{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "$id": "https://schemas.fablepool.org/v1/problem-content.json",
  "title": "FablePool Problem Content",
  "description": "The structured content payload stored in ProblemVersion.content. Covers the statement, the answer specification for all supported answer types, hints and the canonical solution. Metadata (title, tags, difficulty, prerequisites, license) lives in relational columns, not here.",
  "type": "object",
  "required": ["schema", "statement", "answer", "solution"],
  "additionalProperties": false,
  "properties": {
    "schema": { "const": "fablepool.problem/1" },
    "statement": { "$ref": "https://schemas.fablepool.org/v1/content-document.json" },
    "answer": { "$ref": "#/$defs/answer" },
    "hints": {
      "type": "array",
      "maxItems": 10,
      "items": { "$ref": "#/$defs/hint" },
      "default": []
    },
    "solution": { "$ref": "#/$defs/richText" },
    "wrong_answer_feedback": {
      "type": "array",
      "maxItems": 20,
      "description": "Targeted feedback for common wrong answers (misconception handling).",
      "items": {
        "type": "object",
        "required": ["match", "feedback"],
        "additionalProperties": false,
        "properties": {
          "match": {
            "type": "string",
            "maxLength": 500,
            "description": "Exact wrong answer (numeric/expression/choice id) to match."
          },
          "feedback": { "$ref": "#/$defs/richText" }
        }
      }
    }
  },
  "$defs": {
    "richText": {
      "oneOf": [
        { "type": "string", "minLength": 1, "maxLength": 10000 },
        { "$ref": "https://schemas.fablepool.org/v1/content-document.json" }
      ]
    },
    "slug": {
      "type": "string",
      "pattern": "^[a-z0-9][a-z0-9_-]{0,63}$"
    },
    "hint": {
      "type": "object",
      "required": ["content"],
      "additionalProperties": false,
      "properties": {
        "content": { "$ref": "#/$defs/richText" },
        "penalty": {
          "type": "number",
          "minimum": 0,
          "maximum": 1,
          "default": 0,
          "description": "Score fraction deducted when this hint is revealed."
        }
      }
    },
    "answer": {
      "oneOf": [
        { "$ref": "#/$defs/multipleChoice" },
        { "$ref": "#/$defs/numeric" },
        { "$ref": "#/$defs/expression" },
        { "$ref": "#/$defs/freeText" },
        { "$ref": "#/$defs/code" },
        { "$ref": "#/$defs/ordering" },
        { "$ref": "#/$defs/matching" },
        { "$ref": "#/$defs/widget" }
      ]
    },
    "multipleChoice": {
      "type": "object",
      "required": ["type", "choices"],
      "additionalProperties": false,
      "properties": {
        "type": { "const": "multiple_choice" },
        "multiple_select": { "type": "boolean", "default": false },
        "shuffle": { "type": "boolean", "default": true },
        "choices": {
          "type": "array",
          "minItems": 2,
          "maxItems": 12,
          "contains": {
            "type": "object",
            "required": ["correct"],
            "properties": { "correct": { "const": true } }
          },
          "items": {
            "type": "object",
            "required": ["id", "content", "correct"],
            "additionalProperties": false,
            "properties": {
              "id": { "$ref": "#/$defs/slug" },
              "content": { "$ref": "#/$defs/richText" },
              "correct": { "type": "boolean" },
              "feedback": { "$ref": "#/$defs/richText" }
            }
          }
        }
      }
    },
    "numeric": {
      "type": "object",
      "required": ["type", "answer"],
      "additionalProperties": false,
      "properties": {
        "type": { "const": "numeric" },
        "answer": { "type": "number" },
        "tolerance": {
          "oneOf": [
            {
              "type": "object",
              "required": ["mode", "value"],
              "additionalProperties": false,
              "properties": {
                "mode": { "const": "absolute" },
                "value": { "type": "number", "minimum": 0 }
              }
            },
            {
              "type": "object",
              "required": ["mode", "value"],
              "additionalProperties": false,
              "properties": {
                "mode": { "const": "relative" },
                "value": { "type": "number", "exclusiveMinimum": 0, "maximum": 1 }
              }
            },
            {
              "type": "object",
              "required": ["mode", "min", "max"],
              "additionalProperties": false,
              "properties": {
                "mode": { "const": "range" },
                "min": { "type": "number" },
                "max": { "type": "number" }
              }
            }
          ]
        },
        "units": {
          "type": "string",
          "maxLength": 32,
          "description": "Expected unit string, e.g. 'm/s^2'; shown next to the input."
        },
        "precision": {
          "type": "integer",
          "minimum": 0,
          "maximum": 15,
          "description": "Significant decimal places expected from the learner."
        }
      }
    },
    "expression": {
      "type": "object",
      "required": ["type", "answer"],
      "additionalProperties": false,
      "properties": {
        "type": { "const": "expression" },
        "answer": {
          "type": "string",
          "minLength": 1,
          "maxLength": 2000,
          "description": "Canonical answer in LaTeX or SymPy-compatible syntax."
        },
        "equivalence": {
          "enum": ["symbolic", "numeric_sampling"],
          "default": "symbolic"
        },
        "variables": {
          "type": "array",
          "maxItems": 16,
          "items": {
            "type": "object",
            "required": ["name"],
            "additionalProperties": false,
            "properties": {
              "name": { "type": "string", "pattern": "^[A-Za-z][A-Za-z0-9_]{0,15}$" },
              "domain": {
                "type": "object",
                "required": ["min", "max"],
                "additionalProperties": false,
                "properties": {
                  "min": { "type": "number" },
                  "max": { "type": "number" }
                }
              },
              "exclude": {
                "type": "array",
                "maxItems": 32,
                "items": { "type": "number" }
              }
            }
          }
        },
        "trials": {
          "type": "integer",
          "minimum": 1,
          "maximum": 100,
          "default": 20,
          "description": "Number of random evaluations for numeric_sampling equivalence."
        },
        "allowed_functions": {
          "type": "array",
          "maxItems": 64,
          "items": { "type": "string", "maxLength": 32 }
        }
      }
    },
    "freeText": {
      "type": "object",
      "required": ["type", "rubric"],
      "additionalProperties": false,
      "description": "Proof-style or open-ended response, graded by rubric (peer, self, or reviewer grading).",
      "properties": {
        "type": { "const": "free_text" },
        "grading": {
          "enum": ["peer", "self", "reviewer"],
          "default": "self"
        },
        "rubric": {
          "type": "array",
          "minItems": 1,
          "maxItems": 20,
          "items": {
            "type": "object",
            "required": ["criterion", "points"],
            "additionalProperties": false,
            "properties": {
              "criterion": { "type": "string", "minLength": 1, "maxLength": 500 },
              "points": { "type": "number", "minimum": 0, "maximum": 100 },
              "description": { "type": "string", "maxLength": 2000 }
            }
          }
        },
        "min_words": { "type": "integer", "minimum": 0, "maximum": 10000 },
        "max_words": { "type": "integer", "minimum": 1, "maximum": 50000 },
        "exemplar": { "$ref": "#/$defs/richText" }
      }
    },
    "code": {
      "type": "object",
      "required": ["type", "language", "test_cases"],
      "additionalProperties": false,
      "description": "Code challenge executed in an isolated sandbox service, never on the application server.",
      "properties": {
        "type": { "const": "code" },
        "language": {
          "enum": ["python", "javascript", "typescript", "c", "cpp", "java", "rust", "go", "haskell"]
        },
        "starter_code": { "type": "string", "maxLength": 20000, "default": "" },
        "solution_code": { "type": "string", "maxLength": 50000 },
        "entry_point": {
          "type": "string",
          "maxLength": 128,
          "description": "Function or class the harness calls, e.g. 'solve'."
        },
        "harness": {
          "type": "string",
          "maxLength": 50000,
          "description": "Optional grader harness run alongside the submission inside the sandbox."
        },
        "test_cases": {
          "type": "array",
          "minItems": 1,
          "maxItems": 100,
          "items": {
            "type": "object",
            "required": ["input", "expected_output"],
            "additionalProperties": false,
            "properties": {
              "name": { "type": "string", "maxLength": 100 },
              "input": { "type": "string", "maxLength": 20000 },
              "expected_output": { "type": "string", "maxLength": 20000 },
              "hidden": { "type": "boolean", "default": false },
              "points": { "type": "number", "minimum": 0, "default": 1 }
            }
          }
        },
        "limits": {
          "type": "object",
          "additionalProperties": false,
          "properties": {
            "time_limit_ms": { "type": "integer", "minimum": 100, "maximum": 30000, "default": 2000 },
            "memory_limit_mb": { "type": "integer", "minimum": 16, "maximum": 1024, "default": 256 }
          }
        }
      }
    },
    "ordering": {
      "type": "object",
      "required": ["type", "items", "correct_order"],
      "additionalProperties": false,
      "properties": {
        "type": { "const": "ordering" },
        "items": {
          "type": "array",
          "minItems": 2,
          "maxItems": 20,
          "items": {
            "type": "object",
            "required": ["id", "content"],
            "additionalProperties": false,
            "properties": {
              "id": { "$ref": "#/$defs/slug" },
              "content": { "$ref": "#/$defs/richText" }
            }
          }
        },
        "correct_order": {
          "type": "array",
          "minItems": 2,
          "maxItems": 20,
          "uniqueItems": true,
          "items": { "$ref": "#/$defs/slug" }
        },
        "partial_credit": { "type": "boolean", "default": false }
      }
    },
    "matching": {
      "type": "object",
      "required": ["type", "left", "right", "pairs"],
      "additionalProperties": false,
      "properties": {
        "type": { "const": "matching" },
        "left": {
          "type": "array",
          "minItems": 2,
          "maxItems": 20,
          "items": {
            "type": "object",
            "required": ["id", "content"],
            "additionalProperties": false,
            "properties": {
              "id": { "$ref": "#/$defs/slug" },
              "content": { "$ref": "#/$defs/richText" }
            }
          }
        },
        "right": {
          "type": "array",
          "minItems": 2,
          "maxItems": 20,
          "items": {
            "type": "object",
            "required": ["id", "content"],
            "additionalProperties": false,
            "properties": {
              "id": { "$ref": "#/$defs/slug" },
              "content": { "$ref": "#/$defs/richText" }
            }
          }
        },
        "pairs": {
          "type": "array",
          "minItems": 1,
          "maxItems": 40,
          "items": {
            "type": "object",
            "required": ["left", "right"],
            "additionalProperties": false,
            "properties": {
              "left": { "$ref": "#/$defs/slug" },
              "right": { "$ref": "#/$defs/slug" }
            }
          }
        },
        "shuffle": { "type": "boolean", "default": true },
        "partial_credit": { "type": "boolean", "default": true }
      }
    },
    "widget": {
      "type": "object",
      "required": ["type", "widget_id", "grading"],
      "additionalProperties": false,
      "description": "Interactive widget-based task. The widget reports a result over the sandbox postMessage API; server_replay grading re-validates the reported result server-side.",
      "properties": {
        "type": { "const": "widget" },
        "widget_id": { "$ref": "#/$defs/slug" },
        "widget_version": { "type": "string", "pattern": "^\\d+(\\.\\d+){0,2}$" },
        "params": { "type": "object" },
        "grading": {
          "type": "object",
          "required": ["mode"],
          "additionalProperties": false,
          "properties": {
            "mode": { "enum": ["widget_reported", "server_replay"] },
            "max_score": { "type": "number", "minimum": 0, "default": 1 }
          }
        }
      }
    }
  }
}