{
  "#": [
    "The cases are split into multiple sections, all with the same structure.",
    "In all cases, the `expected` key is the resulting stack",
    "after executing the Forth program contained in the `input` key."
  ],
  "parsing and numbers": {
    "cases": [
      {
        "description": "empty input results in empty stack",
        "input": [],
        "expected": []
      },
      {
        "description": "numbers just get pushed onto the stack",
        "input": [
          "1 2 3 4 5"
        ],
        "expected": [1, 2, 3, 4, 5]
      },
      {
        "description": "all non-word characters are separators",
        "input": [
          "1\u00002\u00133\n4\r5 6\t7"
        ],
        "expected": [1, 2, 3, 4, 5, 6, 7]
      }
    ]
  },
  "addition": {
    "cases": [
      {
        "description": "can add two numbers",
        "input": [
          "1 2 +"
        ],
        "expected": [3]
      },
      {
        "description": "errors if there is nothing on the stack",
        "input": [
          "+"
        ],
        "expected": null
      },
      {
        "description": "errors if there is only one value on the stack",
        "input": [
          "1 +"
        ],
        "expected": null
      }
    ]
  },
  "subtraction": {
    "cases": [
      {
        "description": "can subtract two numbers",
        "input": [
          "3 4 -"
        ],
        "expected": [-1]
      },
      {
        "description": "errors if there is nothing on the stack",
        "input": [
          "-"
        ],
        "expected": null
      },
      {
        "description": "errors if there is only one value on the stack",
        "input": [
          "1 -"
        ],
        "expected": null
      }
    ]
  },
  "multiplication": {
    "cases": [
      {
        "description": "can multiply two numbers",
        "input": [
          "2 4 *"
        ],
        "expected": [8]
      },
      {
        "description": "errors if there is nothing on the stack",
        "input": [
          "*"
        ],
        "expected": null
      },
      {
        "description": "errors if there is only one value on the stack",
        "input": [
          "1 *"
        ],
        "expected": null
      }
    ]
  },
  "division": {
    "cases": [
      {
        "description": "can divide two numbers",
        "input": [
          "12 3 /"
        ],
        "expected": [4]
      },
      {
        "description": "performs integer division",
        "input": [
          "8 3 /"
        ],
        "expected": [2]
      },
      {
        "description": "errors if dividing by zero",
        "input": [
          "4 0 /"
        ],
        "expected": null
      },
      {
        "description": "errors if there is nothing on the stack",
        "input": [
          "/"
        ],
        "expected": null
      },
      {
        "description": "errors if there is only one value on the stack",
        "input": [
          "1 /"
        ],
        "expected": null
      }
    ]
  },
  "combined arithmetic": {
    "cases": [
      {
        "description": "addition and subtraction",
        "input": [
          "1 2 + 4 -"
        ],
        "expected": [-1]
      },
      {
        "description": "multiplication and division",
        "input": [
          "2 4 * 3 /"
        ],
        "expected": [2]
      }
    ]
  },
  "dup": {
    "cases": [
      {
        "description": "copies the top value on the stack",
        "input": [
          "1 DUP"
        ],
        "expected": [1, 1]
      },
      {
        "description": "is case-insensitive",
        "input": [
          "1 2 Dup"
        ],
        "expected": [1, 2, 2]
      },
      {
        "description": "errors if there is nothing on the stack",
        "input": [
          "dup"
        ],
        "expected": null
      }
    ]
  },
  "drop": {
    "cases": [
      {
        "description": "removes the top value on the stack if it is the only one",
        "input": [
          "1 drop"
        ],
        "expected": []
      },
      {
        "description": "removes the top value on the stack if it not is the only one",
        "input": [
          "1 2 drop"
        ],
        "expected": [1]
      },
      {
        "description": "errors if there is nothing on the stack",
        "input": [
          "drop"
        ],
        "expected": null
      }
    ]
  },
  "swap": {
    "cases": [
      {
        "description": "swaps the top two values on the stack if they are the only ones",
        "input": [
          "1 2 swap"
        ],
        "expected": [2, 1]
      },
      {
        "description": "swaps the top two values on the stack if they are not the only ones",
        "input": [
          "1 2 3 swap"
        ],
        "expected": [1, 3, 2]
      },
      {
        "description": "errors if there is nothing on the stack",
        "input": [
          "swap"
        ],
        "expected": null
      },
      {
        "description": "errors if there is only one value on the stack",
        "input": [
          "1 swap"
        ],
        "expected": null
      }
    ]
  },
  "over": {
    "cases": [
      {
        "description": "copies the second element if there are only two",
        "input": [
          "1 2 over"
        ],
        "expected": [1, 2, 1]
      },
      {
        "description": "copies the second element if there are more than two",
        "input": [
          "1 2 3 over"
        ],
        "expected": [1, 2, 3, 2]
      },
      {
        "description": "errors if there is nothing on the stack",
        "input": [
          "over"
        ],
        "expected": null
      },
      {
        "description": "errors if there is only one value on the stack",
        "input": [
          "1 over"
        ],
        "expected": null
      }
    ]
  },
  "user-defined words": {
    "cases": [
      {
        "description": "can consist of built-in words",
        "input": [
          ": dup-twice dup dup ;",
          "1 dup-twice"
        ],
        "expected": [1, 1, 1]
      },
      {
        "description": "execute in the right order",
        "input": [
          ": countup 1 2 3 ;",
          "countup"
        ],
        "expected": [1, 2, 3]
      },
      {
        "description": "can override other user-defined words",
        "input": [
          ": foo dup ;",
          ": foo dup dup ;",
          "1 foo"
        ],
        "expected": [1, 1, 1]
      },
      {
        "description": "can override built-in words",
        "input": [
          ": swap dup ;",
          "1 swap"
        ],
        "expected": [1, 1]
      },
      {
        "description": "can consist of arbitrary characters such as Unicode characters",
        "input": [
          ": € 220371 ; €"
        ],
        "expected": [220371]
      },
      {
        "description": "cannot redefine numbers",
        "input": [
          ": 1 2 ;"
        ],
        "expected": null
      },
      {
        "description": "errors if executing a non-existent word",
        "input": [
          "foo"
        ],
        "expected": null
      }
    ]
  }
}