From 9431e981969e392b6c3b8e405d5c736ceed76f60 Mon Sep 17 00:00:00 2001
From: Eduard Urbach <e.urbach@gmail.com>
Date: Thu, 14 Jun 2018 19:46:47 +0900
Subject: [PATCH] Improved question generator

---
 .../multiple-choice-test.ts                   | 59 ++++++++++++++++---
 scripts/levenshtein.ts                        | 42 +++++++++++++
 scripts/randomItem.ts                         |  3 -
 3 files changed, 93 insertions(+), 11 deletions(-)
 create mode 100644 scripts/levenshtein.ts
 delete mode 100644 scripts/randomItem.ts

diff --git a/elements/multiple-choice-test/multiple-choice-test.ts b/elements/multiple-choice-test/multiple-choice-test.ts
index 90c149d..45615b6 100644
--- a/elements/multiple-choice-test/multiple-choice-test.ts
+++ b/elements/multiple-choice-test/multiple-choice-test.ts
@@ -5,9 +5,9 @@ import StatusMessages from "../status-messages/status-messages"
 import QuestionStatistics from "scripts/QuestionStatistics"
 import Statistics from "scripts/Statistics"
 import State from "scripts/State"
-import randomItem from "scripts/randomItem"
 import copyToClipboard from "scripts/copyToClipboard"
 import cloneTemplate from "scripts/cloneTemplate"
+import levenshtein from "scripts/levenshtein"
 
 const preventClicksTimeThreshold = 400
 
@@ -208,13 +208,49 @@ export default class MultipleChoiceTest extends HTMLElement {
 	}
 
 	generateAnswers() {
-		let allKana = [...State.words.values()]
+		let wordPool = [...State.words.values()]
 		let used = new Set<string>()
+		let question = this.kanjiView.kanji
+		let correctAnswer = this.correctAnswer
+
+		// Sort by Levenshtein distance
+		wordPool.sort((a, b) => levenshtein(correctAnswer, a.hiragana) - levenshtein(correctAnswer, b.hiragana))
+
+		// Find kana at the end
+		let i = question.length - 1
+
+		for(; i >= 0; i--) {
+			let charCode = question.charCodeAt(i)
+			let isHiragana = charCode >= 0x3040 && charCode <= 0x309f
+			let isKatakana = charCode >= 0x30a0 && charCode <= 0x30ff
+
+			if(!isHiragana && !isKatakana) {
+				break
+			}
+		}
+
+		let kana = question.slice(i + 1)
+
+		// If there are kana at the end,
+		// only allow answers that end with the given kana.
+		if(kana.length > 0) {
+			let filteredWords = wordPool.filter(x => x.hiragana.endsWith(kana))
+
+			// If the filtered version doesn't have enough options,
+			// combine it with all the other vocab.
+			if(filteredWords.length < this.answers.length) {
+				filteredWords = filteredWords.concat(wordPool)
+			}
+
+			wordPool = filteredWords
+		}
 
 		// Add correct answer
 		let correctAnswerIndex = Math.floor(Math.random() * this.answers.length)
-		this.answers[correctAnswerIndex].innerText = this.correctAnswer
-		used.add(this.correctAnswer)
+		this.answers[correctAnswerIndex].innerText = correctAnswer
+		used.add(correctAnswer)
+
+		let count = 0
 
 		for(let answer of this.answers) {
 			// Skip existing answers
@@ -222,12 +258,19 @@ export default class MultipleChoiceTest extends HTMLElement {
 				continue
 			}
 
-			let text = randomItem(allKana).hiragana
+			if(count >= wordPool.length) {
+				answer.innerText = "-"
+				continue
+			}
+
+			let text = wordPool[count].hiragana
+			count++
 
 			// Avoid duplicate answers
-			if(allKana.length >= this.answers.length) {
-				while(used.has(text)) {
-					text = randomItem(allKana).hiragana
+			if(wordPool.length >= this.answers.length) {
+				while(used.has(text) && count < wordPool.length) {
+					text = wordPool[count].hiragana
+					count++
 				}
 			}
 
diff --git a/scripts/levenshtein.ts b/scripts/levenshtein.ts
new file mode 100644
index 0000000..3de90d6
--- /dev/null
+++ b/scripts/levenshtein.ts
@@ -0,0 +1,42 @@
+export default function levenshtein(a: string, b: string): number {
+	if(a.length === 0) return b.length
+	if(b.length === 0) return a.length
+
+	let tmp, i, j, prev, val, row
+
+	// swap to save some memory O(min(a,b)) instead of O(a)
+	if(a.length > b.length) {
+		tmp = a
+		a = b
+		b = tmp
+	}
+
+	row = Array(a.length + 1)
+
+	// init the row
+	for(i = 0; i <= a.length; i++) {
+		row[i] = i
+	}
+
+	// fill in the rest
+	for(i = 1; i <= b.length; i++) {
+		prev = i
+
+		for (j = 1; j <= a.length; j++) {
+			if (b[i-1] === a[j-1]) {
+				val = row[j-1] // match
+			} else {
+				val = Math.min(row[j-1] + 1, // substitution
+				Math.min(prev + 1,     // insertion
+				row[j] + 1))  // deletion
+			}
+
+			row[j - 1] = prev
+			prev = val
+		}
+
+		row[a.length] = prev
+	}
+
+	return row[a.length]
+}
\ No newline at end of file
diff --git a/scripts/randomItem.ts b/scripts/randomItem.ts
deleted file mode 100644
index 1a9b143..0000000
--- a/scripts/randomItem.ts
+++ /dev/null
@@ -1,3 +0,0 @@
-export default function randomItem<T>(collection: T[]) {
-	return collection[Math.floor(Math.random() * collection.length)]
-}
\ No newline at end of file