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Implement CR Shift 3 properly. #660

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Merged
ratmice merged 1 commit into from
Aug 17, 2026
Merged

Implement CR Shift 3 properly. #660

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47 changes: 24 additions & 23 deletions lrpar/src/lib/cpctplus.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -336,34 +336,13 @@ where
}

fn shift(&self, n: &PathFNode<StorageT>, nbrs: &mut Vec<(u16, PathFNode<StorageT>)>) {
// Forward move rule (ER3)
//
// Note the rule in Corchuelo et al. is confusing and, I think, wrong. It reads:
// (S, I) \rightarrow_{LR*} (S', I')
// \wedge (j = N \vee 0 < j < N \wedge f(q_r, t_{j + 1} \in {accept, error})
// First I think the bracketing would be clearer if written as:
// j = N \vee (0 < j < N \wedge f(q_r, t_{j + 1} \in {accept, error})
// And I think the condition should be:
// j = N \vee (0 <= j < N \wedge f(q_r, t_{j + 1} \in {accept, error})
// because there's no reason that any symbols need to be shifted in order for an accept
// (or, indeed an error) state to be reached.
//
// So the full rule should, I think, be:
// (S, I) \rightarrow_{LR*} (S', I')
// \wedge (j = N \vee (0 <= j < N \wedge f(q_r, t_{j + 1} \in {accept, error}))
//
// That said, as KimYi somewhat obliquely mention, generating multiple shifts in one go is
// a bad idea: it means that we miss out on some minimal cost repairs. Instead, we should
// only generate one shift at a time. So the adjusted rule we implement is:
//
// (S, I) \rightarrow_{LR*} (S', I')
// \wedge 0 <= j < 1 \wedge S != S'
// This is CR Shift 3 from "Don't Panic!".

let laidx = n.laidx;
let (new_laidx, n_pstack) =
self.parser
.lr_cactus(None, laidx, laidx + 1, n.pstack.clone(), &mut None);
if n.pstack != n_pstack {
if new_laidx > laidx || n.pstack != n_pstack {
let n_repairs = if new_laidx > laidx {
n.repairs.child(RepairMerge::Repair(Repair::Shift))
} else {
Expand Down Expand Up @@ -784,6 +763,28 @@ E : 'N'
check_all_repairs(&grm, &lex, &errs[1], &[r#"Insert ")""#]);
}

#[test]
fn stack_need_not_advance() {
let lexs = "
a 'A'
b 'B'
x 'X'
";
let grms = r#"
%start S
%expect-unused Unmatched "X"
%%
S: As "B" ;
As: As "A" | "A" ;
Unmatched: "X" ;
"#;

let us = "xaaab";
let (grm, lex, pr) = do_parse(RecoveryKind::CPCTPlus, lexs, grms, us);
let (_, errs) = pr.unwrap_err();
check_all_repairs(&grm, &lex, &errs[0], &[r#"Delete "x""#]);
}

#[test]
fn test_merge() {
let lexs = "a 'a'
Expand Down