hyperpolymath
diff --git a/‎crates/my-qtt/src/bin/conformance_gen.rs‎
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+// SPDX-License-Identifier: MPL-2.0
+// SPDX-FileCopyrightText: 2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
+//
+// Differential-conformance GENERATOR for the QTT checker coupling.
+//
+// Emits a corpus of random `(ctx, tm)` queries plus the result of the Rust
+// `my_qtt::check` on each, in a canonical text form byte-identical to the one
+// the OCaml ORACLE (extracted from the verified Coq `check`) prints. The
+// harness `conformance/run.sh` then asserts the two streams are equal, i.e.
+//     ∀ (g,t) in corpus.  my_qtt::check g t  ==  (extracted Coq check) g t
+// which is the refinement evidence for "the Rust checker refines Coq `check`".
+//
+// Usage:  conformance_gen <count> <seed> <corpus_out> <rust_results_out>
+//
+// The generator is deterministic in `seed` (a tiny SplitMix64), depends only on
+// std, and deliberately produces a MIX of well-typed and ill-typed terms (out-
+// of-range vars, quantity/type mismatches) so both the `ok` and `none` branches
+// of `check` are exercised on both sides.
+
+use my_qtt::session::{cstep1, Config, Party, Val};
+use my_qtt::{check, step1, Mode, Q, Tm, Ty};
+use std::fs::File;
+use std::io::{BufWriter, Write};
+
+/// normal-form fuel cap — MUST equal the oracle's `eval_nf` cap so a divergent
+/// term yields the same capped term on both sides.
+const NF_FUEL: u32 = 64;
+
+// ---------- deterministic PRNG (SplitMix64) ----------
+struct Rng(u64);
+impl Rng {
+    fn next(&mut self) -> u64 {
+        self.0 = self.0.wrapping_add(0x9E37_79B9_7F4A_7C15);
+        let mut z = self.0;
+        z = (z ^ (z >> 30)).wrapping_mul(0xBF58_476D_1CE4_E5B9);
+        z = (z ^ (z >> 27)).wrapping_mul(0x94D0_49BB_1331_11EB);
+        z ^ (z >> 31)
+    }
+    fn upto(&mut self, n: u64) -> u64 {
+        self.next() % n
+    }
+}
+
+// ---------- random AST ----------
+fn gen_q(r: &mut Rng) -> Q {
+    match r.upto(3) {
+        0 => Q::Zero,
+        1 => Q::One,
+        _ => Q::Omega,
+    }
+}
+fn gen_m(r: &mut Rng) -> Mode {
+    if r.upto(2) == 0 { Mode::Linear } else { Mode::Affine }
+}
+
+fn gen_ty(r: &mut Rng, fuel: u32) -> Ty {
+    if fuel == 0 {
+        return Ty::Unit;
+    }
+    match r.upto(6) {
+        0 => Ty::Unit,
+        1 => Ty::With(Box::new(gen_ty(r, fuel - 1)), Box::new(gen_ty(r, fuel - 1))),
+        2 => Ty::Tensor(Box::new(gen_ty(r, fuel - 1)), Box::new(gen_ty(r, fuel - 1))),
+        3 => Ty::Sum(Box::new(gen_ty(r, fuel - 1)), Box::new(gen_ty(r, fuel - 1))),
+        4 => Ty::Arr(gen_q(r), Box::new(gen_ty(r, fuel - 1)), Box::new(gen_ty(r, fuel - 1))),
+        _ => Ty::Echo(gen_m(r), Box::new(gen_ty(r, fuel - 1)), Box::new(gen_ty(r, fuel - 1))),
+    }
+}
+
+// `scope` = number of binders in scope; Var picks 0..scope+1 so some indices
+// fall out of range (→ `none`), exercising the ill-typed branch.
+fn gen_tm(r: &mut Rng, fuel: u32, scope: usize) -> Tm {
+    if fuel == 0 {
+        return if r.upto(2) == 0 {
+            Tm::Var((r.upto((scope + 1) as u64)) as usize)
+        } else {
+            Tm::UnitT
+        };
+    }
+    match r.upto(15) {
+        0 => Tm::Var((r.upto((scope + 2) as u64)) as usize),
+        1 => Tm::UnitT,
+        2 => Tm::Lam(gen_q(r), gen_ty(r, fuel - 1), Box::new(gen_tm(r, fuel - 1, scope + 1))),
+        3 => Tm::App(Box::new(gen_tm(r, fuel - 1, scope)), Box::new(gen_tm(r, fuel - 1, scope))),
+        4 => Tm::With(Box::new(gen_tm(r, fuel - 1, scope)), Box::new(gen_tm(r, fuel - 1, scope))),
+        5 => Tm::Fst(Box::new(gen_tm(r, fuel - 1, scope))),
+        6 => Tm::Snd(Box::new(gen_tm(r, fuel - 1, scope))),
+        7 => Tm::Tensor(Box::new(gen_tm(r, fuel - 1, scope)), Box::new(gen_tm(r, fuel - 1, scope))),
+        8 => Tm::LetPair(Box::new(gen_tm(r, fuel - 1, scope)), Box::new(gen_tm(r, fuel - 1, scope + 2))),
+        9 => Tm::Inl(gen_ty(r, fuel - 1), Box::new(gen_tm(r, fuel - 1, scope))),
+        10 => Tm::Inr(gen_ty(r, fuel - 1), Box::new(gen_tm(r, fuel - 1, scope))),
+        11 => Tm::Case(
+            Box::new(gen_tm(r, fuel - 1, scope)),
+            Box::new(gen_tm(r, fuel - 1, scope + 1)),
+            Box::new(gen_tm(r, fuel - 1, scope + 1)),
+        ),
+        12 => Tm::Let(gen_q(r), Box::new(gen_tm(r, fuel - 1, scope)), Box::new(gen_tm(r, fuel - 1, scope + 1))),
+        13 => Tm::MkEcho(gen_m(r), gen_ty(r, fuel - 1), gen_ty(r, fuel - 1), Box::new(gen_tm(r, fuel - 1, scope))),
+        _ => Tm::Weaken(Box::new(gen_tm(r, fuel - 1, scope))),
+    }
+}
+
+// ---------- canonical printers (MUST match conformance/oracle.ml) ----------
+fn show_q(q: Q) -> &'static str {
+    match q {
+        Q::Zero => "0",
+        Q::One => "1",
+        Q::Omega => "w",
+    }
+}
+fn show_m(m: Mode) -> &'static str {
+    match m {
+        Mode::Linear => "lin",
+        Mode::Affine => "aff",
+    }
+}
+fn show_ty(t: &Ty) -> String {
+    match t {
+        Ty::Unit => "unit".into(),
+        Ty::With(a, b) => format!("(with {} {})", show_ty(a), show_ty(b)),
+        Ty::Tensor(a, b) => format!("(tensor {} {})", show_ty(a), show_ty(b)),
+        Ty::Sum(a, b) => format!("(sum {} {})", show_ty(a), show_ty(b)),
+        Ty::Arr(q, a, b) => format!("(arr {} {} {})", show_q(*q), show_ty(a), show_ty(b)),
+        Ty::Echo(m, a, b) => format!("(echo {} {} {})", show_m(*m), show_ty(a), show_ty(b)),
+    }
+}
+fn show_uvec(d: &[Q]) -> String {
+    let parts: Vec<&str> = d.iter().map(|&q| show_q(q)).collect();
+    format!("[{}]", parts.join(" "))
+}
+fn show_result(r: &Option<(Ty, Vec<Q>)>) -> String {
+    match r {
+        None => "none".into(),
+        Some((a, d)) => format!("(ok {} {})", show_ty(a), show_uvec(d)),
+    }
+}
+
+// ---------- S-expression query emitters (MUST match the oracle grammar) ----------
+fn sexp_ty(t: &Ty) -> String {
+    show_ty(t) // identical grammar
+}
+fn sexp_tm(t: &Tm) -> String {
+    match t {
+        Tm::Var(n) => format!("(var {})", n),
+        Tm::UnitT => "star".into(),
+        Tm::Lam(q, a, b) => format!("(lam {} {} {})", show_q(*q), sexp_ty(a), sexp_tm(b)),
+        Tm::App(f, x) => format!("(app {} {})", sexp_tm(f), sexp_tm(x)),
+        Tm::With(a, b) => format!("(with {} {})", sexp_tm(a), sexp_tm(b)),
+        Tm::Fst(t) => format!("(fst {})", sexp_tm(t)),
+        Tm::Snd(t) => format!("(snd {})", sexp_tm(t)),
+        Tm::Tensor(a, b) => format!("(tensor {} {})", sexp_tm(a), sexp_tm(b)),
+        Tm::LetPair(a, b) => format!("(letpair {} {})", sexp_tm(a), sexp_tm(b)),
+        Tm::Inl(ty, t) => format!("(inl {} {})", sexp_ty(ty), sexp_tm(t)),
+        Tm::Inr(ty, t) => format!("(inr {} {})", sexp_ty(ty), sexp_tm(t)),
+        Tm::Case(s, l, r) => format!("(case {} {} {})", sexp_tm(s), sexp_tm(l), sexp_tm(r)),
+        Tm::Let(q, a, b) => format!("(let {} {} {})", show_q(*q), sexp_tm(a), sexp_tm(b)),
+        Tm::MkEcho(m, a, b, t) => {
+            format!("(mkecho {} {} {} {})", show_m(*m), sexp_ty(a), sexp_ty(b), sexp_tm(t))
+        }
+        Tm::Weaken(t) => format!("(weaken {})", sexp_tm(t)),
+    }
+}
+fn sexp_ctx(g: &[Ty]) -> String {
+    let mut s = String::from("(ctx");
+    for ty in g {
+        s.push(' ');
+        s.push_str(&sexp_ty(ty));
+    }
+    s.push(')');
+    s
+}
+
+fn show_step(r: &Option<Tm>) -> String {
+    match r {
+        None => "none".into(),
+        Some(t) => format!("(some {})", sexp_tm(t)),
+    }
+}
+
+fn eval_nf(t: &Tm, fuel: u32) -> Tm {
+    let mut cur = t.clone();
+    let mut f = fuel;
+    while f > 0 {
+        match step1(&cur) {
+            None => break,
+            Some(n) => {
+                cur = n;
+                f -= 1;
+            }
+        }
+    }
+    cur
+}
+
+// ---------- closed, redex-rich generators (deep substitution coverage) ----------
+fn gen_value(r: &mut Rng, fuel: u32) -> Tm {
+    if fuel == 0 {
+        return Tm::UnitT;
+    }
+    match r.upto(6) {
+        0 => Tm::UnitT,
+        1 => Tm::Lam(gen_q(r), gen_ty(r, 1), Box::new(gen_tm(r, fuel - 1, 1))),
+        2 => Tm::With(Box::new(gen_value(r, fuel - 1)), Box::new(gen_value(r, fuel - 1))),
+        3 => Tm::Tensor(Box::new(gen_value(r, fuel - 1)), Box::new(gen_value(r, fuel - 1))),
+        4 => Tm::Inl(gen_ty(r, 1), Box::new(gen_value(r, fuel - 1))),
+        _ => Tm::MkEcho(gen_m(r), gen_ty(r, 1), gen_ty(r, 1), Box::new(gen_value(r, fuel - 1))),
+    }
+}
+
+// closed (scope 0) terms built around redexes so reduction proceeds and
+// exercises subst0/subst2/shift in chains.
+fn gen_closed(r: &mut Rng, fuel: u32) -> Tm {
+    if fuel == 0 {
+        return gen_value(r, 1);
+    }
+    match r.upto(8) {
+        0 => Tm::App(
+            Box::new(Tm::Lam(gen_q(r), gen_ty(r, 1), Box::new(gen_tm(r, fuel - 1, 1)))),
+            Box::new(gen_value(r, fuel - 1)),
+        ),
+        1 => Tm::Let(gen_q(r), Box::new(gen_value(r, fuel - 1)), Box::new(gen_tm(r, fuel - 1, 1))),
+        2 => Tm::Fst(Box::new(Tm::With(Box::new(gen_value(r, fuel - 1)), Box::new(gen_value(r, fuel - 1))))),
+        3 => Tm::Snd(Box::new(Tm::With(Box::new(gen_value(r, fuel - 1)), Box::new(gen_value(r, fuel - 1))))),
+        4 => Tm::LetPair(
+            Box::new(Tm::Tensor(Box::new(gen_value(r, fuel - 1)), Box::new(gen_value(r, fuel - 1)))),
+            Box::new(gen_tm(r, fuel - 1, 2)),
+        ),
+        5 => Tm::Case(
+            Box::new(Tm::Inl(gen_ty(r, 1), Box::new(gen_value(r, fuel - 1)))),
+            Box::new(gen_tm(r, fuel - 1, 1)),
+            Box::new(gen_tm(r, fuel - 1, 1)),
+        ),
+        6 => Tm::Weaken(Box::new(Tm::MkEcho(
+            Mode::Linear,
+            gen_ty(r, 1),
+            gen_ty(r, 1),
+            Box::new(gen_value(r, fuel - 1)),
+        ))),
+        _ => gen_value(r, fuel - 1),
+    }
+}
+
+// ---------- session configs (coupling #4) ----------
+fn sval(v: &Val) -> String {
+    match v {
+        Val::Var(n) => format!("(vvar {})", n),
+        Val::Unit => "vunit".into(),
+        Val::Bool(true) => "(vbool t)".into(),
+        Val::Bool(false) => "(vbool f)".into(),
+        Val::Nat(n) => format!("(vnat {})", n),
+    }
+}
+fn sparty(p: &Party) -> String {
+    match p {
+        Party::End => "qend".into(),
+        Party::Send(v, q) => format!("(qsend {} {})", sval(v), sparty(q)),
+        Party::Recv(q) => format!("(qrecv {})", sparty(q)),
+        Party::Sel(l, q) => format!("(qsel {} {})", l, sparty(q)),
+        Party::Bra(bs) => {
+            let mut s = String::from("(qbra");
+            for (l, q) in bs {
+                s.push_str(&format!(" (br {} {})", l, sparty(q)));
+            }
+            s.push(')');
+            s
+        }
+    }
+}
+fn sconfig(c: &Config) -> String {
+    format!("(conf {} {})", sparty(&c.0), sparty(&c.1))
+}
+fn show_cstep(r: &Option<Config>) -> String {
+    match r {
+        None => "none".into(),
+        Some(c) => format!("(some {})", sconfig(c)),
+    }
+}
+
+fn gen_val(r: &mut Rng) -> Val {
+    match r.upto(4) {
+        0 => Val::Var(r.upto(3) as usize),
+        1 => Val::Unit,
+        2 => Val::Bool(r.upto(2) == 0),
+        _ => Val::Nat(r.upto(5)),
+    }
+}
+fn gen_party(r: &mut Rng, fuel: u32) -> Party {
+    if fuel == 0 {
+        return Party::End;
+    }
+    match r.upto(6) {
+        0 | 5 => Party::End,
+        1 => Party::Send(gen_val(r), Box::new(gen_party(r, fuel - 1))),
+        2 => Party::Recv(Box::new(gen_party(r, fuel - 1))),
+        3 => Party::Sel(r.upto(3) as usize, Box::new(gen_party(r, fuel - 1))),
+        _ => {
+            let n = r.upto(3) as usize;
+            Party::Bra((0..n).map(|i| (i, gen_party(r, fuel - 1))).collect())
+        }
+    }
+}
+// bias toward dual pairs so the comm/select steps fire (alongside stuck configs)
+fn gen_config(r: &mut Rng) -> Config {
+    let f = 2 + r.upto(2) as u32;
+    match r.upto(6) {
+        0 => Config(Box::new(Party::Send(gen_val(r), Box::new(gen_party(r, f)))), Box::new(Party::Recv(Box::new(gen_party(r, f))))),
+        1 => Config(Box::new(Party::Recv(Box::new(gen_party(r, f)))), Box::new(Party::Send(gen_val(r), Box::new(gen_party(r, f))))),
+        2 => Config(
+            Box::new(Party::Sel(r.upto(4) as usize, Box::new(gen_party(r, f)))),
+            Box::new(gen_party(r, f + 1)),
+        ),
+        3 => Config(
+            Box::new(gen_party(r, f + 1)),
+            Box::new(Party::Sel(r.upto(4) as usize, Box::new(gen_party(r, f)))),
+        ),
+        _ => Config(Box::new(gen_party(r, f)), Box::new(gen_party(r, f))),
+    }
+}
+
+fn main() {
+    let args: Vec<String> = std::env::args().collect();
+    if args.len() != 5 {
+        eprintln!("usage: conformance_gen <count> <seed> <corpus_out> <rust_results_out>");
+        std::process::exit(2);
+    }
+    let count: u64 = args[1].parse().expect("count");
+    let seed: u64 = args[2].parse().expect("seed");
+    let mut corpus = BufWriter::new(File::create(&args[3]).expect("corpus_out"));
+    let mut results = BufWriter::new(File::create(&args[4]).expect("rust_results_out"));
+
+    let mut r = Rng(seed.wrapping_add(0xD1B5_4A32_D192_ED03));
+    for _ in 0..count {
+        // (a) checker (#1) + one-step (#2) conformance on a random term that
+        //     may be open and may be ill-typed (exercises both decision branches)
+        let k = r.upto(4) as usize;
+        let g: Vec<Ty> = (0..k).map(|_| gen_ty(&mut r, 2)).collect();
+        let fuel = 2 + (r.upto(3) as u32); // depth 2..=4
+        let t = gen_tm(&mut r, fuel, k);
+        writeln!(corpus, "(q {} {})", sexp_ctx(&g), sexp_tm(&t)).unwrap();
+        writeln!(results, "{}", show_result(&check(&g, &t))).unwrap();
+        writeln!(corpus, "(s {})", sexp_tm(&t)).unwrap();
+        writeln!(results, "{}", show_step(&step1(&t))).unwrap();
+
+        // (b) normal-form (#2, deep) conformance on a CLOSED redex-rich term,
+        //     cross-checking subst0/subst2/shift against Coq's extracted subst
+        let ct = gen_closed(&mut r, 3);
+        writeln!(corpus, "(nf {})", sexp_tm(&ct)).unwrap();
+        writeln!(results, "{}", sexp_tm(&eval_nf(&ct, NF_FUEL))).unwrap();
+
+        // (c) session-config step (#4): Rust my_qtt::session::cstep1 vs the
+        //     extracted Coq cstep1 (proved sound+complete vs the cstep relation)
+        let cfg = gen_config(&mut r);
+        writeln!(corpus, "(cstep {})", sconfig(&cfg)).unwrap();
+        writeln!(results, "{}", show_cstep(&cstep1(&cfg))).unwrap();
+    }
+}