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Fix partial-prefix fallback in stream delimiter matching #2

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Fix partial-prefix fallback in stream delimiter matching #2

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32 changes: 23 additions & 9 deletions nxs/dsl/stream.py
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Original file line number Diff line number Diff line change
Expand Up @@ -180,7 +180,7 @@ def read_until(self, delim, max: int = _ASTCompiler.VM_SAMPLE_BUF_SIZE,
em.emit_jmp(Op.JMP, L_TOP)
else:
# Multi-byte delim: `pg` counts delim bytes matched at the tail;
# a mismatch resets it, checking a restart against delim[0].
# a mismatch retains the longest matching prefix suffix.
pg_r = self._regs.get("__match_pg")
em.emit_u32(Op.LOAD_IMM, pg_r, 0)

Expand Down Expand Up @@ -221,14 +221,28 @@ def read_until(self, delim, max: int = _ASTCompiler.VM_SAMPLE_BUF_SIZE,
# pg was 0; stay at 0.
em.emit_jmp(Op.JMP, L_AFTER)
else:
# pg was > 0; reset, but the current byte itself
# might start a fresh match against delim[0].
em.emit_cmp(Op.CMP_EQ, byte_r, delim_bytes[0], match_r)
L_reset_zero = self._fresh_label(f"ru_reset_{i}")
em.emit_jmp(Op.JZ, L_reset_zero, match_r)
em.emit_u32(Op.LOAD_IMM, pg_r, 1)
em.emit_jmp(Op.JMP, L_AFTER)
em.label(L_reset_zero)
# Keep the longest prefix ending at this byte.
# The full delimiter can have no border while an
# interior prefix does: aab must match in aaab.
prefix = delim_bytes[:i]
# The normal-match branch above already handles this
# byte, so it cannot reach the fallback path.
seen_bytes = {delim_bytes[i]}
for keep in range(i, 0, -1):
next_byte = delim_bytes[keep - 1]
if next_byte in seen_bytes or not prefix.endswith(
delim_bytes[:keep - 1]):
continue
# Descending keep: a byte's first fallback wins.
# Smaller keeps for that byte are unreachable;
# emitting them bloats long repeated prefixes.
seen_bytes.add(next_byte)
L_next = self._fresh_label(f"ru_fallback_{i}_{keep}")
em.emit_cmp(Op.CMP_EQ, byte_r, next_byte, match_r)
em.emit_jmp(Op.JZ, L_next, match_r)
em.emit_u32(Op.LOAD_IMM, pg_r, keep)
em.emit_jmp(Op.JMP, L_AFTER)
em.label(L_next)
em.emit_u32(Op.LOAD_IMM, pg_r, 0)
em.emit_jmp(Op.JMP, L_AFTER)
if not is_last:
Expand Down