fix(tools): h5rs diff names its options as h5diff does

-c meant "list at most N differences" in h5rs, but in h5diff -c is
--compare (a flag) and the count is -n/--count=N, so a script moved over
from h5diff behaved differently: `h5diff -r -c 2 A B` exits 2 (the 2 is
taken as a file name) while h5rs exited 1.

The count is now -n/--count, -c/--compare is accepted (h5rs always lists
objects that are not comparable), and the --count=N, --delta=D,
--relative=R forms are accepted; exit codes equal h5diff's on 7 cases.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-tools/README.md

@@ -141,8 +141,13 @@ $ h5rs diff -r a.h5 b.h5 /d          # list every differing element
 $ h5rs diff -d 0.001 a.h5 b.h5       # |a - b| > 0.001 is a difference
 $ h5rs diff -p 0.01 a.h5 b.h5        # |a - b| / |a| > 1% is a difference
 $ h5rs diff --follow-symlinks a.h5 b.h5 /lnk   # what the soft link /lnk leads to
+$ h5rs diff -r -n 10 a.h5 b.h5       # list at most 10 differing elements per object
 ```
 
+The options are named as h5diff's: `-n N`/`--count=N` limits the listed
+elements, `-c`/`--compare` (list objects that are not comparable) is
+accepted and always in effect, and `--delta=D`, `--relative=R` work too.
+
 Integers are compared in integer arithmetic, with or without a tolerance, so
 64-bit values beyond 2^53 lose no precision (`-d 0` tells 2^60 from
 2^60 + 1). A relative tolerance below the f64 epsilon (2.2e-16) compares

crates/clawhdf5-tools/src/cli.rs

@@ -11,30 +11,37 @@ pub struct Out<'a> {
 /// The arguments after the subcommand name.
 pub struct Args {
     cmd: &'static str,
-    rest: std::vec::IntoIter<String>,
+    rest: std::collections::VecDeque<String>,
 }
 
 impl Args {
     pub fn new(cmd: &'static str, rest: Vec<String>) -> Self {
         Self {
             cmd,
-            rest: rest.into_iter(),
+            rest: rest.into(),
         }
     }
 
     #[allow(clippy::should_implement_trait)]
     pub fn next(&mut self) -> Option<String> {
-        self.rest.next()
+        self.rest.pop_front()
+    }
+
+    /// Put an option and its value back, to be read next (for the
+    /// `--name=value` form).
+    pub fn push_front(&mut self, name: String, value: String) {
+        self.rest.push_front(value);
+        self.rest.push_front(name);
     }
 
     /// The value after an option such as `--max-bytes`.
     pub fn value(&mut self) -> Option<String> {
-        self.rest.next()
+        self.rest.pop_front()
     }
 
     /// A numeric option value.
     pub fn number<T: std::str::FromStr>(&mut self) -> Option<T> {
-        self.rest.next().and_then(|s| s.parse().ok())
+        self.rest.pop_front().and_then(|s| s.parse().ok())
     }
 
     /// Report a usage problem; exit status 2.

crates/clawhdf5-tools/src/diff.rs

@@ -29,7 +29,9 @@ soft link is compared as a link, as h5diff does, unless --follow-symlinks).
   -d, --delta D      numbers differ only when |a - b| > D
   -p, --relative R   numbers differ only when |a - b| / |a| > R (an R below
                      the f64 epsilon, 2.2e-16, compares exactly, as h5diff)
-  -c, --count N      list at most N differing elements per object with -r
+  -n, --count N      list at most N differing elements per object with -r
+  -c, --compare      list objects that are not comparable (always done;
+                     accepted for h5diff compatibility)
   --max-bytes N      largest dataset read (default 1 GiB); a larger one is an error
 
 Two NaNs compare equal. Unlike h5diff, objects that cannot be compared
@@ -139,7 +141,9 @@ pub fn run(args: &mut Args, out: &mut Out) -> std::io::Result<i32> {
                 }
                 _ => return args.usage_error(out, "--relative needs a number >= 0", USAGE),
             },
-            "-c" | "--count" => match args.number() {
+            // h5diff's -c: h5rs always lists them.
+            "-c" | "--compare" => {}
+            "-n" | "--count" => match args.number() {
                 Some(n) => opts.count = n,
                 None => return args.usage_error(out, "--count needs a number", USAGE),
             },
@@ -151,6 +155,14 @@ pub fn run(args: &mut Args, out: &mut Out) -> std::io::Result<i32> {
                 writeln!(out.o, "{USAGE}")?;
                 return Ok(0);
             }
+            // h5diff's --count=N, --delta=D, --relative=R forms.
+            s if s.starts_with("--") && s.contains('=') => {
+                let (k, v) = s.split_once('=').unwrap_or((s, ""));
+                if !matches!(k, "--count" | "--delta" | "--relative" | "--max-bytes") {
+                    return args.usage_error(out, &format!("unknown option {a}"), USAGE);
+                }
+                args.push_front(k.to_string(), v.to_string());
+            }
             s if s.starts_with('-') && s.len() > 1 => {
                 return args.usage_error(out, &format!("unknown option {a}"), USAGE);
             }

crates/clawhdf5-tools/tests/h5rs_interop.rs

@@ -380,6 +380,42 @@ fn diff_exit_codes_match_h5diff() {
     assert!(s.contains("2 difference(s) found"), "{s}");
 }
 
+/// The option names are h5diff's: -c is --compare (a flag), the count is
+/// -n/--count, and the --name=value forms are accepted.
+#[test]
+fn diff_options_are_named_like_h5diff() {
+    let Some(f) = generate() else { return };
+    let (b, changed) = (f.p("base.h5"), f.p("changed.h5"));
+    let cases: Vec<(Vec<&str>, i32)> = vec![
+        // "2" is then the first file name, which does not exist.
+        (vec!["-r", "-c", "2", &b, &changed], 2),
+        (vec!["-c", &b, &changed], 1),
+        (vec!["--compare", &b, &b], 0),
+        (vec!["-r", "-n", "1", &b, &changed], 1),
+        (vec!["-r", "--count=1", &b, &changed], 1),
+        (vec!["--delta=0.01", &b, &changed], 0),
+        (vec!["--relative=1e-9", &b, &changed], 1),
+    ];
+    let have_h5diff = tool_available("h5diff");
+    for (args, want) in cases {
+        if have_h5diff {
+            assert_eq!(code(&run("h5diff", &args)), want, "h5diff {args:?}");
+        }
+        let o = h5rs(&[&["diff"], args.as_slice()].concat());
+        assert_eq!(code(&o), want, "h5rs diff {args:?}: {}", stdout(&o));
+    }
+    for count in [vec!["-n", "1"], vec!["--count=1"]] {
+        let o = h5rs(&[&["diff", "-r"], count.as_slice(), &[&b, &changed, "/d"]].concat());
+        let s = stdout(&o);
+        assert!(
+            s.contains("[ 0 2 ]") && !s.contains("[ 2 3 ]"),
+            "{count:?}: {s}"
+        );
+        assert!(s.contains("2 difference(s) found"), "{s}");
+    }
+    assert_eq!(code(&h5rs(&["diff", "--bogus=1", &b, &b])), 2);
+}
+
 /// Tolerances on 64-bit integers one apart, far beyond f64's integer
 /// precision: compared exactly, as h5diff does.
 #[test]