From ed3546759246e690051717c7a880321dda4291fc Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Mon, 10 Nov 2025 16:22:06 +0000
Subject: [PATCH] Add borrowing checkbox for subtraction problems

This commit adds a new checkbox to the print config view that allows users
to generate only subtraction problems that require borrowing.

Changes:
- Added "Only problems that require borrowing?" checkbox to the UI
- Implemented requiresBorrowing() helper function to detect when a
  subtraction problem requires borrowing
- Updated equation generation logic to filter problems based on the
  borrowing constraint
- The filter only applies to subtraction category worksheets

The borrowing detection works by comparing each digit position from right
to left and checking if any minuend digit is less than the corresponding
subtrahend digit.
---
 src/index.html   |  8 ++++++++
 src/js/hw-gen.js | 46 ++++++++++++++++++++++++++++++++++++++--------
 src/js/util.js   | 14 ++++++++++++++
 3 files changed, 60 insertions(+), 8 deletions(-)

diff --git a/src/index.html b/src/index.html
index 4cd7f41..7ed234c 100644
--- a/src/index.html
+++ b/src/index.html
@@ -214,6 +214,14 @@ <h3>How many worksheets?</h3>
         </tr></tbody></table>
         </label>
       </div>
+      <div class="col-md-6 pt-3">
+        <label>
+        <table><tbody><tr>
+          <td><div class="p-3"><input id="borrowingChkbx" class="form-control" type="checkbox"/></div></td>
+          <td><h3 title="Only for subtraction problems. When checked, only generates problems that require borrowing.">Only problems that require borrowing?</h3></td>
+        </tr></tbody></table>
+        </label>
+      </div>
       <div class="col-md-12 pt-3">
         <div class="w-50 ml-auto mr-auto">
           <button class="btn btn-lg btn-primary btn-block p-3" onclick="HwGen.preview()">
diff --git a/src/js/hw-gen.js b/src/js/hw-gen.js
index fc4ab84..eddc1b7 100644
--- a/src/js/hw-gen.js
+++ b/src/js/hw-gen.js
@@ -12,6 +12,7 @@ const HwGen = (() => {
   const worksheetSelectView = document.getElementById("worksheetSelectView");
   const worksheetCountSelect = document.getElementById("worksheetCount");
   const answerKeyChkbx = document.getElementById("answerKeyChkbx");
+  const borrowingChkbx = document.getElementById("borrowingChkbx");
   const printConfigView = document.getElementById("printConfigView");
   const worksheetView = document.getElementById("worksheetView");
   const categorySelectView = document.getElementById("categorySelectView");
@@ -23,28 +24,54 @@ const HwGen = (() => {
     "selectedSet": "",
     "selectedCount": 6,
     "showAnswerKey": true,
+    "onlyBorrowing": false,
   };
   let screenshotInterval = null;
   let screenshotNum = 2;
   let tipScrollInterval = null;
   let tipNum = 0;
-  const genEquation = (xSize, ySize, mathSymbol) => {
-    const x = randRangeByDigits(xSize);
-    const y = randRangeByDigits(ySize);
-    const z = solution(x, y, mathSymbol);
+  const genEquation = (xSize, ySize, mathSymbol, onlyBorrowing) => {
+    let x, y, z;
+    let attempts = 0;
+    const maxAttempts = 1000;
+
+    do {
+      x = randRangeByDigits(xSize);
+      y = randRangeByDigits(ySize);
+      z = solution(x, y, mathSymbol);
+      attempts++;
+
+      // For subtraction with borrowing constraint
+      if (mathSymbol === "+" && onlyBorrowing) {
+        // For subtraction problems, we use mathSymbol "+" but solve for y where z - x = y
+        // Check if z - x requires borrowing
+        if (requiresBorrowing(z, x)) {
+          break;
+        }
+      } else {
+        break;
+      }
+    } while (attempts < maxAttempts);
+
     return { x, y, z };
   };
-  const generate = (xSize, ySize, mathSymbol, count, useAllPossible1Digit, myGenEq) => {
+  const generate = (xSize, ySize, mathSymbol, count, useAllPossible1Digit, myGenEq, onlyBorrowing) => {
     let arr = [];
     if (useAllPossible1Digit) {
       SINGLE_DIGITS.map(x => SINGLE_DIGITS.map(y => arr.push({ x, y, z: solution(x, y, mathSymbol) })));
+
+      // Filter for borrowing if needed
+      if (onlyBorrowing && mathSymbol === "+") {
+        arr = arr.filter(eq => requiresBorrowing(eq.z, eq.x));
+      }
+
       arr.sort(() => Math.random() - 0.5); //shuffle
       arr.sort(() => Math.random() - 0.5); //shuffle
       if (count < arr.length) arr = arr.slice(0, count);
     }
     else {
       for (let i = 0; i < count; i++) {
-        arr.push(myGenEq ? myGenEq() : genEquation(xSize, ySize, mathSymbol));
+        arr.push(myGenEq ? myGenEq() : genEquation(xSize, ySize, mathSymbol, onlyBorrowing));
       }
     }
     return arr;
@@ -119,7 +146,7 @@ const HwGen = (() => {
     worksheetList.innerHTML = `<h3>Select ${data["selectedCat"]} Worksheet</h3>` +
       hwMap[data["selectedCat"]].map((hwSet, i) => {
         const {title, xSize, ySize, mathSymbol, outputFunc, count, name, long, useAllPossible1Digit, myGenEq, myGenEqList} = hwSet;
-        const eq = myGenEq ? myGenEq() : myGenEqList ? myGenEqList()[0] : genEquation(xSize, ySize, mathSymbol);
+        const eq = myGenEq ? myGenEq() : myGenEqList ? myGenEqList()[0] : genEquation(xSize, ySize, mathSymbol, false);
         const eqStr = outputFunc(eq, -1, 0, long);
         return `${i === 0 ? '' : '<hr/>'}<div class="row">
         <div class="col-1 text-right pr-0 text-sm">
@@ -152,10 +179,12 @@ const HwGen = (() => {
     }
     const { title, count, columns, xSize, ySize, mathSymbol, myGenEqList, outputFunc, answerKey, long, answerSpace, useAllPossible1Digit, myGenEq } = hwSet;
     worksheetsDiv.innerHTML = "";
+    // Only apply borrowing filter for subtraction problems
+    const onlyBorrowing = data['onlyBorrowing'] && data['selectedCat'] === 'Subtraction';
     for (let i = 0; i < data['selectedCount']; i++) {
       const worksheet = worksheetOrig.cloneNode(true)
         , output = worksheet.querySelector(".output")
-        , eqList = myGenEqList ? myGenEqList() : generate(xSize, ySize, mathSymbol, count, useAllPossible1Digit, myGenEq)
+        , eqList = myGenEqList ? myGenEqList() : generate(xSize, ySize, mathSymbol, count, useAllPossible1Digit, myGenEq, onlyBorrowing)
         , titleDiv = worksheet.querySelector(".title")
         , outputStr = eqList.map((eq, i) => outputFunc(eq, i, columns, long, answerSpace)).join("")
         , emoji = randArr(emojis)
@@ -206,6 +235,7 @@ const HwGen = (() => {
     }
     data['selectedCount'] = worksheetCountSelect ? parseInt(worksheetCountSelect.value) : 1;
     data['showAnswerKey'] = answerKeyChkbx.checked ? true : false;
+    data['onlyBorrowing'] = borrowingChkbx && borrowingChkbx.checked ? true : false;
   };
   const init = () => {
     const hwSetName = getUrlParam("set");
diff --git a/src/js/util.js b/src/js/util.js
index 293c41c..324efcd 100644
--- a/src/js/util.js
+++ b/src/js/util.js
@@ -24,6 +24,20 @@ const solution = (x, y, mathSymbol) => {
     default: return x + y;
   }
 };
+const requiresBorrowing = (minuend, subtrahend) => {
+  const minuendStr = String(minuend);
+  const subtrahendStr = String(subtrahend);
+  const maxLen = Math.max(minuendStr.length, subtrahendStr.length);
+
+  for (let i = 0; i < maxLen; i++) {
+    const minuendDigit = parseInt(minuendStr[minuendStr.length - 1 - i] || 0);
+    const subtrahendDigit = parseInt(subtrahendStr[subtrahendStr.length - 1 - i] || 0);
+    if (minuendDigit < subtrahendDigit) {
+      return true;
+    }
+  }
+  return false;
+};
 const getUrlParam = param => {
   const paramMap = {};
   const urlArr = document.location.href.split("?");