From 89a840693b7d79a46cb3e16a3bd5750096c70dff Mon Sep 17 00:00:00 2001
From: Yarchik <spoko.dev@gmail.com>
Date: Thu, 18 Jun 2026 13:15:04 +0100
Subject: [PATCH] fix: round ties towards +Infinity in roundTo to match round()

`roundTo` rounded an exact half by magnitude (half away from zero), while
`round` rounds half towards +Infinity (mirroring Math.round, as documented).
Because rounding to the nearest multiple of 1 is the same as integer
rounding, `x.round()` and `x.roundTo(1)` must agree, but they disagreed on
every negative half:

    new Fraction(-1, 2).round()      //  0
    new Fraction(-1, 2).roundTo(1)   // -1   (now 0)
    new Fraction(-3, 2).round()      // -1
    new Fraction(-3, 2).roundTo(1)   // -2   (now -1)

Apply the same tie-breaking `round` uses: only round a half up when the
value is non-negative, so the half is broken towards +Infinity regardless
of the multiple.
---
 dist/fraction.js       |  5 +++--
 dist/fraction.min.js   |  2 +-
 dist/fraction.mjs      |  5 +++--
 src/fraction.js        |  5 +++--
 tests/fraction.test.js | 19 +++++++++++++++++++
 5 files changed, 29 insertions(+), 7 deletions(-)

diff --git a/dist/fraction.js b/dist/fraction.js
index 816d5db..2de186d 100644
--- a/dist/fraction.js
+++ b/dist/fraction.js
@@ -856,9 +856,10 @@ Fraction.prototype = {
     const d = this['d'] * P['n'];
     const r = n % d;
 
-    // round(n / d) = ifloor(n / d) + 2(n % d) >= d ? 1 : 0
+    // Break ties towards +Infinity, like round(): round up on an exact half
+    // only when the value is non-negative, so roundTo(1) matches round().
     let k = ifloor(n / d);
-    if (r + r >= d) {
+    if ((this['s'] >= C_ZERO ? C_ONE : C_ZERO) + r + r > d) {
       k++;
     }
     return newFraction(this['s'] * k * P['n'], P['d']);
diff --git a/dist/fraction.min.js b/dist/fraction.min.js
index 97b02ee..15c872a 100644
--- a/dist/fraction.min.js
+++ b/dist/fraction.min.js
@@ -15,7 +15,7 @@ this.n*e.n,this.d*e.d)},div:function(a,b){q(a,b);return n(this.s*e.s*this.n*e.d,
 this.d,this.n)},pow:function(a,b){q(a,b);if(e.d===h)return e.s<g?n((this.s*this.d)**e.n,this.n**e.n):n((this.s*this.n)**e.n,this.d**e.n);if(this.s<g)return null;a=A(this.n);b=A(this.d);let d=h,c=h;for(let f in a)if("1"!==f){if("0"===f){d=g;break}a[f]*=e.n;if(a[f]%e.d===g)a[f]/=e.d;else return null;d*=BigInt(f)**a[f]}for(let f in b)if("1"!==f){b[f]*=e.n;if(b[f]%e.d===g)b[f]/=e.d;else return null;c*=BigInt(f)**b[f]}return e.s<g?n(c,d):n(d,c)},log:function(a,b){q(a,b);if(this.s<=g||e.s<=g)return null;
 var d=Object.create(null);a=A(e.n);const c=A(e.d);b=A(this.n);const f=A(this.d);for(var k in c)a[k]=(a[k]||g)-c[k];for(var l in f)b[l]=(b[l]||g)-f[l];for(var m in a)"1"!==m&&(d[m]=!0);for(var r in b)"1"!==r&&(d[r]=!0);l=k=null;for(const E in d)if(m=a[E]||g,d=b[E]||g,m===g){if(d!==g)return null}else if(r=y(d,m),d/=r,m/=r,null===k&&null===l)k=d,l=m;else if(d*l!==k*m)return null;return null!==k&&null!==l?n(k,l):null},equals:function(a,b){q(a,b);return this.s*this.n*e.d===e.s*e.n*this.d},lt:function(a,
 b){q(a,b);return this.s*this.n*e.d<e.s*e.n*this.d},lte:function(a,b){q(a,b);return this.s*this.n*e.d<=e.s*e.n*this.d},gt:function(a,b){q(a,b);return this.s*this.n*e.d>e.s*e.n*this.d},gte:function(a,b){q(a,b);return this.s*this.n*e.d>=e.s*e.n*this.d},compare:function(a,b){q(a,b);a=this.s*this.n*e.d-e.s*e.n*this.d;return(g<a)-(a<g)},ceil:function(a){a=t**BigInt(a||0);return n(u(this.s*a*this.n/this.d)+(a*this.n%this.d>g&&this.s>=g?h:g),a)},floor:function(a){a=t**BigInt(a||0);return n(u(this.s*a*this.n/
-this.d)-(a*this.n%this.d>g&&this.s<g?h:g),a)},round:function(a){a=t**BigInt(a||0);return n(u(this.s*a*this.n/this.d)+this.s*((this.s>=g?h:g)+a*this.n%this.d*p>this.d?h:g),a)},roundTo:function(a,b){q(a,b);var d=this.n*e.d;a=this.d*e.n;b=d%a;d=u(d/a);b+b>=a&&d++;return n(this.s*d*e.n,e.d)},divisible:function(a,b){q(a,b);return e.n===g?!1:this.n*e.d%(e.n*this.d)===g},valueOf:function(){return Number(this.s*this.n)/Number(this.d)},toString:function(a=15){let b=this.n,d=this.d;var c;a:{for(c=d;c%p===g;c/=
+this.d)-(a*this.n%this.d>g&&this.s<g?h:g),a)},round:function(a){a=t**BigInt(a||0);return n(u(this.s*a*this.n/this.d)+this.s*((this.s>=g?h:g)+a*this.n%this.d*p>this.d?h:g),a)},roundTo:function(a,b){q(a,b);var d=this.n*e.d;a=this.d*e.n;b=d%a;d=u(d/a);(this.s>=g?h:g)+b+b>a&&d++;return n(this.s*d*e.n,e.d)},divisible:function(a,b){q(a,b);return e.n===g?!1:this.n*e.d%(e.n*this.d)===g},valueOf:function(){return Number(this.s*this.n)/Number(this.d)},toString:function(a=15){let b=this.n,d=this.d;var c;a:{for(c=d;c%p===g;c/=
 p);for(;c%z===g;c/=z);if(c===h)c=g;else{for(var f=t%c,k=1;f!==h;k++)if(f=f*t%c,2E3<k){c=g;break a}c=BigInt(k)}}a:{f=h;k=t;var l=c;let m=h;for(;l>g;k=k*k%d,l>>=h)l&h&&(m=m*k%d);k=m;for(l=0;300>l;l++){if(f===k){f=BigInt(l);break a}f=f*t%d;k=k*t%d}f=0}k=f;f=this.s<g?"-":"";f+=u(b/d);(b=b%d*t)&&(f+=".");if(c){for(a=k;a--;)f+=u(b/d),b%=d,b*=t;f+="(";for(a=c;a--;)f+=u(b/d),b%=d,b*=t;f+=")"}else for(;b&&a--;)f+=u(b/d),b%=d,b*=t;return f},toFraction:function(a=!1){let b=this.n,d=this.d,c=this.s<g?"-":"";
 if(d===h)c+=b;else{const f=u(b/d);a&&f>g&&(c+=f,c+=" ",b%=d);c=c+b+"/"+d}return c},toLatex:function(a=!1){let b=this.n,d=this.d,c=this.s<g?"-":"";if(d===h)c+=b;else{const f=u(b/d);a&&f>g&&(c+=f,b%=d);c=c+"\\frac{"+b+"}{"+d;c+="}"}return c},toContinued:function(){let a=this.n,b=this.d;const d=[];for(;b;){d.push(u(a/b));const c=a%b;a=b;b=c}return d},simplify:function(a=.001){a=BigInt(Math.ceil(1/a));const b=this.abs(),d=b.toContinued();for(let f=1;f<d.length;f++){let k=n(d[f-1],h);for(var c=f-2;0<=
 c;c--)k=k.inverse().add(d[c]);c=k.sub(b);if(c.n*a<c.d)return k.mul(this.s)}return this}};"function"===typeof define&&define.amd?define([],function(){return v}):"object"===typeof exports?(Object.defineProperty(v,"__esModule",{value:!0}),v["default"]=v,v.Fraction=v,module.exports=v):F.Fraction=v})(this);
diff --git a/dist/fraction.mjs b/dist/fraction.mjs
index f6267c4..ee0eb3c 100644
--- a/dist/fraction.mjs
+++ b/dist/fraction.mjs
@@ -856,9 +856,10 @@ Fraction.prototype = {
     const d = this['d'] * P['n'];
     const r = n % d;
 
-    // round(n / d) = ifloor(n / d) + 2(n % d) >= d ? 1 : 0
+    // Break ties towards +Infinity, like round(): round up on an exact half
+    // only when the value is non-negative, so roundTo(1) matches round().
     let k = ifloor(n / d);
-    if (r + r >= d) {
+    if ((this['s'] >= C_ZERO ? C_ONE : C_ZERO) + r + r > d) {
       k++;
     }
     return newFraction(this['s'] * k * P['n'], P['d']);
diff --git a/src/fraction.js b/src/fraction.js
index 4292c48..06c642e 100644
--- a/src/fraction.js
+++ b/src/fraction.js
@@ -862,9 +862,10 @@ Fraction.prototype = {
     const d = this['d'] * P['n'];
     const r = n % d;
 
-    // round(n / d) = ifloor(n / d) + 2(n % d) >= d ? 1 : 0
+    // Break ties towards +Infinity, like round(): round up on an exact half
+    // only when the value is non-negative, so roundTo(1) matches round().
     let k = ifloor(n / d);
-    if (r + r >= d) {
+    if ((this['s'] >= C_ZERO ? C_ONE : C_ZERO) + r + r > d) {
       k++;
     }
     return newFraction(this['s'] * k * P['n'], P['d']);
diff --git a/tests/fraction.test.js b/tests/fraction.test.js
index b65833c..8d85edf 100644
--- a/tests/fraction.test.js
+++ b/tests/fraction.test.js
@@ -1518,6 +1518,25 @@ var tests = [{
   fn: "roundTo",
   param: "1/2",
   expect: "-3.5"
+}, {
+  // a tie must round the same way as round() (half towards +Infinity)
+  label: "-1/2 round to multiple of 1",
+  set: "-1/2",
+  fn: "roundTo",
+  param: "1",
+  expect: "0"
+}, {
+  label: "-3/2 round to multiple of 1",
+  set: "-3/2",
+  fn: "roundTo",
+  param: "1",
+  expect: "-1"
+}, {
+  label: "-1/4 round to multiple of 1/2",
+  set: "-1/4",
+  fn: "roundTo",
+  param: "1/2",
+  expect: "0"
 }, {
   label: "log_2(8)",
   set: "8",