Test cases for https://www.reddit.com/r/gamedev/comments/6ikumc/discrete_physics_on_a_2d_grid_how_hard_can_it_be/

PhysicsTests.kt

    @Test
    fun forceMovesObject() = physicsTest {
        - "   "
        - " A "
        - "   "

        A isPushed right

        A shouldMove right
    }

    @Test
    fun objectPushesObject() = physicsTest {
        - "    "
        - " AB "
        - "    "

        A isPushed right

        A shouldMove right
        B shouldMove right
    }

    @Test
    fun objectBlockedByWall() = physicsTest {
        - "    "
        - " A# "
        - "    "

        A isPushed right

        A shouldMove nowhere
    }

    @Test
    fun forcesCancel() = physicsTest {
        - "    "
        - " AB "
        - "    "

        A isPushed right
        B isPushed left

        A shouldMove nowhere
        B shouldMove nowhere
    }

    @Test
    fun bothPushed() = physicsTest {
        - "    "
        - " AB "
        - "    "

        A isPushed right
        B isPushed right

        A shouldMove right
        B shouldMove right
    }

    @Test
    fun forcesAddUp() = physicsTest {
        - "     "
        - " ABC "
        - "     "

        A isPushed right
        B isPushed right
        C isPushed left

        A shouldMove right
        B shouldMove right
        C shouldMove right
    }

    @Test
    fun forceTransferredToMultipleObjects() = physicsTest {
            - "       "
            - "  B D  "
            - " ABCD  "
            - "    D  "
            - "       "

            A isPushed right

            A shouldMove right
            B shouldMove right
            C shouldMove right
            D shouldMove right
        }

    @Test
    fun multipleObjectsBlockedByWall() = physicsTest {
        - "       "
        - "  B#D  "
        - " ABCD  "
        - "    D  "
        - "       "

        A isPushed right

        A shouldMove nowhere
        B shouldMove nowhere
        C shouldMove nowhere
        D shouldMove nowhere
    }

    @Test
    fun longObject() = physicsTest {
        - "      "
        - " BBBB "
        - " A    "
        - "      "

        A isPushed up

        A shouldMove up
        B shouldMove up
    }

    @Test
    fun interlocked() = physicsTest {
        - "        "
        - "  BBB   "
        - " ABCBC  "
        - "   CCC  "
        - "        "

        A isPushed right

        A shouldMove right
        B shouldMove right
        C shouldMove right
    }

    @Test
    fun blockedHalfway() = physicsTest {
        - "      "
        - "  B#  "
        - " ABC  "
        - "      "

        A isPushed right

        A shouldMove nowhere
        B shouldMove nowhere
        C shouldMove nowhere
    }

    @Test
    fun parallelPushers() = physicsTest {
        - "     "
        - " AB  "
        - "  B  "
        - " CB  "
        - "     "

        A isPushed right
        C isPushed right

        A shouldMove right
        B shouldMove right
        C shouldMove right
    }

    @Test
    fun multipleForces() = physicsTest {
        - "      "
        - " ABC  "
        - "      "

        A isPushed right
        A isPushed right

        A shouldMove right
        B shouldMove right
        C shouldMove right
    }

    @Test
    fun orthogonalForces() = physicsTest {
        - "    "
        - " AB "
        - "  C "
        - "  D "
        - "    "

        A isPushed right
        C isPushed up
        D isPushed up

        A shouldMove nowhere
        B shouldMove up
        C shouldMove up
        D shouldMove up
    }

    @Test
    fun noPulling() = physicsTest {
        - "      "
        - " ZAB  "
        - "   B  "
        - "  CBD "
        - "      "

        Z isPushed right
        A isPushed right
        C isPushed left
        D isPushed left

        Z shouldMove right
        A shouldMove right
        B shouldMove right
        C shouldMove left
        D shouldMove right
    }

    @Test
    fun pushIntoTwoWalls() = physicsTest {
        - "      "
        - "  #A  "
        - "   A  "
        - "  %A  "
        - "      "

        A isPushed left

        A shouldMove nowhere
    }

    @Test
    fun pushIntoWallIndirectly() = physicsTest {
        - "     "
        - " #BA "
        - " ##A "
        - "     "

        A isPushed left

        A shouldMove nowhere
        B shouldMove nowhere
    }

    @Test
    fun wallForceZero() = physicsTest {
        - "      "
        - "  AB  "
        - "  A#  "
        - "      "

        A isPushed right
        B isPushed left

        A shouldMove nowhere
        B shouldMove nowhere
    }

    @Test
    fun noWallPulling() = physicsTest {
        - "      "
        - "  ABC "
        - "  A#  "
        - "      "

        A isPushed right
        B isPushed left
        C isPushed left

        A shouldMove left
        B shouldMove left
        C shouldMove left
    }

    @Test
    fun cancelWithThirdBlock() = physicsTest {
        - "      "
        - "  AB  "
        - "  AC  "
        - "      "

        A isPushed right
        C isPushed left

        A shouldMove nowhere
        B shouldMove nowhere
        C shouldMove nowhere
    }

    @Test
    fun noDiagonalOverlap() = physicsTest {
        - "    "
        - " AB "
        - "    "

        A isPushed right
        B isPushed up

        either {
            A shouldMove right
            B shouldMove right
        }
        or {
            A shouldMove nowhere
            B shouldMove up
        }
    }

    @Test
    fun horizontalVerticalAmbiguity() = physicsTest {
        - "     "
        - "     "
        - " AB  "
        - "  C  "
        - "     "

        A isPushed right
        C isPushed up

        either {
            A shouldMove right
            B shouldMove right
            C shouldMove nowhere
        }
        or {
            A shouldMove nowhere
            B shouldMove up
            C shouldMove up
        }
    }

    @Test
    fun sameDestinationAmbiguity() = physicsTest {
        - "     "
        - " A B "
        - "     "

        A isPushed right
        B isPushed left

        either {
            A shouldMove right
            B shouldMove nowhere
        }
        or {
            A shouldMove nowhere
            B shouldMove left
        }
    }