.96"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5023162dfcd14ef8f32034d8bcd4cc5ddc61ef7a247c024a33e24e1f24d21b56"
+checksum = "d5d0adab1ae378d7f53bdebc67a39f1f151407ef230f0ce2883572f5d8985c80"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -278,94 +163,15 @@ dependencies = [
 
 [[package]]
 name = "unicode-ident"
-version = "1.0.13"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e91b56cd4cadaeb79bbf1a5645f6b4f8dc5bde8834ad5894a8db35fda9efa1fe"
-
-[[package]]
-name = "utf8parse"
-version = "0.2.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "06abde3611657adf66d383f00b093d7faecc7fa57071cce2578660c9f1010821"
-
-[[package]]
-name = "windows-sys"
-version = "0.59.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1e38bc4d79ed67fd075bcc251a1c39b32a1776bbe92e5bef1f0bf1f8c531853b"
-dependencies = [
- "windows-targets",
-]
-
-[[package]]
-name = "windows-targets"
-version = "0.52.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9b724f72796e036ab90c1021d4780d4d3d648aca59e491e6b98e725b84e99973"
-dependencies = [
- "windows_aarch64_gnullvm",
- "windows_aarch64_msvc",
- "windows_i686_gnu",
- "windows_i686_gnullvm",
- "windows_i686_msvc",
- "windows_x86_64_gnu",
- "windows_x86_64_gnullvm",
- "windows_x86_64_msvc",
-]
-
-[[package]]
-name = "windows_aarch64_gnullvm"
-version = "0.52.6"
+version = "1.0.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "32a4622180e7a0ec044bb555404c800bc9fd9ec262ec147edd5989ccd0c02cd3"
-
-[[package]]
-name = "windows_aarch64_msvc"
-version = "0.52.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "09ec2a7bb152e2252b53fa7803150007879548bc709c039df7627cabbd05d469"
-
-[[package]]
-name = "windows_i686_gnu"
-version = "0.52.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8e9b5ad5ab802e97eb8e295ac6720e509ee4c243f69d781394014ebfe8bbfa0b"
-
-[[package]]
-name = "windows_i686_gnullvm"
-version = "0.52.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0eee52d38c090b3caa76c563b86c3a4bd71ef1a819287c19d586d7334ae8ed66"
-
-[[package]]
-name = "windows_i686_msvc"
-version = "0.52.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "240948bc05c5e7c6dabba28bf89d89ffce3e303022809e73deaefe4f6ec56c66"
-
-[[package]]
-name = "windows_x86_64_gnu"
-version = "0.52.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "147a5c80aabfbf0c7d901cb5895d1de30ef2907eb21fbbab29ca94c5b08b1a78"
-
-[[package]]
-name = "windows_x86_64_gnullvm"
-version = "0.52.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "24d5b23dc417412679681396f2b49f3de8c1473deb516bd34410872eff51ed0d"
-
-[[package]]
-name = "windows_x86_64_msvc"
-version = "0.52.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "589f6da84c646204747d1270a2a5661ea66ed1cced2631d546fdfb155959f9ec"
+checksum = "adb9e6ca4f869e1180728b7950e35922a7fc6397f7b641499e8f3ef06e50dc83"
 
 [[package]]
 name = "winnow"
-version = "0.6.20"
+version = "0.6.24"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "36c1fec1a2bb5866f07c25f68c26e565c4c200aebb96d7e55710c19d3e8ac49b"
+checksum = "c8d71a593cc5c42ad7876e2c1fda56f314f3754c084128833e64f1345ff8a03a"
 dependencies = [
  "memchr",
 ]
@@ -373,9 +179,6 @@ dependencies = [
 [[package]]
 name = "xa65"
 version = "0.1.0"
-dependencies = [
- "clap",
-]
 
 [[package]]
 name = "xixanta"
diff --git a/crates/nasm/Cargo.toml b/crates/nasm/Cargo.toml
index 97d5439..f0f11db 100644
--- a/crates/nasm/Cargo.toml
+++ b/crates/nasm/Cargo.toml
@@ -5,5 +5,4 @@ edition = "2021"
 authors = ["Miquel Sabaté Solà <mikisabate@gmail.com>"]
 
 [dependencies]
-clap = { version = "^4", features = ["derive"] }
 xixanta.workspace = true
diff --git a/crates/nasm/src/main.rs b/crates/nasm/src/main.rs
index e757207..ef7c7d4 100644
--- a/crates/nasm/src/main.rs
+++ b/crates/nasm/src/main.rs
@@ -1,37 +1,104 @@
-use clap::Parser as ClapParser;
 use std::fs::File;
 use std::io::{self, Write};
 use std::path::Path;
 use xixanta::assembler::assemble;
 use xixanta::SourceInfo;
 
-/// Assembler for the 6502 microprocessor that targets the NES/Famicom.
-#[derive(ClapParser, Debug)]
-#[command(version, about, long_about = None)]
+/// Version for this program.
+const VERSION: &str = "0.1.0";
+
+#[derive(Default)]
 struct Args {
-    /// Assemble the instructions given on this file.
     file: String,
-
-    /// Linker configuration to be used. This configuration can be an identifier
-    /// for the configurations already baked in into this application, or it can
-    /// be a file path to a configuration of your choosing. See the
-    /// documentation for more information on this format. Defaults to 'nrom'.
-    #[arg(short = 'c', long)]
     config: Option<String>,
-
-    /// Place the output into the given <OUT> file. Ignored if the `stdout` flag
-    /// is provided. Defaults to `out.nes`.
-    #[arg(short = 'o', long)]
     out: Option<String>,
+    werror: bool,
+    stdout: bool,
+}
 
-    /// Treat warnings as errors.
-    #[arg(short = 'W', value_name = "Error")]
-    w: Option<String>,
+// Print the help message and quit.
+fn print_help() {
+    println!("Assembler for the 6502 microprocessor that targets the NES/Famicom.\n");
+    println!("usage: nasm [OPTIONS] <FILE>\n");
+    println!("Options:");
+    println!("  -c, --config <FILE>\tLinker configuration to be used, whether an identifier or a file path.");
+    println!("  -o, --out <FILE>\tFile path where the output should be located after execution.");
+    println!("  --stdout\t\tPrint the output binary to the standard output.");
+    println!("  -Werror\t\tWarnings should be treated as errors.");
+    std::process::exit(0);
+}
 
-    /// Spit the output into the standard output instead. This ignores any given
-    /// `out` flag. Disabled by default.
-    #[arg(long, default_value_t = false)]
-    stdout: bool,
+// Parse the arguments given to the program and returns an Args object with the
+// given information.
+fn parse_arguments() -> Args {
+    let mut args = std::env::args();
+    let mut res = Args::default();
+
+    // Skip command name.
+    args.next();
+
+    while let Some(arg) = args.next() {
+        match arg.as_str() {
+            "-c" | "--config" => match res.config {
+                Some(_) => die("only specify the '-C/--config' flag once".to_string()),
+                None => match args.next() {
+                    Some(v) => res.config = Some(v),
+                    None => {
+                        die("you need to provide a value for the '-C/--config' flag".to_string())
+                    }
+                },
+            },
+            "-h" | "--help" => print_help(),
+            "-o" | "--out" => match res.out {
+                Some(_) => die("only specify the '-o/--out' flag once".to_string()),
+                None => {
+                    if res.stdout {
+                        die("you cannot mix '-o/--out' and '--stdout'".to_string());
+                    }
+                    match args.next() {
+                        Some(v) => res.out = Some(v),
+                        None => {
+                            die("you need to provide a value for the '-o/--out' flag".to_string())
+                        }
+                    }
+                }
+            },
+            "--stdout" => match res.out {
+                Some(_) => die("you cannot mix '-o/--out' and '--stdout'".to_string()),
+                None => {
+                    if res.stdout {
+                        die("only specify the '--stdout' flag once".to_string());
+                    }
+                    res.stdout = true;
+                }
+            },
+            "-v" | "--version" => {
+                println!("nasm {}", VERSION);
+                std::process::exit(0);
+            }
+            "-Werror" => {
+                if res.werror {
+                    die("only specify the '-Werror' flag once".to_string());
+                }
+                res.werror = true;
+            }
+            _ => {
+                if arg.starts_with('-') {
+                    die(format!("don't know how to handle the '{}' flag", arg));
+                }
+                if !res.file.is_empty() {
+                    die("cannot have multiple source files".to_string());
+                }
+                res.file = arg;
+            }
+        }
+    }
+
+    if res.file.is_empty() {
+        die("you need to specify a source file".to_string());
+    }
+
+    res
 }
 
 // Print the given `message` and exit(1).
@@ -41,7 +108,7 @@ fn die(message: String) {
 }
 
 fn main() {
-    let args = Args::parse();
+    let args = parse_arguments();
 
     // Select the input stream and build the source object.
     let path = Path::new(&args.file);
@@ -74,19 +141,6 @@ fn main() {
         }
     };
 
-    // Check if warnings have to be treated as errors.
-    let warn_as_errors = match args.w {
-        Some(value) => {
-            if value.to_lowercase() != "error" {
-                die("the '-W' flag can only be used as '-Werror'".to_string());
-                return;
-            } else {
-                true
-            }
-        }
-        None => false,
-    };
-
     // And assemble.
     let mut error_count = 0;
     let res = assemble(
@@ -98,7 +152,7 @@ fn main() {
     // Print warnings and errors first, while also computing the amount of them
     // that exists.
     for warning in res.warnings {
-        if warn_as_errors {
+        if args.werror {
             eprintln!("error: {}", warning);
             error_count += 1;
         } else {
diff --git a/crates/readrom/Cargo.toml b/crates/readrom/Cargo.toml
index 93d6134..326ed92 100644
--- a/crates/readrom/Cargo.toml
+++ b/crates/readrom/Cargo.toml
@@ -7,5 +7,4 @@ license.workspace = true
 authors.workspace = true
 
 [dependencies]
-clap = "^4"
 header.workspace = true
diff --git a/crates/readrom/src/main.rs b/crates/readrom/src/main.rs
index ae2a44c..e37e8c7 100644
--- a/crates/readrom/src/main.rs
+++ b/crates/readrom/src/main.rs
@@ -1,8 +1,65 @@
-use clap::{arg, Arg, Command};
 use header::{Header, Kind};
 use std::fs::File;
 use std::io::{ErrorKind, Read};
 
+/// Version for this program.
+const VERSION: &str = "0.1.0";
+
+#[derive(Default)]
+struct Args {
+    file: String,
+    header: bool,
+}
+
+fn print_help() {
+    println!("Display information about NES/Famicom ROM files.\n");
+    println!("usage: readrom [OPTIONS] <FILE>\n");
+    println!("Options:");
+    println!("  -H, --header\tJust print the ROM header and quit.");
+    std::process::exit(0);
+}
+
+// Parse the arguments given to the program and returns an Args object with the
+// given information.
+fn parse_arguments() -> Args {
+    let mut args = std::env::args();
+    let mut res = Args::default();
+
+    // Skip command name.
+    args.next();
+
+    for arg in args {
+        match arg.as_str() {
+            "-h" | "--help" => print_help(),
+            "-H" | "--header" => {
+                if res.header {
+                    die("do not specify the '-H/--header' flag twice".to_string());
+                }
+                res.header = true;
+            }
+            "-v" | "--version" => {
+                println!("readrom {}", VERSION);
+                std::process::exit(0);
+            }
+            _ => {
+                if arg.starts_with('-') {
+                    die(format!("don't know how to handle the '{}' flag", arg));
+                }
+                if !res.file.is_empty() {
+                    die("cannot have multiple source files".to_string());
+                }
+                res.file = arg;
+            }
+        }
+    }
+
+    if res.file.is_empty() {
+        die("you need to specify the file to be read".to_string());
+    }
+
+    res
+}
+
 fn print_header(header: &Header) {
     println!("Header:");
 
@@ -47,22 +104,10 @@ fn die(message: String) {
 }
 
 fn main() {
-    let args = Command::new("readrom")
-        .version("0.1.0")
-        .about("Display information about NES/Famicom ROM files.")
-        .arg(Arg::new("FILE").required(true).help("ROM file to be read"))
-        .arg(arg!(-H --header "Just print the ROM header and quit"))
-        .get_matches();
-    let file = match args.get_one::<String>("FILE") {
-        Some(file) => file,
-        None => {
-            die("you have to provide a file".to_string());
-            return;
-        }
-    };
+    let args = parse_arguments();
 
-    let Ok(mut input) = File::open(file) else {
-        die(format!("failed to open the given file '{}'", &file));
+    let Ok(mut input) = File::open(&args.file) else {
+        die(format!("failed to open the given file '{}'", &args.file));
         return;
     };
 
@@ -85,7 +130,7 @@ fn main() {
     };
     print_header(&header);
 
-    if *args.get_one::<bool>("header").unwrap() {
+    if args.header {
         std::process::exit(0);
     }
 
diff --git a/crates/xa65/Cargo.toml b/crates/xa65/Cargo.toml
index 1ecceb0..1a2c52c 100644
--- a/crates/xa65/Cargo.toml
+++ b/crates/xa65/Cargo.toml
@@ -3,6 +3,3 @@ name = "xa65"
 version = "0.1.0"
 edition = "2021"
 authors = ["Miquel Sabaté Solà <mikisabate@gmail.com>"]
-
-[dependencies]
-clap = { version = "^4", features = ["derive"] }
diff --git a/crates/xa65/src/main.rs b/crates/xa65/src/main.rs
index 8a31e7b..cdfce59 100644
--- a/crates/xa65/src/main.rs
+++ b/crates/xa65/src/main.rs
@@ -1,30 +1,101 @@
-use clap::Parser as ClapParser;
 use std::path::PathBuf;
 use std::process::Command;
 
-/// Bridge between 'nasm' and 'ca65'.
-#[derive(ClapParser, Debug)]
-#[command(version, about, long_about = None)]
+/// Version for this program.
+const VERSION: &str = "0.1.0";
+
+// Arguments for this application. See `parse_arguments` on how it's filled.
+#[derive(Default)]
 struct Args {
-    /// Assemble the instructions given on this file.
     file: String,
-
-    /// Linker configuration to be used. This configuration can be an identifier
-    /// for the configurations already baked in into this application, or it can
-    /// be a file path to a configuration of your choosing. See the
-    /// documentation for more information on this format. Defaults to 'nrom'.
-    #[arg(short = 'C', long = "config")]
     config: Option<String>,
-
-    /// Used for compatibility with 'ca65'.
-    #[arg(long)]
     target: Option<String>,
-
-    /// Place the output into the given <OUT> file.
-    #[arg(short = 'o', long)]
     out: String,
 }
 
+// Print the help message and quit.
+fn print_help() {
+    println!("Bridge between 'nasm' and 'ca65'.\n");
+    println!("usage: xa65 [OPTIONS] <FILE>\n");
+    println!("Options:");
+    println!("  -C, --config <FILE>\tLinker configuration to be used, whether an identifier or a file path.");
+    println!("  -o, --out <FILE>\tFile path where the output should be located after execution.");
+    println!("  --target nes\t\tUsed for compatibility with 'ca65'.");
+    std::process::exit(0);
+}
+
+// Parse the arguments given to the program and returns an Args object with the
+// given information.
+fn parse_arguments() -> Args {
+    let mut args = std::env::args();
+    let mut res = Args::default();
+
+    // Skip command name.
+    args.next();
+
+    while let Some(arg) = args.next() {
+        match arg.as_str() {
+            "-C" | "--config" => match res.config {
+                Some(_) => die("only specify the '-C/--config' flag once".to_string()),
+                None => match args.next() {
+                    Some(v) => res.config = Some(v),
+                    None => {
+                        die("you need to provide a value for the '-C/--config' flag".to_string())
+                    }
+                },
+            },
+            "-h" | "--help" => print_help(),
+            "-o" | "--out" => {
+                if res.out.is_empty() {
+                    match args.next() {
+                        Some(v) => res.out = v,
+                        None => {
+                            die("you need to provide a value for the '-o/--out' flag".to_string())
+                        }
+                    }
+                } else {
+                    die("only specify the '-o/--out' flag once".to_string());
+                }
+            }
+            "--target" => match res.target {
+                Some(_) => die("only specify the '--target' flag once".to_string()),
+                None => match args.next() {
+                    Some(v) => {
+                        let real = v.to_lowercase();
+                        if real != "nes" {
+                            die("the '--target' flag only accepts 'nes' as a value".to_string());
+                        }
+                        res.target = Some(real)
+                    }
+                    None => die("you need to provide a value for the '--target' flag".to_string()),
+                },
+            },
+            "-v" | "--version" => {
+                println!("xa65 {}", VERSION);
+                std::process::exit(0);
+            }
+            _ => {
+                if arg.starts_with('-') {
+                    die(format!("don't know how to handle the '{}' flag", arg));
+                }
+                if !res.file.is_empty() {
+                    die("cannot have multiple source files".to_string());
+                }
+                res.file = arg;
+            }
+        }
+    }
+
+    if res.file.is_empty() {
+        die("you need to specify a source file".to_string());
+    }
+    if res.out.is_empty() {
+        die("you need to specify an output file with '-o/--output'".to_string());
+    }
+
+    res
+}
+
 // Print the given `message` and exit(1).
 fn die(message: String) {
     eprintln!("error: {}", message);
@@ -80,18 +151,9 @@ fn main() {
             return;
         }
     };
-    let args = Args::parse();
-
-    // Sanity check on the target flag from 'ca65'.
-    if let Some(target) = args.target {
-        if target != "nes" {
-            die(format!(
-                "the target has to be 'nes', but '{}' was provided instead",
-                target
-            ));
-            return;
-        }
-    }
+
+    // Parse arguments.
+    let args = parse_arguments();
 
     // Generate a temporary directory in which both binary files will be placed
     // as an intermediate step.