1 files changed, 182 insertions, 0 deletions
diff --git a/src/main.rs b/src/main.rs
new file mode 100644
index 0000000..f40604e
--- /dev/null
+++ b/src/main.rs
@@ -0,0 +1,182 @@
+use anyhow::{anyhow, Context};
+use serde_json::Map;
+use toml_edit::{DocumentMut, Table};
+
+/// Apply updates from a JSON file to a TOML file while preserving comments in the latter.
+#[derive(argh::FromArgs)]
+struct Args {
+    #[argh(positional)]
+    toml_file: String,
+
+    #[argh(positional)]
+    json_file: String,
+
+    /// overwrites the given TOML file rather than printing to stdout
+    #[argh(switch)]
+    write: bool,
+}
+
+fn main() -> anyhow::Result<()> {
+    let args: Args = argh::from_env();
+
+    let toml = std::fs::read_to_string(&args.toml_file)?;
+    let json = std::fs::read_to_string(&args.json_file)?;
+
+    let mut doc: DocumentMut = toml.parse()?;
+    let patch: Map<String, serde_json::Value> =
+        serde_json::from_str(&json).context("failed to parse JSON")?;
+    apply(&patch, doc.as_table_mut())?;
+    if args.write {
+        std::fs::write(args.toml_file, doc.to_string())?;
+    } else {
+        print!("{doc}");
+    }
+    Ok(())
+}
+
+fn apply(patch: &Map<String, serde_json::Value>, table: &mut Table) -> anyhow::Result<()> {
+    for (key, val) in patch {
+        match val {
+            serde_json::Value::Object(map) => {
+                if !table.contains_key(key) || !matches!(table[key], toml_edit::Item::Table(_)) {
+                    table.insert(key, toml_edit::table());
+                }
+                apply(map, table[key].as_table_mut().unwrap())?;
+            }
+            other => match json_value_to_toml_value(other)? {
+                None => {
+                    table.remove(key);
+                }
+                Some(new_value) => {
+                    if let Some((_, value)) = table.get_key_value_mut(key) {
+                        *value = new_value.into();
+                    } else {
+                        table.insert(key, new_value.into());
+                    }
+                }
+            },
+        }
+    }
+    Ok(())
+}
+
+fn json_value_to_toml_value(value: &serde_json::Value) -> anyhow::Result<Option<toml_edit::Value>> {
+    Ok(match value {
+        serde_json::Value::Null => None,
+        serde_json::Value::Bool(bool) => Some((*bool).into()),
+        serde_json::Value::Number(number) => {
+            if number.is_u64() {
+                Some(i64::try_from(number.as_u64().unwrap()).unwrap().into())
+            } else if number.is_i64() {
+                Some(number.as_i64().unwrap().into())
+            } else if number.is_f64() {
+                Some(number.as_f64().unwrap().into())
+            } else {
+                unreachable!();
+            }
+        }
+        serde_json::Value::String(str) => Some(str.into()),
+        serde_json::Value::Array(vec) => {
+            let mut new = toml_edit::Array::new();
+            for value in vec {
+                match json_value_to_toml_value(value)? {
+                    None => return Err(anyhow!("arrays must not contain null")),
+                    Some(value) => new.push(value),
+                }
+            }
+            Some(new.into())
+        }
+        serde_json::Value::Object(_) => unreachable!(),
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use indoc::indoc;
+    use serde_json::json;
+    use toml_edit::DocumentMut;
+
+    fn apply(new_value: serde_json::Value, toml: &str) -> anyhow::Result<String> {
+        let mut doc = toml.parse::<DocumentMut>().unwrap();
+        super::apply(new_value.as_object().unwrap(), doc.as_table_mut())?;
+        Ok(doc.to_string())
+    }
+
+    #[test]
+    fn patch_object() {
+        assert_eq!(
+            apply(
+                json!({"x": {"y": {"foo": true}}}),
+                indoc! {r#"
+                [x.y]
+                # This is x.y.z.
+                z = 3
+                # End of x.y section.
+            "#}
+            )
+            .unwrap(),
+            indoc! {r#"
+                [x.y]
+                # This is x.y.z.
+                z = 3
+                foo = true
+                # End of x.y section.
+        "#}
+        );
+    }
+
+    #[test]
+    fn null() {
+        assert_eq!(apply(json!({"x": null}), "x = 0").unwrap(), "");
+    }
+
+    #[test]
+    fn object() {
+        assert_eq!(
+            apply(json!({"x": {"y": {"z": true}}}), "x = 0").unwrap(),
+            indoc! {r#"
+            [x]
+
+            [x.y]
+            z = true
+        "#}
+        );
+    }
+
+    #[test]
+    fn array_may_not_contain_null() {
+        assert_eq!(
+            apply(json!({"x": [null]}), "x = 0")
+                .unwrap_err()
+                .to_string(),
+            "arrays must not contain null"
+        );
+    }
+
+    macro_rules! test_apply {
+        ($name:ident, $json_value:expr, $toml_value:expr) => {
+            #[test]
+            fn $name() {
+                assert_eq!(apply(json!({"x": $json_value}), "x = 0").unwrap(), format!("x = {}\n", $toml_value));
+
+                // FUTURE: move these assertions to separate functions once https://github.com/rust-lang/rust/issues/124225 is stable
+                assert_eq!(
+                    apply(json!({"x": $json_value}), "# before\nx = 0").unwrap(),
+                    format!("# before\nx = {}\n", $toml_value)
+                );
+
+                assert_eq!(
+                    apply(json!({"x": $json_value}), "x = 0\n# after").unwrap(),
+                    format!("x = {}\n# after", $toml_value)
+                );
+            }
+        };
+    }
+
+    test_apply!(bool, true, "true");
+    test_apply!(u64, 1, "1");
+    test_apply!(i64, -1, "-1");
+    test_apply!(f64, 1.0, "1.0");
+    test_apply!(str, "foo", "\"foo\"");
+    test_apply!(array, ["foo", "bar"], "[\"foo\", \"bar\"]");
+}