nasm: Allow file paths for the configuration

This opens up the door for developers to pass their own configuration
files.

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

3 files changed, 36 insertions, 16 deletions

diff --git a/README.md b/README.md
index 7d29b1a..bbc281e 100644
--- a/README.md
+++ b/README.md
@@ -30,12 +30,21 @@ By default it will assume the configuration for an `NROM` mapper, but this can
 be changed with the `-c/--configuration` flag, which accepts the following
 values:
 
-- `empty`: just `HEADER` and `CODE`. Useful for one-liners (e.g. "how is this
-  instruction encoded in binary?").
-- `nrom`: the default configuration. It has the following segments: `HEADER`,
-  `CODE`, `VECTORS`, and `CHARS`.
-- `nrom65`: same as `nrom` but it also has `STARTUP` for compatibility with the
-  default linker configuration from [cc65](https://github.com/cc65/cc65).
+- [empty](./lib/xixanta/src/mappings/empty.toml): just `HEADER` and `CODE`.
+  Useful for one-liners (e.g. "how is this instruction encoded in binary?").
+- [nrom](./lib/xixanta/src/mappings/nrom.toml): the default configuration.
+  It has the following segments: `HEADER`, `CODE`, `VECTORS`, and `CHARS`.
+- [nrom65](./lib/xixanta/src/mappings/nrom65.toml): same as `nrom` but it
+  also has `STARTUP` for compatibility with the default linker configuration
+  from [cc65](https://github.com/cc65/cc65).
+- [unrom or uxrom](./lib/xixanta/src/mappings/unrom.toml): a configuration for
+  UxROM chips, with seven swappable banks bound at 0x8000 and 16KB of size
+  (`PRG0`..`PRG6`), and a fixed bank on 0xC000 and 16KB of size as well
+  (`FIXED`).
+
+Alternatively, you can also pass a path to a configuration of your own. Check
+out the [ones already bundled](./lib/xixanta/src/mappings) on this application
+for reference.
 
 ## `readrom`
 
diff --git a/crates/nasm/src/main.rs b/crates/nasm/src/main.rs
index c93dda0..ff8bcae 100644
--- a/crates/nasm/src/main.rs
+++ b/crates/nasm/src/main.rs
@@ -14,7 +14,10 @@ struct Args {
     /// when this argument is not given.
     file: Option<String>,
 
-    /// Linker configuration to be used. Defaults to 'nrom'.
+    /// 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>,
 
diff --git a/lib/xixanta/src/mapping.rs b/lib/xixanta/src/mapping.rs
index a196c8e..12b64c8 100644
--- a/lib/xixanta/src/mapping.rs
+++ b/lib/xixanta/src/mapping.rs
@@ -97,18 +97,26 @@ pub struct Mapping {
 }
 
 /// Returns a vector corresponding to the configuration of mappings that is
-/// expected for the given `name`. Returns an error if the configuration cannot
-/// be parsed or there's something wrong about it.
+/// expected for the given `name`. This `name` can either be an already known
+/// identifier (e.g. "nrom"), or a file path. Returns an error if the
+/// configuration cannot be parsed or there's something wrong about it.
 pub fn get_mapping_configuration(name: &str) -> Result<Vec<Mapping>, String> {
-    let text = match name.to_lowercase().as_str() {
-        "empty" => EMPTY_CONFIG,
-        "nrom" => NROM_CONFIG,
-        "nrom65" => NROM65_CONFIG,
-        "uxrom" | "unrom" => UXROM_CONFIG,
-        _ => return Err("mapper configuration is not known".to_string()),
+    let configuration = if std::fs::exists(name).unwrap_or(false) {
+        match std::fs::read_to_string(name) {
+            Ok(contents) => load_configuration_for(contents.as_str())?,
+            Err(_) => return Err(format!("could not read '{}'", name)),
+        }
+    } else {
+        let text = match name.to_lowercase().as_str() {
+            "empty" => EMPTY_CONFIG,
+            "nrom" => NROM_CONFIG,
+            "nrom65" => NROM65_CONFIG,
+            "uxrom" | "unrom" => UXROM_CONFIG,
+            _ => return Err("mapper configuration is not known".to_string()),
+        };
+        load_configuration_for(text)?
     };
 
-    let configuration = load_configuration_for(text)?;
     validate_configuration(&configuration)?;
 
     Ok(configuration)