{
  "exercise": "forth",
  "version": "1.1.0",
  "comments": [
    "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."
  ],
  "cases": [
    {
      "description": "parsing and numbers",
      "cases": [
        {
          "description": "empty input results in empty stack",
          "property": "evaluate",
          "input": [],
          "expected": []
        },
        {
          "description": "numbers just get pushed onto the stack",
          "property": "evaluate",
          "input": ["1 2 3 4 5"],
          "expected": [1, 2, 3, 4, 5]
        }
      ]
    },
    {
      "description": "addition",
      "cases": [
        {
          "description": "can add two numbers",
          "property": "evaluate",
          "input": ["1 2 +"],
          "expected": [3]
        },
        {
          "description": "errors if there is nothing on the stack",
          "property": "evaluate",
          "input": ["+"],
          "expected": null
        },
        {
          "description": "errors if there is only one value on the stack",
          "property": "evaluate",
          "input": ["1 +"],
          "expected": null
        }
      ]
    },
    {
      "description": "subtraction",
      "cases": [
        {
          "description": "can subtract two numbers",
          "property": "evaluate",
          "input": ["3 4 -"],
          "expected": [-1]
        },
        {
          "description": "errors if there is nothing on the stack",
          "property": "evaluate",
          "input": ["-"],
          "expected": null
        },
        {
          "description": "errors if there is only one value on the stack",
          "property": "evaluate",
          "input": ["1 -"],
          "expected": null
        }
      ]
    },
    {
      "description": "multiplication",
      "cases": [
        {
          "description": "can multiply two numbers",
          "property": "evaluate",
          "input": ["2 4 *"],
          "expected": [8]
        },
        {
          "description": "errors if there is nothing on the stack",
          "property": "evaluate",
          "input": ["*"],
          "expected": null
        },
        {
          "description": "errors if there is only one value on the stack",
          "property": "evaluate",
          "input": ["1 *"],
          "expected": null
        }
      ]
    },
    {
      "description": "division",
      "cases": [
        {
          "description": "can divide two numbers",
          "property": "evaluate",
          "input": ["12 3 /"],
          "expected": [4]
        },
        {
          "description": "performs integer division",
          "property": "evaluate",
          "input": ["8 3 /"],
          "expected": [2]
        },
        {
          "description": "errors if dividing by zero",
          "property": "evaluate",
          "input": ["4 0 /"],
          "expected": null
        },
        {
          "description": "errors if there is nothing on the stack",
          "property": "evaluate",
          "input": ["/"],
          "expected": null
        },
        {
          "description": "errors if there is only one value on the stack",
          "property": "evaluate",
          "input": ["1 /"],
          "expected": null
        }
      ]
    },
    {
      "description": "combined arithmetic",
      "cases": [
        {
          "description": "addition and subtraction",
          "property": "evaluate",
          "input": ["1 2 + 4 -"],
          "expected": [-1]
        },
        {
          "description": "multiplication and division",
          "property": "evaluate",
          "input": ["2 4 * 3 /"],
          "expected": [2]
        }
      ]
    },
    {
      "description": "dup",
      "cases": [
        {
          "description": "copies the top value on the stack",
          "property": "evaluate",
          "input": ["1 DUP"],
          "expected": [1, 1]
        },
        {
          "description": "is case-insensitive",
          "property": "evaluate",
          "input": ["1 2 Dup"],
          "expected": [1, 2, 2]
        },
        {
          "description": "errors if there is nothing on the stack",
          "property": "evaluate",
          "input": ["dup"],
          "expected": null
        }
      ]
    },
    {
      "description": "drop",
      "cases": [
        {
          "description": "removes the top value on the stack if it is the only one",
          "property": "evaluate",
          "input": ["1 drop"],
          "expected": []
        },
        {
          "description": "removes the top value on the stack if it is not the only one",
          "property": "evaluate",
          "input": ["1 2 drop"],
          "expected": [1]
        },
        {
          "description": "errors if there is nothing on the stack",
          "property": "evaluate",
          "input": ["drop"],
          "expected": null
        }
      ]
    },
    {
      "description": "swap",
      "cases": [
        {
          "description": "swaps the top two values on the stack if they are the only ones",
          "property": "evaluate",
          "input": ["1 2 swap"],
          "expected": [2, 1]
        },
        {
          "description": "swaps the top two values on the stack if they are not the only ones",
          "property": "evaluate",
          "input": ["1 2 3 swap"],
          "expected": [1, 3, 2]
        },
        {
          "description": "errors if there is nothing on the stack",
          "property": "evaluate",
          "input": ["swap"],
          "expected": null
        },
        {
          "description": "errors if there is only one value on the stack",
          "property": "evaluate",
          "input": ["1 swap"],
          "expected": null
        }
      ]
    },
    {
      "description": "over",
      "cases": [
        {
          "description": "copies the second element if there are only two",
          "property": "evaluate",
          "input": ["1 2 over"],
          "expected": [1, 2, 1]
        },
        {
          "description": "copies the second element if there are more than two",
          "property": "evaluate",
          "input": ["1 2 3 over"],
          "expected": [1, 2, 3, 2]
        },
        {
          "description": "errors if there is nothing on the stack",
          "property": "evaluate",
          "input": ["over"],
          "expected": null
        },
        {
          "description": "errors if there is only one value on the stack",
          "property": "evaluate",
          "input": ["1 over"],
          "expected": null
        }
      ]
    },
    {
      "description": "user-defined words",
      "cases": [
        {
          "description": "can consist of built-in words",
          "property": "evaluate",
          "input": [
            ": dup-twice dup dup ;",
            "1 dup-twice"
          ],
          "expected": [1, 1, 1]
        },
        {
          "description": "execute in the right order",
          "property": "evaluate",
          "input": [
            ": countup 1 2 3 ;",
            "countup"
          ],
          "expected": [1, 2, 3]
        },
        {
          "description": "can override other user-defined words",
          "property": "evaluate",
          "input": [
            ": foo dup ;",
            ": foo dup dup ;",
            "1 foo"
          ],
          "expected": [1, 1, 1]
        },
        {
          "description": "can override built-in words",
          "property": "evaluate",
          "input": [
            ": swap dup ;",
            "1 swap"
          ],
          "expected": [1, 1]
        },
        {
          "description": "cannot redefine numbers",
          "property": "evaluate",
          "input": [": 1 2 ;"],
          "expected": null
        },
        {
          "description": "errors if executing a non-existent word",
          "property": "evaluate",
          "input": ["foo"],
          "expected": null
        }
      ]
    }
  ]
}