ppsfp finished

kyupy

@@ -1 +1 @@
-Subproject commit f5af7ec3d9554c35f72ed42be6719f49438201c2
+Subproject commit 7a69ce901680164ad392a303f11b9f8067df5728

main.py

@@ -12,13 +12,17 @@ from kyupy.techlib import techlib_by_name, KYUPY
 from fsim.static import LineRoles, FaultSet
 from fsim.simple import SAFSimSimple
 from fsim.incremental import SAFSimIncremental
+from fsim.ppsfp import SAFSimPPSFP
 
 
 def main():
 
+    algorithms = {'simple': SAFSimSimple, 'incr': SAFSimIncremental, 'ppsfp': SAFSimPPSFP}
+
     parser = argparse.ArgumentParser(description='A basic stuck-at fault simulator.')
     parser.add_argument('-t', '--tlib', default='SKY130', help=f'Techlib for verilog circuit. Default: SKY130, available: {sorted(techlib_by_name.keys())}.')
     parser.add_argument('-p', '--patterns', default='1024', help='Pattern file or number of random patterns to simulate. Default: 1024.')
+    parser.add_argument('-a', '--algorithm', default='simple', help=f'Fault simulation algorithm. Default: simple, available: {sorted(algorithms.keys())}.')
     parser.add_argument('-o', '--output', default=None, help='')
     parser.add_argument('--seed', type=int, default=42, help='Random seed for reproducibility. Default: 42.')
     parser.add_argument('circuit', help='Gate-level verilog, bench, or nix package to import. See available packages: "nix flake show github:s-holst/benchmark-circuits".')
@@ -76,8 +80,8 @@ def main():
     saf_collapsed = np.array(list(fs.saf_equiv_classes.keys()), dtype=np.uint32)
     rng.shuffle(saf_collapsed)
 
-    safsim = SAFSimSimple(c_resolved, min(patterns.shape[1], 10240))
+    safsim = algorithms[args.algorithm](c_resolved, min(patterns.shape[1], 10240))
-    log.info(f'{safsim.sim=}')
+    log.info(f'{args.algorithm} {safsim.sim=}')
 
     fclasses = safsim.classify_faults(saf_collapsed, patterns)
 

src/fsim/ppsfp.py

@@ -0,0 +1,146 @@
+import time
+from collections.abc import Collection
+
+import numpy as np
+
+from kyupy import log, batchrange, cdiv, Timers, logic
+from kyupy.circuit import Circuit, Node
+from kyupy.logic_sim import LogicSim2V
+from kyupy.techlib import KYUPY
+
+class SAFSimPPSFP:
+    """A stuck-at fault simulator that uses explicit simulations only at roots of fan-out free regions.
+    """
+
+    def __init__(self, circuit_resolved: Circuit, batch_size: int):
+        self.sim = LogicSim2V(circuit_resolved, sims=batch_size)
+        self.timers = Timers()
+
+        self.ffr_stem2idx = {}
+        self.lines2ffr_stem: list[Node|None] = [None] * len(circuit_resolved.lines)
+        for stem, nodes in circuit_resolved.fanout_free_regions(KYUPY):
+            self.ffr_stem2idx[stem] = len(self.ffr_stem2idx)
+            for n in nodes + [stem]:
+                for il in n.ins.without_nones():
+                    self.lines2ffr_stem[il.index] = stem
+
+    def _gate_sensitivity(self, node: Node, pin: int) -> np.ndarray:
+        """Patterns (as a packed bit-vector) for which `node`'s output is
+        sensitive to a change on input `pin` (the Boolean difference d_o/d_pin),
+        evaluated bit-parallel from the current signal values in ``self.sim.c``.
+
+        For simple gates an input is sensitised iff all the other inputs hold
+        their non-controlling value: 1 for AND/NAND, 0 for OR/NOR.  XOR/XNOR and
+        single-input gates (INV/BUF) are always sensitive.  For the AND-OR/OR-AND
+        complex cells the condition follows from their structure; output
+        inversion (the AOI/OAI variants) does not change sensitivity.
+        """
+        kind = node.kind
+        v = [self.sim.c[self.sim.c_locs[il.index]][0] for il in node.ins]
+
+        if kind.startswith('AND') or kind.startswith('NAND'):
+            sens = np.full(self.sim.c.shape[2], 0xff, dtype=np.uint8)
+            for j, vj in enumerate(v):
+                if j != pin: sens &= vj
+            return sens
+        if kind.startswith('OR') or kind.startswith('NOR'):
+            sens = np.full(self.sim.c.shape[2], 0xff, dtype=np.uint8)
+            for j, vj in enumerate(v):
+                if j != pin: sens &= ~vj
+            return sens
+        if (kind.startswith('XOR') or kind.startswith('XNOR')
+                or kind.startswith('INV') or kind.startswith('BUF')):
+            return np.full(self.sim.c.shape[2], 0xff, dtype=np.uint8)
+
+        shape = ''.join(ch for ch in kind if ch.isdigit())
+        if kind.startswith('AO'):  # AOxx / AOIxx, sum-of-products
+            if shape == '21':      # (i0&i1) | i2
+                return [v[1] & ~v[2], v[0] & ~v[2], ~(v[0] & v[1])][pin]
+            if shape == '22':      # (i0&i1) | (i2&i3)
+                return [v[1] & ~(v[2] & v[3]), v[0] & ~(v[2] & v[3]),
+                        v[3] & ~(v[0] & v[1]), v[2] & ~(v[0] & v[1])][pin]
+            if shape == '211':     # (i0&i1) | i2 | i3
+                return [v[1] & ~v[2] & ~v[3], v[0] & ~v[2] & ~v[3],
+                        ~(v[0] & v[1]) & ~v[3], ~(v[0] & v[1]) & ~v[2]][pin]
+        if kind.startswith('OA'):  # OAxx / OAIxx, product-of-sums
+            if shape == '21':      # (i0|i1) & i2
+                return [~v[1] & v[2], ~v[0] & v[2], v[0] | v[1]][pin]
+            if shape == '22':      # (i0|i1) & (i2|i3)
+                return [~v[1] & (v[2] | v[3]), ~v[0] & (v[2] | v[3]),
+                        ~v[3] & (v[0] | v[1]), ~v[2] & (v[0] | v[1])][pin]
+            if shape == '211':     # (i0|i1) & i2 & i3
+                return [~v[1] & v[2] & v[3], ~v[0] & v[2] & v[3],
+                        (v[0] | v[1]) & v[3], (v[0] | v[1]) & v[2]][pin]
+        if kind.startswith('MUX'):  # MUX21(i0,i1,i2) = i2 ? i1 : i0
+            return [~v[2], v[2], v[0] ^ v[1]][pin]
+
+        raise NotImplementedError(f'gate sensitivity for cell kind {kind!r}')
+
+    def classify_faults(self, faults: Collection[int], patterns: np.ndarray):
+
+        with self.timers['startup']:
+            self.sim.c_prop()  # trigger jit
+            c_golden = np.zeros_like(self.sim.c)
+            fclass_DS = set()
+            nbatches = cdiv(patterns.shape[1], self.sim.sims)
+            nfaults = len(faults)
+            ffr_obs = np.zeros((len(self.ffr_stem2idx), self.sim.c.shape[2]), dtype=np.uint8)
+            ffr_obs_valid = np.zeros(len(self.ffr_stem2idx), dtype=np.uint8)
+            obs_mask = np.packbits(np.full(patterns.shape[1], 1, dtype=np.uint8), bitorder='little')
+            fault_to_stem_obs = obs_mask.copy()
+            fault_act = obs_mask.copy()
+
+        with self.timers['sim'], log.progress() as p:
+            for bidx, (bo, bs) in enumerate(batchrange(patterns.shape[1], self.sim.sims)):
+                self.sim.s_assign[:, :bs] = patterns[:, bo:bo+bs]
+                self.sim.s_to_c()
+                self.sim.c_dirty[...] = 1
+                with self.timers['sim_full_prop']:
+                    self.sim.c_prop()
+                c_golden[...] = self.sim.c
+                c_golden_poppo = c_golden[self.sim.poppo_c_locs]
+                ffr_obs_valid[...] = 0
+
+                for fidx, fault in enumerate(faults):
+                    fault_site = fault//2
+                    fault_polarity = fault&1
+                    ffr_stem = self.lines2ffr_stem[fault_site]
+                    assert ffr_stem is not None
+                    ffr_idx = self.ffr_stem2idx[ffr_stem]
+
+                    # compute FFR observability vector ffr_obs[ffr_idx] if necessary
+                    if not ffr_obs_valid[ffr_idx]:
+                        if len(ffr_stem.outs) > 1:
+                            self.sim.c_dirty[...] = 0
+                            with self.timers['sim_ffr_prop']:
+                                self.sim.c_prop(fault_line=ffr_stem.ins[0].index, fault_model=2)
+                            with self.timers['sim_ffr_out_reduce']:
+                                ffr_obs[ffr_idx] = np.bitwise_or.reduce(c_golden_poppo ^ self.sim.c[self.sim.poppo_c_locs], axis=0) & obs_mask
+                            with self.timers['sim_ffr_reset']:
+                                self.sim.c[...] = c_golden # clear fault
+                        else:  # primary output, completely observable
+                            ffr_obs[ffr_idx] = obs_mask
+                        ffr_obs_valid[ffr_idx] = 1
+
+                    # observability of the fault site at the FFR stem by tracing
+                    # the (unique, fan-out free) path from the site to the stem
+                    # and requiring every gate along it to be sensitised.
+                    with self.timers['sim_sens']:
+                        fault_to_stem_obs[...] = obs_mask
+                        line = self.sim.circuit.lines[fault_site]
+                        while line.reader is not ffr_stem:
+                            n = line.reader
+                            if n.kind != '__fork__':  # forks pass the value through
+                                fault_to_stem_obs &= self._gate_sensitivity(n, line.reader_pin)
+                            line = n.outs[0]
+
+                    # compute fault activation
+                    fault_act[...] = self.sim.c[self.sim.c_locs[fault_site]] & obs_mask
+                    if fault_polarity:  # stuck-at 1?
+                        fault_act ^= obs_mask  # activated iff signal is 0
+
+                    if np.bitwise_or.reduce(ffr_obs[ffr_idx] & fault_to_stem_obs & fault_act) != 0:
+                        fclass_DS.add(fault)
+                    p.update(((fidx+1) / nfaults) * ((bidx+1) / nbatches))
+
+        return {'DS': fclass_DS, 'NO': set(faults) - fclass_DS}

tests/all_kyupy_simprims.v

@@ -0,0 +1,53 @@
+module all_kyupy_primitives (i0, i1, i2, i3, o);
+    input i0;
+    input i1;
+    input i2;
+    input i3;
+    output [32:0] o;
+
+    BUF1 buf1_0 (.i0(i0), .o(o[0]));
+    INV1 inv1_0 (.i0(i1), .o(o[1]));
+
+    AND2 and2_0 (.i0(i0), .i1(i1), .o(o[2]));
+    AND3 and3_0 (.i0(i0), .i1(i1), .i2(i2), .o(o[3]));
+    AND4 and4_0 (.i0(i0), .i1(i1), .i2(i2), .i3(i3), .o(o[4]));
+
+    NAND2 nand2_0 (.i0(i0), .i1(i1), .o(o[5]));
+    NAND3 nand3_0 (.i0(i0), .i1(i1), .i2(i2), .o(o[6]));
+    NAND4 nand4_0 (.i0(i0), .i1(i1), .i2(i2), .i3(i3), .o(o[7]));
+
+    OR2 or2_0 (.i0(i0), .i1(i1), .o(o[8]));
+    OR3 or3_0 (.i0(i0), .i1(i1), .i2(i2), .o(o[9]));
+    OR4 or4_0 (.i0(i0), .i1(i1), .i2(i2), .i3(i3), .o(o[10]));
+
+    NOR2 nor2_0 (.i0(i0), .i1(i1), .o(o[11]));
+    NOR3 nor3_0 (.i0(i0), .i1(i1), .i2(i2), .o(o[12]));
+    NOR4 nor4_0 (.i0(i0), .i1(i1), .i2(i2), .i3(i3), .o(o[13]));
+
+    XOR2 xor2_0 (.i0(i0), .i1(i1), .o(o[14]));
+    XOR3 xor3_0 (.i0(i0), .i1(i1), .i2(i2), .o(o[15]));
+    XOR4 xor4_0 (.i0(i0), .i1(i1), .i2(i2), .i3(i3), .o(o[16]));
+
+    XNOR2 xnor2_0 (.i0(i0), .i1(i1), .o(o[17]));
+    XNOR3 xnor3_0 (.i0(i0), .i1(i1), .i2(i2), .o(o[18]));
+    XNOR4 xnor4_0 (.i0(i0), .i1(i1), .i2(i2), .i3(i3), .o(o[19]));
+
+    AO21 ao21_0 (.i0(i0), .i1(i1), .i2(i2), .o(o[20]));
+    AO22 ao22_0 (.i0(i0), .i1(i1), .i2(i2), .i3(i3), .o(o[21]));
+    OA21 oa21_0 (.i0(i0), .i1(i1), .i2(i2), .o(o[22]));
+    OA22 oa22_0 (.i0(i0), .i1(i1), .i2(i2), .i3(i3), .o(o[23]));
+
+    AOI21 aoi21_0 (.i0(i0), .i1(i1), .i2(i2), .o(o[24]));
+    AOI22 aoi22_0 (.i0(i0), .i1(i1), .i2(i2), .i3(i3), .o(o[25]));
+    OAI21 oai21_0 (.i0(i0), .i1(i1), .i2(i2), .o(o[26]));
+    OAI22 oai22_0 (.i0(i0), .i1(i1), .i2(i2), .i3(i3), .o(o[27]));
+
+    AO211 ao211_0 (.i0(i0), .i1(i1), .i2(i2), .i3(i3), .o(o[28]));
+    OA211 oa211_0 (.i0(i0), .i1(i1), .i2(i2), .i3(i3), .o(o[29]));
+
+    AOI211 aoi211_0 (.i0(i0), .i1(i1), .i2(i2), .i3(i3), .o(o[30]));
+    OAI211 oai211_0 (.i0(i0), .i1(i1), .i2(i2), .i3(i3), .o(o[31]));
+
+    MUX21 mux21_0 (.i0(i0), .i1(i1), .i2(i2), .o(o[32]));
+
+endmodule

tests/test_safsim.py

@@ -0,0 +1,219 @@
+"""Unit tests for the stuck-at fault simulators (the SAF* classes).
+
+Each fault is identified by ``line.index * 2 + polarity`` where polarity 0 is
+stuck-at-0 and polarity 1 is stuck-at-1.  ``classify_faults`` returns a dict
+with the detected-by-simulation set under ``'DS'`` and the not-observed set
+under ``'NO'``.
+
+The three simulators are exercised side-by-side and must agree exactly:
+  * ``SAFSimSimple``      - brute force, exact.
+  * ``SAFSimIncremental`` - incremental cone re-simulation, exact.
+  * ``SAFSimPPSFP``       - FFR-based: explicit simulation only at FFR stems,
+    with fault-to-stem observability from bit-parallel path sensitisation.
+"""
+
+import itertools
+
+import numpy as np
+import pytest
+
+from kyupy import bench, verilog, logic
+from kyupy.techlib import KYUPY
+from fsim.static import FaultSet
+from fsim.simple import SAFSimSimple
+from fsim.incremental import SAFSimIncremental
+from fsim.ppsfp import SAFSimPPSFP
+
+
+# All three simulators are exact and must classify every fault identically.
+ALL_ALGS = [SAFSimSimple, SAFSimIncremental, SAFSimPPSFP]
+
+
+def make_patterns(*specs):
+    """Build a 2D pattern array from per-pattern strings via ``logic.mvarray``.
+
+    Each string holds one character per s-node, ordered as the inputs and
+    outputs are defined in the netlist (output positions are don't-cares here,
+    overwritten by simulation).  The result has shape ``(n_s_nodes, n_patterns)``
+    as expected by the simulators; a lone pattern is reshaped to a single column.
+    """
+    pat = logic.mvarray(*specs)
+    return pat if pat.ndim == 2 else pat.reshape(-1, 1)
+
+
+def classify(alg, circuit, faults, patterns):
+    # PPSFP requires the batch size to match the pattern count (as main.py does).
+    sim = alg(circuit, patterns.shape[1])
+    return sim.classify_faults(list(faults), patterns)
+
+
+def saf(line, polarity):
+    return line.index * 2 + polarity
+
+
+# --------------------------------------------------------------------------- #
+# Inline single-gate circuits with hand-computed expectations.
+# --------------------------------------------------------------------------- #
+
+@pytest.mark.parametrize('alg', ALL_ALGS)
+def test_inv_single_pattern(alg):
+    # s-nodes are 'i','o'.  i=0 => golden o=1.  Detect faults that flip o off 1.
+    c = bench.parse('input(i) output(o) o=INV(i)')
+    oline = c.forks['o'].ins[0]
+    iline = oline.driver.ins[0]
+    pat = make_patterns('0-')  # i=0 (o position is a don't-care)
+    faults = [saf(oline, 0), saf(oline, 1), saf(iline, 0), saf(iline, 1)]
+    r = classify(alg, c, faults, pat)
+    # o s-a-0 forces 0 (!=1) -> detected; i s-a-1 forces i=1 -> o=0 -> detected.
+    assert r['DS'] == {saf(oline, 0), saf(iline, 1)}
+    # o s-a-1 already matches golden; i s-a-0 keeps i=0 -> nothing changes.
+    assert r['NO'] == {saf(oline, 1), saf(iline, 0)}
+
+
+@pytest.mark.parametrize('alg', ALL_ALGS)
+def test_inv_both_polarities_detected(alg):
+    # Applying both i=0 and i=1 sensitises every fault of the inverter.
+    c = bench.parse('input(i) output(o) o=INV(i)')
+    oline = c.forks['o'].ins[0]
+    iline = oline.driver.ins[0]
+    pat = make_patterns('00', '10')  # i=0 and i=1
+    faults = [saf(oline, 0), saf(oline, 1), saf(iline, 0), saf(iline, 1)]
+    r = classify(alg, c, faults, pat)
+    assert r['DS'] == set(faults)
+    assert r['NO'] == set()
+
+
+@pytest.mark.parametrize('alg', ALL_ALGS)
+def test_and2_output(alg):
+    # s-nodes are 'i0','i1','o'.
+    c = bench.parse('input(i0,i1) output(o) o=AND2(i0,i1)')
+    oline = c.forks['o'].ins[0]
+    o_sa0, o_sa1 = saf(oline, 0), saf(oline, 1)
+
+    # i0=i1=1 => golden o=1: only o s-a-0 is observable.
+    r = classify(alg, c, [o_sa0, o_sa1], make_patterns('110'))
+    assert r['DS'] == {o_sa0}
+    assert r['NO'] == {o_sa1}
+
+    # i0=0,i1=1 => golden o=0: only o s-a-1 is observable.
+    r = classify(alg, c, [o_sa0, o_sa1], make_patterns('010'))
+    assert r['DS'] == {o_sa1}
+    assert r['NO'] == {o_sa0}
+
+
+@pytest.mark.parametrize('alg', ALL_ALGS)
+def test_two_level_nand_propagation(alg):
+    # o = NAND2(NAND2(a,b), c); s-nodes 'a','b','c','o'.  a=b=c=1: n=0, o=1.
+    c = bench.parse('input(a,b,c) output(o) n=NAND2(a,b) o=NAND2(n,c)')
+    nline = c.forks['n'].ins[0]
+    oline = c.forks['o'].ins[0]
+    faults = [saf(nline, 0), saf(nline, 1), saf(oline, 0), saf(oline, 1)]
+    r = classify(alg, c, faults, make_patterns('1110'))
+    # n already 0 (s-a-0 invisible); n s-a-1 -> o=0; o s-a-0 visible; o already 1.
+    assert r['DS'] == {saf(nline, 1), saf(oline, 0)}
+    assert r['NO'] == {saf(nline, 0), saf(oline, 1)}
+
+
+# --------------------------------------------------------------------------- #
+# c17 fixture: small, fully testable combinational benchmark.
+# --------------------------------------------------------------------------- #
+
+def _c17_exhaustive_patterns():
+    """All 2**5 input combinations of c17.
+
+    s-nodes are inputs 1,2,3,6,7 then outputs 22,23 (netlist order); the two
+    output positions are don't-cares appended after the five input bits.
+    """
+    return make_patterns(*[''.join(combo) + '--'
+                           for combo in itertools.product('01', repeat=5)])
+
+
+@pytest.mark.parametrize('alg', ALL_ALGS)
+def test_c17_fully_detected(alg, c17_bench, c17_resolved):
+    # c17 has no redundant faults, so exhaustive patterns detect every fault.
+    fs = FaultSet(c17_bench, KYUPY, c17_resolved)
+    faults = list(fs.saf_equiv_classes.keys())
+    pat = _c17_exhaustive_patterns()
+    r = classify(alg, c17_resolved, faults, pat)
+    assert r['DS'] == set(faults)
+    assert r['NO'] == set()
+
+
+@pytest.mark.parametrize('alg', ALL_ALGS)
+def test_c17_output_fault_class(alg, c17_resolved):
+    # All-zero inputs: output 22 settles to 0, so only its s-a-1 is observable.
+    o22 = c17_resolved.forks['22'].ins[0]
+    pat = make_patterns('00000--')  # all inputs 0
+    r = classify(alg, c17_resolved, [saf(o22, 0), saf(o22, 1)], pat)
+    assert r['DS'] == {saf(o22, 1)}
+    assert r['NO'] == {saf(o22, 0)}
+
+
+# --------------------------------------------------------------------------- #
+# s27 fixture: exposes the exact-vs-optimistic difference between simulators.
+# --------------------------------------------------------------------------- #
+
+def _s27_setup(s27_bench, s27_resolved):
+    fs = FaultSet(s27_bench, KYUPY, s27_resolved)
+    faults = list(fs.saf_equiv_classes.keys())
+    rng = np.random.default_rng(1)
+    pat = rng.choice([logic.ZERO, logic.ONE],
+                     size=(len(s27_resolved.s_nodes(KYUPY)), 64)).astype(np.uint8)
+    return faults, pat
+
+
+def test_s27_exact_simulators_agree(s27_bench, s27_resolved):
+    faults, pat = _s27_setup(s27_bench, s27_resolved)
+    assert len(faults) == 32  # collapsed fault count, see test_fault_set.test_s27
+
+    simple = classify(SAFSimSimple, s27_resolved, faults, pat)
+    incr = classify(SAFSimIncremental, s27_resolved, faults, pat)
+
+    # The two exact simulators must classify every fault identically.
+    assert simple['DS'] == incr['DS']
+    assert simple['NO'] == incr['NO']
+    # This deterministic pattern set leaves exactly one fault unobserved.
+    assert len(simple['DS']) == 31
+    assert len(simple['NO']) == 1
+
+
+def test_s27_ppsfp_matches_exact(s27_bench, s27_resolved):
+    faults, pat = _s27_setup(s27_bench, s27_resolved)
+
+    exact = classify(SAFSimSimple, s27_resolved, faults, pat)
+    ppsfp = classify(SAFSimPPSFP, s27_resolved, faults, pat)
+
+    # With exact FFR-stem observability, PPSFP classifies identically to the
+    # brute-force simulator -- no longer an over-approximation.
+    assert ppsfp['DS'] == exact['DS']
+    assert ppsfp['NO'] == exact['NO']
+
+
+@pytest.mark.parametrize('alg', ALL_ALGS)
+def test_ppsfp_multibranch_stem_observability(alg):
+    # Stem `s` fans out to two AND gates; with sel1=0,sel2=1 the stem is
+    # observable only through the o2 branch (the stem's *second* fanout line).
+    c = bench.parse('input(a,b,sel1,sel2) output(o1,o2) '
+                    's=NAND2(a,b) o1=AND2(s,sel1) o2=AND2(s,sel2)')
+    a_sa0 = saf(c.forks['s'].ins[0].driver.ins[0], 0)
+    # a=1,b=1,sel1=0,sel2=1: a s-a-0 flips s, propagating through o2 only.
+    pat = make_patterns('1101--')
+    r = classify(alg, c, [a_sa0], pat)
+    assert r['DS'] == {a_sa0}
+
+@pytest.mark.parametrize('alg', [SAFSimIncremental, SAFSimPPSFP])
+def test_all_simprims(mydir, alg):
+    c = verilog.load(mydir / 'all_kyupy_simprims.v')
+    faults = [saf(line, polarity) for line in c.lines for polarity in (0, 1)]
+    s_nodes = c.s_nodes(KYUPY)
+    in_positions = [i for i, n in enumerate(s_nodes) if len(n.ins) == 0]
+
+    for combo in itertools.product('01', repeat=len(in_positions)):
+        chars = ['-'] * len(s_nodes)
+        for pos, bit in zip(in_positions, combo):
+            chars[pos] = bit
+        pat = make_patterns(''.join(chars))
+        ref = classify(SAFSimSimple, c, faults, pat)
+        res = classify(alg, c, faults, pat)
+        assert res['DS'] == ref['DS']
+        assert res['NO'] == ref['NO']