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| const COMMAND_PRINT = '3'; | |
| const COMMAND_UNDO = '4'; | |
| const COMMAND_TO_FUNCTION = { | |
| '1': appendToString, | |
| '2': deleteLastNLetters, | |
| [COMMAND_PRINT]: printNthLetter, | |
| [COMMAND_UNDO]: undo | |
| }; | |
| const WHITESPACE = ' '; | |
| function processData(input = '') { | |
| const commands = input | |
| .split('\n') | |
| .slice(1) | |
| .map((rawCommand) => parseCommand(rawCommand)); | |
| const initialEditorStateHistory = ['']; | |
| const editorStateHistory = commands.reduce( | |
| (stateHistory, [command, argument]) => reduceToNextEditorState(stateHistory, command, argument), | |
| initialEditorStateHistory | |
| ); | |
| } | |
| function parseCommand(rawCommand) { | |
| return rawCommand | |
| .split(WHITESPACE); | |
| } | |
| function reduceToNextEditorState(stateHistory, command, argument) { | |
| const newEditorString = COMMAND_TO_FUNCTION[command](stateHistory, argument); | |
| // For performance optimizations, we mutate the state instead of being "pure" and | |
| // creating a new array for the state each time. | |
| if (command !== COMMAND_PRINT && command !== COMMAND_UNDO) { | |
| stateHistory.push(newEditorString); | |
| } | |
| return stateHistory; | |
| } | |
| function appendToString(stateHistory, suffix) { | |
| return stateHistory[stateHistory.length - 1] + suffix; | |
| } | |
| function deleteLastNLetters(stateHistory, numOfLettersToDrop) { | |
| return stateHistory[stateHistory.length - 1].slice(0, -numOfLettersToDrop); | |
| } | |
| function printNthLetter(stateHistory, n) { | |
| const index = parseInt(n, 10) - 1; | |
| const editorString = stateHistory[stateHistory.length - 1]; | |
| console.log(editorString[index]); | |
| return editorString; | |
| } | |
| function undo(stateHistory) { | |
| return stateHistory.pop(); | |
| } | |
| process.stdin.resume(); | |
| process.stdin.setEncoding('ascii'); | |
| _input = ''; | |
| process.stdin.on('data', function (input) { | |
| _input += input; | |
| }); | |
| process.stdin.on('end', function () { | |
| processData(_input); | |
| }); |
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