instances/langford_n7__v11_nh.json

{
  "name": "langford_n7__v11_nh",
  "problem_type": "langford",
  "params": {
    "n": 7
  },
  "prompt": "Construct a Langford sequence L(2,7): a sequence of length 14 containing exactly 2 copies of each integer from 1 to 7, such that the two occurrences of each value v are exactly v+1 positions apart (so copies of 1 are 1 position apart, copies of 2 are 2 positions apart, etc.). Example: L(2,3) has solution [2, 3, 1, 2, 1, 3].\n\nReturn seq as a list of 14 integers in 1..7, or state \"UNSATISFIABLE\" if no such sequence exists.",
  "satisfiable": true,
  "solution": {
    "seq": [
      1,
      7,
      1,
      2,
      5,
      6,
      2,
      3,
      4,
      7,
      5,
      3,
      6,
      4
    ]
  },
  "difficulty": {
    "solve_time_ms": 1875.0,
    "search_space": 87178291200,
    "num_variables": 14,
    "num_constraints": 110,
    "num_edges": -1,
    "backend": "pycsp",
    "solve_tier": "hard",
    "solve_pct_global": 90.79,
    "solve_pct_type": 56.25
  },
  "partial_assignment": null
}