{"abstract":"Move-to-front decoder updates the wrong position.","category":"Compression format semantics","checks":6,"contract":"Decode valid ranks against a unique-symbol alphabet; emit selected symbol and move it to the front after each selection.","evaluation_group":"model-3067dd818bc86056","failed_approach":"Moving a decoded symbol to the back applies the opposite update rule.","family":"z-compression-move-to-front-rank","id":"FA-11631","implementations":{"attempt":{"sha256":"a8b97745f030a21947297d20fc99b2753faf42845bc92482e3ad75a7d0a867c1","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(alphabet, ranks):\n    a=list(alphabet); out=''\n    for i in ranks:\n        s=a.pop(i); out+=s; a.append(s)\n    return out\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('repeat front',solve('abc',[2]+[0]*N),'c'*(N+1))\ncheck('empty',solve('abc',[]),'')\ncheck('first',solve('abc',[0]),'a')\ncheck('middle twice',solve('abc',[1,1]),'ba')\ncheck('last twice',solve('abc',[2,2]),'cb')\ncheck('single alphabet',solve('x',[0,0]),'xx')\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"broken":{"sha256":"c307a1580c00fbb6cee945fce0a59e93d6c58980bd2b53ac421e376c7a7d0d73","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(alphabet, ranks):\n    return ''.join(alphabet[i] for i in ranks)\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('repeat front',solve('abc',[2]+[0]*N),'c'*(N+1))\ncheck('empty',solve('abc',[]),'')\ncheck('first',solve('abc',[0]),'a')\ncheck('middle twice',solve('abc',[1,1]),'ba')\ncheck('last twice',solve('abc',[2,2]),'cb')\ncheck('single alphabet',solve('x',[0,0]),'xx')\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"fixed":{"sha256":"ccf333118a98efa13ef7dca6882d678af6f98c4fcc3d30e25ee45f6671024064","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(alphabet, ranks):\n    a=list(alphabet); out=''\n    for i in ranks:\n        s=a.pop(i); out+=s; a.insert(0,s)\n    return out\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('repeat front',solve('abc',[2]+[0]*N),'c'*(N+1))\ncheck('empty',solve('abc',[]),'')\ncheck('first',solve('abc',[0]),'a')\ncheck('middle twice',solve('abc',[1,1]),'ba')\ncheck('last twice',solve('abc',[2,2]),'cb')\ncheck('single alphabet',solve('x',[0,0]),'xx')\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"}},"limitations":"Controlled educational model, not a complete implementation of a production compression format. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.","method":"Deterministic executable model with adversarial boundary fixtures.","provenance":{"created_by":"Failure Map","dependencies":"Python standard library","family":"z-compression-move-to-front-rank","generated_at":"2026-09-29T14:38:49.658574+00:00","license":"CC0-1.0","python":"3.12.14","seed":1,"split":"open-access"},"relevance":"A small offline codec model isolates a compression-specific failure without external files or libraries.","repair":"Remove the symbol at the requested current rank and insert that same symbol at the front before decoding the next rank.","root_cause":"The alphabet remains static after emitted symbols.","sha256":"6d4ab19b615e6772ab3355f67e997091b3263490cd0670655714069aab479ee1","title":"Move-to-front decoder updates the wrong position · case 01","variant":1,"variant_policy":"Five numbered records share a model and may reuse boundary fixtures.","verification":{"attempt":{"elapsed_ms":40.962,"exit_code":1,"observations":[{"actual":"ca","check":"repeat front","expected":"cc","passed":false},{"actual":"","check":"empty","expected":"","passed":true},{"actual":"a","check":"first","expected":"a","passed":true},{"actual":"bc","check":"middle twice","expected":"ba","passed":false},{"actual":"cc","check":"last twice","expected":"cb","passed":false},{"actual":"xx","check":"single alphabet","expected":"xx","passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"repeat front\", \"actual\": \"ca\", \"expected\": \"cc\", \"passed\": false}, {\"check\": \"empty\", \"actual\": \"\", \"expected\": \"\", \"passed\": true}, {\"check\": \"first\", \"actual\": \"a\", \"expected\": \"a\", \"passed\": true}, {\"check\": \"middle twice\", \"actual\": \"bc\", \"expected\": \"ba\", \"passed\": false}, {\"check\": \"last twice\", \"actual\": \"cc\", \"expected\": \"cb\", \"passed\": false}, {\"check\": \"single alphabet\", \"actual\": \"xx\", \"expected\": \"xx\", \"passed\": true}], \"passed\": false}\n"},"broken":{"elapsed_ms":41.709,"exit_code":1,"observations":[{"actual":"ca","check":"repeat front","expected":"cc","passed":false},{"actual":"","check":"empty","expected":"","passed":true},{"actual":"a","check":"first","expected":"a","passed":true},{"actual":"bb","check":"middle twice","expected":"ba","passed":false},{"actual":"cc","check":"last twice","expected":"cb","passed":false},{"actual":"xx","check":"single alphabet","expected":"xx","passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"repeat front\", \"actual\": \"ca\", \"expected\": \"cc\", \"passed\": false}, {\"check\": \"empty\", \"actual\": \"\", \"expected\": \"\", \"passed\": true}, {\"check\": \"first\", \"actual\": \"a\", \"expected\": \"a\", \"passed\": true}, {\"check\": \"middle twice\", \"actual\": \"bb\", \"expected\": \"ba\", \"passed\": false}, {\"check\": \"last twice\", \"actual\": \"cc\", \"expected\": \"cb\", \"passed\": false}, {\"check\": \"single alphabet\", \"actual\": \"xx\", \"expected\": \"xx\", \"passed\": true}], \"passed\": false}\n"},"fixed":{"elapsed_ms":38.625,"exit_code":0,"observations":[{"actual":"cc","check":"repeat front","expected":"cc","passed":true},{"actual":"","check":"empty","expected":"","passed":true},{"actual":"a","check":"first","expected":"a","passed":true},{"actual":"ba","check":"middle twice","expected":"ba","passed":true},{"actual":"cb","check":"last twice","expected":"cb","passed":true},{"actual":"xx","check":"single alphabet","expected":"xx","passed":true}],"passed":true,"stderr":"","stdout":"{\"observations\": [{\"check\": \"repeat front\", \"actual\": \"cc\", \"expected\": \"cc\", \"passed\": true}, {\"check\": \"empty\", \"actual\": \"\", \"expected\": \"\", \"passed\": true}, {\"check\": \"first\", \"actual\": \"a\", \"expected\": \"a\", \"passed\": true}, {\"check\": \"middle twice\", \"actual\": \"ba\", \"expected\": \"ba\", \"passed\": true}, {\"check\": \"last twice\", \"actual\": \"cb\", \"expected\": \"cb\", \"passed\": true}, {\"check\": \"single alphabet\", \"actual\": \"xx\", \"expected\": \"xx\", \"passed\": true}], \"passed\": true}\n"}},"verified":true,"visibility":"public"}