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fix: round ties towards +Infinity in roundTo to match round() #94

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fix: round ties towards +Infinity in roundTo to match round() #94

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5 changes: 3 additions & 2 deletions dist/fraction.js
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Original file line number Diff line number Diff line change
Expand Up @@ -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']);
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2 changes: 1 addition & 1 deletion dist/fraction.min.js
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5 changes: 3 additions & 2 deletions dist/fraction.mjs
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Original file line number Diff line number Diff line change
Expand Up @@ -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']);
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5 changes: 3 additions & 2 deletions src/fraction.js
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Original file line number Diff line number Diff line change
Expand Up @@ -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']);
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19 changes: 19 additions & 0 deletions tests/fraction.test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -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",
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