Implemented a half-way okay-ish user interaction pattern: mouse drag.

src/App.js

@@ -1,27 +1,38 @@
-import React, {useState} from 'react';
+import React, {useReducer} from 'react';
 import Maze from "./Maze";
 import InputForm from "./InputForm";
+import reduce from "./reducer";
+import MessageBanner from "./MessageBanner";
 
 export default function App() {
-    const [maze, setMaze] = useState({});
+    const [state, dispatch] = useReducer(reduce, {
-    const [showSolution, setShowSolution] = useState(false);
+            maze: null,
-    const hasValidMaze = !!maze.width &&
+            loading: false,
-        !!maze.height &&
+            errorMessage: null,
-        !!maze.id &&
+            showSolution: false,
-        !!maze.grid;
+            userPath: []
+        },
+        undefined);
+    const hasValidMaze = !!state.maze;
     return (
         <>
-            <InputForm handleResult={setMaze}/>
+            <MessageBanner state={state}
+                           dispatch={dispatch}/>
+            <InputForm state={state}
+                       dispatch={dispatch}/>
             {hasValidMaze &&
                 <>
-                    <h1>The Maze ({maze.width}x{maze.height}, ID: {maze.id})</h1>
+                    <h1>The Maze ({state.maze.width}x{state.maze.height}, ID: {state.maze.id})</h1>
                     <input type={"checkbox"}
                            onChange={(e) => {
-                               setShowSolution(e.target.checked);
+                               dispatch({
+                                   type: 'toggled_show_solution',
+                                   value: e.target.checked
+                               });
                            }}
                            id={"showSolution"}/><label htmlFor="showSolution">Show Solution</label>
-                    <Maze labyrinth={maze}
+                    <Maze state={state}
-                          showSolution={showSolution}/>
+                          dispatch={dispatch}/>
                 </>
             }
         </>

src/Cell.js

@@ -1,20 +1,30 @@
-import React, {useState} from 'react';
+import React from 'react';
 
-export default function Cell({spec, rowIndex, cellIndex, showSolution}) {
+function isMarked(x, y, marked) {
-    const [mark, setMark] = useState(false);
+    return !!marked.find(e => e.x === x && e.y === y);
-    let classes = "cell r" + rowIndex + " c" + cellIndex;
+}
-    if (spec.top) classes += " top";
+
-    if (spec.right) classes += " right";
+export default function Cell({x, y, state, dispatch}) {
-    if (spec.bottom) classes += " bottom";
+    const cell = state.maze.grid[y][x];
-    if (spec.left) classes += " left";
+    let classes = "cell r" + y + " c" + x;
-    if (spec.solution && showSolution) classes += " solution";
+    if (cell.top) classes += " top";
-    if (mark) classes += " user";
+    if (cell.right) classes += " right";
+    if (cell.bottom) classes += " bottom";
+    if (cell.left) classes += " left";
+    if (cell.solution && state.showSolution) classes += " solution";
+    const marked = isMarked(x, y, state.userPath);
+    if (marked) classes += " user";
     return (
         <div className={classes}
              onMouseEnter={(e) => {
                  const leftPressed = e.buttons & 0x1;
                  if (leftPressed) {
-                     setMark(!mark);
+                     dispatch({
+                         type: 'clicked_cell',
+                         x,
+                         y,
+                         e
+                     });
                  }
              }}
              onMouseLeave={(e) => {
@@ -23,7 +33,13 @@ export default function Cell({spec, rowIndex, cellIndex, showSolution}) {
                  }
              }}
              onClick={(e) => {
-                 setMark(!mark);
+                 dispatch({
-             }}/>
+                     type: 'clicked_cell',
+                     x,
+                     y,
+                     e
+                 });
+             }}>
+        </div>
     );
 }

src/InputForm.js

@@ -1,39 +1,34 @@
 import React, {useState} from 'react';
 import ValidatingInputNumberField from "./ValidatingInputNumberField";
 
-export default function InputForm({handleResult}) {
+export default function InputForm({state, dispatch}) {
     const [width, setWidth] = useState(10);
     const [height, setHeight] = useState(10);
     const [id, setId] = useState(null);
-    const [status, setStatus] = useState("ready"); // "ready", "submitting"
 
-    if (status === "submitted") {
+    const handleSubmit = (e) => {
-        return <span/>;
+        e.preventDefault();
-    }
+        dispatch({
-    const callAPI = () => {
+            type: 'started_loading'
-        handleResult({});
+        });
         const url = `https://manuel.friedli.info/labyrinth/create/json?w=${width}&h=${height}&id=${id || ''}`;
         fetch(url)
             .then(response => response.json())
             // .then(result => new Promise(resolve => setTimeout(resolve, 600, result)))
             .then(result => {
-                handleResult(result);
+                dispatch({
-                setId(_ => result.id);
+                    type: 'loaded_maze',
+                    maze: result
+                });
             })
             .catch(reason => {
                 console.error("Failed to fetch maze data.", reason);
-                // FIXME alert is not user friendly
+                dispatch({
-                alert(`Failed to fetch maze data: ${reason}`);
+                    type: 'loading_failed',
-            })
+                    reason
-            .finally(() => {
+                });
-                setStatus("ready");
             });
     };
-    const handleSubmit = (e) => {
-        e.preventDefault();
-        setStatus("submitting");
-        callAPI();
-    };
     const validateWidthHeightInput = value => {
         if (isNaN(value) || "" === value || (Math.floor(value) !== Number(value))) {
             return {
@@ -77,7 +72,7 @@ export default function InputForm({handleResult}) {
                                             min: 2
                                         }}
                                         validatorFn={validateWidthHeightInput}
-                                        disabled={status === "submitting"}
+                                        disabled={state.loading}
                                         onChange={setWidth}
             /><br/>
             <ValidatingInputNumberField id={"height"}
@@ -87,21 +82,21 @@ export default function InputForm({handleResult}) {
                                             min: 2
                                         }}
                                         validatorFn={validateWidthHeightInput}
-                                        disabled={status === "submitting"}
+                                        disabled={state.loading}
                                         onChange={setHeight}
             /><br/>
             <ValidatingInputNumberField id={"id"}
                                         label={"ID (optional)"}
                                         value={id}
                                         validatorFn={validateIdInput}
-                                        disabled={status === "submitting"}
+                                        disabled={state.loading}
                                         onChange={setId}
             /><br/>
             <button type={"submit"}
-                    disabled={status === "submitting"
+                    disabled={state.loading
                         || isNaN(width)
                         || isNaN(height)
-                    }>{status === "ready" ? "Create" : "Loading ..."}
+                    }>{state.loading ? "Loading ..." : "Create Maze!"}
             </button>
         </form>
     );

src/Maze.js

@@ -1,16 +1,20 @@
 import React from 'react';
-import Row from "./Row";
+import Cell from "./Cell";
 
 
-export default function Maze({labyrinth, showSolution = false}) {
+export default function Maze({state, dispatch}) {
-    if (!labyrinth.grid) {
+    if (!state.maze) {
         return <div>No valid maze.</div>
     }
 
-    const maze = labyrinth.grid.map((row, rowIdx) => <Row key={rowIdx}
+    let maze = [];
-                                                          spec={row}
+    for (let y = 0; y < state.maze.height; y++) {
-                                                          index={rowIdx}
+        let row = [];
-                                                          showSolution={showSolution}/>);
+        for (let x = 0; x < state.maze.width; x++) {
+            row.push(<Cell key={`${x}x${y}`} x={x} y={y} state={state} dispatch={dispatch}/>)
+        }
+        maze.push(<div key={`r${y}`} className={"row"}>{row}</div>);
+    }
     return (
         <div className={"maze"}>
             {maze}

src/MessageBanner.js

@@ -0,0 +1,17 @@
+import React from "react";
+
+export default function MessageBanner({state, dispatch}) {
+    function handleClose() {
+        dispatch({
+            type: 'closed_message_banner'
+        })
+    }
+
+    if (!!state.errorMessage) {
+        return (<div className={"message-banner"}>
+            {state.errorMessage}
+            <button onClick={handleClose}>Dismiss message</button>
+        </div>);
+    }
+    return <></>;
+}

src/Row.js

@@ -1,15 +0,0 @@
-import React from 'react';
-import Cell from "./Cell";
-
-export default function Row({spec, index, showSolution}) {
-    const cells = spec.map((cell, cellIdx) => <Cell key={`${index}-${cellIdx}`}
-                                                    spec={cell}
-                                                    rowIndex={index}
-                                                    cellIndex={cellIdx}
-                                                    showSolution={showSolution}/>)
-    return (
-        <div className={"row"}>
-            {cells}
-        </div>
-    );
-}

src/reducer.js

@@ -0,0 +1,48 @@
+import handleUserClicked from "./userpathhandler";
+
+export default function reduce(state, action) {
+    switch (action.type) {
+        case 'started_loading': {
+            return {
+                ...state,
+                maze: null,
+                loading: true,
+                errorMessage: null
+            }
+        }
+        case 'loaded_maze': {
+            return {
+                ...state,
+                loading: false,
+                maze: action.maze,
+                userPath: []
+            }
+        }
+        case 'loading_failed': {
+            return {
+                ...state,
+                loading: false,
+                errorMessage: `Failed to load maze. Reason: ${action.reason}`
+            }
+        }
+        case 'toggled_show_solution': {
+            return {
+                ...state,
+                showSolution: action.value
+            }
+        }
+        case 'closed_message_banner': {
+            return {
+                ...state,
+                errorMessage: null
+            }
+        }
+        case 'clicked_cell': {
+            // There's so much logic involved, externalize that into its own file.
+            return handleUserClicked(state, action.x, action.y);
+        }
+        default: {
+            throw new Error(`Unknown action: ${action.type}`);
+        }
+    }
+}

src/styles.css

@@ -18,7 +18,6 @@ div.cell {
     border: 1px solid transparent;
     height: 2em;
     width: 2em;
-    margin: 0;
     padding: 0;
 }
 
@@ -41,15 +40,28 @@ div.cell.bottom {
 div.cell.left {
     border-left-color: #000;
 }
+
 div.cell.user {
     background-color: hotpink;
 }
+
+div.cell.user2 {
+    border-radius: 0.5em;
+}
+
 div.cell.solution.user {
     background-color: darkred;
 }
+
+div.message-banner {
+    border: 1px solid red;
+    background-color: #ff000044;
+}
+
 input:invalid {
     border-color: #f00;
 }
+
 body {
     font-family: sans-serif;
     margin: 20px;

src/userpathhandler.js

@@ -0,0 +1,73 @@
+export default function handleUserClicked(state, x, y) {
+    if (isClickAllowed(x, y, state)) {
+        // Okay, we clicked a cell that's adjacent to the end of the userpath (or which IS the end of the userpath)
+        // and that's not blocked by a wall. Now let's see.
+        if (-1 === state.userPath.findIndex(step => step.x === x && step.y === y)) {
+            // The clicked cell is not yet part of the userpath --> add it.
+            return {
+                ...state,
+                userPath: [...state.userPath, {x: x, y: y}]
+            };
+        } else {
+            // The clicked cell IS part of the userpath. Is it the last cell of it?
+            const lastCoordsFromUserPath = getLastCoordsFromUserPath(state);
+            if (lastCoordsFromUserPath.x === x && lastCoordsFromUserPath.y === y) {
+                // Yes, it's the last cell of the userpath --> remove it.
+                return {
+                    ...state,
+                    userPath: state.userPath.filter(step => step.x !== x || step.y !== y)
+                }
+            }
+        }
+    }
+    // Not allowed to toggle that cell. Don't apply any change to the state.
+    return state;
+}
+
+function isClickAllowed(x, y, state) {
+    const lastCoordsFromUserPath = getLastCoordsFromUserPath(state);
+    if (!lastCoordsFromUserPath) {
+        // when nothing has been marked yet, we can only toggle the starting position
+        return x === state.maze.start.x && y === state.maze.start.y;
+    }
+    if (lastCoordsFromUserPath.x === x && lastCoordsFromUserPath.y === y) {
+        // toggling the last position in the path is always allowed
+        return true;
+    }
+
+    if (Math.abs(x - lastCoordsFromUserPath.x) + Math.abs(y - lastCoordsFromUserPath.y) !== 1) {
+        // It's not a neighbor. So it's not allowed.
+        return false;
+    }
+
+    const lastCell = getCellAt(lastCoordsFromUserPath, state);
+    if (x === lastCoordsFromUserPath.x + 1) {
+        // There must be no wall to the right of the last cell.
+        return !lastCell.right;
+    }
+    if (x === lastCoordsFromUserPath.x - 1) {
+        // There must be no wall to the left of the last cell.
+        return !lastCell.left;
+    }
+    if (y === lastCoordsFromUserPath.y + 1) {
+        // There must be no wall below the last cell.
+        return !lastCell.bottom;
+    }
+    if (y === lastCoordsFromUserPath.y - 1) {
+        // There must be no wall above the last cell.
+        return !lastCell.top;
+    }
+
+    return false;
+}
+
+function getCellAt(coords, state) {
+    return state.maze.grid[coords.y][coords.x];
+}
+
+function getLastCoordsFromUserPath(state) {
+    if (state.userPath.length > 0) {
+        return state.userPath[state.userPath.length - 1];
+    }
+    return null;
+}